- Email: [email protected]
Ten Years of Cassini Science & More to Come
Saturn, its complex rings, the amazing assortment of moons and the planet’s dynamic magnetic environment. A cooperative project of NASA, ESA and the Italian Space Agency, and the most distant planetary orbiter ever sent by humanity, Cassini arrived at Saturn in June 2004 after a seven-year flight from Earth. The school bus-sized spacecraft, Cassini, dropped a parachuted probe, Huygens, to study the atmosphere and surface of Saturn’s big moon Titan, and commenced making astonishing discoveries that continue today. Icy jets shoot from the tiny moon Enceladus. Titan’s hydrocarbon lakes and seas are dominated by liquid ethane and methane, and complex prebiotic chemicals form in the atmosphere and rain to the surface. Three-dimensional structures tower above Saturn’s rings, and a giant Saturn storm circled the entire planet for most of 2011. Cassini’s findings at Saturn have also fundamentally altered many of our concepts of how planets form around stars.
Representatives from space science-related international organizations, space agencies, and other relevant institutions participated in this meeting. The Symposium participants mandated the Programme on Space Applications to continue the Basic Space Science Initiative beyond 2015. The presentations made at the Symposium are available in form of an archive (.zip) file from
www.unoosa.org/oosa/en/SAP/act2014/gra z/index.html. In 2015 the UN/Japan Workshop on Space Weather will take place in Fukuoka, Japan, as part of the BSSI. Please see
www.unoosa.org/oosa/en/SAP/act2015/jap an/index.html. On the basis of the discussions at the Graz Symposium and at the forthcoming Japan Workshop, activities beyond 2016 will be planned in close cooperation with all stakeholders. The final report of the UN/Austria Symposium will be released towards the end of the year and will be posted at the Graz Symposium webpage.
In 2014 the international Cassini-Huygens mission team is celebrating the mission's tenth anniversary in orbit around Saturn. The spacecraft is still healthy and generating exciting new scientific results. Ten years of orbiting Saturn has already yielded an amazing list of accomplishments and discoveries, and one can only imagine what the final three years, including a paradigm-shifting end-ofmission set of orbits will reveal.
Research Highlights Ten Years of Cassini Science and More to Come
The final orbit sends Cassini deep into Saturn’s atmosphere, ending the mission in September 2017. Too numerous to be described in one article, what follows is a summary of some of the most important highlights of the 10-year mission followed by an overview of its upcoming grand finale.
[Scott G. Edgington, Linda Spilker, NASA/JPL]
Highlights of the Past Ten Years The Huygens probe makes first landing on a moon in the outer solar system (Figure 1) On 21 January 2005, mankind made an amazing achievement: the landing of ESA’s Huygens probe on the surface of Titan. Huygens' historic landing was the most distant in our solar system to date and the only one on
The findings of the Cassini-Huygens mission have revolutionized our understanding of
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a moon in the outer solar system. The probe’s two-hour and 27-minute descent revealed Titan to be remarkably like Earth before life evolved -- with methane rain, erosion, drainage channels, and liquid hydrocarbon lakes at the moon's poles.
became even more important when Cassini found evidence of water and simple organics in the plume. Life as we know it relies on water, so the search for life suddenly found a new focus on this small, icy moon. The recent discovery of a subsurface ocean from gravity measurements makes Enceladus, in addition to Titan, one of the most exciting places to search for life in our solar system.
Figure 2 (Image: NASA/JPL-Caltech/Space Science Institute)
Saturn's rings revealed as active and dynamic – a laboratory for how planets and moons form (Figure 3) Cassini’s decade-long mission made it possible to watch changes in Saturn’s dynamic ring system. The spacecraft discovered propellerlike formations in the A ring and has observed what may be one of the most active, chaotic rings in our solar system, the F ring.
Figure 1 (Image: NASA/JPL-Caltech/ESA/University of Arizona)
The probe found a soup of complex hydrocarbons, including benzene, and nitriles in Titan's atmosphere. Huygens also provided the first in-situ measurements of atmospheric temperatures and aerosols. Discovery of active, icy jets on the Saturnian moon Enceladus (Figure 2) The discovery of Enceladus' massive plume, comprised of over 100 jets, was such a surprise that Cassini scientists immediately asked mission designers to reshape the spacecraft's trajectory to get a better look. Their discovery
Figure 3 (Image: NASA/JPL-Caltech/Space Science Institute)
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Quite possibly, Cassini is also witnessing the birth of a new moon in the outer edges of the main rings! All of these observations provide potential clues about how solar systems form. The rings have also been shown to be a detector of meteoritic material and even a seismometer for determining the nature of Saturn’s interior.
Titan revealed as an Earth-like world with rain, rivers, lakes and seas (Figure 4) Imaging with radar, visible and infrared wavelengths shows that Titan has many geologic and atmospheric processes similar to those of Earth.
