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On the Origin and Thermal Stability of Arrokoths and Plutos Ices

We discuss in a thermodynamic, geologically empirical way the long-term nature of the stable majority ices that could be present in Kuiper Belt Object 2014 MU69 after its 4.6 Gyr residence in the EKB as a cold classical object. Considering the stability versus sublimation into vacuum for the suite of ices commonly found on comets, Centaurs, and KBOs at the average ~40K sunlit surface temperature of MU69 over Myr to Gyr, we find only 3 common ices that are truly refractory: HCN, CH3OH, and H2O (in order of increasing stability). NH3 and H2CO ices are marginally stable and may be removed by any positive temperature excursions in the EKB, as produced every 1e8 - 1e9 yrs by nearby supernovae and passing O/B stars. To date the NH team has reported the presence of abundant CH3OH and evidence for H2O on MU69s surface (Lisse et al. 2017, Grundy et al. 2020). NH3 has been searched for, but not found. We predict that future absorption feature detections will be due to an HCN or poly-H2CO based species. Consideration of the conditions present in the EKB region during the formation era of MU69 lead us to infer that it formed"in the dark", in an optically thick mid-plane, unable to see the nascent, variable, highly luminous Young Stellar Object-TTauri Sun, and that KBOs contain HCN and CH3OH ice phases in addition to the H2O ice phases found in their Short Period comet descendants. Finally, when we apply our ice thermal stability analysis to bodies/populations related to MU69, we find that methanol ice may be ubiquitous in the outer solar system; that if Pluto is not a fully differentiated body, then it must have gained its hypervolatile ices from proto-planetary disk sources in the first few Myr of the solar systems existence; and that hypervolatile rich, highly primordial comet C/2016 R2 was placed onto an Oort Cloud orbit on a similar timescale.

On 01 Jan 2019, the New Horizons spacecraft visited for the first time ever a small, pristine cold classical Kuiper Belt object (KBO), Arrokoth (2014 MU69; hereafter MU69). Resolved multiband photometric imaging and near-infrared (NIR) spectroscopy were obtained of MU69's surface, producing a picture of a dark, highly reddened object containing solid water ice, methanol, and tholins (Stern et al. 2019) and no detectable gas in any surrounding coma.
Because its orbit is nearly circular and uncoupled to the solar system's planets, MU69 is thought to have formed in place 4.6 Gyrs ago at ~45 AU, and could contain one of the most primitive collections of early solar system material studied to date. Naively, one might think that its current low ambient dayside average temperature of ~42 K should have kept most icy species in "deep freeze", leaving them unreacted and unchanged since their formation. The ice species of interest include those detected on planetary satellites, Centaurs, Pluto, and KBOs in the outer Solar System like H2O, CH4, N2, CO, CO2, CH3OH (methanol), HCN (hydrogen cyanide), NH3•nH2O (ammonia hydrate), and C2H6 (ethane); and those detected in cometary comae as the products of nuclear mass loss, such as C2H2, C3H8, SO2, and O2 (Bieler et al. 2015, Mall et al. 2016).
However, while CH3OH was found from two well-defined absorption bands at 2.27 and 2.34 µm by the New Horizons Linear Etalon Imaging Spectral Array LEISA (LEISA) 1.25 -2.5 µm mapping spectrometer (λ/Δλ = 240 and MU69 spatial resolution ~1.8 km px -1 for a, 33 x 17 km object ;Reuter et al. 2008, Stern et al. 2019; Figure 1), H2O signatures were only weakly present if at all (as seen by a broad H2O absorption band centered at 2.0 µm; Lisse et al. 2017, Grundy et al. 2020. As for CH4, none of the multiple prominent absorption bands in the LEISA spectral range were seen. Nor were the extremely "hypervolatile" species CO and N2 detected. Among the weakly absorbing species, the important candidate molecule HCN has three small absorption bands in the LEISA range (2υ3 at 2.432 µm, (υ1 + υ3) at 1.866 µm, and 2υ1 at 1.535 µm; Quirico et al. 1999), that could not be discerned within the noise pattern of the spectra obtained.
Similarly, CO2 was not detected; its principal signature in the LEISA spectral range consists of three narrow and relatively weak bands near 2.0 µm which, if present, could not be discerned at the noise level achieved in the data. ((The returned spectra, while revolutionary for such a small KBO, were also at the same time limited by the finite pixel scale and rapid scan pattern of the fast New Horizons flyby.) Figure 1 -Ices found on Arrokoth (top) and Pluto (bottom) by New Horizons' Ralph/LEISA instrument suite to create spectrophotometric maps of the bodies. MU69's surface is dominated by strong, nearly uniform signatures of CH3OH and a reddish-orange tholin, and weak signatures of H2O. In stark contrast, Pluto's surface is dominated by CH4, N2, CO, and reddish "Pluto tholin".
