Internal structure and cryovolcanism on Trans-Neptunian objects

C H A P T E R

8 Internal structure and cryovolcanism on Trans-Neptunian objects Aurélie Guilbert-Lepoutrea, Dina Prialnikb, Robin Métayera a LGL-TPE,

UMR 5276, CNRS, Claude Bernard Lyon 1 University, ENS Lyon, Villeurbanne Cedex, France b Department of Geosciences, Tel Aviv University, Tel Aviv, Israel

8.1 Introduction The Trans-Neptunian population includes objects with sizes ranging from a few kilometers to a few 103 km (Petit et al., 2008), but most of our knowledge relates to large-size (∼2000 km) and mid-size (500–1000 km) objects, which are more easily observable. The bulk of the population, that is, the smaller objects, are thought to have undergone less processing than their larger counterparts, and any processing would be preferentially at their surface. In contrast, large- and mid-size Trans-Neptunian objects (TNOs), like planets, could have been strongly altered by geological activity that modified both their internal structure and their surface.

8.1.1 Cryovolcanism Similar to volcanism on the Earth, which plays an important role in reshaping the surface of our planet, cryovolcanism may have acted in shaping the largest TNOs as we observe them today. Cryovolcanism is a process by which material from the internal or subsurface layers of an icy object reaches the surface as liquids or vapors, where they would otherwise be frozen (Geissler, 2015). Active cryovolcanism is known to occur on moons of the giant planets, and past activity has been recently evidenced in the Pluto-Charon system by the NASA/New Horizons probe (Stern et al., 2015). Because the liquid material is emplaced from below the surface, cryovolcanism is generally associated with the occurrence of melts inside an object. Hence, evidence for cryovolcanism can be regarded as evidence for conditions favorable to the physical and chemical differentiation of icy bodies.

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8.1.2 Observational motivation With time, several observational features have been proposed as suggestive of past activity, or at least of past surface renewal. The first was the detection of crystalline water ice at the surface of Charon and Quaoar (Brown and Calvin, 2000; Buie and Grundy, 2000; Dumas et al., 2001; Jewitt and Luu, 2004), later observed at the surface of many other TNOs (Barkume et al., 2008; Guilbert et al., 2009; Brown, 2012). The reason for the presence of large quantities of crystalline water ice at the surface of TNOs remains to be fully understood. Whether water ice is formed amorphous or crystalline, once crystalline water ice is present at the surface of an atmosphere-less object, it is subject to irradiation, which is effective in converting it to amorphous water ice in several million years (Cooper et al., 2003; Mastrapa and Brown, 2006; Cook et al., 2007). Therefore, there are at least two reasons why crystalline water ice may be observed at the surface of TNOs today. First, it may require some replenishment process, including cryovolcanism, bringing fresh material to the surface (e.g., Cook et al., 2007). Second, as indicated by laboratory experiments, an equilibrium between the amorphous and crystalline phases may be reached at temperatures higher than 30 K (Leto and Baratta, 2003; Mastrapa and Brown, 2006; Zheng et al., 2009). Holler et al. (2017a) showed that at a surface temperature typical of TNOs, this equilibrium could be reached in less than 1 Gyr, and the amount of crystalline water ice would increase with temperature. The second observational evidence for internal processing was the detection of ammonia and ammonia hydrates, first at the surface of Charon (Brown and Calvin, 2000; Buie and Grundy, 2000; Dumas et al., 2001; Cook et al., 2007; Merlin et al., 2010a; Cruikshank et al., 2015; DeMeo et al., 2015; Holler et al., 2017a), then Orcus (Barucci et al., 2008; Delsanti et al., 2010; DeMeo et al., 2010; Carry et al., 2011). Similar to crystalline water ice, ammonia and its hydrates should be destroyed at the surface in about 20 Myr in the radiation environment of TNOs (Cooper et al., 2003; Cook et al., 2007). Contrarily to water ice, for which conversion between amorphous to crystalline phases is possible, ammonia is not undergoing any phase transition under irradiation, but is rather destroyed by the process (Moore et al., 2007). Therefore, in order to be present at the surface, ammonia needs to be replenished from a subsurface reservoir. For Charon, no mechanism other than the recently localized emplacement at the surface by cryovolcanism has been able to explain the observations (Cook et al., 2007; McKinnon et al., 2008; Desch et al., 2009) at the time these were done. However, we now know that though cryovolcanism did shape parts of the surface of Charon, it must have occurred very early in the history of the satellite, as the surface is estimated to be as old as 4 Gyr (Singer et al., 2019). Alternate explanations, which do not require the presence of liquids inside TNOs, have been proposed, such as the diffusion of ammonia through a layer of water ice, resulting in the hydration of ammonia (Cruikshank et al., 2005; Holler et al., 2017a). In this chapter, we review the current knowledge on the thermal processing of TNOs, in particular, the possible occurrence of liquid water and its consequences on the internal structure and on the rejuvenation of the surface (Section 8.2). We then review the most recent modeling results obtained for Charon, and show how the evidence for its past geological activity obtained by the NASA/New Horizons probe can be used to study TNOs in general (Section 8.3). Finally, we review the observed physical properties that may be used to

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constrain the actual processing of TNOs and propose ways to improve our understanding of their differentiation state, their internal structure, and the production of liquid water in the outer solar system (Section 8.4). We conclude (Section 8.5) by indicating future lines of research.

8.2 Evolution of large- and mid-size TNOs Early spectral observations mentioned earlier have rapidly prompted studies of the thermal evolution of TNOs. If the presence of crystalline water ice or ammonia hydrates does not require past cryovolcanism, it nonetheless implies some sort of replenishment from subsurface material. This, in turn, implies the presence of liquid water and differentiation. Ammonia in particular can play a significant role in the differentiation of icy objects since it depresses the melting point of the ice mixture and lowers the thermal conductivity and the viscosity of melts. Eventually, it can allow for a liquid phase to be preserved in TNOs above a rocky core and below a thermally insulating crust of unprocessed material (Desch et al., 2009). The early studies have been reviewed by Coradini et al. (2008), McKinnon et al. (2008), and Prialnik et al. (2008). We thus refer to these reviews for details, and summarize the findings later. Given the uncertainties in TNOs’ thermophysical properties, and the unknowns regarding their formation mechanism, time or location, the goal of these works has been to identify key characteristics of the thermal evolution of TNOs, and to propose statistically significant outcomes for the overall population.

8.2.1 Early processing: Accretion and short-lived radiogenic heating The effect of early radiogenic heating remains largely unknown, since it depends not only on the formation epoch but also on the formation timescale of TNOs, hence on the formation mechanism, which may not be unique to all TNOs, given the range of heliocentric distances involved for forming the entire TNO population. Radiogenic heating during accretion was investigated by Merk and Prialnik (2003, 2006). Their model considered a simple composition of mixed amorphous water ice and dust and allowed for crystallization and melting of ice, though no gas or liquid flow was taken into account. Depending on free initial parameters such as initial composition, size, or heliocentric distance, pristine compositions could be largely retained. However, the authors showed that some TNOs might have undergone significant processing, including crystallization of amorphous water ice, and production of liquid water. Interestingly, the potential outcomes included objects which could hold liquid layers in their innermost parts while retaining a pristine uppermost layer of amorphous water ice.

8.2.2 Chemical differentiation: Sublimation and crystallization Regardless of the effects of early heating, long-term radiogenic heating should be inevitable. In general, radiogenic heating results in a temperature gradient from the warm interior to the cold surface, in contrast to solar heating, which results in a warm surface relative to the interior.

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De Sanctis et al. (2001), Choi et al. (2002), or McKinnon (2002), for example, studied the effect of radiogenic heating, in particular with respect to the survival of volatile species such as CO or CO2 . They showed that TNOs’ surface layers remain cold enough to retain volatiles in icy form, although CO—and by analogy, all volatiles sublimating below ∼50 K initially present as pure ices—should be lost at the surface regardless of radiogenic heating (De Sanctis et al., 2001; Choi et al., 2002), due to insolation alone. Radiogenic heating may also trigger crystallization at various depths. In general, largescale crystallization is expected to occur, especially for the larger objects. Crystallization is accompanied by the migration of released volatiles and their possible recondensation in appropriately cold regions of the nucleus. The outcome depends on many free parameters that are extremely temperature dependent (Prialnik, 2000). Furthermore, these processes have similar characteristic timescales and hence they are expected to create a complex pattern of chemical differentiation, with highly volatile species closer to the surface, and less volatile ones layered toward the interior in order of volatility.