Figure 4 (Image: NASA/JPL-Caltech/University of Arizona/University of Idaho)
These processes generate methane and ethane clouds, from which fall methane and ethane rain. These rains build river channels, which then flow into lakes and seas containing these hydrocarbons that don’t immediately evaporate.
hydrocarbons then complete the cycle, replenishing a portion of the methane in the atmosphere. Studies of Saturn's great northern storm of 2010-2011 (Figure 5)
Models suggest that subsurface “alkanofers” convert liquid methane into more complex hydrocarbon liquids, which may add further complexity to the composition of Titan's lakes and seas. Evaporation of methane and other
Late in 2010, Cassini watched as Saturn’s relatively tranquil atmosphere erupted with a storm of gigantic proportions. Typically Saturn
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endures a storm of this magnitude every 30 years, but this one arrived 10 years early, giving Cassini a front-row seat. Within months, the storm grew to encircle the planet with a swirling band. The largest temperature increases ever recorded for any planet were measured in Saturn’s stratosphere.
variation of the SKR is different in the northern and southern hemispheres for unknown reasons. Inferred rotation rates appear to change with the Saturnian seasons and the rates for the different hemispheres have actually swapped around the time of equinox. Scientists are hoping that the close periapse passages at the end of the mission will fulfil the, thus far, elusive quest to determine the true rotation rate and distinguish it from a multitude of other similar periodic signals within the Saturn system.
Figure 5 (Image: NASA/JPL-Caltech/Space Science Institute)
Researchers detected molecules never before seen in Saturn’s upper atmosphere. The storm activity, including, lightning discharges, diminished shortly after the tempest's head collided with its tail, a bit less than a year after it began.
Figure 6 (Image: NASA/JPL-Caltech/University of Iowa)
Vertical structures in the rings imaged for the first time (Figure 7) Once about every 15 years, at equinox, the Sun shines on the edge of the ring plane and the northern and southern sides of the rings receive little sunlight. Vertical clumps, undulations, and ridges were known to stick out of the ring plane, but their heights and widths could not be directly measured until Saturn's equinox revealed their shadows to Cassini.
Studies reveal radio-wave patterns are not tied to Saturn's interior rotation, as previously thought (Figure 6) Saturn emits radio waves known as Saturn Kilometric Radiation (SKR). A similar radio wave pattern was measured at Jupiter to deduce the length of that planet's day. However, determining Saturn's daily rotation rate has turned out to be much more complicated due to a currently undetectable tilt of the magnetic field axis relative to the spin axis, which was thought to modulate the SKR as is does at Jupiter.
Mission scientists measured those long shadows during this rare event to determine the heights of structures within the rings. Some of these structures were observed to be giant mountains of ice particles as much as 3 km tall. In addition, clouds of dust kicked up in collisions between small space debris and ring particles were easier to see under the low-light
Data from Cassini's Radio Wave and Plasma Science (RPWS) instrument show that the
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conditions of equinox than under normal conditions, allowing researchers to study their evolution.
Cassini himself. The Cassini spacecraft has solved this puzzle. Charcoal-dark, reddish dust in Iapetus's orbital path is swept up by the moon and lands on its leading surface. These darker areas absorb more sunlight and become warmer, while the uncontaminated areas remain cooler. This causes water molecules to migrate from the darker (leading) side to brighten the cooler (trailing) side. The moon’s long rotation period (1,904 hours), distance from the Sun, and small surface gravity are all factors in creating the yin-yang effect that makes Iapetus a contender for Saturn's most visually striking moon. 1st complete view of the north polar hexagon and discovery of giant hurricanes at both of Saturn's poles (Figure 10)
Figure 7 (Image: NASA/JPL-Caltech/Space Science Institute)
Analyses of data from Voyager’s Saturn flybys are credited with the discovery of a hexagonshaped jet stream (with wavenumber 6) surrounding the planet's north pole. The recovery of this jet stream by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) has shown that it is a relatively long-lived structure.
Study of prebiotic chemistry on Titan (Figure 8) Titan’s atmosphere is a veritable zoo teaming with a host of organic molecules and is currently the most chemically complex known in the solar system.
Cassini’s inclined orbits have enabled scientists to track and study the clouds and haze that circulate within this pattern. These efforts have revealed a lack of large haze particles within the hexagon compared to the atmosphere outside of it, which is similar to Earth’s ozone hole. Hurricane-like vortices, discovered by Cassini, are locked to both poles.
Beginning with sunlight and methane, ever more complex molecules—hydrocarbons and nitriles—are formed until they become large enough to condense into the haze that shrouds the surface of the giant moon. Nearer to the surface, methane, ethane, and other organics condense into clouds and fall to the surface to fill the lakes and seas where likely other prebiotic chemistry can take place.
Their driving forces are a mystery and, in the remaining three years of Cassini’s mission, scientists hope to learn more of their properties and the conditions for their existence.
Mystery of the dual, bright-dark surface of the moon Iapetus solved (Figure9) The origin of Iapetus' two-faced surface has been a mystery for more than 300 years, since the moon’s discovery by Giovanni Domenico
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Figure 8 (Image: ESA/ATG medialab
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Figure 9 (Image: NASA/JPL/Space Science Institute)
Figure 10 (Image: NASA/JPL-Caltech/SSI/Hampton University)
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rings, ring propellers and ring moons. Highresolution imaging will provide some of the best views of ring particle interactions and clumping.
