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Ocean worlds exploration
Author’s Accepted Manuscript Ocean Worlds Exploration Jonathan I. Lunine
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S0094-5765(16)31151-1 http://dx.doi.org/10.1016/j.actaastro.2016.11.017 AA6081
To appear in: Acta Astronautica Received date: 4 November 2016 Accepted date: 7 November 2016 Cite this article as: Jonathan I. Lunine, Ocean Worlds Exploration, Acta Astronautica, http://dx.doi.org/10.1016/j.actaastro.2016.11.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
67th International Astronautical Congress (IAC), Guadalajara, Mexico, 26-30 September 2016. Copyright ©2016 by the International Astronautical Federation (IAF). All rights reserved.
Ocean Worlds Exploration Jonathan I. Luninea* a Department of Astronomy and Carl Sagan Institute, Cornell University, 122 Sciences Drive, Ithaca NY 14853, USA. * Corresponding Author. Abstract Ocean worlds is the label given to objects in the solar system that host stable, globe-girdling bodies of liquid water—“oceans”. Of these, the Earth is the only one to support its oceans on the surface, making it a model for habitable planets around other stars but not for habitable worlds elsewhere in the solar system. Elsewhere in the solar system, three objects—Jupiter’s moon Europa, and Saturn’s moons Enceladus and Titan—have subsurface oceans whose existence has been detected or inferred by two independent spacecraft techniques. A host of other bodies in the outer solar system are inferred by a single type of observation or by theoretical modeling to have subsurface oceans. This paper focusses on the three best-documented water oceans beyond Earth: those within Europa, Titan and Enceladus. Of these, Europa’s is closest to the surface (less than 10 km and possibly less than 1 km in places), and hence potentially best suited for eventual direct exploration. Enceladus’ ocean is deeper—5-40 km below its surface—but fractures beneath the south pole of this moon allow ice and gas from the ocean to escape to space where it has been sampled by mass spectrometers aboard the Cassini Saturn Orbiter. Titan’s ocean is the deepest—perhaps 50-100 km—and no evidence for plumes or ice volcanism exist on the surface. In terms of the search for evidence of life within these oceans, the plume of ice and gas emanating from Enceladus makes this the moon of choice for a fast-track program to search for life. If plumes exist on Europa—yet to be confirmed—or places can be located where ocean water is extruded onto the surface, then the search for life on this lunar-sized body can also be accomplished quickly by the standards of outer solar system exploration. Keywords: planetary exploration, moons, Saturn, Jupiter, exobiology 1. Introduction: The meaning of “Ocean Worlds” The post-Renaissance perception of the solar system’s planets (and our Moon) as abodes for life was driven first in the West by the Copernican concept that Earth is not unique in any astronomical sense and then by 20 th century science fiction aided by marginal and sometimes misinterpreted telescopic observations [1], [2]. This perception has largely been dashed by the past half-century of increasingly detailed astronomical observations and growing sophistication of planetary exploration. In particular, Mars has been largely relegated to a long-past abode of life, with speculations of remnant microbial organisms eking out an existence today deep within its rocky crust [3]. Exploration of the rocky and icy moons of the giant planets beginning in 1979 with Voyagers 1 and 2 at Jupiter changed this perception of a geologically and biologically inactive solar system by showing that such bodies— previously imagined to be cold, dead and geologically uninteresting—possess diverse levels of geological activity up to and including Io, the most volcanically active world in the solar system [4]. What was not understood until 1979 was the importance of tidal heating—the transformation of potential energy of an eccentric orbit into frictional heating within the body—in the multi-moon systems of the giant planets [5]. Subsequent discoveries by the Voyagers 1 and 2 in the 1980’s[6], the Galileo Jupiter Orbiter in the 1990’s [7], the Cassini Saturn Orbiter over the past 12 years and its Huygens Probe which landed on Titan in 2005 [8], the New Horizons Pluto flyby in 2015 [9], and the Dawn rendezvous with the asteroid Ceres [10] have provided evidence for liquid water oceans beneath the surfaces of multiple bodies in the solar system—from Mars, to the asteroid belt and the vast realm beyond. In a few cases the evidence comes from multiple observations of diverse types, in others a single measurement, and in many cases only theoretical assertions can be made. But in at least one instance (Saturn’s moon Enceladus), there is strong evidence that ocean material has been directly sampled by mass spectrometry in a plume of gas and ice emanating from fractures in the south polar region of that moon. The planetary science community and NASA itself have therefore begun to recognize the existence of a class of objects—“Ocean Worlds”—which are solar system bodies that definitely, provisionally, or potentially host globegirdling layers of liquid water within their interiors. Since liquid water is essential for life as we know it—life composed of various organic acids, sugars, and similar molecules—the presence of liquid water within a body makes it a candidate in the search for life. The present paper will summarize the evidence for oceans within each of the bodies currently included in the ocean worlds list as used by many planetary scientists, then focus on three objects--Jupiter’s Europa, Saturn’s Enceladus and Titan—where the evidence is strongest. It will describe what we know about these oceans and how we know it, and then move on to the prospects for determining more about their suitability for life (“habitability”)
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and testing whether life is actually present. It will be argued that a robust search for life in the solar system beyond Earth requires a program that targets multiple bodies—but not too many—and a sustained determination to do so given the long (5-10 year) flight times. Before proceeding, the reader is cautioned about a potential confusion in terminology associated with the nearcorrespondence of the term “ocean worlds” to “ocean planet” and “water world”, both of which refer to exoplanets (planets orbiting other stars) with substantial mass fractions of water in their bulk compositions. While most of the solar system objects considered in the present paper are made up of comparable amounts of rock and water ice, and are therefore “like” extrasolar water worlds in this respect, one should be careful to refer to them as ocean worlds and reserve the other two terms for objects outside our solar system. 2. Ocean worlds: which bodies, and why There is as yet no official set of solar system objects considered to be “ocean worlds”. Table 1 lists the objects currently included by many planetary scientists; figure 1 in the paper by Sherwood et al. at this conference [11] provides a pictorial representation of these objects at their approximate relative sizes, and puts them in the following categories: “Ocean relics” (Mars, Ceres), “Jovian Moons” (Europa, Ganymede, Callisto), “Saturnian Moons” (Mimas, Enceladus, Dione, Titan), and “Kuiper Belt Objects” (Triton, Pluto, Charon). Earth is implictly on this list and we will discuss aspects of its ocean in the context of describing that of Enceladus. This section examines the evidence for each ocean in turn. 2.1 Ocean relics These are bodies for which evidence exists of oceans in the past that have now disappeared. On Mars, multiple orbiters and rovers have provided remote sensing and direct chemical evidence for standing bodies of liquid water on the surface in the ancient past [12]. That water is now gone, and occasional small amounts of meltwater notwithstanding [13], the surface of Mars is largely dry. Subsurface layers of ice or permafrost have been mapped from orbit [14], but any claim of liquid water beneath the Martian surface is based on theoretical calculations only [15]. The detection of methane in the Martian atmosphere from Earth [16] has recently been supported by Curiosity rover measurements [17], but its significance for biological activity beneath the surface is uncertain. Ceres is the largest asteroid and the first to be discovered; spectral observations from the Dawn mission indicate the presence of ammonia-bearing minerals and water of hydration [18], which suggest the presence of liquid water today or at one time in the interior. That water in the interior is not present today or is deep is only weakly suggested by a lack of jets or plumes, but overall the state of water in the interior of Ceres remains poorly constrained. Figure 1: Europa’s surface with crisscrossing fractures. Galileo image about 200 x 200 km. (NASA/JPL/University of Arizona). 2.2 Jovian moons Of the four large Galilean moons that orbit Jupiter, three show evidence for internal oceans of liquid water. The most detailed and convincing is that of Europa, the lunar-sized moon that orbits between Io and Ganymede. While measurements of Europa’s density show that it is mostly made of rock, determination of the moon’s moment of inertia by the Galileo orbiter shows that Europa’s water ice surface is the top of an icy crust that extends downward some 100-200 km to meet the silicates [19]. But it is Europa’s appearance that led since Voyager to speculations of a liquid water layer beneath the ice crust, possibly the predominant phase of water below the surface. The surface geology of Europa, in contrast to many of the moons of the solar system including our own moon, shows few craters. Images from Voyager and the Galileo Jupiter orbiter show fractures crisscrossing much of the water ice surface (figure 1), and in places it appears that blocks of the crust were pulled apart and rotated in a slushy or liquid medium. Today, the entire surface is frozen, but the geology is consistent with a liquid water layer underneath the ice present today or geologically recently. Galileo spectral measurements of the surface were noisy because of the intense
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radiation environment generated by Jupiter’s magnetic field, but indications of salts in places on the surface [20] are consistent with recent extrusion of liquid water from the interior. The most compelling evidence for an ocean, one which is completely independent of geological interpretation, comes from the magnetometer measurements made by the Galileo Orbiter during five close (< 2000 km) flybys of Europa from 1996-2000. The magnetometer observed deformations in the geometry of the Jovian magnetic field consistent with the movement of a conducting body relative to the field [21]. In this case, it is Europa orbiting Jupiter as the field lines pass rapidly through the Jovian moon at the spin rate (one rotation per ~ 10 hours) of the planet. One can visualize the effect as the field “standing off” from the conducting body but what the magnetometer actually measured were perturbations to the field that could be fitted to the volume and conductivity of the conducting layer. The result was that this layer could not be at the center of Europa or deep in the silicates; it must be within kilometers of the surface with an electrical conductivity implying a salinity with a large uncertainty of two orders of magnitude (dependent on ocean depth) up to and including that of terrestrial seawater. The pressure at the base of the water layer on Europa remains within the field of normal ice I, and hence the ocean is not sandwiched between low- and high-pressure ice phases as it can be on other bodies. Therefore, the base of the ocean is at the top of the silicate interior of Europa, and one can imagine therefore interactions of water with rock akin to that in hydrothermal systems on the Earth. Because long-lived radioactive elements are expected to be present in the silicates as they are within the Earth, one source of energy for heating the interior is radioactive decay, along with residual heat trapped from the original formation of Europa from smaller bodies. But these two sources together are insufficient to maintain an ocean of the volume inferred from the Galileo data [22]; instead, energy must be extracted from the Europa’s orbit through tidal dissipation. The commensurability in orbit periods around Jupiter of Io, Europa and Ganymede (Europa’s is twice Io’s and half Ganymede’s) permits the eccentricity of Europa’s orbit to be maintained over long periods of geologic time as the tidal extraction of energy circularizes the orbit [23]. In contrast to Europa, where two separate lines of evidence support an ocean below its surface, the cases for Ganymede and Callisto each ride on a single type of observation. For Callisto, it is again the fluctuations in the Jovian magnetic field as measured by the Galileo magnetometer, but the perturbations are weaker for Callisto than for Europa [24]. Ganymede, on the other hand, shows evidence for its own magnetic field generated by an internal dynamo, likely in a metallic core. This makes more difficult the definitive measurement of an induced field, so that while evidence for one has been seen, other explanation are also possible [25]. The presence of an ocean within Ganymede has been supported by Hubble Space Telescope observations of weak auroral belts possessed by Ganymede, which are more stable in the Jovian magnetic field than would be predicted in the absence of a subsurface electric conductor that partly screens the external field [26]. Both Ganymede and Callisto have surfaces dominated by impact craters, (figure 2) and even the so-called “grooved terrain” on Ganymede indicative of geologic activity seems very ancient [27]. To support the ancient craters requires thick, cold crusts, so that their oceans are likely deep below the surface—perhaps 100 km or more. Because both objects are more massive than Europa and possess as much ice as rock, some of the ice must be in the form of the high pressure phases that are denser than liquid water. Thus their oceans are likely sandwiched between layers of ice above and below, making contact with the underlying silicate (thought to be important for life) unlikely.