In this paper, we seek to provide a thermal stability interpretation for the observed composition of MU69's surface. To this end, we utilize thermodynamical studies of laboratory analogue ice vapor pressures and find that almost all of the primitive ices known to be in outer solar system icy bodies should have been removed long ago from a nigh-gravitationless MU69. Our analysis suggests that only the high boiling temperature (hereafter "refractory"), hydrogen-bonded ice species H2O, poly-H2CO, CH3OH, and HCN, as well as minority impurities in refractory-ice phases, should be present if accreted by MU69 4.6 Gyr ago. Our analysis also reproduces the observed mix of solid, stable ices on Pluto's surface and the N2/CO/CH4 found in its atmosphere (Grundy et al. 2016; Figure 1).
We interpret the surface composition to also reflect the bulk composition of MU69 and other distant small bodies with a similar dynamical history. Although in principle the New Horizons LEISA spectra and MVIC 4-color photometric maps (Stern et al. 2019, Grundy et al. 2020 1). Yet when we follow these objects in, we see them become active and lose their abundant methanol, and convert to the water ice dominated inner centaurs and short period comets. None of these objects show any evidence for abundant (vs water) hypervolatiles such as the CO, N2, and CH4 seen on Pluto and the larger KBOs. There has, however been one recent small icy body counterexample that does appear to be just as expected for a small, primordial hypervolatile-rich object -Comet C/2016 R2, known for its large outgassing of CO and N2 but no water. As we discuss below, this makes sense if R2 has retained its hypervolatile ices to the present day, and is only now subliming some of them while keeping the rest of its volatile ices ultra-cold, stable, and rock hard via sublimative cooling.
Section 1 of this paper is intended to present the known observational results for ices in MU69 from the New Horizons flyby. Each alone is moderately useful, but the combination of surface spectral mapping, UV coma non-detection, and rotationally resolved body imaging provide powerful constraints on the nature of the body. In Section 2 we present the details of our thermodynamic calculations and the resulting saturation vapor pressure versus temperature (Psat vs T) curves. 1 In this Section we also calculate the upper limits for the surface area of any currently exposed sublimating ices presented by the non-detection of a gas coma around MU69 by New Horizons.
1 Section 2 and the SOM are important for explicitly writing down for both the KBO and the Comet communities' use the temperature dependence of ice vapor pressure for the dominant ice species expected in outer solar system icy bodies. In doing this, we employ the measured heats of vaporization for the pure ice species which are readily obtainable from terrestrial laboratory measurements. Real KBO ices are likely to be some sort of mixture of pure and compounded (or alloyed, or mixed; in this paper we use the terms interchangeably) ices, due to the chaotic messiness of planetesimal formation in the early solar system, with a PPD likely dominated by number by simple H2O, CO, and N2 molecules plus heavier species created via heterogenous catalysis on grain surfaces (e.g. Nuth et al. 2008). Add to this the vagaries of the temperature and density structure of the convecting PPD, the effects of protoplanet disk stirring, and the behavior of stochastic solar FU Ori/EX Lupi accretion events and XUV stellar outbursts over time, and it is quite difficult to state with any certainty the exact nature (pure, mixed, alloyed, or crystalline) of the ices first incorporated into proto-KBO bodies. Until we have a cryogenic core sample of these ices in a terrestrial laboratory for measurement, we will not know which of a nearly infinite possible mixed phases we should measure; fortunately the properties of many possible combinations of mixed ices have been studied by the Ames group in the 90's [c.f. Sandford et al. 1988Sandford et al. , 1990Sandford et al. , 1993, and they grossly tend to follow the dominant ice species' behavior, which we study here. Investigations on the subject of the stability of EKB ices have been conducted intermittently in the literature over the last few decades. We summarize the major efforts here in order to provide the reader with a useful summary of previous thought and likely avenues for future research and progress. (This work, for example, was inspired by our 2019 understanding that updated laboratory cryogenic heat of sublimation measurements produced by KBO astronomers have not yet been applied to the small icy body sublimation models produced by cometary astronomers or the hot core ice models of astrophysicists.) 2 Caveat: By no means is this treatment meant to replace detailed time-dependent modeling of a 3-dimensional representation of KBO, which is why we do not delve deeply in this paper into the details of the first ~1 Myrs of Arrokoth's life -which must have been very interesting and variable, although transient. The purpose of this paper is to set up the problem of the possible icy constituents of KBO MU69 and Pluto as seen today after 4.56 Gyr of thermal evolution. I.e., to state what we know for sure from simple thermodynamic and geophysical considerations, while avoiding the uncertainties in important parameters like the runs of interior density, thermal heat capacity, and thermal conductivity through the body that produce a variety of outcomes in more detailed models. We hope that this work and its given material abundance restrictions inspires future detailed modeling work that will fill in more of the picture, especially the short-term time dependent behavior of KBO interiors over the first few Myr, by studying the extent of modeling phase space in the manner of Merk et al. 2006 andPrialnik 2009. Starting in the late 1980s, astronomical infrared spectral studies investigating the composition of icy molecular cloud cores, the precursors to solar systems and their icy planetesimals, detected absorption features indicative of ices containing H2O, CO, CO2, CH4, H2CO, NH3, and CH3OH (Tielens et al. 1989, Allamandola et al. 1992, Lacy et al. 1998. The spectral and physical properties of these ices, including their sublimation and condensation behaviors, were studied in the laboratory (e.g., Schutte et al. 1993a,b) to investigate the possible makeup of these clouds, and the plausibility that the laboratory ice analogues could be present in the molecular cloud at the ambient temperatures estimated from their spectroscopy. This work produced surface binding energy estimates for the various ices, useful for producing stability ages for their solid, icy phases assuming the relative "sticking probabilities" of molecules as a function of temperature.