8.2.3 Physical differentiation: Internal ocean formation Large objects are expected to have undergone more internal processing than their smaller counterparts, simply because they may attain higher internal temperatures due to radiogenic heating. Indeed, central temperatures high enough to reach the water-ammonia, watermethanol, or water-methanol-ammonia eutectics can be obtained in objects larger than 500 km (McKinnon et al., 2008, and references therein). The larger the object, the larger the internal volume reaching the temperature thresholds required for producing a liquid phase. Such a liquid phase should be buoyant with respect to the surrounding solid material made of water

FIG. 8.1 Times after formation at which all liquid freezes within a TNO with initial ammonia content X = 1% (left) and 5% (right). TNOs with mean densities and radii that place them to the right of the rightmost curve potentially can retain subsurface liquid to the present day. The dashed curves on the right represent models where convection has been shut off. From Desch, S.J., Cook, J.C., Doggett, T.C., Porter, S.B., 2009. Thermal evolution of Kuiper belt objects, with implications for cryovolcanism. Icarus 202, 694–714. https://doi.org/10.1016/j.icarus.2009.03.009.

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ice and rock, and is expected to percolate upward, possibly making its way up to the surface and erupt (Stevenson, 1982). Hussmann et al. (2006) studied the formation of internal oceans within TNOs. In general, they found that in order to sustain an internal ocean, the composition should include some sort of antifreeze, like ammonia, methanol, or salts, so as to overcome solid-state convection. The key to the production of internal oceans, according to their modeling results, was the amount of antifreeze. The effect of ammonia on the internal structure of medium-size TNOs (Desch et al., 2009) is shown in Fig. 8.1. For objects larger than ∼900–1000 km, it is entirely possible that the water ice melting point is reached even when no ammonia, methanol or other antifreeze is involved, and even without the contribution of early radiogenic heating. More recently, Malamud and Prialnik (2015) studied the effect of serpentinization on the thermal evolution of TNOs. They applied their model to Charon, Orcus, and Salacia, and showed that the final internal structure should be differentiated, with an inner rocky core, and an outer ice-rich mantle, the degree of differentiation and exact final structure depending on the mass of the object (see Fig. 8.2).

FIG. 8.2 Compositional cross-section of the final differentiated state of three TNO models after 4.6 Gyr of evolution. The color coding is: black (pores); white (crystalline ice); pink (amorphous ice); brown (unprocessed rock); and olive (processed rock). The initial models differ only in mass, while all other parameters physical properties and composition are identical. The different densities of these objects, as derived from observations, may be uniquely determined by their different masses. (A) Salacia, (B) Orcus, (C) Charon. From Malamud, U., Prialnik, D., 2015. Modeling Kuiper belt objects Charon, Orcus and Salacia by means of a new equation of state for porous icy bodies. Icarus 246, 21–36. https://doi.org/10.1016/j.icarus.2014.02.027.

In summary, large- and mid-size TNOs are expected to have a complex internal structure, resulting from chemical stratification and physical differentiation, either or both. Finally, it is entirely possible that the larger objects may have experienced cryovolcanic events, which should be reflected in their surface properties (such as composition and albedo). Table 8.1 summarizes the processes and factors that are bound to affect the internal structure and evolution of TNOs. All of them have been studied by different groups, but no model to date includes all of these factors.

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TABLE 8.1 Processes and factors affecting internal and surface structure. Process/factor

Effect

Crystallization of amorphous ice

Heat and volatile release; thermal conductivity change

Serpentinization

Heat release; density and thermal conductivity change

Dehydration

Heat absorption; density and thermal conductivity change

Sublimation/condensation

Heat absorption/release; porosity change; composition change

Self-gravity

Compaction; settling; differentiation; energetic changes

Accretion

Size change; heat release

Water flow

Differentiation; ocean formation

Volatile flow

Composition alteration; formation of atmosphere

Cryovolcanism

Resurfacing; formation of geological structures

Irradiation by energetic protons

Surface composition changes (amorphization)

Ammonia

Lowering freezing point

Porosity

Lowering material strength and thermal conductivity

Short-lived radionuclides

Intense heat release during early evolution

Long-lived radionuclides

Slow heating during long-term evolution

8.3 Evolution of the Pluto-Charon system The aforementioned studies have highlighted the fact that any evolutionary track—from entirely pristine to heavily processed—is plausible, given the uncertainties related to TNO formation, and the lack of constraints on many of their thermophysical properties. For the large- and mid-size TNOs, it is thus relevant to consider each object individually, with its own thermal and collisional history. In this regard, the Pluto-Charon system, recently studied in great detail by the NASA/New Horizons probe (see Chapters 13 and 14), is most informative on the processes responsible for shaping TNOs as we observe them today. Pluto, being one of the largest TNOs, was expected to be at least partially differentiated, and some studies even predicted its complete differentiation, accompanied by the presence of liquid today (McKinnon et al., 2008; Robuchon and Nimmo, 2011; Barr and Collins, 2015). Images provided by the NASA/New Horizons probe (Stern et al., 2015) have brought the evidence for such past geological activity. A variety of geological features can be observed, including impact craters, tectonics, signs of cryovolcanism, or surface-atmosphere interactions (Stern et al., 2015; McKinnon et al., 2016; Trowbridge et al., 2016; Trilling, 2016; Buhler and Ingersoll, 2018).

8.3.1 Pre-New Horizons: Thermal history of Charon after formation Ever since its discovery, Charon has been considered as too small to sustain any cryovolcanic activity, or even to differentiate (Stern, 1989; Durand-Manterola, 2003; Hussmann et al.,

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2006; Schubert et al., 2010). Therefore, there was no reason to suspect that any geological activity would be seen on Charon. However, as stated in Section 8.1, near-infrared spectra of Charon’s observed separately from Pluto showed the presence of crystalline water and ammonia or ammonia hydrates are clearly identified (Brown and Calvin, 2000; Buie and Grundy, 2000; Dumas et al., 2001; Cook et al., 2007; Merlin et al., 2010a; Cruikshank et al., 2015; DeMeo et al., 2015; Holler et al., 2017a). A series of dedicated models, with an increasing degree of complexity, has been developed before New Horizons to study Charon’s thermal history in link with possible cryovolcanism (Desch et al., 2009; Neveu et al., 2015; Desch, 2015; Malamud and Prialnik, 2015). We note again that alternate explanations (which do not require the presence of liquids inside TNOs) have been proposed to explain the presence of ammonia at its surface, such as the diffusion of ammonia through a layer of water ice, resulting in the hydration of ammonia (Cruikshank et al., 2005; Holler et al., 2017a). New Horizons taught us that though cryovolcanism did shape parts of the surface of Charon, it must have occurred very early in the history of the satellite, as the surface is estimated to be as old as 4 Gyr (Singer et al., 2019). Also, the link between ammonia and cryovolcanic features is not so clear, and could require impacts excavating subsurface material (Grundy et al., 2016; Dalle Ore et al., 2018). Today, Charon does not display any sign of active cryovolcanism (Moore et al., 2016), but its surface clearly shows evidence for past processing (Moore et al., 2016; Beyer et al., 2017). Therefore, now that New Horizons has provided the evidence for surface renewal processes such as tectonics and cryovolcanism, these models are of particular significance for studying other TNOs with intermediate sizes similar to Charon’s.

8.3.2 Post-New Horizons: Forming the Pluto-Charon system The new constraints brought by New Horizons allow for even more detailed models of the internal structure and evolution of Charon to be developed. For example, McKinnon et al. (2017) argued that in order to achieve the density contrast observed between Pluto and Charon without any initial compositional difference, the entire icy shell of Pluto needed to melt. Bierson et al. (2018, see Fig. 8.3) tried to explain it by constraining the evolution of the internal porosity. They showed that porosity variations alone cannot explain the density contrast, and that Pluto needed to have accreted more silicates than Charon. This appears to be consistent with a formation scenario in which Pluto formed quickly enough to lose a significant fraction of its original water ice content. The matter of how the system formed is still not clear though. Stern et al. (2015) suggested that the impactors at the origin of the Pluto-Charon system should have been moderately differentiated or not differentiated at all, a scenario in which Charon tends to form more or less intact from its precursor, rather than from the disk of debris resulting from the collision (Canup, 2005, 2011). However, Desch (2015) and Desch and Neveu (2017) argue that Charon should instead have formed from the collisional debris disk, and that it accreted not only material from the precursor icy mantle but also from the undifferentiated inner parts of the progenitors. Such progenitors are in this framework formed within 5 Myr after CAI formation in order to sustain early radiogenic heating discussed in Section 8.2.