Fig.2. Galileo image of much of Ganymede, showing substantial numbers of impact craters across the entire surface. (NASA/JPL). 2.3 Saturnian moons In contrast to the Jovian system, the evidence for oceans in four of the moons of Saturn is largely gravitational rather than magnetic in nature (with one exception, at Titan). The most compelling evidence for a liquid water subsurface ocean in the Saturn system, if not the solar system overall, is that for Saturn’s small moon Enceladus. Over 200 times smaller in volume than Europa, Enceladus was found by the Cassini Saturn Orbiter soon after its 2004 arrival to possess a plume of gas and ice emanating from the south polar region—a phenomenon that does not
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in itself imply a subsurface ocean [28]. The density of the moon indicates it is a mixed rock and water ice body, though with more rock than ice. Given its bright ice-rich surface, it is most likely that like Europa the silicates lie beneath the water ice crust. During three flybys from 2010 to 2012, Cassini accumulated data on the shape of Enceladus’ gravitational field using doppler tracking—measuring the change in frequency of a spacecraft’s transmitted signal to Earth caused by variations in the relative velocity of the Earth and the spacecraft as the latter’s trajectory is altered by a nearby object of interest. Part of that signal comes from deviations in the gravitational field of the body from that of a simple sphere, generated both by internal structure and the spin of the body. For Enceladus, the various contributions to this deviation are complex, but when combined with the shape of the moon derived from Cassini images the data require a subsurface lens of material that is slightly higher density than water ice, some 30-40 km under the high southern latitudes [28]. Only liquid water has the combination of the right density to lie under the ice crust and the right melting point to be stable under the expected conditions. Subsequent analysis of the doppler data including the effects of Enceladus’ rotation around Saturn and spin indicate that this liquid water ocean is global, thickest at the south pole and becoming thinner away from the southern high latitudes [29]. Given Enceladus’ small size and slightly higher amount of rock than water, the base of the ocean would be in contact with the silicate core as is the case for Europa. Enceladus on average keeps the same face toward Saturn, but because its orbit is not circular, the moon’s speed varies along the orbit. This in turn means that a given point on Enceladus does not see Saturn fixed in the sky: the satellite nods back and forth relative to a body possessed of instantaneous synchroneity between spin and orbital motion, an example of libration akin to that for the Earth’s Moon. Cassini imaging of Enceladus over a period of years revealed a libration amplitude too large to be consistent with the entire mass of the Enceladus nodding back and forth. Instead, it requires that a thin outer layer—a decoupled shell—slide back and forth over an underlying liquid [30]. Although it provides no information on composition, this result supports with an independent data set the doppler determination of a liquid water ocean beneath the ice. Cassini’s direct sampling of Enceladus’ plume, described later, confirms that the liquid is water with an admixture of salt, and other species including organics. Titan is the second-largest moon in the solar system, just slightly smaller than Ganymede, and both larger than the planet Mercury. A roughly equal mixture of rock and water ice, Titan is the only natural satellite to possess a dense atmosphere—some four times denser at the surface of Titan than is the density of air at sea level on Earth. The air is mostly nitrogen, like our own, but the very weak sunlight at the distance of Saturn dictates a frigid surface temperature—94 K at the equator even with a modest greenhouse effect—which in turn means water is frozen out of the atmosphere. In its place, methane plays the role of weathermaker: Cassini and the probe Huygens that it carried for a descent to Titan’s surface detected clouds, rain, surface dew, gulleys and river valleys [31]. Most strikingly, the Cassini Saturn Orbiter discovered lakes and seas in the high latitude regions of Titan, mostly in the north, and the radar system it carried was used to determine that the composition of these bodies of liquids is mostly methane with smaller amounts of ethane and nitrogen [32]. Several of these bodies of liquid are large; one is the size of the Caspian sea. Whether they might support an exotic type of life that could form and survive in liquid methane is an open question [33], but deep beneath the frigid surface The Cassini orbiter and Huygens Lander found evidence in two ways for a much warmer, liquid water ocean.
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Fig. 3. North polar image of Titan showing numerous lakes and several large seas. The sea near the middle of the frame is the size of Earth’s Lake Superior. (NASA/ JPL-Caltech/Space Science Institute).
The first of these came from the Huygens Probe, which was equipped with sensors for measuring the ambient electric field as the probe descended under parachute to the surface of Titan. One of the results of this experiment was the determination that the atmospheric electric field, generated in an ionosphere near the top of the atmosphere, does not go to zero at the surface. The orientation of the electric field and its behavior can be modeled as requiring a lower boundary, some 55-80 km below Titan’s surface, at which point the material transitions from a dielectric to one of conductivity exceeding that of pure liquid water [34]. Salty water or water mixed with ammonia provides good fits to the conductivity. The second, more diagnostic, detection of a subsurface ocean came from the radio science doppler tracking used as well to find an ocean on Enceladus. Here, however, the great distance of the spacecraft from Titan’s surface (1000 km) required by the moon’s thick, extended atmosphere made impractical the detection of layers of density discontinuities within the crust. Instead, the experiment sought to measure the change in shape of Titan as it moves in its eccentric orbit around Saturn. As the moon passes its periapse—closest point—to Saturn, tidal forces are strongest and will elongate it more than at the farthest point in the orbit. The difference corresponds to surface height variations of meters that cannot be detected because of the hazy atmosphere, but it also corresponds to mass distribution changes that are measurable as changes in the gravity field. The measurements indicate shape changes of sufficient magnitude to require that Titan’s ice crust be decoupled from the deep interior—hence by a liquid layer— roughly 100 km below the surface [35].
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Fig. 4: New Horizons image (approximately 75 km across) of Pluto showing blocks of water ice that appear to have been pushed by tectonic forces to make the mountains in the upper left of the image. Adjoining terrain is a snow or ice made mostly of frozen nitrogen. (NASA/JHUAPL/SwRI).