The Next Three Years The Cassini spacecraft has spent the last several years orbiting Saturn at high inclinations, which provide great views of Saturn’s poles and rings, as well as sampling of unique regions of the magnetosphere. Cassini’s orbital ballet is guided by numerous Titan flybys (18) over the next three years, each with its own unique coverage of the giant moon. During late 2014 and early 2015, Cassini’s orbits will gradually decrease in inclination until they lie nearly in the equator for the remainder of 2015. At the same time, we are swinging Cassini’s orbit orientation around Saturn such that its most distant point from Saturn, or apoapse, lies within Saturn’s magnetotail. This equatorial set of orbits will provide a number of close satellite flybys of Enceladus and Dione as well as the opportunity for other good, but more distant satellite observations. On one of the close flybys of Enceladus, Cassini will fly through the plume one last time to directly sample it and look for any variations in plume output over a timescale of years.
An Exciting End of Mission The last six months of Cassini’s journey are composed of 22 orbits, which begin with a final Titan flyby in April 2017. Titan’s gravitational kick allows the spacecraft’s periapse to hop from outside Saturn's main rings into the gap between the innermost D ring and the planet’s upper atmosphere. On the last of these orbits, on September 15, 2017, Cassini will plunge into Saturn’s atmosphere, becoming permanently a part of the gas giant. These orbits, dubbed the Grand Finale or Proximal Orbits, present a unique, unprecedented opportunity for the Cassini mission to gather data on fundamental science questions about Saturn, the rings, and innermost plasma environment. This phase of the mission will yield science objectives similar to that of the Juno mission at Jupiter, allowing for comparative investigations of these two gas giants.
During this entire period, the angle of the Sun with respect to Saturn’s ring plane, the solar elevation, will be higher than at any other time in the mission, allowing for further exploration of seasonal change on Saturn and Titan, in the rings and magnetosphere, and on the icy satellites. Numerous opportunities for observing Saturn and Titan using solar and stellar occultations will enable Cassini to observe a variety of latitudes and longitudes on these bodies during late spring for comparison to Saturn seasons observed earlier in the mission. Occultations by the rings will reveal long-time-scale temporal changes within the rings. At the same time, we make our last probe deep within Saturn’s magnetotail.
Grand Finale science objectives include: (1) determinations of the higher order gravity and magnetic field moments to constrain Saturn’s interior structure and possibly determine its currently unknown internal rotation rate; (2) measurement of Saturn’s ring mass, currently uncertain by about an order of magnitude; (3) in-situ measurements of Saturn’s ionosphere, innermost radiation belts, dust environment, and auroral acceleration region; (4) highresolution studies of the main rings; and (5) high-resolution Saturn atmospheric observations. The periapse orientation near local solar noon and slightly south of the ring plane allow for optimized gravitation measurements, in addition to high-phase, highresolution imaging and occultations of the main rings. The approach to periapse is over the northern hemisphere for inbound observations of the sunlit rings and Saturn’s north polar regions. Outbound trajectories will provide excellent views of Saturn’s southern aurorae and the unlit side of the rings.
Starting in early 2016, Cassini’s orbits will once again ascend in inclination, providing new, on-high observations of Saturn’s poles, rings and magnetosphere at the end of Saturnian spring. At the end of this rise in inclination, Cassini’s orbits will move periapse very near to the F ring for a series of 20 orbits that will allow for close study of the outmost 87
Knowing the mass of Saturn’s rings, which is currently uncertain by an order of magnitude, will help settle important questions regarding their age and evolution. It is thought, for instance, that a low mass for the rings implies a younger ring system, while a much larger mass implies that the rings may be as old as Saturn itself. The ring mass will help to discriminate between competing theories about how Saturn’s rings formed
Saturn’s Interior and the Magnetic and Gravitational Fields The Grand Finale orbits will allow Cassini to study Saturn’s magnetic and gravity fields while in close proximity to the planet. The magnetometer will make high-resolution measurements of the magnetic field as close as 1.02 Saturn radii to determine the higher-order coefficients of the magnetic field to degree-9 and possibly degree-11. This will allow a determination of the depth of Saturn’s metallic hydrogen core, which is important for understanding the dynamo mechanism in the interior.
These orbits also allow for in-situ sampling of smaller ring grains. Deep within Saturn’s inner magnetosphere, the rings are continually eroded by magnetospheric plasma. Mutual collisions and impacts from micrometeoroids also slowly grind down ring particles. Some of these micron-sized grains develop an electric charge and migrate inward, where they can be sampled directly by Cassini’s Cosmic Dust Analyzer (CDA).
A tenet of dynamo theory is that a planetary dynamo cannot sustain an axisymmetric field, and yet Cassini measurements of Saturn’s intrinsic magnetic field to date have not shown any significant deviations from axisymmetry. The Grand Finale orbit measurements are thus Cassini's best chance for determining the degree to which Saturn’s magnetic field axis deviates from Saturn’s rotation axis, and hence the rotation rate of the planetary interior.