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FIG. 8.3 Thermal histories of Pluto and Charon, assuming an initial porosity of 0.3, and a silicate mass fraction of 0.69 and density of 3500 kg m−3 . Temperature is contoured at a 100 K interval. The horizontal brown line is the top of the silicate core, the dashed magenta line contours the bottom of the porous layer, and the thick black line is the top of the ocean layer. In this model, Pluto forms a substantial ocean that persists to the present day, while Charon forms an ocean 40 km thick that then refreezes. Charon maintains a larger porous layer than Pluto due to lower temperatures and pressure in the ice mantle. At the end of evolution, Pluto and Charon have radii of 1191 and 605.2 km, respectively. From Bierson, C.J., Nimmo, F., McKinnon, W.B., 2018. Implications of the observed Pluto-Charon density contrast. Icarus 309, 207–219. https://doi.org/10.1016/j.icarus.2018.03.007.

8.3.3 Evolution, geological activity, and internal structure of Charon Desch and Neveu (2017) proposed an even more detailed model in which Pluto and Charon formed from the collision of two partially differentiated precursors. Charon and the other small satellites would form from the collisional debris disk. Then, as described in Section 8.2, heating due to the decay of long-lived radionuclides, assisted by the antifreeze effect of ammonia, allowed the material to reach the temperature threshold for melting the ice, thus creating a subsurface ocean that eventually refroze 1.7–2.5 Gyr ago. The freezing of that ocean should have caused extensional stresses at the origin of Serenity Chasma, and should have led to widespread resurfacing (perhaps explaining the younger age of Vulcan Planum). Subsequently, radiogenic heat would have been able to build up to the point of creating a second ocean which refroze 0.5–1.7 Gyr ago. The freezing enabled cryovolcanism, resulting in the creation of Kubric Mons (Desch and Neveu, 2017).

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In addition, Malamud et al. (2017) highlighted the role that serpentinization may play in causing the extensional tectonics detected on Charon, observed by Beyer et al. (2017). In this model, the tectonic belt observed at the surface of Charon would be due to contraction and expansion episodes which are related to ice compaction, differentiation, and chemical reactions in the interior of Charon. The authors propose a complex history, resulting in a stratified internal structure, which also includes the formation and freezing of a subsurface ocean. The strength of their previous model (Malamud and Prialnik, 2015), also applied to Orcus and Salacia, was to explain the density contrasts between those three objects with a similar set of initial properties. While the scenarios described by Desch and Neveu (2017) and Malamud et al. (2017) need to be confirmed and may not be unique in explaining the features observed at the surface of Charon, they deserve attention since the same processes may take place on other TNOs with properties similar to Charon’s.

8.4 Constraining cryovolcanism and the internal structure of TNOs From the breadth of thermophysical properties involved in the numerical calculations of thermal evolution, as described in Sections 8.2 and 8.3, key characteristics can be identified, which should help constrain the actual evolutionary track of each TNO.

8.4.1 Density Density, more than the mass or the size of an object, is crucial for understanding the formation and evolution of large- and mid-size TNOs (Brown and Butler, 2017). It is one of the most important measurements for determining the first-order structure and composition of any object. Many of the large objects do have satellites (Table 8.2; Brown et al., 2006a), hence the mass of the system can be determined to assess the density of the primary object. Though no correlation could initially be found between the density and the size of a TNO (Brown, 2008), we now know that a trend exists, where larger objects tend to be denser than the smaller ones (McKinnon et al., 2017). This trend may be understood by considering the combined thermal and collisional history of these systems. Indeed, large-scale collisions could have played a role in changing the density of large objects, especially if those were differentiated beforehand (Brown et al., 2007; Brown, 2008, 2013b). Barr and Schwamb (2016) argued that TNOs may fall in two categories: systems with large satellites (like Pluto-Charon and Orcus-Vanth) could be formed from the low-speed collision between undifferentiated (or moderately differentiated) objects, while systems with small satellites (like Eris-Dysnomia) could be formed from the high-speed collision between differentiated objects. Further constraints on the density of TNOs and their satellites are obviously desirable for understanding the formation and early evolution of these worlds.

8.4.2 Shape The stratified internal structure of TNOs might be reflected in their general shape (due to their rotation), and may thus be constrained without any dedicated space mission or in situ

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TABLE 8.2 General characteristics of large- and mid-size TNOs. Number

Name

 (g cm−3 )

D (km)

Surface

1.854 ± 0.006

2374 ± 8

H2 O, N2 , CH4 , CO, C2 H6 , evidence for complex geology: craters, tectonics, mountains, etc.

Charon

1.702 ± 0.021

1212 ± 6

H2 O, NH3 , NH4 + , CH4 , NH3 :H2 O, evidence for tectonics and surface renewal

2326 ± 12

CH4 , CO, N2

1430 ± 9

CH4 partially diluted in N2 , C2 H6 , C2 H2 ; C2 H4 , C3 H8

Satellite Styx

134340

Plutoa

Nix Kerberos Hydra

136199

Eris

Dysnomia

2.52 ± 0.05

136472

Makemake

Yesb

–

136108

Haumea

Hi’iaka

2.551+0.115 −0.010

Namaka 225088

2007 OR10

Yes

50000

Quaoar

Weywot

90377

90482

Sedna

Orcus

?

Vanth

–

1434 ± 14 1922 × 1536 × 998 2322 × 1704 × 1026 1142+647 −467 1535+75 −225

2.13 ± 0.29

1070 ± 38

1.99 ± 0.46

1110 ± 5

<1.82 ± 0.28

>1138 ± 25

–

906+314 −258

2.47 ± 0.89

917 ± 25

1.65+0.35 −0.24

906 ± 72

1.53+0.15 −0.13

910+50 −40

1.854 ± 0.006

2374 ± 8

1.29+0.29 −0.23

829 ± 30

1.26 ± 0.16

866 ± 37

1420+110 −180

H2 O, almost pure

H2 O, CH4 ?, CH3 OH?

H2 O, CH4 , C2 H6 , NH3 :H2 O?, N2 ?, CO?

H2 O, CH4 , C2 H6 ?

H2 O, NH3 ?, CH4 , NH3 :H2 O?, NH4 + ?, C2 H6 ?

Styx 134340

Plutoa

Nix Kerberos

H2 O, N2 , CH4 , CO, C2 H6 , evidence for complex geology: craters, tectonics, mountains, etc.

Hydra 120347

Salacia

Actaea

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Inconclusive

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TABLE 8.2 General characteristics of large- and mid-size TNOs—cont’d Number

Name

Satellite

 (g cm−3 )

D (km)

Surface

174567

Varda

Ilmarë

1.27+0.41 −0.44

705+81 −75

1.24+0.50 −0.35

722+82 −76

Inconclusive

2013 FY27

Yes

–

740+85 −90

2003 AZ84 c

Yes

0.87 ± 0.01

–

724 ± 64 208996

764 ± 6

H2 O, CH3 OH?

692 ± 12 670 ± 34 55637

2002 UX25

Yes

0.80 ± 0.13

659 ± 38

Inconclusive

695+30 −29 a Values for the Pluto/Charon system are from the NASA/New Horizons mission (Stern et al., 2015). b Parker et al. (2016) noted that Makemake has a satellite but the tracking of the orbit was too sparse to provide a reliable estimate of the

system’s mass (currently estimated at 3.15 × 1021 kg, leading to a density of 2.1). c Plutino 2003 AZ may have a nonspherical shape as revealed by stellar occultations, which provide the a size of (470 ± 20) × (383 ± 10) 84 × (245 ± 8) km (Dias-Oliveira et al., 2017), equivalent to a spherical radius of 383 ± 6 km, comparable to the thermal measurements (364 ± 33 km; Mommert et al., 2012). The two solutions given in this table are related to the presence of a surface feature, which can be interpreted as a canyon or a depression. Notes: Diameters given with a gray background are from the “TNOs are Cool” key program on ESA/Herschel for the primary object (available at http://public-tnosarecool.lesia.obspm.fr/Published-results.html). Measurements obtained with different techniques are given for comparison, in particular stellar occultation results are given in bold. References are given in Table 8.3 to keep the table readable. Mid-size object 2015 RR245 has not been included in this table due to few constraints existing on its physical parameters. From an absolute magnitude of 3.6 ± 0.1 and an assumed albedo of 12%, its diameter is estimated at ∼670 km (Bannister et al., 2016).

measurements. The hydrostatic equilibrium shape may indicate whether an object is uniform or stratified. Rambaux et al. (2017) studied the shape of TNOs Makemake, Quaoar, Orcus, Salacia, and Sedna. They showed that differences between the equatorial and polar axes for a homogeneous and a heterogeneous internal structure could be significant enough to be measured by ground-based techniques like stellar occultations. For instance, they predict that for their best case examples, Makemake and Salacia, variations would be of the order of 40 km between a uniform internal structure and a structure with two layers. This difference would further increase if an additional layer is considered (Rambaux personal communication). For the other TNOs studied, the differences are of the order of the size of topographic features possibly present at their surface.