For a gaseous species in chemical equilibrium with its solid phase the dependence of its saturation vapor pressure is given by the Clausius-Clapeyron thermodynamic equation: (1) 1/P dPsat/dT = µHsubl/RgT 2 with solution Psat = C*exp (-µHsubl/RgT) . Here, Psat is the saturation vapor pressure above a patch of ice, T is the ice temperature, µ is the mass of 1 mole of gas, Hsubl denotes the enthalpy (i.e., latent heat) of sublimation per kg, and Rg = NAvogadro*kBoltzmann is the ideal gas constant. Assuming that Hsubl is independent of temperature, , following up on earlier laboratory work in forming and evolving analogue icy cometary materials (Yamamoto 1983;Bar-Nun et al. 1987;Prialnik et al. 1987Prialnik et al. , 1990Schmitt 1989Schmitt , 1992, used this relation together with the empirically tabulated Psat(T) = A * exp (-B/T) fits known since the 1920s to obtain and tabulate its exponential coefficient in terms of the latent heat (SOM Table 1). The form of these Psat curves is such that they all have a universally similar structure with rapidly rising pressures and "knees" controlled by the value of (Hsubl/kBoltzmann), i.e. the heat of sublimation expressed in Kelvin. Figure 2 shows examples of these curves, where it can be seen that ices which sublime easily (like hypervolatile molecular solids N2, CO, and CH4 with only van der Waals molecule-molecule interactions) have small values of Hsubl/kBoltzmann, on the order of 10 2 K, while refractory ices (like the hydrogen-bonded species H2O, H2CO, CH3OH, HCN, and NH3 with relatively strong inter-molecular bonds) have Hsubl/kBoltzmann values in the many-thousands (e.g. Sandford & Allamandola 1993a. The flux rate at which an icy cometary material loses molecules into a surrounding medium follows from gas kinetic theory ) as (2) Z (molecules/m 2 /s) = rgas*vgas = (Psat-Pambient)/kT*vthermal, z+ = (Psat-Pambient)/(2pkTm) 0.5 where Z is the mean loss rate per unit normal surface area, the ideal gas law has been used to express the gas number density rgas = (N/V) = P/kT, vthermal,z+ = (kT/2pm) 0.5 is the mean Maxwell-Boltzmann velocity at temperature T for gas molecules leaving perpendicular to the ice surface, m is the molecular gas mass, and k is Boltzmann's constant, respectively.
Figure 2 -Saturation pressure Psat as a function of temperature for 12 common species found in small solar system body ices. The range of saturation pressure curve behavior ranges from hypervolatile (N2, CO, O2, Ar, CH4) at upper left to highly refractory (HCN, CH3OH, H2O) at lower right. In the mid-range are a host of medium-volatile hydrocarbon species. Also shown is the range of temperatures (37-47 o K) and pressures (~10 µbar) on Pluto's surface determined by New Horizons (intersection of horizontal and vertical gold bars). By comparing to our model Psat curves, it can be seen that only the hypervolatile ices will be in the gas phase on Pluto, and that in some regions they will be stable solids.
Once the proper local equilibrium temperature T is calculated (see Section 4.1), two important special cases of Eqn. 2 occur: (1) When Psat < Pambient, as for a body with a stable atmosphere, no net molecules are actively emitted into the medium and the ice is thermally stable and remains condensed. This is the case studied in the seminal paper of  predicting which of the largest KBOs could retain atmospheres and thus finite surface pressures, and which ices would be stable given the KBOs ambient temperature and pressure , Brown et al. 2011. We can use this approach to predict the ices which should occur on Pluto's surface and in its atmosphere given the ~10 µbar of ambient surface pressure (as measured by New Horizons, Stern et al. 2015, Gladstone et al. 2017).
(2) When Pambient ≈ 0, in which case the ice is exposed to vacuum, and the derived sublimative flux rates can be compared to the ~10 15 /cm 2 molar surface densities of ices 3 to determine how long it takes for a patch of exposed ice to fully sublimate away. This case is analogous to the residence lifetime arguments of Langmuir (1916), Frenkel (1924, and Sandford & Allamandola (1993b), and can be used to predict the lifetime (versus thermal sublimation) of exposed ices on comet surfaces and in interstellar and interplanetary dust grains. These lifetimes are upper limits, as other removal processes, such as photon irradiation, stellar wind particle sputtering, and micrometeorite impact gardening can also remove mass from exposed ice.