The sputtering of ring particles by magnetospheric plasma also releases ions and neutral molecules that can be detected by the Ion and Neutral Mass Spectrometer (INMS) as they drift inward, permitting the direct measurement of ring particle composition. Though it has long been known that the rings are largely composed of contaminated water ice, ‘rings scientists’ do not yet know what the contaminants are or from where they originate. Cassini observations will bring invaluable new data to bear on this question.
The low-order harmonics of the gravity field give information about the mass of the core and possible internal layering, while the highorder harmonics give information about the winds deep in the fluid interior. Radio science data from a number of dedicated gravity passes will allow estimation of Saturn's zonal gravity harmonics up to degree-12 with an accuracy of less than 2x10-7 -- at least two orders of magnitude better than current model values for J10. This will allow determination of how deep the observed winds of the upper atmosphere extend into the interior.
When Cassini entered orbit about Saturn, the spacecraft flew directly over the unlit side of the main rings, coming closer to them than it ever has since. In so doing, its imaging cameras and spectrometers caught glimpses of new structures in the rings, such as density waves and features called “straw.”
Mass, Composition and Structure of Saturn’s Rings The Grand Finale orbits, along with the 20 preceding F ring orbits, will enable observations that can provide new insight into several key questions surrounding Saturn’s rings. During these orbits, radio science will measure the ring mass directly at the same time that it is making measurements of Saturn’s gravity field. These measurements are performed in a nearly ideal orbital alignment.
The unique Grand Finale orbit viewing geometry of both the lit and unlit sides of the rings may very well reveal additional new ring features and new details about the rings. In addition, the radar instrument will take advantage of the unprecedented spatial resolution to carry out unique observations at microwave wavelengths.
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our planet. Cassini will also set the stage for future missions to explore this magnificent world, leaving an incredibly rich legacy of discoveries that have changed our views of the Saturn system, how solar systems form from protoplanetary disks, and the potential for life elsewhere in our universe.
In-situ Atmospheric and Radiation Field Studies For the first time, Saturn’s ionospheric composition will be investigated with in-situ measurements of electron density and temperature by the Langmiur probe (part of the RPWS investigation), as well as dust composition by CDA. INMS will be able to, for the first time, sample in-situ the neutral and ion composition and density of Saturn’s ionosphere and thermosphere. Acquiring these data will allow for a comparison of in-situ measurements of composition and density to that derived from previous spectral observations and occultation experiments.
Cassini-Huygens is a multidisciplinary, international planetary mission consisting of an orbiting spacecraft and a probe. The Huygens probe successfully landed on Titan’s surface on January 14, 2005, while the orbiter has performed observations in the Saturn system since it entered orbit around Saturn on July 1, 2004. NASA has provided funds for an extended Cassini mission that will last until 2017, the year of Saturn Solstice.
During Saturn Orbit Insertion in 2004, the Magnetospheric Imaging Instrument (MIMI) detected the signature of a radiation belt inside the D ring, but has not been able to further study the belt because of the spacecraft’s orbital geometry. Flying interior to the D ring, MIMI and RPWS will characterize the radiation environment that exists in the gap between the D ring and the upper atmosphere with in-situ measurements. Finally, these orbits will also provide the best opportunity for studying the properties of lightning by searching for their characteristic whistler radio waves.
Cassini-Huygens is a cooperative mission by NASA, European Space Agency (ESA) and the Italian space agency, with NASA supplying the Cassini orbiter, and ESA supplying the Huygens probe. The Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, manages the mission for NASA's Science Mission Directorate in Washington.
In Closing… Cassini’s final three years and Grand Finale orbits, are like a brand new mission. Building upon past achievements and observations, Cassini will explore a season of the Saturn system totally unexplored by an outer planet spacecraft.
WANTED: COSPAR Website Volunteers The COSPAR Secretariat needs your help to improve the COSPAR website with: - suggestions to improve the content; - technical assistance to improve the presentation, structure and navigation. The COSPAR website was developed under Drupal and is operated within the CNES firewall and in accordance to CNES IT safety regulations. All voluntary contributions will be welcome, acknowledged, and the most significant contribution will be rewarded. Please contact the Secretariat at: [email protected]
In her final year, Cassini will go where no spacecraft has gone before, diving between the innermost ring and the top of Saturn’s atmosphere. Cassini’s final Earth-bound transmission will occur on Sept. 15, 2017, as she plunges into the planet and vaporizes. This protective measure will ensure that any hardy microbes that might have survived on board Cassini could not inadvertently contaminate either Titan or Enceladus, both worlds that Cassini discovered to harbour liquid water oceans beneath their icy crusts making them potential habitats for life beyond 89
About the Authors
Linda Spilker
Scott G. Edgington
Dr. Linda Spilker is a NASA research scientist at JPL. She is currently the Cassini Project Scientist and a Co-Investigator on the Cassini Composite Infrared Spectrometer team. She has worked on Cassini since 1988. Since joining JPL in 1977 she has worked on the Voyager Project, the Cassini Project and conducted independent research on the origin and evolution of planetary ring systems.
Dr. Scott G. Edgington is a NASA research scientist at JPL. He is currently the Cassini Deputy Project Scientist and the Investigation Scientist for the Cassini Composite Infrared Spectrometer team. He has worked at JPL since 2001 when he joined the Cassini Science Planning team. His research interests include photochemistry and spectroscopy of the giant planets and radiation transport in atmospheres and rings.