8.4.3 Geomorphological features and surface composition Cryovolcanic activity may be identified from specific geomorphological features, thermal anomalies, or spectroscopic features related to the composition of cryolava. Since it is unlikely that thermal anomalies can be identified in a near future, given the spatial resolution required, we must turn to surface composition and geomorphological features which can be seen from ground- or space-based telescopes, in order to constrain past activity. Interestingly, stellar

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TABLE 8.3 References corresponding to Table 8.1. Number

Name

References

134340

Pluto/Charon

Values from the NASA/New Horizons mission (Stern et al., 2015)

136199

Eris

Sicardy et al. (2011), Brown et al. (2005, 2006b), Dumas et al. (2007), Merlin et al. (2009), Santos-Sanz et al. (2012), Tegler et al. (2010, 2012), and Alvarez-Candal et al. (2011)

136472

Makemake

136108

Haumea

Ortiz et al. (2012), Brown (2013a), Licandro et al. (2006), Brown et al. (2007, 2015), Parker et al. (2016), Lim et al. (2010), Lellouch et al. (2017), and Lorenzi et al. (2015) Lacerda and Jewitt (2007), Barkume et al. (2006), Trujillo et al. (2007), Merlin et al. (2007), Dumas et al. (2011), Pinilla-Alonso et al. (2009), Lellouch et al. (2010), Lockwood et al. (2014), Ortiz et al. (2017), and Rabinowitz et al. (2006)

225088

2007 OR10

Kiss et al. (2017), Santos-Sanz et al. (2012), Pál et al. (2016), Brown et al. (2011, 2012), and Holler et al. (2017b)

50000

Quaoar

Brown and Butler (2017), Braga-Ribas et al. (2013), Davis et al. (2014), Fornasier et al. (2013), Jewitt and Luu (2004), Schaller and Brown (2007), Guilbert et al. (2009), Dalle Ore et al. (2009), and Barucci et al. (2015)

90377

Sedna

Lellouch et al. (2013), Lykawka and Mukai (2005), Trujillo et al. (2005), Emery et al. (2007), and Barucci et al. (2010)

90482

Orcus

Carry et al. (2011), Brown and Butler (2017, 2018), Fornasier et al. (2004, 2013), Thirouin et al. (2014), de Bergh et al. (2005), Trujillo et al. (2005), Barucci et al. (2008), Delsanti et al. (2010), and DeMeo et al. (2010)

120347

Salacia

Fornasier et al. (2013) and Brown and Butler (2017)

2013 FY27

Sheppard et al. (2018)

174567

Varda

Vilenius et al. (2014) and Grundy et al. (2015)

208996

2003 AZ84

Brown and Suer (2007), Dias-Oliveira et al. (2017), Thirouin et al. (2014), Guilbert et al. (2009), Merlin et al. (2010b), and Trujillo et al. (2011)

55637

2002 UX25

Brown and Suer (2007), Brown and Butler (2017), Vilenius et al. (2014), and Lellouch et al. (2017)

occultations of Plutino 2003 AZ84 observed by Dias-Oliveira et al. (2017) reveal a topographic feature along the object’s limb that can be interpreted as a canyon-like feature, perhaps similar to that observed at close range on the surface of Charon. Surface composition alone may not be sufficient to offer a diagnostic for past activity, but the presence of volatiles easily destroyed under irradiation may help identify objects for which surface renewal is likely. Although the presence of methane may effectively hide it, ammonia (or hydrates) has been detected at the surface of Charon, Orcus (see Table 8.2), and possibly Quaoar. In addition, methanol (another antifreeze) has been observed at the surface of some smaller objects, like Pholus or 2002 VE95 (Cruikshank et al., 1998; Barucci et al., 2006). Merlin et al. (2012) considered the formation and destruction processes of methanol, and suggested that at least part of the surface of these objects is younger than the age of the solar system. McKinnon et al. (2008) discussed the fact that although it would be tempting to attribute the presence of methanol on the surface to cryovolcanism, these bodies are significantly smaller

II. Properties and structure

References

195

than any prediction discussed in Section 8.2. Dedicated studies would thus be required to address this specific issue.

8.5 Where should we go from here? The key question we face when coming to investigate the internal structure of TNOs is whether surface and bulk properties, which may be obtained from observations, are sufficient for inferring internal properties and more ambitiously, the evolutionary course of a TNO from formation to present day. If a robust and comprehensive evolutionary model could be constructed and if the initial parameters were known, the answer to this question would be in the affirmative. The problem is that the physical characteristics of these objects are poorly known and theories about formation scenarios vary. Therefore, observations performed to date can only weakly constrain interior models. In the near future, promising observations will be performed to advance this field. Stellar occultations in the ESA/Gaia era can be expected to allow precise constraints on the shapes of TNOs, and reveal possible topographic features at their surface. JWST/NIRSpec observations will access the 3–5 μm wavelength range, where the fundamental absorption features of ices can be observed, thus allowing for the composition of possible cryolava to be identified (Metayer et al., 2019). In addition, extreme adaptive optics systems, such as the ESO/VLT SPHERE instrument (Beuzit et al., 2008), are now able to image binaries and multiple systems, providing us with new reliable constraints on TNOs’ densities. Modeling the evolution of mid-size TNOs is strongly dependent on the details of the model itself, such as the many free parameters, or the thermal, physical, and chemical processes accounted for. Studies summarized in this chapter have taught us that cryovolcanism is enabled by radiogenic heating, aided by the presence of ammonia, methanol, and other antifreeze compounds like salts. In this regard, advance can be achieved by further constraining the effect of CH3 OH, for example, which has been poorly studied so far, or the role of geochemical reactions, like serpentinization. The complexity of the problem stems from the fact that each object may need to be treated as a unique individual world. Eventually, however, consistent initial parameters will be required for the overall population. We may not reach as detailed empirical constraints for each large- and mid-size TNO as for Pluto or Charon, but we should be able to model a large enough number of well-observed objects, using similar processes and initial properties. Thus, thermal evolution modeling remains a powerful tool for studying TNOs in general and cryovolcanism in particular, which should lead to significant progress in our understanding of how the entire TNO population formed and evolved.

References Alvarez-Candal, A., Pinilla-Alonso, N., Licandro, J., Cook, J., Mason, E., Roush, T., Cruikshank, D.P., Gourgeot, F., et al., 2011. The spectrum of (136199) Eris between 350 and 2350 nm: results with X-Shooter. Astron. Astrophys. 532, A130. https://doi.org/10.1051/0004-6361/201117069. Bannister, M.T., Alexandersen, M., Benecchi, S.D., Chen, Y.-T., Delsanti, A., Fraser, W.C., Gladman, B.J., Granvik, M., et al., 2016. OSSOS. IV. Discovery of a dwarf planet candidate in the 9:2 resonance with Neptune. Astron. J. 152, 212. https://doi.org/10.3847/0004-6256/152/6/212.