Unlike Enceladus and Titan, the remaining two Saturnian moons classified as ocean words do not have two independent detections of their subsurface oceans. Indeed, for both the evidence is tenuous. Mimas, an Enceladussized satellite closer to Saturn, exhibits a libration indicative of a decoupled shell—an ice crust sliding over a liquid layer [36]. But images of this moon show only a heavily cratered surface, and no evidence of plumes. For a small, geologically inactive moon to have a liquid water ocean seems surprising, and an alternative explanation is that the interior is strongly non-hydrostatic—that is, not relaxed to a shape determined only by gravity and its spin. Dione, further from Saturn but larger, exhibits no rotational or gravitational evidence for an ocean; instead, early Cassini magnetometer and plasma measurements hinted at a weak plume or plumes of material emanating from its interior (and that of another moon Tethys) [37], but could not be confirmed with other instruments [38]. Cassini images do suggest, however, that warm ice or liquid water have flooded parts of the surface in the recent past [39]. 2.4 Kuiper Belt objects Very little is known of the interiors of these intriguing bodies which range in size from Pluto and its near sizetwin Eris (roughly 2300 km in diameter) down to sub-100 km objects. Pluto and its moon Charon were visited by the New Horizons spacecraft in 2015 and were found to have diverse geological features suggestive of internal activity (figure 4). In the case of Pluto, various molecules like carbon monoxide and nitrogen outgassed from the interior and now exist as snow deposits on the surface [9]. Thermal models of these relatively rock-rich bodies combined with the evidence for geologic activity suggest that internal layers of liquid water could be present. Triton is a moon of Neptune on a retrograde orbit that suggests it was captured from the Kuiper Belt early in the history of the solar system. Larger than Pluto by several hundred kilometers, it possesses geysers of nitrogen imaged by Voyager 2 in 1989. While their origin has been suggested the result of solar heating of relatively transparent ice [40], it is also possible that they indicate internal activity. Hence, like Pluto, the circumstantial evidence of geology coupled with interior models [41] hint at the present of a subsurface water ocean. It will be a long time before more is known for any of these bodies. 3. What we know of the oceans of Titan, Europa, and Enceladus For only three of the worlds listed above—Enceladus, Europa, and Titan—are there multiple lines of evidence for a subsurface ocean, and information on the nature of the oceans. This section outlines the information available for each of the objects. 3.1 Titan Of the three, we know the least about the subsurface ocean within Titan. More detailed analysis of Titan’s gravity and its topography constrain the subsurface ocean to have a density slightly higher than that of pure liquid water, suggesting it is charged with salts [42], and refines the mean depth of the crust to be roughly 70km, with an upper limit of 100 km to the thickness. A very recent study of the spin of Titan—axial tilt and period—suggest an even thinner crust of 10-50 km [43], and it is as yet unclear how to reconcile these estimates of the crustal thickness. As for the base of the ocean, standard models place the ocean between the upper low pressure ice phase and denser high
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pressure ices beneath [44], although variants on this model might allow some limited contact between ocean and rocky core [45]. 3.2 Europa Relatively little is known definitively about the Europa ocean. The presence of salts on the surface [20] coupled with the magnetic field data [19] indicate that the water is salty, but the amount of salt and its composition is very poorly known. Observations using the Keck telescope in Hawaii [46] point to magnesium sulfate being present, but only on the side of Europa facing Io, which might supply both the sulfur and the magnesium as an external coating. We do not know whether the ocean contains organic molecules, or nitrogen-bearing compounds—both crucial to life, exist in the ocean: the Galileo instruments were not capable of detecting small-scale deposits of such compounds.
Fig. 5. Jets of ice are seen emanating from fractures in the south polar region of Enceladus in an image from the Cassini Saturn Orbiter. (NASA/JPL/Space Science Institute). The thickness of Europa’s crust is also poorly known and may be spatially highly variable. Parts of Europa exhibit features, including but not limited to the so-called “cycloidal ridges” that suggest a thin crust in those locations—perhaps a kilometer or less [47]. Other regions have features indicative of a much thicker crust. In particular, impact craters seen on Europa require ice crusts many tens of kilometers thick in order to support their topography [48]. 3.3 Enceladus The last of the three oceans to be discovered, more is known about Enceladus’ than the others thanks to direct sampling of the south polar plume by instruments aboard the Cassini Saturn Orbiter and supporting remote sensing observations. The gas mass spectrometer called the Ion Neutral Mass Spectrometer (INMS), has detected water vapor, nitrogen-bearing molecules and carbon-bearing molecules including organics (those containing carbon and hydrogen) [49]. The Cosmic Dust Analyzer (CDA), a mass spectrometer optimized to analyze ice grains lofted in the plume, has found these contain sodium and potassium salts—including sodium chloride-- with abundances in the 0.5-2% range relative to water for the largest grains [50]. The amount of salt in the largest grains and the known solubility of salt in liquid water and ice strongly argue for these being frozen droplets of seawater. CDA also detected a population of very tiny silica grains—SiO2—whose size and composition point to circulation of water inside warm rock at the base of the ocean [51]. Analysis of the abundances of the various elements detected by INMS and CDA point to an alkaline hydrothermal system at the base of the ocean akin to the “low temperature off axis hydrothermal systems” seen in several seafloor locations on Earth [52]. Remote sensing data relevant to the ocean includes Cassini imaging system observation of jet positions and angles along the fracture zones from which they are emitted (figure 5) [53], infrared observations showing the amount of power radiated as heat from the active regions [54, 55], and supporting ultraviolet observations confirming the presence of water vapor in the plume [56]. In sum, the evidence is extremely compelling that much if not all of the material sampled in the plume of Enceladus comes directly or nearly so from the subsurface ocean, and that the ocean is habitable in the sense of having organic molecules, salts, sources of heat and circulation within the underlying rock. That Cassini was able to follow up on its own discovery of the plume by analyzing its nature and inferring the habitability of Enceladus’ ocean was the fortuitous result of carrying a large instrument payload and two mass spectrometers intended to observe other targets in the Saturn system. 4. The next steps in exploring Titan, Europa and Enceladus
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4.1 Titan The primary issue for the analysis of Titan’s subsurface ocean is access to ocean material. With the ocean tens of kilometers below the surface, and a density at or exceeding that of liquid water which is denser than ice I, extrusion of liquid to the surface is difficult. Geologic evidence of such resurfacing is present in a few places on Titan’s surface based on Cassini Saturn orbiter data [57]. The right strategy for identifying sites from orbit through the thick atmospheric haze, landing with sufficient precision, and sampling underneath the pervasive blanket of deposits from sedimenting hazes, have yet to be fully developed. There is also competition between this type of mission and one that would land on and float across a surface hydrocarbon sea [58], and others that might survey large areas of the surface at high resolution from balloon or aircraft [59].
4.2 Europa Efforts in the scientific community to mount a mission to orbit Europa began shortly after repeated Galileo magnetometer measurements indicated the presence of an ocean [60]. The general science goals of such a mission have been to confirm the presence of an ocean and measure its properties, to determine the thickness of the crust and whether ocean material is extruded onto the surface, and to identify sites where one might safely land and sample ocean material. Any such mission is technically very challenging because of the strong radiation environment generated by Jupiter’s powerful magnetic field. However, more daunting has been the political and fiscal environment over the past nearly two decades, which delayed a start for such a mission until mandated by Congress last year. Since then, NASA has competitively selected an instrument suite and the mission, variously called the Europa Clipper or Europa Mission, is in Phase A development leading to a launch in the 2020’s. To reduce the radiation load on the instruments, and hence cost, the mission will make repeated flybys of Europa from Jovian orbit, rather than orbit Europa itself. Conducting flybys will reduce somewhat the science, particularly geodetic measurements, but the vast majority of science objectives associated with the goals outlined above can be achieved.
Fig. 6. Schematic cross section of the Europan crust and ocean with Europa Mission instruments (see text) shown in rough relation to their targets. RMS group is the radiation monitoring systems. Background image: NASA/JPLCaltech. The Mass Spectrometer for Planetary Exploration/Europa (MASPEX) and Surface Dust Mass Analyzer (SUDA) are mass spectrometers of advanced design compared to those on Cassini, able to sample material over a greater mass range and with superior sensitivity and resolution. They will sample plumes should such be present on Europa, but can also detect molecules and ions evaporated and sputtered off the surface. Imaging and near infrared spectroscopy of the surface will be provided by the Europa Imaging System (EIS) and Mapping Imaging Spectrometer for Europa (MISE), while the Ultraviolet Spectrograph/Europa (UVS) determines the density and composition of the atmosphere and any plumes that might be detected. Below the surface, the Europa Thermal Emission Imaging System (E-THEMIS) detects hot spots associated with a thin crust or buoyant warm upwelling ice or liquid, while the Radar for Europa Assessment and Sounding Ocean to Near Surface (REASON) probes the structure, thickness and electrical properties of the ice crust. Working together, the Interior Characterization of Europa using Magnetometry (ICEMAG) magnetometer and the Plasma Instrument for Magnetic Sounding (PIMS) provide a much more sensitive determination of the salinity of the ocean than did Galileo, by removing effects of the external magnetic field and plasmas.