She received her PhD from UCLA in geophysics and space physics.
He received his PhD in atmospheric and space physics from the University of Michigan in 1997 and his BSE in Engineering Physics from the University of Pittsburgh in 1992. COSPAR 2nd SYMPOSIUM: Water and Life in the Universe Foz do Iguaçu, Brazil, 9-13 November 2015 IMPORTANT DATES Abstract submission: 2 March-31 May 2015 Early Registration: 1 June-31 July 2015 Regular Registration: 1 August -31 October Registration on Site: 9 – 13 November 2015 www.cosparbrazil2015.org/.
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Representatives from space science-related international organizations, space agencies, and other relevant institutions participated in this meeting. The Symposium participants mandated the Programme on Space Applications to continue the Basic Space Science Initiative beyond 2015. The presentations made at the Symposium are available in form of an archive (.zip) file from
www.unoosa.org/oosa/en/SAP/act2014/gra z/index.html. In 2015 the UN/Japan Workshop on Space Weather will take place in Fukuoka, Japan, as part of the BSSI. Please see
www.unoosa.org/oosa/en/SAP/act2015/jap an/index.html. On the basis of the discussions at the Graz Symposium and at the forthcoming Japan Workshop, activities beyond 2016 will be planned in close cooperation with all stakeholders. The final report of the UN/Austria Symposium will be released towards the end of the year and will be posted at the Graz Symposium webpage.
In 2014 the international Cassini-Huygens mission team is celebrating the mission's tenth anniversary in orbit around Saturn. The spacecraft is still healthy and generating exciting new scientific results. Ten years of orbiting Saturn has already yielded an amazing list of accomplishments and discoveries, and one can only imagine what the final three years, including a paradigm-shifting end-ofmission set of orbits will reveal.
Research Highlights Ten Years of Cassini Science and More to Come
The final orbit sends Cassini deep into Saturn’s atmosphere, ending the mission in September 2017. Too numerous to be described in one article, what follows is a summary of some of the most important highlights of the 10-year mission followed by an overview of its upcoming grand finale.
[Scott G. Edgington, Linda Spilker, NASA/JPL]
Highlights of the Past Ten Years The Huygens probe makes first landing on a moon in the outer solar system (Figure 1) On 21 January 2005, mankind made an amazing achievement: the landing of ESA’s Huygens probe on the surface of Titan. Huygens' historic landing was the most distant in our solar system to date and the only one on
The findings of the Cassini-Huygens mission have revolutionized our understanding of
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a moon in the outer solar system. The probe’s two-hour and 27-minute descent revealed Titan to be remarkably like Earth before life evolved -- with methane rain, erosion, drainage channels, and liquid hydrocarbon lakes at the moon's poles.
became even more important when Cassini found evidence of water and simple organics in the plume. Life as we know it relies on water, so the search for life suddenly found a new focus on this small, icy moon. The recent discovery of a subsurface ocean from gravity measurements makes Enceladus, in addition to Titan, one of the most exciting places to search for life in our solar system.
Figure 2 (Image: NASA/JPL-Caltech/Space Science Institute)
Saturn's rings revealed as active and dynamic – a laboratory for how planets and moons form (Figure 3) Cassini’s decade-long mission made it possible to watch changes in Saturn’s dynamic ring system. The spacecraft discovered propellerlike formations in the A ring and has observed what may be one of the most active, chaotic rings in our solar system, the F ring.
Figure 1 (Image: NASA/JPL-Caltech/ESA/University of Arizona)
The probe found a soup of complex hydrocarbons, including benzene, and nitriles in Titan's atmosphere. Huygens also provided the first in-situ measurements of atmospheric temperatures and aerosols. Discovery of active, icy jets on the Saturnian moon Enceladus (Figure 2) The discovery of Enceladus' massive plume, comprised of over 100 jets, was such a surprise that Cassini scientists immediately asked mission designers to reshape the spacecraft's trajectory to get a better look. Their discovery
Figure 3 (Image: NASA/JPL-Caltech/Space Science Institute)
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Quite possibly, Cassini is also witnessing the birth of a new moon in the outer edges of the main rings! All of these observations provide potential clues about how solar systems form. The rings have also been shown to be a detector of meteoritic material and even a seismometer for determining the nature of Saturn’s interior.
Titan revealed as an Earth-like world with rain, rivers, lakes and seas (Figure 4) Imaging with radar, visible and infrared wavelengths shows that Titan has many geologic and atmospheric processes similar to those of Earth.