In the next section, we show that if bulk majority phase hypervolatile ices were initially incorporated into MU69, then subsequent short-lived radioactive driven evolution would relegate them to near-surface regions, and that these regions will then become depleted within ~1 Myr of the present-day surface temperatures being established. We go on to state that the hypervolatile ice species CO and CH4 we see outgassing from icy bodies today must have been stabilized over time by residing as a minority component in a non-pure ice whose thermal stability was controlled by a more refractory ice like CH3OH or H2O.
Figure 3 -Plot of total Qgas, the total production rate of gas molecules for a given ice species from MU69, vs ambient temperature, assuming the ice species is uniformly distributed across MU69's sunlit side. The non-detection for gas production produced using the NH/ALICE spectrometer's measurements (of H2) is shown by the dashed line. If the surface of MU69 contained appreciable N2, CO, O2, Ar, or CH4 icy material at ~42K in thermal contact with the surface, we would have detected its gas molecule emission rate above the limiting sensitivity of the ALICE measurements.
Observational support for the lack of hypervolatile ices (except as minority impurities in dominantly refractory ice phases) in MU69 is shown in Figure 3. Here we have compared the 3sigma upper limit on the gas production rate, Qgas, supporting the gravitationally unbound atmosphere, or coma, surrounding MU69 as determined by NH/ALICE UV spectrometer solar Lyman-alpha airglow measurements during the 01 Jan 2019 flyby to our calculated net outgassing rate for ices mixed uniformly on the surface of a TLTE = 42 K object of MU69's measured surface area. The published upper limit for Qgas is ~ 10 24 molecules/sec (Stern et al. 2019), a relatively high rate of gas release at 45 au when compared to other actively subliming solar system bodies like comets at ~1 au (Lisse 2002, Bockelee-Morvan et al. 2004). However, this value is sensitive enough to rule out uniform layers of hypervolatile CH4 ice, and, by extension, (N2, CO, O2, and Ar), on direct measurement grounds, as these species would produce gas flux rates far above the upper limit rate (dashed line) in Fig. 3.
Further, if we note that MU69 had a special orientation geometry during the NH flyby such that one-half of its highly flattened surface was pointed nearly sunward while the other half was in near-total shadow, so that the sunlit side temperature was closer to Tss = 59K, then we see that even the moderately volatile organic ice species C2H2, C2H4, C2H6, and C6H6 are ruled out.

Discussion & Analysis.
More stringent constraints can be placed on ice stability if we consider not only their current sublimation loss rate, but their loss rate over the age of the Solar System and the age of MU69.
Using the outflux rates of Fig. 3, as a most-conservative case we can assume their stability over time in the EKB during their Gyrs-long post-aggregation phase, and see how long it takes a molar surface density's worth 1 (~10 15 /cm 2 = 10 19 /m 2 ) of material to be removed via thermal sublimation.

4.1
Negligible Sublimative Cooling After ~1 Myr. We can safely make an approximate calculation using simple thermodynamic and energy balance arguments on Gyr timescales for the following reason. The energy balance equation (neglecting any endogenic heat flux) for heating of a unit slab of KBO material (1m wide by 1 m long by dz thick) of Bond albedo A and effective thermal emissivity ε oriented at angle ξ at rh distance from a 1 Lsun luminosity star is : where the first term is any input insolation, the second is the re-emitted thermal radiation from the object, and the third is the cooling derived from sublimation of the object's volatile species. We look at the relative magnitudes of these terms by calculating the maximal possible rate of sublimative cooling vs. temperature possible for an icy species, using the gas production rate (Qgas, in molecules/m 2 /s) curves of Fig. 3 multiplied by the heat of sublimation (Hsubl,i, in J/molecule sublimed) for a species. Figure 4 shows the result, from which it can be seen that sublimative cooling is a negligible term for an object like MU69 in the Edgeworth-Kuiper Belt compared to the ~0.7 W/m 2 received from the Sun, except in the case of the hypervolatile species (CO, N2, CH4, Ar, and O2). This result is also consistent with other ice sublimation modeling work, like that of Steckloff et al. (2015Steckloff et al. ( , 2020. Figure 4 -Plot of the maximal possible sublimative cooling power per unit surface area for a given ice species vs. ambient temperature, assuming the ice species is uniformly distributed across MU69's sunlit side. Only for the hypervolatile species (N2, CO, O2, Ar, or CH4) does the sublimative power achieve or exceed the input insolation power.