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The instrument package has the potential for characterizing the habitability of the Europan ocean to a more detailed extent than Cassini did for Enceladus, particularly if plumes are present. It will also map out where organic deposits may be present, and determine whether oxidants are fed to the Europa ocean as has been speculated since the results of the Galileo mission [61]. Although the Europa Mission is preparatory to a mission to land on Europa and attempt to access ocean material, in its Consolidated Appropriations Act of 2016 Congress directed NASA to initiate designs for a lander mission. The current concept is one that could be launched contemporaneously with the flyby mission on a separate launch vehicle, and then land on the surface to sample ocean deposits after a suitable landing site has been found by the Clipper payload. A Science Definition Team chartered by NASA is examining science objectives and payload options at the time of this writing. 4.3 Enceladus Cassini completed its last plume passage of Enceladus, and final close flyby, October 21, 2015, leaving open what the next steps in Enceladus exploration ought to be. The determination that the ocean satisfies the basic criteria for “habitability” (the ability to support life) has focussed attention on missions that will further probe the composition of the ocean and even search for evidence of living organisms. Two detailed proposals for future exploration of Enceladus have been written: a plume sample return mission, called the Life Investigation for Enceladus (LIFE) [62], and Enceladus Life Finder (ELF), a mission proposed unsuccessfully to NASA’s Discovery program to do in situ analysis of the plume with mass spectrometers [63]. A third mission, to bring a Clipper-like payload to orbit Enceladus, was studied at a JPL summer school in 2015 [64]. The ELF concept builds on the success of two spaceborne mass spectrometers on Cassini in characterizing details of the subsurface ocean properties (salinity, general composition, pH, contact with hot rock) that are essential to establishing the habitability of the ocean, even though those instruments are neither state-of-the-art nor designed specifically to analyse the plume. Two state-of-the-art mass spectrometers (one for gas, one for dust) using the timeof-flight technique to provide orders of magnitude better mass resolution and sensitivity, and an order of magnitude better mass range, can seek the molecular signature of active biological processes while better constraining the chemical and physical properties of the ocean. The spacecraft need do nothing other than what Cassini successfully did seven times, which was to fly through the plume of Enceladus (five flythroughs are needed to complete the ELF objectives). The proposed mission was cited by a NASA panel as “a pioneering example for future missions to other planetary bodies with known sources of water.” The LIFE sample return concept allows plume material to be analysed in terrestrial laboratories, along with a limited amount of in situ sampling. Unknowns for this mission include the technologies required to preserve delicate organic traces of life for the return journey to Earth (very low temperatures and low acceleration), and the unknown costs of ensuring no accidental release of biologically viable material from the sample during entry and landing on Earth. Managing public perceptions in regard to the latter is also an issue. These and more ambitious mission to enter the ocean directly are not mutually exclusive; instead they potentially form the elements of an Enceladus exploration program that seeks habitability and life through progressively more complex missions, with off-ramps and alternative pathways provided to take account of intermediate results [65]. 5. Discussion: why an Ocean Worlds program? The description of potential opportunities for furthering our understanding of the nature and habitability of oceans in the outer solar system, and indeed looking for life in those environments, was organized object by object. While it is tempting to focus on a single object and throw as many resources as possible there before moving on to others, a multi-object ocean worlds program carries with it several advantages: (i). Interleaved missions allow lessons learned to be applied from one object to another: the experience of Cassini at Enceladus has greatly informed the next step in the exploration of Europa, including details of the types of instruments to be carried and what signatures to look for. Technologies and instruments developed for one body may be useful on another. (ii). Interleaved missions make better use of fiscal and technical resources: Trip times to Europa are 5 years and to Enceladus/Titan 10 years on medium-class launch vehicles; investment in heavy lift vehicles to shorten trip times is possible but speculative. With the long trip times to the outer solar system, waiting for the results of one mission at one object in order to plan the next mission leaves long gaps in a scientific campaign to look for habitability and life. Interleaving missions among three bodies (at two giant planets) allows a leapfrog approach where new discoveries are always being made at one ocean world while plans for future efforts are being brought to fruition at others. It also allows for a more level funding profile and hence the possibility of a more stable program.
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(iii) A program hedges the bet: The disappointing results from Viking with respect to life on Mars had a strongly negative effect on the program because there was no plan B for what to do next. With multiple ocean worlds, strongly negative results on one target simply means that it is time to refocus on the remaining one or ones. At the same time, because of persistent limitations in planetary science funding, goals will generally take longer to accomplish than expected. The Europa Orbiter mission originally proposed in 1999 is only now under development in the guise of the Europa Multiple-flyby Mission. There will be only a handful of missions to the outer solar system over the next two decades. This argues for the number of targets in an Ocean Worlds Program to be limited to the most promising, which means Enceladus, Europa and Titan. However, international collaboration can help to extend our reach to a broader class of targets; the European Space Agency’s Jupiter Icy Moons Explorer (JUICE) will focus on Ganymede and Callisto, with a more cursory look at Europa [66]. 6. Conclusions A remarkable two-decade robotic campaign in the outer solar system has led to the identification of multiple worlds with liquid water oceans in their interiors. Of these, Europa, Enceladus and Titan have oceans detected in multiple independent ways, and for Enceladus the properties of the ocean are known to a substantial degree thanks to ice and gas derived from the ocean jetting from its surface into space. The pliancy of the outer solar system in yielding up information on environments that might support life rejuvenates the search for extant microorganisms in our own solar system that once stopped at Mars. While their distance is a disadvantage in terms of long trip times, it is an advantage in ensuring that any life we might find had its origin there, separate from the origin of life on Earth. Should we be successful, it would bolster the view that we may be only one of countless inhabited worlds in the cosmos—propelling us onward to search for our equals among the stars. Acknowledgements The author was generously supported in the preparation of this paper as the David Baltimore Distinguished Visiting Scientist at the Jet Propulsion Laboratory, managed for NASA by the California Institute of Technology. References [1] K. Guthkie, The Last Frontier, Cornell University Press, Ithaca, 1990. [2] P. Lowell, Mars as an Abode for Life, Macmillan, New York, 1908. [3] M.D. Max, S.M. Clifford, The state, potential distribution, and biological implications of methane in the Martian crust, J. Geophys. Res. Planets 105(2000) 4165-4171. [4] J.R. Spencer and N.M. 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[17] C. Webster, P.R. Mahaffy, S. Atreya, G. Flesch, M. Mischna, P-Y Meslin, K. Farley, P. Conrad, L. Christensen, A. Pavlov, L. Martin-Torres, M-P Zorzano, T. McConnochie, T. Owen, J. Eigenbrode, D. Glavin, A. Steele, C. Malespin, P.D. Archer, B. Sutter, P. Coll, C. Freissinet, C.P. McKay, J. Moores, S. Schwenzer, J. Bridges, R. Navarro-Gonzalez, R. Gellert, M. Lemmon, MSL Team, Mars methane detection and variability at Gale crater, Science 347 (2015), 415-417. [18] E. Ammannito, M.C. De Sanctis, M. Ciarnello, A. A. Frigeri, F.G. Carrozzo, J-B. Combe, B. Ehlmann, S. Marchi, H.Y. McSween, A. Raponi, M. Toplis, F. Tosi, J. Castillo-Rogez, F. Capaccioni, M.T. Capria, S. Fonte, M. Giardino, R. Jauman, A. Longobardo, S.P. Joy, G. Magni, T.B. McCord, L.A. McFadden, E. Palomba, C.M. Pieters, C. Polansky, M.D. Raymond, C.A. Raymond, P. Schenk, F. Zambon, C.T. Russell, Distribution of phyllosilicates on the surface of Ceres, Science 353 (2016) aaf4279. [19] F. Nimmo and M. Manga, Geodynamics of Europa’s icy shell, in: R. Pappalardo, W.B. McKinnon and K. Khurana (Eds.), Europa, Univ. of Arizona Press, Tucson, 2009, pp. 381-404. [20] J. B. Dalton, O. Prieto-Ballesteros, J.S. Kargel, R. Quinn, Spectral comparison of heavily hydrated salts with disrupted terrains on Europa, Icarus 177 (2005) 472-490. [21] K. K. Khurana, M.G. Kivelson, K.P. Hand, C.T. Russell, Electromagnetic induction from Europa’s ocean and the deep interior, in: R. Pappalardo, W.B. McKinnon and K. Khurana (Eds.), Europa, Univ. of Arizona Press, Tucson, 2009, pp. 571-588. [22]G. Schubert, F. Sohl, H. Hussman, Interior of Europa, in: R. Pappalardo, W.B. McKinnon and K. Khurana (Eds.), Europa, Univ. of Arizona Press, Tucson, 2009, pp. 353-368. [23]C. Sotin, G. Tobie, J. Wahr, W.B. McKinnon, Tides and tidal heating on Europa, in: R. Pappalardo, W.B. McKinnon and K. Khurana (Eds.), Europa, Univ. of Arizona Press, Tucson, 2009, pp. 85-118. [24] K.K. Khurana, M.G. Kivelson, D.J. Stevenson, G. Schubert, C.Russell, R.J. Walker, C. Polansky, Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto, Nature 395 (1998) 777-80. [25] M. G. Kivelson, K.K. Kurana, M. Volwerk, The permanent and inductive magnetic moments of Ganymede, Icarus 157 (2002) 507-522. [26] J. Sauer, S. Duling, L. Roth, X. Jia, D.F. Strobel, P. Feldman, U.R., Christensen, K.D., Retherford, M.A., McGrath, F. Musacchio, A. Wennmacher, F.M. Neubauer, S., Simon, O. Hartkorn, The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals, JGR Space Physics 120 (2015),1715-1737. [27] S.W. Kieffer, X. Lu, C.M. Bethke, J.R. Spencer, S. Marshak, A. Navrotsky, A clathrate reservoir hypothesis for Enceladus’ south polar plume, Science 314 (2006), 1764-1766. [28] L. Iess, D.J. Stevenson, M. Parisi, D. Hemingway, R. A. Jacobson, J. Lunine, F. Nimmo, J.Armstrong, S.Asmar,M.Ducci, P Tortora, Gravity field and interior structure of Enceladus, Science 344 (2014) 78-80. [29] W.B. McKinnon, Effect of Enceladus’ rapid synchronous spin on interpretation of Cassini gravity, Geophysical Research Letters 42 (2015) 2137-2143.. [30] P.C. Thomas, R. Tajeddine, M.S. Tiscareno, J.A. Burns, J. Joseph, T.J. Loredo, P. Helfenstein, C. Porco, Enceladus’ measured physical libration requires a global subsurface ocean, Icarus 264 (2016) 37-47. [31] J.I. Lunine, R.L. Lorenz, Rivers, lakes dunes and rain: crustal processes in Titan’s methane cycle, Ann. Rev Earth Planet. Sci. 37 (2009) 299-320. . [32] A.G. Hayes, The lakes and seas of Titan, Ann. Rev. Earth Planet. Sci. 44 ( 2016) 57-83. [33] J. Lunine, Proc. Am. Phil. Soc. 153 (2008) 403-18. [34] C. Béghin, O. Randriamboarison, M. Hamelin, E. Karkoschka, R.C. Whitten, J.J. Berthelier, R. Grard, F. Simões, Analytic theory of Titan’s Schumann resonance: Constraints on ionospheric conductivity and buried water ocean, Icarus 218 (2012) 1028-1042. [35] l. Iess, R.A. Jacobson, M. Ducci, D.J. Stevenson, J.I. Lunine, J.W. Armstrong, S.W. Asmar, P. Racioppa, N.J. Rappaport, P. Tortora, The tides of Titan, Science 337 (2012) 457-459. [36] R. Tajeddine, N. Rambaux, V. Lainey, S. Charnoz, A. Richard, A. Rivoldini, B. Noyelles, Contraints on Mimas’ interior from Cassini ISS libration measurements, Science 346 (2014) 322-324. [37] J.L. Burch, J. Goldstein, W.S. Lewis, D.T. Young, A.J. Coats, M.K. Dougherty, N. Andre, Tethus and Dione as sources of outward-flowing plasma in Saturn’s magnetosphere, Nature 447 (2007) 833-835. [38] B. Buratti, S.P. Faulk, J. Mosher, K.H. Baines, R.H. Brown, R.N. Clark, P.D. Nicholson, Search for and limits on plume activity on Mimas, Tethys, and Dione with the Cassini Visual Infrared Mapping Spectrometer (VIMS), Icarus 214 (2011), 534-540. [39] T. Roatsch, M. Wahlisch, A. Hoffmeister, K.-D Matz, F. Scholten, E. Kersten, R. Wagner, T. Denk, G. Neukum, C. Porco, High-resolution Dione atlas derived from Cassini-ISS images.