Figure 4 (Image: NASA/JPL-Caltech/University of Arizona/University of Idaho)
These processes generate methane and ethane clouds, from which fall methane and ethane rain. These rains build river channels, which then flow into lakes and seas containing these hydrocarbons that don’t immediately evaporate.
hydrocarbons then complete the cycle, replenishing a portion of the methane in the atmosphere. Studies of Saturn's great northern storm of 2010-2011 (Figure 5)
Models suggest that subsurface “alkanofers” convert liquid methane into more complex hydrocarbon liquids, which may add further complexity to the composition of Titan's lakes and seas. Evaporation of methane and other
Late in 2010, Cassini watched as Saturn’s relatively tranquil atmosphere erupted with a storm of gigantic proportions. Typically Saturn
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endures a storm of this magnitude every 30 years, but this one arrived 10 years early, giving Cassini a front-row seat. Within months, the storm grew to encircle the planet with a swirling band. The largest temperature increases ever recorded for any planet were measured in Saturn’s stratosphere.
variation of the SKR is different in the northern and southern hemispheres for unknown reasons. Inferred rotation rates appear to change with the Saturnian seasons and the rates for the different hemispheres have actually swapped around the time of equinox. Scientists are hoping that the close periapse passages at the end of the mission will fulfil the, thus far, elusive quest to determine the true rotation rate and distinguish it from a multitude of other similar periodic signals within the Saturn system.
Figure 5 (Image: NASA/JPL-Caltech/Space Science Institute)
Researchers detected molecules never before seen in Saturn’s upper atmosphere. The storm activity, including, lightning discharges, diminished shortly after the tempest's head collided with its tail, a bit less than a year after it began.
Figure 6 (Image: NASA/JPL-Caltech/University of Iowa)
Vertical structures in the rings imaged for the first time (Figure 7) Once about every 15 years, at equinox, the Sun shines on the edge of the ring plane and the northern and southern sides of the rings receive little sunlight. Vertical clumps, undulations, and ridges were known to stick out of the ring plane, but their heights and widths could not be directly measured until Saturn's equinox revealed their shadows to Cassini.
Studies reveal radio-wave patterns are not tied to Saturn's interior rotation, as previously thought (Figure 6) Saturn emits radio waves known as Saturn Kilometric Radiation (SKR). A similar radio wave pattern was measured at Jupiter to deduce the length of that planet's day. However, determining Saturn's daily rotation rate has turned out to be much more complicated due to a currently undetectable tilt of the magnetic field axis relative to the spin axis, which was thought to modulate the SKR as is does at Jupiter.
Mission scientists measured those long shadows during this rare event to determine the heights of structures within the rings. Some of these structures were observed to be giant mountains of ice particles as much as 3 km tall. In addition, clouds of dust kicked up in collisions between small space debris and ring particles were easier to see under the low-light
Data from Cassini's Radio Wave and Plasma Science (RPWS) instrument show that the
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conditions of equinox than under normal conditions, allowing researchers to study their evolution.
Cassini himself. The Cassini spacecraft has solved this puzzle. Charcoal-dark, reddish dust in Iapetus's orbital path is swept up by the moon and lands on its leading surface. These darker areas absorb more sunlight and become warmer, while the uncontaminated areas remain cooler. This causes water molecules to migrate from the darker (leading) side to brighten the cooler (trailing) side. The moon’s long rotation period (1,904 hours), distance from the Sun, and small surface gravity are all factors in creating the yin-yang effect that makes Iapetus a contender for Saturn's most visually striking moon. 1st complete view of the north polar hexagon and discovery of giant hurricanes at both of Saturn's poles (Figure 10)
Figure 7 (Image: NASA/JPL-Caltech/Space Science Institute)
Analyses of data from Voyager’s Saturn flybys are credited with the discovery of a hexagonshaped jet stream (with wavenumber 6) surrounding the planet's north pole. The recovery of this jet stream by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) has shown that it is a relatively long-lived structure.
Study of prebiotic chemistry on Titan (Figure 8) Titan’s atmosphere is a veritable zoo teaming with a host of organic molecules and is currently the most chemically complex known in the solar system.
Cassini’s inclined orbits have enabled scientists to track and study the clouds and haze that circulate within this pattern. These efforts have revealed a lack of large haze particles within the hexagon compared to the atmosphere outside of it, which is similar to Earth’s ozone hole. Hurricane-like vortices, discovered by Cassini, are locked to both poles.
Beginning with sunlight and methane, ever more complex molecules—hydrocarbons and nitriles—are formed until they become large enough to condense into the haze that shrouds the surface of the giant moon. Nearer to the surface, methane, ethane, and other organics condense into clouds and fall to the surface to fill the lakes and seas where likely other prebiotic chemistry can take place.
Their driving forces are a mystery and, in the remaining three years of Cassini’s mission, scientists hope to learn more of their properties and the conditions for their existence.
Mystery of the dual, bright-dark surface of the moon Iapetus solved (Figure9) The origin of Iapetus' two-faced surface has been a mystery for more than 300 years, since the moon’s discovery by Giovanni Domenico
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Figure 8 (Image: ESA/ATG medialab
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Figure 9 (Image: NASA/JPL/Space Science Institute)
Figure 10 (Image: NASA/JPL-Caltech/SSI/Hampton University)
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rings, ring propellers and ring moons. Highresolution imaging will provide some of the best views of ring particle interactions and clumping.