II. Properties and structure

196

8. Internal structure and cryovolcanism on Trans-Neptunian objects

Barkume, K.M., Brown, M.E., Schaller, E.L., 2006. Water ice on the satellite of Kuiper belt object 2003 EL61 . Astrophys. J. Lett. 640, L87–L89. https://doi.org/10.1086/503159. Barkume, K.M., Brown, M.E., Schaller, E.L., 2008. Near-infrared spectra of centaurs and Kuiper belt objects. Astron. J. 135, 55–67. https://doi.org/10.1088/0004-6256/135/1/55. Barr, A.C., Collins, G.C., 2015. Tectonic activity on Pluto after the Charon-forming impact. Icarus 246, 146–155. https://doi.org/10.1016/j.icarus.2014.03.042. Barr, A.C., Schwamb, M.E., 2016. Interpreting the densities of the Kuiper belt’s dwarf planets. Mon. Not. R. Astron. Soc. 460, 1542–1548. https://doi.org/10.1093/mnras/stw1052. Barucci, M.A., Merlin, F., Dotto, E., Doressoundiram, A., de Bergh, C., 2006. TNO surface ices. Observations of the TNO 55638 (2002 VE95 ). Astron. Astrophys. 455, 725–730. https://doi.org/10.1051/0004-6361:20064951. Barucci, M.A., Merlin, F., Guilbert, A., de Bergh, C., Alvarez-Candal, A., Hainaut, O., Doressoundiram, A., Dumas, C., et al., 2008. Surface composition and temperature of the TNO Orcus. Astron. Astrophys. 479, L13–L16. https://doi.org/10.1051/0004-6361:20079079. Barucci, M.A., Dalle Ore, C.M., Alvarez-Candal, A., de Bergh, C., Merlin, F., Dumas, C., Cruikshank, D.P., 2010. (90377) Sedna: investigation of surface compositional variation. Astron. J. 140, 2095–2100. https://doi.org/10.1088/00046256/140/6/2095. Barucci, M.A., Dalle Ore, C.M., Perna, D., Cruikshank, D.P., Doressoundiram, A., Alvarez-Candal, A., Dotto, E., Nitschelm, C., 2015. (50000) Quaoar: surface composition variability. Astron. Astrophys. 584, A107. https://doi.org/10.1051/0004-6361/201526119. Beuzit, J.-L., Feldt, M., Dohlen, K., Mouillet, D., Puget, P., Wildi, F., Abe, L., Antichi, J., et al., 2008. SPHERE: a “Planet Finder” instrument for the VLT. In: Ground-Based and Airborne Instrumentation for Astronomy II, Proc. SPIE, vol. 7014, p. 701418. Beyer, R.A., Nimmo, F., McKinnon, W.B., Moore, J.M., Binzel, R.P., Conrad, J.W., Cheng, A., Ennico, K., et al., 2017. Charon tectonics. Icarus 287, 161–174. https://doi.org/10.1016/j.icarus.2016.12.018. Bierson, C.J., Nimmo, F., McKinnon, W.B., 2018. Implications of the observed Pluto-Charon density contrast. Icarus 309, 207–219. https://doi.org/10.1016/j.icarus.2018.03.007. Braga-Ribas, F., Sicardy, B., Ortiz, J.L., Lellouch, E., Tancredi, G., Lecacheux, J., Vieira-Martins, R., Camargo, J.I.B., et al., 2013. The size, shape, albedo, density, and atmospheric limit of transneptunian object (50000) Quaoar from multi-chord stellar occultations. Astrophys. J. 773, 26. https://doi.org/10.1088/0004-637X/773/1/26. Brown, M.E., 2008. The largest Kuiper belt objects. In: Barucci, M.A., Boehnhardt, H., Cruikshank, D.P., Morbidelli, A. (Eds.), The Solar System Beyond Neptune. University of Arizona Press, Tucson, pp. 335–344. Brown, M.E., 2012. The compositions of Kuiper belt objects. Annu. Rev. Earth Planet. Sci. 40, 467–494. https://doi.org/10.1146/annurev-earth-042711-105352. Brown, M.E., 2013a. On the size, shape, and density of dwarf planet Makemake. Astrophys. J. Lett. 767 (1), Article ID: L7. Brown, M.E., 2013b. The density of mid-sized Kuiper belt object 2002 UX25 and the formation of the dwarf planets. Astrophys. J. Lett. 778 (2), Article ID: L34. Brown, M.E., Butler, B.J., 2017. The density of mid-sized Kuiper belt objects from ALMA thermal observations. Astron. J. 154, 19. https://doi.org/10.3847/1538-3881/aa6346. Brown, M.E., Butler, B.J., 2018. Medium-sized satellites of large Kuiper belt objects. Astron. J. 156, 164. https://doi.org/10.3847/1538-3881/aad9f2. Brown, M.E., Calvin, W.M., 2000. Evidence for crystalline water and ammonia ices on Pluto’s satellite Charon. Science 287, 107–109. https://doi.org/10.1126/science.287.5450.107. Brown, M.E., Suer, T.-A., 2007. Satellites of 2003 AZ_84, (50000), (55637), and (90482). Int. Astron. Union Circ. 8812, 1. Brown, M.E., Trujillo, C.A., Rabinowitz, D.L., 2005. Discovery of a planetary-sized object in the scattered Kuiper belt. Astrophys. J. Lett. 635, L97–L100. https://doi.org/10.1086/499336. Brown, M.E., van Dam, M.A., Bouchez, A.H., Le Mignant, D., Campbell, R.D., Chin, J.C.Y., Conrad, A., Hartman, S.K., et al., 2006a. Satellites of the largest Kuiper belt objects. Astrophys. J. Lett. 639, L43–L46. https://doi.org/10.1086/501524. Brown, M.E., Schaller, E.L., Roe, H.G., Rabinowitz, D.L., Trujillo, C.A., 2006b. Direct measurement of the size of 2003 UB313 from the Hubble Space Telescope. Astrophys. J. Lett. 643, L61–L63. https://doi.org/10.1086/504843. Brown, M.E., Barkume, K.M., Blake, G.A., Schaller, E.L., Rabinowitz, D.L., Roe, H.G., Trujillo, C.A., 2007. Methane and ethane on the bright Kuiper belt object 2005 FY9. Astron. J. 133, 284. Brown, M.E., Burgasser, A.J., Fraser, W.C., 2011. The surface composition of large Kuiper belt object 2007 OR10 . Astrophys. J. Lett. 738, L26. https://doi.org/10.1088/2041-8205/738/2/L26.

II. Properties and structure

References

197

Brown, M.E., Schaller, E.L., Fraser, W.C., 2012. Water ice in the Kuiper belt. Astron. J. 143, 146. https://doi.org/10.1088/0004-6256/143/6/146. Brown, M.E., Schaller, E.L., Blake, G.A., 2015. Irradiation products on dwarf planet Makemake. Astron. J. 149, 105. https://doi.org/10.1088/0004-6256/149/3/105. Buhler, P.B., Ingersoll, A.P., 2018. Sublimation pit distribution indicates convection cell surface velocities of ∼10 cm per year in Sputnik Planitia, Pluto. Icarus 300, 327–340. https://doi.org/10.1016/j.icarus.2017.09.018. Buie, M.W., Grundy, W.M., 2000. The distribution and physical state of H2 O on Charon. Icarus 148, 324–339. https://doi.org/10.1006/icar.2000.6509. Canup, R.M., 2005. A giant impact origin of Pluto-Charon. Science 307, 546–550. https://doi.org/10.1126/science. 1106818. Canup, R.M., 2011. On a giant impact origin of Charon, Nix, and Hydra. Astron. J. 141, 35. https://doi.org/10.1088/ 0004-6256/141/2/35. Carry, B., Hestroffer, D., DeMeo, F.E., Thirouin, A., Berthier, J., Lacerda, P., Sicardy, B., Doressoundiram, A., et al., 2011. Integral-field spectroscopy of (90482) Orcus-Vanth. Astron. Astrophys. 534, A115. https://doi.org/10.1051/0004-6361/201117486. Choi, Y.-J., Cohen, M., Merk, R., Prialnik, D., 2002. Long-term evolution of objects in the Kuiper belt zone—effects of insolation and radiogenic heating. Icarus 160, 300–312. https://doi.org/10.1006/icar.2002.6976. Cook, J.C., Desch, S.J., Roush, T.L., Trujillo, C.A., Geballe, T.R., 2007. Near-infrared spectroscopy of Charon: possible evidence for cryovolcanism on Kuiper belt objects. Astrophys. J. 663, 1406–1419. https://doi.org/10.1086/ 518222. Cooper, J.F., Christian, E.R., Richardson, J.D., Wang, C., 2003. Proton irradiation of centaur, Kuiper belt, and Oort cloud objects at plasma to cosmic ray energy. Earth Moon Planets 92, 261–277. https://doi.org/10.1023/B:MOON. 0000031944.41883.80. Coradini, A., Capria, M.T., de Sanctis, M.C., McKinnon, W.B., 2008. The structure of Kuiper belt bodies: link with comets. In: Barucci, M.A., Boehnhardt, H., Cruikshank, D.P., Morbidelli, A. (Eds.), The Solar System Beyond Neptune. University of Arizona Press, Tucson, pp. 243–256. Cruikshank, D.P., Roush, T.L., Bartholomew, M.J., Geballe, T.R., Pendleton, Y.J., White, S.M., Bell, J.F., Davies, J.K., et al., 1998. The composition of centaur 5145 Pholus. Icarus 135, 389–407. https://doi.org/10.1006/icar.1998.5997. Cruikshank, D.P., Owen, T.C., Dalle Ore, C.M., Geballe, T.R., Roush, T.L., de Bergh, C., Sandford, S.A., Poulet, F., et al., 2005. A spectroscopic study of the surfaces of Saturn’s large satellites: H2 O ice, tholins, and minor constituents. Icarus 175, 268–283. https://doi.org/10.1016/j.icarus.2004.09.003. Cruikshank, D.P., Grundy, W.M., DeMeo, F.E., Buie, M.W., Binzel, R.P., Jennings, D.E., Olkin, C.B., Parker, J.W., et al., 2015. The surface compositions of Pluto and Charon. Icarus 246, 82–92. https://doi.org/10.1016/j.icarus.2014. 05.023. Dalle Ore, C.M., Barucci, M.A., Emery, J.P., Cruikshank, D.P., Dalle Ore, L.V., Merlin, F., Alvarez-Candal, A., de Bergh, C., et al., 2009. Composition of KBO (50000) Quaoar. Astron. Astrophys. 501, 349–357. https://doi.org/10.1051/0004-6361/200911752. Dalle Ore, C.M., Protopapa, S., Cook, J.C., Grundy, W.M., Cruikshank, D.P., Verbiscer, A.J., et al., 2018. Ices on Charon: distribution of H2 O and NH3 from new horizons LEISA observations. Icarus 300, 21–32. https://doi.org/10.1016/j.icarus.2017.08.026. Davis, A.B., Pasachoff, J.M., Babcock, B.A., Person, M.J., Zuluaga, C.A., Bosh, A.S., Levine, S., Naranjo, O.A., et al., 2014. Observation and analysis of a single-chord stellar occultation by Kuiper belt object (50000) Quaoar. In: American Astronomical Society Meeting Abstracts No. 223. de Bergh, C., Delsanti, A., Tozzi, G.P., Dotto, E., Doressoundiram, A., Barucci, M.A., 2005. The surface of the transneptunian object 90482 Orcus. Astron. Astrophys. 437, 1115–1120. https://doi.org/10.1051/0004-6361:20042533. De Sanctis, M.C., Capria, M.T., Coradini, A., 2001. Thermal evolution and differentiation of Edgeworth-Kuiper belt objects. Astron. J. 121, 2792–2799. https://doi.org/10.1086/320385. Delsanti, A., Merlin, F., Guilbert-Lepoutre, A., Bauer, J., Yang, B., Meech, K.J., 2010. Methane, ammonia, and their irradiation products at the surface of an intermediate-size KBO? A portrait of Plutino (90482) Orcus. Astron. Astrophys. 520, A40. https://doi.org/10.1051/0004-6361/201014296. DeMeo, F.E., Barucci, M.A., Merlin, F., Guilbert-Lepoutre, A., Alvarez-Candal, A., Delsanti, A., Fornasier, S., de Bergh, C., 2010. A spectroscopic analysis of Jupiter-coupled object (52872) Okyrhoe, and TNOs (90482) Orcus and (73480) 2002 PN34 . Astron. Astrophys. 521, A35. https://doi.org/10.1051/0004-6361/201014042. DeMeo, F.E., Dumas, C., Cook, J.C., Carry, B., Merlin, F., Verbiscer, A.J., Binzel, R.P., 2015. Spectral variability of Charon’s 2.21-μm feature. Icarus 246, 213–219. https://doi.org/10.1016/j.icarus.2014.04.010.