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[40] L. A. Soderblom, S. Kieffer, T. Becker, R. Brown, A. Cook 2nd, C. Hansen, T. Johnson, R. Kirk, E.M. Shoemaker, Triton’s geyser-like plumes: Discovery and basic characterization, Science 250 (1990) 410-415. [41] G. Robuchon and F. Nimmo, Thermal evolution of Pluto and implications for surface tectonics and a subsurface ocean, Icarus 216 (2011) 426–439. [42] G. Mitri, R. Meriggiola, A. Hayes, A. Lefevre, G. Tobie, A. Genova, J.I. Lunine, H. Zebker, Shape, topography, gravity anomalies and tidal deformation of Titan, Icarus 236 (2014) 169-177. [43] R. Meriggiola, L. Iess, B.W. Stiles, J.I. Lunine, G. Mitri, The rotational dynamics of Titan from Cassini RADAR images, Icarus 275 (2016) 183-192. [44] G. Tobie, J.I. Lunine, J. Monteux, O. Mousis, F. Nimmo, The origin and evolution of Titan, in: G. MüllerWodarg, E. Lellouch & T. Cravens (Eds.), Titan: Interior, Surface, Atmosphere and Space Environment. Cambridge Univ. Press, Cambridge, 2013, pp. 24-55. [45] J.C. Castillo-Rogez, J.I. 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Srama, A salt-water reservoir as the source of a compositionally stratified plume on Enceladus Nature 474 (2011) 620-622. [51] H-W. Hsu, F. Postberg, Y. Sekine, T. Shibuya, S. Kempf, M. Horanyi, A. Juhasz, N. Altobelli, K. Suzuki, Y. Masaki, T. Kuwatani, S. Tachibana, S-I Sirono, G. Moragas-Klostermeyer, R. Srama, Ongoing hydro- thermal activities within Enceladus. Nature 519 (2015) 207-210. [52] C. Glein, J. Barross, H.W. Waite, The pH of Enceladus’ ocean, Geochim. Cosmochim Acta. 162 (2015) 202-219. [53] P. Helfenstein, C.C. Porco, Enceladus’ geysers: Relation to geologic features, Astron. J. 150 (2016) 96. [54] C.J.A. Howett, J.R. Spencer, J. Pearl, M. Segura, High heat flow from Enceladus’ south polar region measured using 10-600 cm-1Cassini/CIRS data, J. Geophys. Res. Planets, 116 (2011) E03003. [55] J.D. Goguen, B.J. Buratti, R.H. Brown, R.H.,R.N. Clark, P.D. Nicholson, M.M. Hedman, R.R., Howell, C. Sotin, D.P. Cruikshank, K.H. Baines, K.J. Lawrence, J.R. Spencer, D.G. 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[64] S.M. MacKenzie, T.E. Caswell, C.M., Phillips-Lander, E.N. Stavros, J.D., Hofgartner, V.Z. Sun, K.E. Powell, C.J. , Steuer, J.G. O'Rourke, J.K. Dhaliwal, C.W.S. Leung, E.M. Petro, J.J. Wynne, S. Phan,, M. Crismani, A. Krishnamurthy, K.K. John, K. DeBruin, C.J. Budney, K.L. Mitchell, THEO concept mission: Testing the habitability of Enceladus's ocean, Advances in Space Research 58 (2016) 1117-1137. [65] B. Sherwood, Strategic map for exploring the ocean-world Enceladus, Acta Astronautica 126 (2016) 52-58. [66] ESA Science and Technology JUICE site, http://sci.esa.int/juice/.
Table 1 Categorization of “ocean world” solar system bodies other than the Earth, in terms of whether the ocean exists today and how many lines of evidence are present.
Name Mars Ceres Europa Ganym ede Callisto Mimas Encela dus Dione Titan Triton Pluto Charon
R Exta elic nt 2 ✔ ✔ ✔
Exta nt 1
The ory
✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ (✔ )
Notes: Relic means ocean was present in the past and has left evidence; extant (2,1) means exists based on (two, one) line(s) of evidence; theory means inferred primarily from theoretical studies. The possibility of an ocean within Charon is at present highly speculative.
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To appear in: Acta Astronautica Received date: 4 November 2016 Accepted date: 7 November 2016 Cite this article as: Jonathan I. Lunine, Ocean Worlds Exploration, Acta Astronautica, http://dx.doi.org/10.1016/j.actaastro.2016.11.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Ocean Worlds Exploration Jonathan I. Luninea* a Department of Astronomy and Carl Sagan Institute, Cornell University, 122 Sciences Drive, Ithaca NY 14853, USA. * Corresponding Author. Abstract Ocean worlds is the label given to objects in the solar system that host stable, globe-girdling bodies of liquid water—“oceans”. Of these, the Earth is the only one to support its oceans on the surface, making it a model for habitable planets around other stars but not for habitable worlds elsewhere in the solar system. Elsewhere in the solar system, three objects—Jupiter’s moon Europa, and Saturn’s moons Enceladus and Titan—have subsurface oceans whose existence has been detected or inferred by two independent spacecraft techniques. A host of other bodies in the outer solar system are inferred by a single type of observation or by theoretical modeling to have subsurface oceans. This paper focusses on the three best-documented water oceans beyond Earth: those within Europa, Titan and Enceladus. Of these, Europa’s is closest to the surface (less than 10 km and possibly less than 1 km in places), and hence potentially best suited for eventual direct exploration. Enceladus’ ocean is deeper—5-40 km below its surface—but fractures beneath the south pole of this moon allow ice and gas from the ocean to escape to space where it has been sampled by mass spectrometers aboard the Cassini Saturn Orbiter. Titan’s ocean is the deepest—perhaps 50-100 km—and no evidence for plumes or ice volcanism exist on the surface. In terms of the search for evidence of life within these oceans, the plume of ice and gas emanating from Enceladus makes this the moon of choice for a fast-track program to search for life. If plumes exist on Europa—yet to be confirmed—or places can be located where ocean water is extruded onto the surface, then the search for life on this lunar-sized body can also be accomplished quickly by the standards of outer solar system exploration. Keywords: planetary exploration, moons, Saturn, Jupiter, exobiology 1. Introduction: The meaning of “Ocean Worlds” The post-Renaissance perception of the solar system’s planets (and our Moon) as abodes for life was driven first in the West by the Copernican concept that Earth is not unique in any astronomical sense and then by 20 th century science fiction aided by marginal and sometimes misinterpreted telescopic observations [1], [2]. This perception has largely been dashed by the past half-century of increasingly detailed astronomical observations and growing sophistication of planetary exploration. In particular, Mars has been largely relegated to a long-past abode of life, with speculations of remnant microbial organisms eking out an existence today deep within its rocky crust [3]. Exploration of the rocky and icy moons of the giant planets beginning in 1979 with Voyagers 1 and 2 at Jupiter changed this perception of a geologically and biologically inactive solar system by showing that such bodies— previously imagined to be cold, dead and geologically uninteresting—possess diverse levels of geological activity up to and including Io, the most volcanically active world in the solar system [4]. What was not understood until 1979 was the importance of tidal heating—the transformation of potential energy of an eccentric orbit into frictional heating within the body—in the multi-moon systems of the giant planets [5]. Subsequent discoveries by the Voyagers 1 and 2 in the 1980’s[6], the Galileo Jupiter Orbiter in the 1990’s [7], the Cassini Saturn Orbiter over the past 12 years and its Huygens Probe which landed on Titan in 2005 [8], the New Horizons Pluto flyby in 2015 [9], and the Dawn rendezvous with the asteroid Ceres [10] have provided evidence for liquid water oceans beneath the surfaces of multiple bodies in the solar system—from Mars, to the asteroid belt and the vast realm beyond. In a few cases the evidence comes from multiple observations of diverse types, in others a single measurement, and in many cases only theoretical assertions can be made. But in at least one instance (Saturn’s moon Enceladus), there is strong evidence that ocean material has been directly sampled by mass spectrometry in a plume of gas and ice emanating from fractures in the south polar region of that moon. The planetary science community and NASA itself have therefore begun to recognize the existence of a class of objects—“Ocean Worlds”—which are solar system bodies that definitely, provisionally, or potentially host globegirdling layers of liquid water within their interiors. Since liquid water is essential for life as we know it—life composed of various organic acids, sugars, and similar molecules—the presence of liquid water within a body makes it a candidate in the search for life. The present paper will summarize the evidence for oceans within each of the bodies currently included in the ocean worlds list as used by many planetary scientists, then focus on three objects--Jupiter’s Europa, Saturn’s Enceladus and Titan—where the evidence is strongest. It will describe what we know about these oceans and how we know it, and then move on to the prospects for determining more about their suitability for life (“habitability”)