We have shown in Section 3 that there are no dominantly hypervolatile ice phases in abundance today on the surface of MU69, and argued that it is unlikely that any majority hypervolatile ices ever did condense into the KBO unless it formed in a cold, optically thick mid-plane region of the PPD. In this eventuality, the effects of sublimative cooling due to hypervolatile evaporation would be short-lived. E.g., if we were to construct MU69 entirely out of albedo = 0.16 N2 or CO ice with Hsubl ~10 -20 J/molecule , Fray & Schmitt 2009), it would take ~4x10 28 molecules/sec (~10 3 kg/sec) subliming (close to the typical activity level for an inner system SP comet near perihelion) to match the (0.7 W/m 2 * 6x10 8 m 2 surface area) of MU69 total input solar power. The mass of MU69 is about (6x10 8 ) 3/2 m 3 * 500 kg/m 3 = 2x10 15 kg. Since the 10 3 kg/sec mass loss is always "on" for a body in a near-circular, stable orbit like MU69's, this means an MU69 consisting of solid N2/CO would be fully sublimated in ~10 5 yrs, and any hypervolatiles providing substantive cooling would be quickly exhausted. (N.B. -The vast majority of small icy bodies known in the solar system are in such a hypervolatile-depleted condition. But one strong counterexample of an object temperature-controlled by apparent hypervolatile sublimation -that of comet C/2016 R2 -has been recently seen, and we discuss its case further in Section 5.5) Figure 5 -Plot of Qgas, the production rate of gas molecules for a given ice species, vs. ambient temperature, for a patch of unit surface area. This plot is useful in comparing to the molar surface density of an ice, ~10 15 /cm 2 (=10 19 /m 2 ). For an assumed time interval over which this outgassing occurs (e.g., 10 3 yrs (=3x10 10 sec), 10 6 yrs (= 3x10 13 sec), or 10 9 yrs (= 3x10 16 sec)), the maximal Qgas for which any appreciable solid ice remains (rather than sublimating) can be calculated (dashed horizontal lines). While this line of analysis involves assuming constant levels of gas mass loss over large periods of time, we have assumed the lowest possible loss rates (thermal only) and the method produces much more stringent limits on the possible presence of any ices. As a result, it immediately shows that only the highly refractory hydrogen-cross bonded ices (NH3, HCN, CH3OH, H2O) can be present for a surface at today's sunlit value of T = ~40 K, and that any N2, CO, O2, Ar, or CH4 ice incorporated by MU69 when it first formed cold (at T~15K) in the optically dense mid-plane of the solar system's proto-planetary disk (PPD) was removed within ~1 Myrs of the mid-plane clearing and MU69 warming up to ~40K.
We can now go back to Eqn. 3 and ask what the effect of any heating by short-lived radioactive elements like Al 26 would be on our unit slab, and answer that they would offset the effects of any hypervolatile sublimative cooling by causing substantive heating in the first few Myr and reducing the timespan over which sublimative cooling can keep the object below Local Thermodynamic Equilibrium (LTE). The short-lived radionuclides are especially efficient at doing this deep inside the KBO through additional volume radioactive heating decay and thermal conduction terms on the left-hand side of Eqn. 3 (Prialnik et al. , 2006.
We are thus back to safely ignoring the effects of sublimative cooling on timescales of > 1 Myr, and with this zeroth-order approximation can produce a plot like that shown in Fig. 5. The same general family-of-curves shape is apparent in Fig. 5 as for the Qgas production rates in Fig. 3  Once the short-lived radionuclides decayed and the PPD cleared, allowing direct insolation at MU69's location within the first ~10 Myr (i.e., "morning arrived in the EKB"), MU69's subsolar surface rapidly warmed to ~50-60 K. This would have caused rapid sublimation of the surface hypervolatile ices (Figs. 4 & 5) and near-surface mass loss, which may be evidenced by the pit chains seen on MU69's limb (Stern et al. 2019, Spencer et al. 2020, Schenk et al. 2020 as well as by possible instances of scarp retreat seen on the small lobe (Moore et al. 2019). Other structures on MU69, like the rumpled surface texture and the delineated joints between apparent subunits on the larger lobe, could also be due to significant subsurface mass wasting "deflation" of its initial structure (Spencer et al. 2020, Schenk et al. 2020. The local temperature would have fallen from this peak towards ~40 K as the Sun finished its gravitational condensation and commenced nuclear fusion burning, appearing on the main sequence within the first ~100 Myr.
Over the next ~4 Gyr, the Sun would have been a consistent heat source for MU69, although the MU69 environment at ~45 AU was likely subjected to a variety of other time-dependent energetic processes. Every 10 8 -10 9 yrs, the transient effects of nearby passing O/B stars and supernovae acting on 10 1 -10 3 yr timescales may have dramatically heated MU69's surface down to 10's of meters (Stern 2003), removing all but the most refractory of ices (Fig. 5). This refreshed and modified surface will also have been dosed by photons (down to µm depths), solar wind particles (down to cm), micrometeorites (down to cm), and galactic cosmic rays (down to m) to create the optical surface we see today. We can thus expect that the optical surface of MU69 observed with New Horizons is likely only 10 8 -10 9 yrs old, having been processed by irradiation and micrometeorite bombardment. Since the colored surfaces of both lobes appeared to be compositionally very uniform to New Horizons (Stern et al. 2019, Grundy et al. 2020, we can infer that both lobes were compositionally uniform and similar to begin with. As evidence of potential surface radiolytic effects, it is worth noting that the spectral signature of CH3OH in the wavelength region in which it was detected with New Horizons (Fig. 1) Fig. 2, giving us confidence in this line of reasoning.) We therefore infer that H2CO in its pure ice form will have thermal meta-stability over the age of the solar system similar to NH3. This is consistent with its ability to form weak, but finite hydrogen bonds between the carbonyl oxygen and an adjacent aliphatic hydrogen.