II. Properties and structure

198

8. Internal structure and cryovolcanism on Trans-Neptunian objects

Desch, S.J., 2015. Density of Charon formed from a disk generated by the impact of partially differentiated bodies. Icarus 246, 37–47. https://doi.org/10.1016/j.icarus.2014.07.034. Desch, S.J., Neveu, M., 2017. Differentiation and cryovolcanism on Charon: a view before and after New Horizons. Icarus 287, 175–186. https://doi.org/10.1016/j.icarus.2016.11.037. Desch, S.J., Cook, J.C., Doggett, T.C., Porter, S.B., 2009. Thermal evolution of Kuiper belt objects, with implications for cryovolcanism. Icarus 202, 694–714. https://doi.org/10.1016/j.icarus.2009.03.009. Dias-Oliveira, A., Sicardy, B., Ortiz, J.L., Braga-Ribas, F., Leiva, R., Vieira-Martins, R., Benedetti-Rossi, G., Camargo, J.I.B., et al., 2017. Study of the Plutino object (208996) 2003 AZ84 from stellar occultations: size, shape, and topographic features. Astron. J. 154, 22. https://doi.org/10.3847/1538-3881/aa74e9. Dumas, C., Terrile, R.J., Brown, R.H., Schneider, G., Smith, B.A., 2001. Hubble space telescope NICMOS spectroscopy of Charon’s leading and trailing hemispheres. Astron. J. 121, 1163–1170. https://doi.org/10.1086/318747. Dumas, C., Merlin, F., Barucci, M.A., de Bergh, C., Hainault, O., Guilbert, A., Vernazza, P., Doressoundiram, A., 2007. Surface composition of the largest dwarf planet 136199 Eris (2003 UB313 ). Astron. Astrophys. 471, 331–334. https://doi.org/10.1051/0004-6361:20066665. Dumas, C., Carry, B., Hestroffer, D., Merlin, F., 2011. High-contrast observations of (136108) Haumea. A crystalline water-ice multiple system. Astron. Astrophys. 528, A105. https://doi.org/10.1051/0004-6361/201015011. Durand-Manterola, H.J., 2003. Internal structure of planetary icy bodies. In: EGS-AGU-EUG Joint Assembly, p. 7389. Emery, J.P., Dalle Ore, C.M., Cruikshank, D.P., Fernández, Y.R., Trilling, D.E., Stansberry, J.A., 2007. Ices on (90377) Sedna: confirmation and compositional constraints. Astron. Astrophys. 466, 395–398. https://doi.org/10.1051/ 0004-6361:20067021. Fornasier, S., Dotto, E., Barucci, M.A., Barbieri, C., 2004. Water ice on the surface of the large TNO 2004 DW. Astron. Astrophys. 422, L43–L46. https://doi.org/10.1051/0004-6361:20048004. Fornasier, S., Lellouch, E., Müller, T., Santos-Sanz, P., Panuzzo, P., Kiss, C., Lim, T., Mommert, M., et al., 2013. TNOs are Cool: a survey of the Trans-Neptunian region. VIII. Combined Herschel PACS and SPIRE observations of nine bright targets at 70–500 μm. Astron. Astrophys. 555, A15. https://doi.org/10.1051/0004-6361/201321329. Geissler, P.E., 2015. Cryovolcanism in the outer solar system. In: Sigurdsson, H. (Ed.), The Encyclopedia of Volcanoes, pp. 763–776. Grundy, W.M., Porter, S.B., Benecchi, S.D., Roe, H.G., Noll, K.S., Trujillo, C.A., Thirouin, A., Stansberry, J.A., et al., 2015. The mutual orbit, mass, and density of the large transneptunian binary system Varda and Ilmarë. Icarus 257, 130–138. https://doi.org/10.1016/j.icarus.2015.04.036. Grundy, W.M., Binzel, R.P., Buratti, B.J., Cook, J.C., Cruikshank, D.P., Dalle Ore, C.M., et al., 2016. Surface compositions across Pluto and Charon. Science 351, aad9189. https://doi.org/10.1126/science.aad9189. Guilbert, A., Alvarez-Candal, A., Merlin, F., Barucci, M.A., Dumas, C., de Bergh, C., Delsanti, A., 2009. ESO-large program on TNOs: near-infrared spectroscopy with SINFONI. Icarus 201, 272–283. https://doi.org/10.1016/j. icarus.2008.12.023. Holler, B.J., Young, L.A., Buie, M.W., Grundy, W.M., Lyke, J.E., Young, E.F., Roe, H.G., 2017a. Measuring temperature and ammonia hydrate ice on Charon in 2015 from Keck/OSIRIS spectra. Icarus 284, 394–406. https://doi.org/10.1016/j.icarus.2016.12.003. Holler, B.J., Young, L.A., Bus, S.J., Protopapa, S., 2017b. Methanol ice on Kuiper belt objects 2007 OR10 and Salacia: implications for formation and dynamical evolution. Eur. Planet. Sci. Congr. 11, EPSC2017-330. Hussmann, H., Sohl, F., Spohn, T., 2006. Subsurface oceans and deep interiors of medium-sized outer planet satellites and large Trans-Neptunian objects. Icarus 185, 258–273. https://doi.org/10.1016/j.icarus.2006.06.005. Jewitt, D.C., Luu, J., 2004. Crystalline water ice on the Kuiper belt object (50000) Quaoar. Nature 432, 731–733. https://doi.org/10.1038/nature03111. Kiss, C., Marton, G., Farkas-Takács, A., Stansberry, J., Müller, T., Vinkó, J., Balog, Z., Ortiz, J.-L., et al., 2017. Discovery of a satellite of the large Trans-Neptunian object (225088) 2007 OR10 . Astrophys. J. Lett. 838, L1. https://doi.org/10. 3847/2041-8213/aa6484. Lacerda, P., Jewitt, D.C., 2007. Densities of solar system objects from their rotational light curves. Astron. J. 133, 1393. https://doi.org/10.1086/511772. Lellouch, E., Kiss, C., Santos-Sanz, P., Müller, T.G., Fornasier, S., Groussin, O., Lacerda, P., Ortiz, J.L., et al., 2010. “TNOs are Cool”: a survey of the Trans-Neptunian region. II. The thermal lightcurve of (136108) Haumea. Astron. Astrophys. 518, L147. https://doi.org/10.1051/0004-6361/201014648.