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and testing whether life is actually present. It will be argued that a robust search for life in the solar system beyond Earth requires a program that targets multiple bodies—but not too many—and a sustained determination to do so given the long (5-10 year) flight times. Before proceeding, the reader is cautioned about a potential confusion in terminology associated with the nearcorrespondence of the term “ocean worlds” to “ocean planet” and “water world”, both of which refer to exoplanets (planets orbiting other stars) with substantial mass fractions of water in their bulk compositions. While most of the solar system objects considered in the present paper are made up of comparable amounts of rock and water ice, and are therefore “like” extrasolar water worlds in this respect, one should be careful to refer to them as ocean worlds and reserve the other two terms for objects outside our solar system. 2. Ocean worlds: which bodies, and why There is as yet no official set of solar system objects considered to be “ocean worlds”. Table 1 lists the objects currently included by many planetary scientists; figure 1 in the paper by Sherwood et al. at this conference [11] provides a pictorial representation of these objects at their approximate relative sizes, and puts them in the following categories: “Ocean relics” (Mars, Ceres), “Jovian Moons” (Europa, Ganymede, Callisto), “Saturnian Moons” (Mimas, Enceladus, Dione, Titan), and “Kuiper Belt Objects” (Triton, Pluto, Charon). Earth is implictly on this list and we will discuss aspects of its ocean in the context of describing that of Enceladus. This section examines the evidence for each ocean in turn. 2.1 Ocean relics These are bodies for which evidence exists of oceans in the past that have now disappeared. On Mars, multiple orbiters and rovers have provided remote sensing and direct chemical evidence for standing bodies of liquid water on the surface in the ancient past [12]. That water is now gone, and occasional small amounts of meltwater notwithstanding [13], the surface of Mars is largely dry. Subsurface layers of ice or permafrost have been mapped from orbit [14], but any claim of liquid water beneath the Martian surface is based on theoretical calculations only [15]. The detection of methane in the Martian atmosphere from Earth [16] has recently been supported by Curiosity rover measurements [17], but its significance for biological activity beneath the surface is uncertain. Ceres is the largest asteroid and the first to be discovered; spectral observations from the Dawn mission indicate the presence of ammonia-bearing minerals and water of hydration [18], which suggest the presence of liquid water today or at one time in the interior. That water in the interior is not present today or is deep is only weakly suggested by a lack of jets or plumes, but overall the state of water in the interior of Ceres remains poorly constrained. Figure 1: Europa’s surface with crisscrossing fractures. Galileo image about 200 x 200 km. (NASA/JPL/University of Arizona). 2.2 Jovian moons Of the four large Galilean moons that orbit Jupiter, three show evidence for internal oceans of liquid water. The most detailed and convincing is that of Europa, the lunar-sized moon that orbits between Io and Ganymede. While measurements of Europa’s density show that it is mostly made of rock, determination of the moon’s moment of inertia by the Galileo orbiter shows that Europa’s water ice surface is the top of an icy crust that extends downward some 100-200 km to meet the silicates [19]. But it is Europa’s appearance that led since Voyager to speculations of a liquid water layer beneath the ice crust, possibly the predominant phase of water below the surface. The surface geology of Europa, in contrast to many of the moons of the solar system including our own moon, shows few craters. Images from Voyager and the Galileo Jupiter orbiter show fractures crisscrossing much of the water ice surface (figure 1), and in places it appears that blocks of the crust were pulled apart and rotated in a slushy or liquid medium. Today, the entire surface is frozen, but the geology is consistent with a liquid water layer underneath the ice present today or geologically recently. Galileo spectral measurements of the surface were noisy because of the intense
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radiation environment generated by Jupiter’s magnetic field, but indications of salts in places on the surface [20] are consistent with recent extrusion of liquid water from the interior. The most compelling evidence for an ocean, one which is completely independent of geological interpretation, comes from the magnetometer measurements made by the Galileo Orbiter during five close (< 2000 km) flybys of Europa from 1996-2000. The magnetometer observed deformations in the geometry of the Jovian magnetic field consistent with the movement of a conducting body relative to the field [21]. In this case, it is Europa orbiting Jupiter as the field lines pass rapidly through the Jovian moon at the spin rate (one rotation per ~ 10 hours) of the planet. One can visualize the effect as the field “standing off” from the conducting body but what the magnetometer actually measured were perturbations to the field that could be fitted to the volume and conductivity of the conducting layer. The result was that this layer could not be at the center of Europa or deep in the silicates; it must be within kilometers of the surface with an electrical conductivity implying a salinity with a large uncertainty of two orders of magnitude (dependent on ocean depth) up to and including that of terrestrial seawater. The pressure at the base of the water layer on Europa remains within the field of normal ice I, and hence the ocean is not sandwiched between low- and high-pressure ice phases as it can be on other bodies. Therefore, the base of the ocean is at the top of the silicate interior of Europa, and one can imagine therefore interactions of water with rock akin to that in hydrothermal systems on the Earth. Because long-lived radioactive elements are expected to be present in the silicates as they are within the Earth, one source of energy for heating the interior is radioactive decay, along with residual heat trapped from the original formation of Europa from smaller bodies. But these two sources together are insufficient to maintain an ocean of the volume inferred from the Galileo data [22]; instead, energy must be extracted from the Europa’s orbit through tidal dissipation. The commensurability in orbit periods around Jupiter of Io, Europa and Ganymede (Europa’s is twice Io’s and half Ganymede’s) permits the eccentricity of Europa’s orbit to be maintained over long periods of geologic time as the tidal extraction of energy circularizes the orbit [23]. In contrast to Europa, where two separate lines of evidence support an ocean below its surface, the cases for Ganymede and Callisto each ride on a single type of observation. For Callisto, it is again the fluctuations in the Jovian magnetic field as measured by the Galileo magnetometer, but the perturbations are weaker for Callisto than for Europa [24]. Ganymede, on the other hand, shows evidence for its own magnetic field generated by an internal dynamo, likely in a metallic core. This makes more difficult the definitive measurement of an induced field, so that while evidence for one has been seen, other explanation are also possible [25]. The presence of an ocean within Ganymede has been supported by Hubble Space Telescope observations of weak auroral belts possessed by Ganymede, which are more stable in the Jovian magnetic field than would be predicted in the absence of a subsurface electric conductor that partly screens the external field [26]. Both Ganymede and Callisto have surfaces dominated by impact craters, (figure 2) and even the so-called “grooved terrain” on Ganymede indicative of geologic activity seems very ancient [27]. To support the ancient craters requires thick, cold crusts, so that their oceans are likely deep below the surface—perhaps 100 km or more. Because both objects are more massive than Europa and possess as much ice as rock, some of the ice must be in the form of the high pressure phases that are denser than liquid water. Thus their oceans are likely sandwiched between layers of ice above and below, making contact with the underlying silicate (thought to be important for life) unlikely.