One of the difficulties in establishing the sublimation temperature of H2CO is likely due to the fact that it is highly reactive even at temperatures as low as 40 K (Schutte et al. 1993a,b). In pure form it can polymerize into polyoxymethylene (paraformaldehyde) and if other species like H2O, NH3 and CH3OH are also present, it can polymerize to form a host of compounds having polyoxymethylene backbones with some H atoms substituted for by other functional groups like -NH2, -CH3, and -OH (Schutte et al. 1993b). H2CO is also known to easily cross-link via ion-radical and photon-induced polymerization. Once polymerized, H2CO is no longer present as a lightweight molecule but is instead associated with structures having much higher molecular weights, i.e., it is no longer a simple ice component, but is instead a complex macromolecule. In this macromolecular form the H2CO moiety is expected to be very thermally stable vs. sublimation.

Polymerized H2CO could be stored in KBOs and their cometary descendants in this form and
would not be expected to be released until exposed to very high temperature excursions (T > 300 K) during inner system passages (e.g. Fray et al. 2004).

Implications and Predictions.
In the above sections, we presented the results of simple thermodynamic calculations for the stability and lifetime of different ice species on the cold classical KBO body MU69. The results for a flattened, 4.6 Gyr old KBO like MU69, with obliquity near 90 o and spinning around its short axis in a stable rh= 45 AU orbit suggest that only the refractory strongly hydrogen bonded ices (H2O, CH3OH, and HCN, and poly-H2CO) should have remained stable throughout until the present day.
The somewhat less refractory and more volatile species NH3 and CO2 could possibly also exist in the cooler "deep interior regions", if further detailed modeling (including realistic ranges of hypervolatiles, short-lived radioactives, and initial temperature conditions) bears out their survival past the first few Myr. All other ice species, especially the hypervolatiles (N2, CO, CH4, Ar, etc.) should be long gone. The importance of refractory hydrogen bonded species like methanol ice in outer solar system bodies is a relatively new concept, as is the rapid removal of hypervolatile ices.
In this section we discuss how these results fit into the bigger picture of other KBOs, the Pluto System, and of the KBOs' dynamical descendants, the Centaurs and Short Period (SP) comets.
5.1 KBOs to Centaurs to Comets. To begin with, we present the state of current observational knowledge of ices on the KBOs, Centaurs, and SP comets. Water and methanol ice, but also ofttimes ammonia and the hypervolatile ices, are known to be present on the largest KBOs (Barucci et al. 2008(Barucci et al. , 2011Brown 2012). By contrast, the Centaurs show absorptions only due to water ice and methanol (for a few of the most distant objects; in fact the first detection of methanol on a small icy solar system body was for Centaur 5145 Pholus by Cruikshank et al. 1998).
Correlations of the outgassing activity of 23 Centaurs with their perihelion distance from the Sun led Jewitt (2009) to conclude that the activity of the inner (rh < 10 AU) Centaurs is driven not by CO or CO2 ice sublimation, but instead by crystallization of amorphous water ice and the "squeezing out" of trapped molecules no longer able to fit into the reduced lattice pore space of the newly crystallized ice. SP comet surface spectra do not show any obvious absorption features due to abundant ices, except for the rare small patch of water ice (Sunshine et al. 2006;Quirico et al. 2015Quirico et al. , 2016Lisse et al. 2017). However, their comae, produced largely by water ice sublimation, show an abundant range of icy species ( if they were ever incorporated into an MU69-sized, near zero-gravity body, they were stably bound at EKB temperatures for less than 1 Myr. This immediately implies that if these largest EKB bodies formed from small KBOs or the same material as smaller KBOs, then (1) they did so within 1 Myr of the PPD mid-plane clearing; or (2) that there was abundant CO, N2, and CH4 gas in the PPD when they became large enough to gravitationally bind these species; or ( On the other hand, the N2/CO >> 1 ice abundance ratio found on Pluto's surface (Grundy et al. 2016, Protopapa et al. 2017, Schmitt et al. 2017) is counter to the N2/CO ~ 0.10 abundance ratio expected for these ices trapped in water ice phases; CO with its small but finite dipole moment is much more efficiently trapped in polar water ice than homonuclear, zero dipole N2 from a starting equal mix of the two gases (Yokochi et al. 2012;K. Öberg 2019, priv. commun.). However, Kamata et al. (2019) argue to the contrary that differential N2 versus CO trapping ability being the case, geochemical and geophysical mechanisms at work on a Pluto surface-devolatilized by a Charon forming impact (and subsequently re-volatilized from deep interior stores of hypervolatiles) that could allow for N2 re-concentration versus CO on its surface.