The Next Three Years The Cassini spacecraft has spent the last several years orbiting Saturn at high inclinations, which provide great views of Saturn’s poles and rings, as well as sampling of unique regions of the magnetosphere. Cassini’s orbital ballet is guided by numerous Titan flybys (18) over the next three years, each with its own unique coverage of the giant moon. During late 2014 and early 2015, Cassini’s orbits will gradually decrease in inclination until they lie nearly in the equator for the remainder of 2015. At the same time, we are swinging Cassini’s orbit orientation around Saturn such that its most distant point from Saturn, or apoapse, lies within Saturn’s magnetotail. This equatorial set of orbits will provide a number of close satellite flybys of Enceladus and Dione as well as the opportunity for other good, but more distant satellite observations. On one of the close flybys of Enceladus, Cassini will fly through the plume one last time to directly sample it and look for any variations in plume output over a timescale of years.
An Exciting End of Mission The last six months of Cassini’s journey are composed of 22 orbits, which begin with a final Titan flyby in April 2017. Titan’s gravitational kick allows the spacecraft’s periapse to hop from outside Saturn's main rings into the gap between the innermost D ring and the planet’s upper atmosphere. On the last of these orbits, on September 15, 2017, Cassini will plunge into Saturn’s atmosphere, becoming permanently a part of the gas giant. These orbits, dubbed the Grand Finale or Proximal Orbits, present a unique, unprecedented opportunity for the Cassini mission to gather data on fundamental science questions about Saturn, the rings, and innermost plasma environment. This phase of the mission will yield science objectives similar to that of the Juno mission at Jupiter, allowing for comparative investigations of these two gas giants.
During this entire period, the angle of the Sun with respect to Saturn’s ring plane, the solar elevation, will be higher than at any other time in the mission, allowing for further exploration of seasonal change on Saturn and Titan, in the rings and magnetosphere, and on the icy satellites. Numerous opportunities for observing Saturn and Titan using solar and stellar occultations will enable Cassini to observe a variety of latitudes and longitudes on these bodies during late spring for comparison to Saturn seasons observed earlier in the mission. Occultations by the rings will reveal long-time-scale temporal changes within the rings. At the same time, we make our last probe deep within Saturn’s magnetotail.
Grand Finale science objectives include: (1) determinations of the higher order gravity and magnetic field moments to constrain Saturn’s interior structure and possibly determine its currently unknown internal rotation rate; (2) measurement of Saturn’s ring mass, currently uncertain by about an order of magnitude; (3) in-situ measurements of Saturn’s ionosphere, innermost radiation belts, dust environment, and auroral acceleration region; (4) highresolution studies of the main rings; and (5) high-resolution Saturn atmospheric observations. The periapse orientation near local solar noon and slightly south of the ring plane allow for optimized gravitation measurements, in addition to high-phase, highresolution imaging and occultations of the main rings. The approach to periapse is over the northern hemisphere for inbound observations of the sunlit rings and Saturn’s north polar regions. Outbound trajectories will provide excellent views of Saturn’s southern aurorae and the unlit side of the rings.
Starting in early 2016, Cassini’s orbits will once again ascend in inclination, providing new, on-high observations of Saturn’s poles, rings and magnetosphere at the end of Saturnian spring. At the end of this rise in inclination, Cassini’s orbits will move periapse very near to the F ring for a series of 20 orbits that will allow for close study of the outmost 87
Knowing the mass of Saturn’s rings, which is currently uncertain by an order of magnitude, will help settle important questions regarding their age and evolution. It is thought, for instance, that a low mass for the rings implies a younger ring system, while a much larger mass implies that the rings may be as old as Saturn itself. The ring mass will help to discriminate between competing theories about how Saturn’s rings formed
Saturn’s Interior and the Magnetic and Gravitational Fields The Grand Finale orbits will allow Cassini to study Saturn’s magnetic and gravity fields while in close proximity to the planet. The magnetometer will make high-resolution measurements of the magnetic field as close as 1.02 Saturn radii to determine the higher-order coefficients of the magnetic field to degree-9 and possibly degree-11. This will allow a determination of the depth of Saturn’s metallic hydrogen core, which is important for understanding the dynamo mechanism in the interior.
These orbits also allow for in-situ sampling of smaller ring grains. Deep within Saturn’s inner magnetosphere, the rings are continually eroded by magnetospheric plasma. Mutual collisions and impacts from micrometeoroids also slowly grind down ring particles. Some of these micron-sized grains develop an electric charge and migrate inward, where they can be sampled directly by Cassini’s Cosmic Dust Analyzer (CDA).
A tenet of dynamo theory is that a planetary dynamo cannot sustain an axisymmetric field, and yet Cassini measurements of Saturn’s intrinsic magnetic field to date have not shown any significant deviations from axisymmetry. The Grand Finale orbit measurements are thus Cassini's best chance for determining the degree to which Saturn’s magnetic field axis deviates from Saturn’s rotation axis, and hence the rotation rate of the planetary interior.
The sputtering of ring particles by magnetospheric plasma also releases ions and neutral molecules that can be detected by the Ion and Neutral Mass Spectrometer (INMS) as they drift inward, permitting the direct measurement of ring particle composition. Though it has long been known that the rings are largely composed of contaminated water ice, ‘rings scientists’ do not yet know what the contaminants are or from where they originate. Cassini observations will bring invaluable new data to bear on this question.