Fig.2. Galileo image of much of Ganymede, showing substantial numbers of impact craters across the entire surface. (NASA/JPL). 2.3 Saturnian moons In contrast to the Jovian system, the evidence for oceans in four of the moons of Saturn is largely gravitational rather than magnetic in nature (with one exception, at Titan). The most compelling evidence for a liquid water subsurface ocean in the Saturn system, if not the solar system overall, is that for Saturn’s small moon Enceladus. Over 200 times smaller in volume than Europa, Enceladus was found by the Cassini Saturn Orbiter soon after its 2004 arrival to possess a plume of gas and ice emanating from the south polar region—a phenomenon that does not
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in itself imply a subsurface ocean [28]. The density of the moon indicates it is a mixed rock and water ice body, though with more rock than ice. Given its bright ice-rich surface, it is most likely that like Europa the silicates lie beneath the water ice crust. During three flybys from 2010 to 2012, Cassini accumulated data on the shape of Enceladus’ gravitational field using doppler tracking—measuring the change in frequency of a spacecraft’s transmitted signal to Earth caused by variations in the relative velocity of the Earth and the spacecraft as the latter’s trajectory is altered by a nearby object of interest. Part of that signal comes from deviations in the gravitational field of the body from that of a simple sphere, generated both by internal structure and the spin of the body. For Enceladus, the various contributions to this deviation are complex, but when combined with the shape of the moon derived from Cassini images the data require a subsurface lens of material that is slightly higher density than water ice, some 30-40 km under the high southern latitudes [28]. Only liquid water has the combination of the right density to lie under the ice crust and the right melting point to be stable under the expected conditions. Subsequent analysis of the doppler data including the effects of Enceladus’ rotation around Saturn and spin indicate that this liquid water ocean is global, thickest at the south pole and becoming thinner away from the southern high latitudes [29]. Given Enceladus’ small size and slightly higher amount of rock than water, the base of the ocean would be in contact with the silicate core as is the case for Europa. Enceladus on average keeps the same face toward Saturn, but because its orbit is not circular, the moon’s speed varies along the orbit. This in turn means that a given point on Enceladus does not see Saturn fixed in the sky: the satellite nods back and forth relative to a body possessed of instantaneous synchroneity between spin and orbital motion, an example of libration akin to that for the Earth’s Moon. Cassini imaging of Enceladus over a period of years revealed a libration amplitude too large to be consistent with the entire mass of the Enceladus nodding back and forth. Instead, it requires that a thin outer layer—a decoupled shell—slide back and forth over an underlying liquid [30]. Although it provides no information on composition, this result supports with an independent data set the doppler determination of a liquid water ocean beneath the ice. Cassini’s direct sampling of Enceladus’ plume, described later, confirms that the liquid is water with an admixture of salt, and other species including organics. Titan is the second-largest moon in the solar system, just slightly smaller than Ganymede, and both larger than the planet Mercury. A roughly equal mixture of rock and water ice, Titan is the only natural satellite to possess a dense atmosphere—some four times denser at the surface of Titan than is the density of air at sea level on Earth. The air is mostly nitrogen, like our own, but the very weak sunlight at the distance of Saturn dictates a frigid surface temperature—94 K at the equator even with a modest greenhouse effect—which in turn means water is frozen out of the atmosphere. In its place, methane plays the role of weathermaker: Cassini and the probe Huygens that it carried for a descent to Titan’s surface detected clouds, rain, surface dew, gulleys and river valleys [31]. Most strikingly, the Cassini Saturn Orbiter discovered lakes and seas in the high latitude regions of Titan, mostly in the north, and the radar system it carried was used to determine that the composition of these bodies of liquids is mostly methane with smaller amounts of ethane and nitrogen [32]. Several of these bodies of liquid are large; one is the size of the Caspian sea. Whether they might support an exotic type of life that could form and survive in liquid methane is an open question [33], but deep beneath the frigid surface The Cassini orbiter and Huygens Lander found evidence in two ways for a much warmer, liquid water ocean.
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Fig. 3. North polar image of Titan showing numerous lakes and several large seas. The sea near the middle of the frame is the size of Earth’s Lake Superior. (NASA/ JPL-Caltech/Space Science Institute).
The first of these came from the Huygens Probe, which was equipped with sensors for measuring the ambient electric field as the probe descended under parachute to the surface of Titan. One of the results of this experiment was the determination that the atmospheric electric field, generated in an ionosphere near the top of the atmosphere, does not go to zero at the surface. The orientation of the electric field and its behavior can be modeled as requiring a lower boundary, some 55-80 km below Titan’s surface, at which point the material transitions from a dielectric to one of conductivity exceeding that of pure liquid water [34]. Salty water or water mixed with ammonia provides good fits to the conductivity. The second, more diagnostic, detection of a subsurface ocean came from the radio science doppler tracking used as well to find an ocean on Enceladus. Here, however, the great distance of the spacecraft from Titan’s surface (1000 km) required by the moon’s thick, extended atmosphere made impractical the detection of layers of density discontinuities within the crust. Instead, the experiment sought to measure the change in shape of Titan as it moves in its eccentric orbit around Saturn. As the moon passes its periapse—closest point—to Saturn, tidal forces are strongest and will elongate it more than at the farthest point in the orbit. The difference corresponds to surface height variations of meters that cannot be detected because of the hazy atmosphere, but it also corresponds to mass distribution changes that are measurable as changes in the gravity field. The measurements indicate shape changes of sufficient magnitude to require that Titan’s ice crust be decoupled from the deep interior—hence by a liquid layer— roughly 100 km below the surface [35].
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Fig. 4: New Horizons image (approximately 75 km across) of Pluto showing blocks of water ice that appear to have been pushed by tectonic forces to make the mountains in the upper left of the image. Adjoining terrain is a snow or ice made mostly of frozen nitrogen. (NASA/JHUAPL/SwRI).
Unlike Enceladus and Titan, the remaining two Saturnian moons classified as ocean words do not have two independent detections of their subsurface oceans. Indeed, for both the evidence is tenuous. Mimas, an Enceladussized satellite closer to Saturn, exhibits a libration indicative of a decoupled shell—an ice crust sliding over a liquid layer [36]. But images of this moon show only a heavily cratered surface, and no evidence of plumes. For a small, geologically inactive moon to have a liquid water ocean seems surprising, and an alternative explanation is that the interior is strongly non-hydrostatic—that is, not relaxed to a shape determined only by gravity and its spin. Dione, further from Saturn but larger, exhibits no rotational or gravitational evidence for an ocean; instead, early Cassini magnetometer and plasma measurements hinted at a weak plume or plumes of material emanating from its interior (and that of another moon Tethys) [37], but could not be confirmed with other instruments [38]. Cassini images do suggest, however, that warm ice or liquid water have flooded parts of the surface in the recent past [39]. 2.4 Kuiper Belt objects Very little is known of the interiors of these intriguing bodies which range in size from Pluto and its near sizetwin Eris (roughly 2300 km in diameter) down to sub-100 km objects. Pluto and its moon Charon were visited by the New Horizons spacecraft in 2015 and were found to have diverse geological features suggestive of internal activity (figure 4). In the case of Pluto, various molecules like carbon monoxide and nitrogen outgassed from the interior and now exist as snow deposits on the surface [9]. Thermal models of these relatively rock-rich bodies combined with the evidence for geologic activity suggest that internal layers of liquid water could be present. Triton is a moon of Neptune on a retrograde orbit that suggests it was captured from the Kuiper Belt early in the history of the solar system. Larger than Pluto by several hundred kilometers, it possesses geysers of nitrogen imaged by Voyager 2 in 1989. While their origin has been suggested the result of solar heating of relatively transparent ice [40], it is also possible that they indicate internal activity. Hence, like Pluto, the circumstantial evidence of geology coupled with interior models [41] hint at the present of a subsurface water ocean. It will be a long time before more is known for any of these bodies. 3. What we know of the oceans of Titan, Europa, and Enceladus For only three of the worlds listed above—Enceladus, Europa, and Titan—are there multiple lines of evidence for a subsurface ocean, and information on the nature of the oceans. This section outlines the information available for each of the objects. 3.1 Titan Of the three, we know the least about the subsurface ocean within Titan. More detailed analysis of Titan’s gravity and its topography constrain the subsurface ocean to have a density slightly higher than that of pure liquid water, suggesting it is charged with salts [42], and refines the mean depth of the crust to be roughly 70km, with an upper limit of 100 km to the thickness. A very recent study of the spin of Titan—axial tilt and period—suggest an even thinner crust of 10-50 km [43], and it is as yet unclear how to reconcile these estimates of the crustal thickness. As for the base of the ocean, standard models place the ocean between the upper low pressure ice phase and denser high
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pressure ices beneath [44], although variants on this model might allow some limited contact between ocean and rocky core [45]. 3.2 Europa Relatively little is known definitively about the Europa ocean. The presence of salts on the surface [20] coupled with the magnetic field data [19] indicate that the water is salty, but the amount of salt and its composition is very poorly known. Observations using the Keck telescope in Hawaii [46] point to magnesium sulfate being present, but only on the side of Europa facing Io, which might supply both the sulfur and the magnesium as an external coating. We do not know whether the ocean contains organic molecules, or nitrogen-bearing compounds—both crucial to life, exist in the ocean: the Galileo instruments were not capable of detecting small-scale deposits of such compounds.
Fig. 5. Jets of ice are seen emanating from fractures in the south polar region of Enceladus in an image from the Cassini Saturn Orbiter. (NASA/JPL/Space Science Institute). The thickness of Europa’s crust is also poorly known and may be spatially highly variable. Parts of Europa exhibit features, including but not limited to the so-called “cycloidal ridges” that suggest a thin crust in those locations—perhaps a kilometer or less [47]. Other regions have features indicative of a much thicker crust. In particular, impact craters seen on Europa require ice crusts many tens of kilometers thick in order to support their topography [48]. 3.3 Enceladus The last of the three oceans to be discovered, more is known about Enceladus’ than the others thanks to direct sampling of the south polar plume by instruments aboard the Cassini Saturn Orbiter and supporting remote sensing observations. The gas mass spectrometer called the Ion Neutral Mass Spectrometer (INMS), has detected water vapor, nitrogen-bearing molecules and carbon-bearing molecules including organics (those containing carbon and hydrogen) [49]. The Cosmic Dust Analyzer (CDA), a mass spectrometer optimized to analyze ice grains lofted in the plume, has found these contain sodium and potassium salts—including sodium chloride-- with abundances in the 0.5-2% range relative to water for the largest grains [50]. The amount of salt in the largest grains and the known solubility of salt in liquid water and ice strongly argue for these being frozen droplets of seawater. CDA also detected a population of very tiny silica grains—SiO2—whose size and composition point to circulation of water inside warm rock at the base of the ocean [51]. Analysis of the abundances of the various elements detected by INMS and CDA point to an alkaline hydrothermal system at the base of the ocean akin to the “low temperature off axis hydrothermal systems” seen in several seafloor locations on Earth [52]. Remote sensing data relevant to the ocean includes Cassini imaging system observation of jet positions and angles along the fracture zones from which they are emitted (figure 5) [53], infrared observations showing the amount of power radiated as heat from the active regions [54, 55], and supporting ultraviolet observations confirming the presence of water vapor in the plume [56]. In sum, the evidence is extremely compelling that much if not all of the material sampled in the plume of Enceladus comes directly or nearly so from the subsurface ocean, and that the ocean is habitable in the sense of having organic molecules, salts, sources of heat and circulation within the underlying rock. That Cassini was able to follow up on its own discovery of the plume by analyzing its nature and inferring the habitability of Enceladus’ ocean was the fortuitous result of carrying a large instrument payload and two mass spectrometers intended to observe other targets in the Saturn system. 4. The next steps in exploring Titan, Europa and Enceladus
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4.1 Titan The primary issue for the analysis of Titan’s subsurface ocean is access to ocean material. With the ocean tens of kilometers below the surface, and a density at or exceeding that of liquid water which is denser than ice I, extrusion of liquid to the surface is difficult. Geologic evidence of such resurfacing is present in a few places on Titan’s surface based on Cassini Saturn orbiter data [57]. The right strategy for identifying sites from orbit through the thick atmospheric haze, landing with sufficient precision, and sampling underneath the pervasive blanket of deposits from sedimenting hazes, have yet to be fully developed. There is also competition between this type of mission and one that would land on and float across a surface hydrocarbon sea [58], and others that might survey large areas of the surface at high resolution from balloon or aircraft [59].