5.2
No "Top-Down" KBO formation. If we turn the problem around, and note that we do not see nebular abundances of N2 and CO in the comets or small KBOs, this argues against these objects forming as large bodies and incorporating everything available in the PPD via gravity stabilization, then collisionally fragmenting and retaining these ices. Similarly, we can expect Pluto and Charon to have large amounts of stable methanol ice content; methanol ice should be almost as strong and as stable as water ice on these bodies. If the primordial KBO methanol: water ratio is on the order of unity, as implied from the NH flyby of MU69, then Pluto & Charon, with rock:ice ratios on the order of unity, should contain copious amounts of methanol "rock" in their interiors (and dissolved methanol in their subsurface oceans, if any exist) that need to be considered when modeling their geological structure and history. A search for exposed methanol ice in the NH LEISA data to verify this assertion is difficult on Pluto, however, due to the abundant methane ice coverage on its surface, as methane shares similar 1-2.5 µsm absorption features to methanol.

5.4
Non-Zero Amounts of Hypervolatiles Observed in Comets. The fact that there are low, but finite hypervolatile species seen in SP comets derived from the EKB, implies that there must be some reservoir for them in these recent EKB escapees. The current crop of SP comets have been in the inner system for < 10 5 yr (Levison & Duncan 1994, Lisse 2002, so their deep interiors are still warming from the recent increase in surface insolation (Benkhoff & Huebner 1996, Huebner 2002) and the fate of any core ices will be dominated by the interior composition established 4.6 Gyr ago. However, our calculations above show that except for H2O, CH3OH, HCN, or polymerized H2CO compounds, this reservoir cannot contain hypervolatile ice phases, unless they have deeply buried regions held at ~15 K since the beginning of the Solar System. This is highly unlikely given that the models of Choi et al. 2001, Merk & Prialnik 2006, and Prialnik 2009 show that KBO cores are always as warm or warmer than their surface; the lack of substantial hypervolatile emission from end member objects 45P/Honda-Mrkos-Pajdušáková,

2020's new outer Centaur activity survey) that the hypervolatiles and moderately-volatile species
in the Centaurs and SP comets are protectively stored in H2O ice matrices -first at high concentrations in cold (T < 80K) amorphous water ice composites, then in lower concentrations in warmer T > 100K crystalline water ice matrices limited by the maximum interlattice "pore space" trapping capability of the crystallite. This is consistent with the early work of Prialnik et al. (1987) who argued that the presence of amorphous ice within the subsurface of comets -inferred from observations of outgassing at surprisingly large heliocentric distances (5.8 -11.4 AU) and attributed to the annealing of amorphous ice as comets first enter the inner Solar System (Prialnik & Bar-Nun, 1990;1992;Meech et al. 2009 H2O ice reservoirs for remnant icy molecules also explain the seeming disconnect between the strong CH3OH vs H2O signature on the surface of MU69, the weak CH3OH:H2O signature of several Centaurs, and the low CH3OH:H2O abundance ratio seen in the atmospheres of the active SP comets. Any CH3OH (and maybe HCN) ice phases that are stable at MU69's T=42 K temperature in the heart of the EKB sublimes within the few Myrs of time it takes to scatter the KBO past Neptune (rh ~ 30 AU, T ~ 51 K), then Uranus (rh ~ 19 AU, T ~ 64 K), and into Saturn's dynamical regime (rh ~ 9.6 AU, T ~91 K; Fig. 4). What is left after this process are the molecules mixed in with H2O ice, and subject to any changes in the state of the H2O ice, like the amorphous -> crystalline water ice transitions at T = 80 -120 K (Blake et al. 1991), and water ice sublimation at T > 140 K (Sandford & Allamandola 1993b). (Note that both of these processes proceed at a temperature-dependent rate; Schmitt et al. (1989) gives the timescale for the amorphous -> crystalline water ice transition as t = (3x10 -21 yrs) * exp (E a /kT)  The presence of water ice reservoirs for remnant volatile ice molecules also makes a strong prediction about Qgas, the gas production trends for comets. Species that are sourced from amorphous water ice composites do not need to track Qwater, only the rate at which amorphous water ice crystallizes to produce solid crystalline water + gaseous minor species. Amorphous water ice minor species can also be present in much greater numbers vs water than the limited amount (~20%) possibly carried in the pore spaces of cubic crystalline water, i.e., as H2O ices warm through ~ 80 K, the rearrangement of H2O molecules during the conversion of H2O from one amorphous phase to another will allow some hypervolatiles to escape Blake et al. 1991). Additional hypervolatile loss can occur when the ice is warmed to temperatures that convert the amorphous H2O ice to its cubic crystalline form (Schmitt et al. 1989, Jenneskins & Blake 1996 44 . It is only upon subsequent sublimation of the cubic crystalline H2O (as seen for comets inside 3 AU) that the emission rate of remaining (~20% total vs water) minority volatiles will track Qwater. where it is undergoing intense sublimation of its hypervolatiles, which as we saw in Section 4.1 is enough to sublimatively cool the body to very low temperatures where water ice is rock-like and inactive sublimation-wise.