II. Properties and structure

References

199

Lellouch, E., Santos-Sanz, P., Lacerda, P., Mommert, M., Duffard, R., Ortiz, J.L., Müller, T.G., Fornasier, S., et al., 2013. “TNOs are Cool”: a survey of the Trans-Neptunian region. IX. Thermal properties of Kuiper belt objects and Centaurs from combined Herschel and Spitzer observations. Astron. Astrophys. 557, A60. https://doi.org/10. 1051/0004-6361/201322047. Lellouch, E., Moreno, R., Müller, T., Fornasier, S., Santos-Sanz, P., Moullet, A., Gurwell, M., Stansberry, J., et al., 2017. The thermal emission of Centaurs and Trans-Neptunian objects at millimeter wavelengths from ALMA observations. Astron. Astrophys. 608, A45. https://doi.org/10.1051/0004-6361/201731676. Leto, G., Baratta, G.A., 2003. Ly-alpha photon induced amorphization of IC water ice at 16 Kelvin. Effects and quantitative comparison with ion irradiation. Astron. Astrophys. 397, 7–13. https://doi.org/10.1051/0004-6361:20021473. Licandro, J., Pinilla-Alonso, N., Pedani, M., Oliva, E., Tozzi, G.P., Grundy, W.M., 2006. The methane ice rich surface of large TNO 2005 FY_9: a Pluto-twin in the Trans-Neptunian belt? Astron. Astrophys. 445, L35–L38. https://doi.org/10.1051/0004-6361:200500219. Lim, T.L., Stansberry, J., Müller, T.G., Mueller, M., Lellouch, E., Kiss, C., Santos-Sanz, P., Vilenius, E., et al., 2010. “TNOs are Cool”: a survey of the Trans-Neptunian region. III. Thermophysical properties of 90482 Orcus and 136472 Makemake. Astron. Astrophys. 518, L148. https://doi.org/10.1051/0004-6361/201014701. Lockwood, A.C., Brown, M.E., Stansberry, J., 2014. The size and shape of the oblong dwarf planet Haumea. Earth Moon Planets 111, 127–137. https://doi.org/10.1007/s11038-014-9430-1. Lorenzi, V., Pinilla-Alonso, N., Licandro, J., 2015. Rotationally resolved spectroscopy of dwarf planet (136472) Makemake. Astron. Astrophys. 577, A86. https://doi.org/10.1051/0004-6361/201425575. Lykawka, P.S., Mukai, T., 2005. Higher albedos and size distribution of large transneptunian objects. Planet. Space Sci. 53, 1319–1330. https://doi.org/10.1016/j.pss.2005.06.004. Malamud, U., Prialnik, D., 2015. Modeling Kuiper belt objects Charon, Orcus and Salacia by means of a new equation of state for porous icy bodies. Icarus 246, 21–36. https://doi.org/10.1016/j.icarus.2014.02.027. Malamud, U., Perets, H.B., Schubert, G., 2017. The contraction/expansion history of Charon with implications for its planetary-scale tectonic belt. Mon. Not. R. Astron. Soc. 468, 1056–1069. https://doi.org/10.1093/mnras/stx546. Mastrapa, R.M.E., Brown, R.H., 2006. Ion irradiation of crystalline H2 O-ice: effect on the 1.65-μm band. Icarus 183, 207–214. https://doi.org/10.1016/j.icarus.2006.02.006. McKinnon, W.B., 2002. On the initial thermal evolution of Kuiper belt objects. In: Warmbein, B. (Ed.), Asteroids, Comets, and Meteors: ACM 2002, vol. 500. ESA Special Publication, pp. 29–38. McKinnon, W.B., Prialnik, D., Stern, S.A., Coradini, A., 2008. Structure and evolution of Kuiper belt objects and dwarf planets. In: Barucci, M.A., Boehnhardt, H., Cruikshank, D.P., Morbidelli, A. (Eds.), The Solar System Beyond Neptune. University of Arizona Press, Tucson, pp. 213–241. McKinnon, W.B., Nimmo, F., Wong, T., Schenk, P.M., White, O.L., Roberts, J.H., Moore, J.M., Spencer, J.R., et al., 2016. Convection in a volatile nitrogen-ice-rich layer drives Pluto’s geological vigour. Nature 534, 82–85. https://doi.org/10.1038/nature18289. McKinnon, W.B., Stern, S.A., Weaver, H.A., Nimmo, F., Bierson, C.J., Grundy, W.M., Cook, J.C., Cruikshank, D.P., et al., 2017. Origin of the Pluto-Charon system: constraints from the new horizons flyby. Icarus 287, 2–11. https://doi.org/10.1016/j.icarus.2016.11.019. Merk, R., Prialnik, D., 2003. Early thermal and structural evolution of small bodies in the Trans-Neptunian zone. Earth Moon Planets 92, 359–374. https://doi.org/10.1023/B:MOON.0000031952.89891.a4. Merk, R., Prialnik, D., 2006. Combined modeling of thermal evolution and accretion of Trans-Neptunian objects—occurrence of high temperatures and liquid water. Icarus 183, 283–295. https://doi.org/10.1016/j.icarus.2006.02.011. Merlin, F., Guilbert, A., Dumas, C., Barucci, M.A., de Bergh, C., Vernazza, P., 2007. Properties of the icy surface of the TNO 136108 (2003 EL{61}). Astron. Astrophys. 466, 1185–1188. https://doi.org/10.1051/0004-6361:20066866. Merlin, F., Alvarez-Candal, A., Delsanti, A., Fornasier, S., Barucci, M.A., DeMeo, F.E., de Bergh, C., Doressoundiram, A., et al., 2009. Stratification of methane ice on Eris’ surface. Astron. J. 137, 315–328. https://doi.org/10.1088/00046256/137/1/315. Merlin, F., Barucci, M.A., de Bergh, C., DeMeo, F.E., Alvarez-Candal, A., Dumas, C., Cruikshank, D.P., 2010a. Chemical and physical properties of the variegated Pluto and Charon surfaces. Icarus 210, 930–943. https://doi.org/10. 1016/j.icarus.2010.07.028. Merlin, F., Barucci, M.A., de Bergh, C., Fornasier, S., Doressoundiram, A., Perna, D., Protopapa, S., 2010b. Surface composition and physical properties of several Trans-Neptunian objects from the Hapke scattering theory and Shkuratov model. Icarus 208, 945–954. https://doi.org/10.1016/j.icarus.2010.03.014.