The low-order harmonics of the gravity field give information about the mass of the core and possible internal layering, while the highorder harmonics give information about the winds deep in the fluid interior. Radio science data from a number of dedicated gravity passes will allow estimation of Saturn's zonal gravity harmonics up to degree-12 with an accuracy of less than 2x10-7 -- at least two orders of magnitude better than current model values for J10. This will allow determination of how deep the observed winds of the upper atmosphere extend into the interior.
When Cassini entered orbit about Saturn, the spacecraft flew directly over the unlit side of the main rings, coming closer to them than it ever has since. In so doing, its imaging cameras and spectrometers caught glimpses of new structures in the rings, such as density waves and features called “straw.”
Mass, Composition and Structure of Saturn’s Rings The Grand Finale orbits, along with the 20 preceding F ring orbits, will enable observations that can provide new insight into several key questions surrounding Saturn’s rings. During these orbits, radio science will measure the ring mass directly at the same time that it is making measurements of Saturn’s gravity field. These measurements are performed in a nearly ideal orbital alignment.
The unique Grand Finale orbit viewing geometry of both the lit and unlit sides of the rings may very well reveal additional new ring features and new details about the rings. In addition, the radar instrument will take advantage of the unprecedented spatial resolution to carry out unique observations at microwave wavelengths.
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our planet. Cassini will also set the stage for future missions to explore this magnificent world, leaving an incredibly rich legacy of discoveries that have changed our views of the Saturn system, how solar systems form from protoplanetary disks, and the potential for life elsewhere in our universe.
In-situ Atmospheric and Radiation Field Studies For the first time, Saturn’s ionospheric composition will be investigated with in-situ measurements of electron density and temperature by the Langmiur probe (part of the RPWS investigation), as well as dust composition by CDA. INMS will be able to, for the first time, sample in-situ the neutral and ion composition and density of Saturn’s ionosphere and thermosphere. Acquiring these data will allow for a comparison of in-situ measurements of composition and density to that derived from previous spectral observations and occultation experiments.
Cassini-Huygens is a multidisciplinary, international planetary mission consisting of an orbiting spacecraft and a probe. The Huygens probe successfully landed on Titan’s surface on January 14, 2005, while the orbiter has performed observations in the Saturn system since it entered orbit around Saturn on July 1, 2004. NASA has provided funds for an extended Cassini mission that will last until 2017, the year of Saturn Solstice.
During Saturn Orbit Insertion in 2004, the Magnetospheric Imaging Instrument (MIMI) detected the signature of a radiation belt inside the D ring, but has not been able to further study the belt because of the spacecraft’s orbital geometry. Flying interior to the D ring, MIMI and RPWS will characterize the radiation environment that exists in the gap between the D ring and the upper atmosphere with in-situ measurements. Finally, these orbits will also provide the best opportunity for studying the properties of lightning by searching for their characteristic whistler radio waves.
Cassini-Huygens is a cooperative mission by NASA, European Space Agency (ESA) and the Italian space agency, with NASA supplying the Cassini orbiter, and ESA supplying the Huygens probe. The Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, manages the mission for NASA's Science Mission Directorate in Washington.
In Closing… Cassini’s final three years and Grand Finale orbits, are like a brand new mission. Building upon past achievements and observations, Cassini will explore a season of the Saturn system totally unexplored by an outer planet spacecraft.
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In her final year, Cassini will go where no spacecraft has gone before, diving between the innermost ring and the top of Saturn’s atmosphere. Cassini’s final Earth-bound transmission will occur on Sept. 15, 2017, as she plunges into the planet and vaporizes. This protective measure will ensure that any hardy microbes that might have survived on board Cassini could not inadvertently contaminate either Titan or Enceladus, both worlds that Cassini discovered to harbour liquid water oceans beneath their icy crusts making them potential habitats for life beyond 89
About the Authors
Linda Spilker
Scott G. Edgington
Dr. Linda Spilker is a NASA research scientist at JPL. She is currently the Cassini Project Scientist and a Co-Investigator on the Cassini Composite Infrared Spectrometer team. She has worked on Cassini since 1988. Since joining JPL in 1977 she has worked on the Voyager Project, the Cassini Project and conducted independent research on the origin and evolution of planetary ring systems.
Dr. Scott G. Edgington is a NASA research scientist at JPL. He is currently the Cassini Deputy Project Scientist and the Investigation Scientist for the Cassini Composite Infrared Spectrometer team. He has worked at JPL since 2001 when he joined the Cassini Science Planning team. His research interests include photochemistry and spectroscopy of the giant planets and radiation transport in atmospheres and rings.
She received her PhD from UCLA in geophysics and space physics.
He received his PhD in atmospheric and space physics from the University of Michigan in 1997 and his BSE in Engineering Physics from the University of Pittsburgh in 1992. COSPAR 2nd SYMPOSIUM: Water and Life in the Universe Foz do Iguaçu, Brazil, 9-13 November 2015 IMPORTANT DATES Abstract submission: 2 March-31 May 2015 Early Registration: 1 June-31 July 2015 Regular Registration: 1 August -31 October Registration on Site: 9 – 13 November 2015 www.cosparbrazil2015.org/.
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