4.2 Europa Efforts in the scientific community to mount a mission to orbit Europa began shortly after repeated Galileo magnetometer measurements indicated the presence of an ocean [60]. The general science goals of such a mission have been to confirm the presence of an ocean and measure its properties, to determine the thickness of the crust and whether ocean material is extruded onto the surface, and to identify sites where one might safely land and sample ocean material. Any such mission is technically very challenging because of the strong radiation environment generated by Jupiter’s powerful magnetic field. However, more daunting has been the political and fiscal environment over the past nearly two decades, which delayed a start for such a mission until mandated by Congress last year. Since then, NASA has competitively selected an instrument suite and the mission, variously called the Europa Clipper or Europa Mission, is in Phase A development leading to a launch in the 2020’s. To reduce the radiation load on the instruments, and hence cost, the mission will make repeated flybys of Europa from Jovian orbit, rather than orbit Europa itself. Conducting flybys will reduce somewhat the science, particularly geodetic measurements, but the vast majority of science objectives associated with the goals outlined above can be achieved.
Fig. 6. Schematic cross section of the Europan crust and ocean with Europa Mission instruments (see text) shown in rough relation to their targets. RMS group is the radiation monitoring systems. Background image: NASA/JPLCaltech. The Mass Spectrometer for Planetary Exploration/Europa (MASPEX) and Surface Dust Mass Analyzer (SUDA) are mass spectrometers of advanced design compared to those on Cassini, able to sample material over a greater mass range and with superior sensitivity and resolution. They will sample plumes should such be present on Europa, but can also detect molecules and ions evaporated and sputtered off the surface. Imaging and near infrared spectroscopy of the surface will be provided by the Europa Imaging System (EIS) and Mapping Imaging Spectrometer for Europa (MISE), while the Ultraviolet Spectrograph/Europa (UVS) determines the density and composition of the atmosphere and any plumes that might be detected. Below the surface, the Europa Thermal Emission Imaging System (E-THEMIS) detects hot spots associated with a thin crust or buoyant warm upwelling ice or liquid, while the Radar for Europa Assessment and Sounding Ocean to Near Surface (REASON) probes the structure, thickness and electrical properties of the ice crust. Working together, the Interior Characterization of Europa using Magnetometry (ICEMAG) magnetometer and the Plasma Instrument for Magnetic Sounding (PIMS) provide a much more sensitive determination of the salinity of the ocean than did Galileo, by removing effects of the external magnetic field and plasmas.
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The instrument package has the potential for characterizing the habitability of the Europan ocean to a more detailed extent than Cassini did for Enceladus, particularly if plumes are present. It will also map out where organic deposits may be present, and determine whether oxidants are fed to the Europa ocean as has been speculated since the results of the Galileo mission [61]. Although the Europa Mission is preparatory to a mission to land on Europa and attempt to access ocean material, in its Consolidated Appropriations Act of 2016 Congress directed NASA to initiate designs for a lander mission. The current concept is one that could be launched contemporaneously with the flyby mission on a separate launch vehicle, and then land on the surface to sample ocean deposits after a suitable landing site has been found by the Clipper payload. A Science Definition Team chartered by NASA is examining science objectives and payload options at the time of this writing. 4.3 Enceladus Cassini completed its last plume passage of Enceladus, and final close flyby, October 21, 2015, leaving open what the next steps in Enceladus exploration ought to be. The determination that the ocean satisfies the basic criteria for “habitability” (the ability to support life) has focussed attention on missions that will further probe the composition of the ocean and even search for evidence of living organisms. Two detailed proposals for future exploration of Enceladus have been written: a plume sample return mission, called the Life Investigation for Enceladus (LIFE) [62], and Enceladus Life Finder (ELF), a mission proposed unsuccessfully to NASA’s Discovery program to do in situ analysis of the plume with mass spectrometers [63]. A third mission, to bring a Clipper-like payload to orbit Enceladus, was studied at a JPL summer school in 2015 [64]. The ELF concept builds on the success of two spaceborne mass spectrometers on Cassini in characterizing details of the subsurface ocean properties (salinity, general composition, pH, contact with hot rock) that are essential to establishing the habitability of the ocean, even though those instruments are neither state-of-the-art nor designed specifically to analyse the plume. Two state-of-the-art mass spectrometers (one for gas, one for dust) using the timeof-flight technique to provide orders of magnitude better mass resolution and sensitivity, and an order of magnitude better mass range, can seek the molecular signature of active biological processes while better constraining the chemical and physical properties of the ocean. The spacecraft need do nothing other than what Cassini successfully did seven times, which was to fly through the plume of Enceladus (five flythroughs are needed to complete the ELF objectives). The proposed mission was cited by a NASA panel as “a pioneering example for future missions to other planetary bodies with known sources of water.” The LIFE sample return concept allows plume material to be analysed in terrestrial laboratories, along with a limited amount of in situ sampling. Unknowns for this mission include the technologies required to preserve delicate organic traces of life for the return journey to Earth (very low temperatures and low acceleration), and the unknown costs of ensuring no accidental release of biologically viable material from the sample during entry and landing on Earth. Managing public perceptions in regard to the latter is also an issue. These and more ambitious mission to enter the ocean directly are not mutually exclusive; instead they potentially form the elements of an Enceladus exploration program that seeks habitability and life through progressively more complex missions, with off-ramps and alternative pathways provided to take account of intermediate results [65]. 5. Discussion: why an Ocean Worlds program? The description of potential opportunities for furthering our understanding of the nature and habitability of oceans in the outer solar system, and indeed looking for life in those environments, was organized object by object. While it is tempting to focus on a single object and throw as many resources as possible there before moving on to others, a multi-object ocean worlds program carries with it several advantages: (i). Interleaved missions allow lessons learned to be applied from one object to another: the experience of Cassini at Enceladus has greatly informed the next step in the exploration of Europa, including details of the types of instruments to be carried and what signatures to look for. Technologies and instruments developed for one body may be useful on another. (ii). Interleaved missions make better use of fiscal and technical resources: Trip times to Europa are 5 years and to Enceladus/Titan 10 years on medium-class launch vehicles; investment in heavy lift vehicles to shorten trip times is possible but speculative. With the long trip times to the outer solar system, waiting for the results of one mission at one object in order to plan the next mission leaves long gaps in a scientific campaign to look for habitability and life. Interleaving missions among three bodies (at two giant planets) allows a leapfrog approach where new discoveries are always being made at one ocean world while plans for future efforts are being brought to fruition at others. It also allows for a more level funding profile and hence the possibility of a more stable program.
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(iii) A program hedges the bet: The disappointing results from Viking with respect to life on Mars had a strongly negative effect on the program because there was no plan B for what to do next. With multiple ocean worlds, strongly negative results on one target simply means that it is time to refocus on the remaining one or ones. At the same time, because of persistent limitations in planetary science funding, goals will generally take longer to accomplish than expected. The Europa Orbiter mission originally proposed in 1999 is only now under development in the guise of the Europa Multiple-flyby Mission. There will be only a handful of missions to the outer solar system over the next two decades. This argues for the number of targets in an Ocean Worlds Program to be limited to the most promising, which means Enceladus, Europa and Titan. However, international collaboration can help to extend our reach to a broader class of targets; the European Space Agency’s Jupiter Icy Moons Explorer (JUICE) will focus on Ganymede and Callisto, with a more cursory look at Europa [66]. 6. Conclusions A remarkable two-decade robotic campaign in the outer solar system has led to the identification of multiple worlds with liquid water oceans in their interiors. Of these, Europa, Enceladus and Titan have oceans detected in multiple independent ways, and for Enceladus the properties of the ocean are known to a substantial degree thanks to ice and gas derived from the ocean jetting from its surface into space. The pliancy of the outer solar system in yielding up information on environments that might support life rejuvenates the search for extant microorganisms in our own solar system that once stopped at Mars. 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Table 1 Categorization of “ocean world” solar system bodies other than the Earth, in terms of whether the ocean exists today and how many lines of evidence are present.
Name Mars Ceres Europa Ganym ede Callisto Mimas Encela dus Dione Titan Triton Pluto Charon
R Exta elic nt 2 ✔ ✔ ✔
Exta nt 1
The ory
✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ (✔ )
Notes: Relic means ocean was present in the past and has left evidence; extant (2,1) means exists based on (two, one) line(s) of evidence; theory means inferred primarily from theoretical studies. The possibility of an ocean within Charon is at present highly speculative.
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