II. Properties and structure

200

8. Internal structure and cryovolcanism on Trans-Neptunian objects

Merlin, F., Quirico, E., Barucci, M.A., de Bergh, C., 2012. Methanol ice on the surface of minor bodies in the solar system. Astron. Astrophys. 544, A20. https://doi.org/10.1051/0004-6361/201219181. Metayer, R., Guilbert-Lepoutre, A., Ferruit, P., Merlin, F., Holler, B.J., Cabral, N., et al., 2019. JWST/NIRSpec prospects on transneptunian objects. Front. Astron. Space Sci. 6, A8. https://doi.org/10.3389/fspas.2019.00008. Mommert, M., Harris, A.W., Kiss, C., Pál, A., Santos-Sanz, P., Stansberry, J., Delsanti, A., Vilenius, E., et al., 2012. TNOs are Cool: a survey of the Trans-Neptunian region. V. Physical characterization of 18 Plutinos using Herschel-PACS observations. Astron. Astrophys. 541, A93. https://doi.org/10.1051/0004-6361/201118562. Moore, M.H., Ferrante, R.F., Hudson, R.L., Stone, J.N., 2007. Ammonia water ice laboratory studies relevant to outer Solar System surfaces. Icarus 190, 260–273. https://doi.org/10.1016/j.icarus.2007.02.020. Moore, J.M., McKinnon, W.B., Spencer, J.R., Howard, A.D., Schenk, P.M., Beyer, R.A., Nimmo, F., Singer, K.N., et al., 2016. The geology of Pluto and Charon through the eyes of new horizons. Science 351, 1284–1293. https://doi.org/10.1126/science.aad7055. Neveu, M., Desch, S.J., Shock, E.L., Glein, C.R., 2015. Prerequisites for explosive cryovolcanism on dwarf planet-class Kuiper belt objects. Icarus 246, 48–64. https://doi.org/10.1016/j.icarus.2014.03.043. Ortiz, J.L., Sicardy, B., Braga-Ribas, F., Alvarez- Candal, A., Lellouch, E., Duffard, R., Pinilla-Alonso, N., Ivanov, V.D., et al., 2012. Albedo and atmospheric constraints of dwarf planet Makemake from a stellar occultation. Nature 491, 566–569. https://doi.org/10.1038/nature11597. Ortiz, J.L., Santos-Sanz, P., Sicardy, B., Benedetti-Rossi, G., Bérard, D., Morales, N., Duffard, R., Braga-Ribas, F., et al., 2017. The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation. Nature 550, 219–223. https://doi.org/10.1038/nature24051. Pál, A., Kiss, C., Müller, T.G., Molnár, L., Szabó, R., Szabó, G.M., Sárneczky, K., Kiss, L.L., 2016. Large size and slow rotation of the Trans-Neptunian object (225088) 2007 OR10 discovered from Herschel and K2 observations. Astron. J. 151, 117. https://doi.org/10.3847/0004-6256/151/5/117. Parker, A.H., Buie, M.W., Grundy, W.M., Noll, K.S., 2016. Discovery of a Makemakean Moon. Astrophys. J. Lett. 825, L9. https://doi.org/10.3847/2041-8205/825/1/L9. Petit, J.-M., Kavelaars, J.J., Gladman, B., Loredo, T., 2008. Size distribution of multikilometer transneptunian objects. In: Barucci, M.A., Boehnhardt, H., Cruikshank, D.P., Morbidelli, A. (Eds.), The Solar System Beyond Neptune. University of Arizona Press, Tucson, pp. 71–87. Pinilla-Alonso, N., Brunetto, R., Licandro, J., Gil-Hutton, R., Roush, T.L., Strazzulla, G., 2009. The surface of (136108) Haumea (2003 EL{61}), the largest carbon-depleted object in the Trans-Neptunian belt. Astron. Astrophys. 496, 547–556. https://doi.org/10.1051/0004-6361/200809733. Prialnik, D., 2000. Physical characteristics of distant comets. In: Fitzsimmons, A., Jewitt, D., West, R.M. (Eds.), Minor Bodies in the Outer Solar System ESO Astrophysics Symposia (European Southern Observatory). Springer, Berlin, Heidelberg, p. 33. Prialnik, D., Sarid, G., Rosenberg, E.D., Merk, R., 2008. Thermal and chemical evolution of comet nuclei and Kuiper belt objects. Space Sci. Rev. 138, 147–164. https://doi.org/10.1007/s11214-007-9301-4. Rabinowitz, D.L., Barkume, K., Brown, M.E., Roe, H., Schwartz, M., Tourtellotte, S., Trujillo, C., 2006. Photometric observations constraining the size, shape, and albedo of 2003 EL61, a rapidly rotating, Pluto-sized object in the Kuiper belt. Astrophys. J. 639, 1238–1251. https://doi.org/10.1086/499575. Rambaux, N., Baguet, D., Chambat, F., Castillo-Rogez, J.C., 2017. Equilibrium shapes of large Trans-Neptunian objects. Astrophys. J. Lett. 850, L9. https://doi.org/10.3847/2041-8213/aa95bd. Robuchon, G., Nimmo, F., 2011. Thermal evolution of Pluto and implications for surface tectonics and a subsurface ocean. Icarus 216, 426–439. https://doi.org/10.1016/j.icarus.2011.08.015. Santos-Sanz, P., Lellouch, E., Fornasier, S., Kiss, C., Pál, A., Müller, T.G., Vilenius, E., Stansberry, J., et al., 2012. “TNOs are Cool”: a survey of the Trans-Neptunian region. IV. Size/albedo characterization of 15 scattered disk and detached objects observed with Herschel-PACS. Astron. Astrophys. 541, A92. https://doi.org/10.1051/0004-6361/201118541. Schaller, E.L., Brown, M.E., 2007. Detection of methane on Kuiper belt object (50000) Quaoar. Astrophys. J. Lett. 670, L49–L51. https://doi.org/10.1086/524140. Schubert, G., Hussmann, H., Lainey, V., Matson, D.L., McKinnon, W.B., Sohl, F., Sotin, C., Tobie, G., et al., 2010. Evolution of icy satellites. Space Sci. Rev. 153, 447–484. https://doi.org/10.1007/s11214-010-9635-1. Sheppard, S.S., Fernandez, Y.R., Moullet, A., 2018. The albedos, sizes, colors, and satellites of dwarf planets compared with newly measured dwarf planet 2013 FY27. Astron. J. 156, 270. https://doi.org/10.3847/1538-3881/aae92a.

II. Properties and structure

References

201

Sicardy, B., Ortiz, J.L., Assafin, M., Jehin, E., Maury, A., Lellouch, E., Hutton, R.G., Braga-Ribas, F., et al., 2011. A Pluto-like radius and a high albedo for the dwarf planet Eris from an occultation. Nature 478, 493–496. https://doi.org/10.1038/nature10550. Singer, K.N., McKinnon, W.B., Gladman, B., Greenstreet, S., Bierhaus, E.B., Stern, S.A., et al., 2019. Impact craters on Pluto and Charon indicate a deficit of small Kuiper belt objects. Science 363, 955–959. https://doi.org/10.1126/ science.aap8628. Stern, S.A., 1989. Pluto—comments on crustal composition, evidence for global differentiation. Icarus 81, 14–23. https://doi.org/10.1016/0019-1035(89)90121-8. Stern, S.A., Bagenal, F., Ennico, K., Gladstone, G.R., Grundy, W.M., McKinnon, W.B., Moore, J.M., Olkin, C.B., et al., 2015. The Pluto system: initial results from its exploration by New Horizons. Science 350, aad1815. https://doi.org/10.1126/science.aad1815. Stevenson, D.J., 1982. Volcanism and igneous processes in small icy satellites. Nature 298, 142–144. https://doi.org/10. 1038/298142a0. Tegler, S.C., Cornelison, D.M., Grundy, W.M., Romanishin, W., Abernathy, M.R., Bovyn, M.J., Burt, J.A., Evans, D.E., et al., 2010. Methane and nitrogen abundances on Pluto and Eris. Astrophys. J. 725, 1296–1305. https://doi.org/10. 1088/0004-637X/725/1/1296. Tegler, S.C., Grundy, W.M., Olkin, C.B., Young, L.A., Romanishin, W., Cornelison, D.M., Khodadadkouchaki, R., 2012. Ice mineralogy across and into the surfaces of Pluto, Triton, and Eris. Astrophys. J. 751, 76. https://doi.org/10.1088/0004-637X/751/1/76. Thirouin, A., Noll, K.S., Ortiz, J.L., Morales, N., 2014. Rotational properties of the binary and non-binary populations in the Trans-Neptunian belt. Astron. Astrophys. 569, A3. https://doi.org/10.1051/0004-6361/201423567. Trilling, D.E., 2016. The surface age of Sputnik Planum, Pluto, must be less than 10 million years. PLoS ONE 11, e0147386. https://doi.org/10.1371/journal.pone.0147386. Trowbridge, A.J., Melosh, H.J., Steckloff, J.K., Freed, A.M., 2016. Vigorous convection as the explanation for Pluto’s polygonal terrain. Nature 534, 79–81. https://doi.org/10.1038/nature18016. Trujillo, C.A., Brown, M.E., Rabinowitz, D.L., Geballe, T.R., 2005. Near-infrared surface properties of the two intrinsically brightest minor Planets: (90377) Sedna and (90482) Orcus. Astrophys. J. 627, 1057–1065. https://doi.org/10.1086/430337. Trujillo, C.A., Brown, M.E., Barkume, K.M., Schaller, E.L., Rabinowitz, D.L., 2007. The surface of 2003 EL61 in the near-infrared. Astrophys. J. 655, 1172–1178. https://doi.org/10.1086/509861. Trujillo, C.A., Sheppard, S.S., Schaller, E.L., 2011. A photometric system for detection of water and methane ices on Kuiper belt objects. Astrophys. J. 730, 105. https://doi.org/10.1088/0004-637X/730/2/105. Vilenius, E., Kiss, C., Müller, T., Mommert, M., Santos-Sanz, P., Pál, A., Stansberry, J., Mueller, M., et al., 2014. “TNOs are Cool”: a survey of the Trans-Neptunian region. X. Analysis of classical Kuiper belt objects from Herschel and Spitzer observations. Astron. Astrophys. 564, A35. https://doi.org/10.1051/0004-6361/201322416. Zheng, W., Jewitt, D., Kaiser, R.I., 2009. On the state of water ice on Saturn’s Moon Titan and implications to icy bodies in the outer solar system. J. Phys. Chem. A 113, 11174–11181. https://doi.org/10.1021/jp903817y.

II. Properties and structure