Is this a viable picture? It takes a major dynamical event to put an object onto an Oort Cloud orbit.
KBO-KBO collisions, like the Haumea family formation event or the Pluto-Charon formation event, cannot do this as Pluto's escape velocity (vesc) and EKB relative velocities are too low (Canup 2005, Stern et al. 2006, Sekine et al. 2017. Dynamical scattering by a giant planet with a large vesc, as in the initial phases of planetary core formation via planetesimal-aggregation, or during proto-Neptune's later migration through the inner proto-EKB, can do this. 5 The latter case would require Neptune's migration through the inner proto-EKB to have occurred no more than a few Myr after the midplane clears for R2's hypervolatiles to have remained stable (Section 4), contrary to the early Nice models (e.g. Gomes et al. 2005), but consistent with later Nice scenarios including ejected outer planets (e.g. Nesvorny & Morbidelli 2012).
For the sake of completeness, another possibility, that R2 somehow retained significant deep interior hypervolatiles in the inner solar system until being scattered into the Oort Cloud, should also be entertained. Following the work of Choi et al. 2001, Merk & Prialnik 2006, and Prialnik 2009, this would require an unusually large overabundance of initial hypervolatiles and/or underabundance of short-lived radionuclides. While this seems very unlikely, given the evidence  Russo et al. 2007) and 17P/(Dello Russo et al. 2008), and the hypervolatile-devoid KBO compositions discussed in this paper, it is perhaps possible that this likelihood is no less extreme than requiring an object to be scattered into the Oort Cloud during the era of giant planet formation/migration. Further modeling work will be required to assess the relative likelihoods of these two scenarios. 5 The main issue with this picture is how many times R2 can pass through perihelion and still remain stable against evaporative disintegration. A 15 km radius object of ~0.5g/cm 3 density masses on the order of 7 x 10 15 kg. At the Qgas ~ 10 29 molecules/sec outgassing levels seen during R2's current apparition for its current perihelion distance of 2.6 AU it will lose ~ 3 x 10 7 sec [1 yr]*(28 amu for CO/N2*1.67 x 10 -27 kg/amu)*1 x 10 29 molecules/sec ~ 1 x 10 11 kg. Thus it should be able to endure ~7 x 10 4 more of these kind of passages before dissipating, which should take ~2 x 10 4 yrs/passage * 7 x 10 4 passages = 1.4 x 10 9 yrs, or 1.4 Gyrs. It is thus unlikely to have been on its present orbit for more than 1 Gyr. Note that a 10 29 molecules/sec level of hypervolatile outgassing is reasonable, and can be supported by a 15 km radius object that is losing molar amounts of ice surface every second : (2*pi*r 2 cm 2 )*(2x10 15 /cm 2 /sec) = 3x10 28 mol/sec. From section 4.1 and Tables 1 & 2, the evaporation of 1 mole of hypervolatile ice requires ~7.3 kJ of heat, thus the amount of hypervolatile cooling from the emission rate of 10 29 molecules/sec = 1.6 x 10 5 moles/sec is ~ 1.4 x 10 9 J/sec, the same order of magnitude, (1-0.9) * pRnucleus 2 * (0.1W/cm 2 * (1.0 AU/2.6 AU) 2 ) = 1 x 10 10 W, as the insolation heating of the Sun is delivering to R2's nucleus (with assumed albedo = 0.90) at 2.6 AU. So the observed mass loss rate of R2 being attributed to hypervolatile sublimation makes rough sense if R2 is feverishly sublimating from its entire sunlit surface during the small portion of each 20,000 yr long orbital cycle where it is intensely heated.
In summary, we find that in the ice-sense, MU69 is "more primordial" than SP comets, and as or
In this paper we have found, from simple thermodynamic arguments independent of detailed formation scenario assumptions (e.g., formation location in the proto-solar nebula or the PPD, amount of Al 26 incorporated & when, interior detailed structure), that New Horizons should not have seen any typical cometary icy material on the surface of MU69 other than the highly refractory ices H2O, CH3OH, HCN, and poly-H2CO. These are the maximal possible set for the current low-level insolation conditions. Any positive temperature excursions, due to nearby passing hot stars, nearby supernovae, impacts, meteorite gardening, etc. could only remove more ice, and reduce this possible set. The New Horizons science team has already announced the detection and CH3OH and evidence for H2O on this object (Lisse et al. 2017, Stern et al. 2019, Grundy et al. 2020. We predict that further absorption feature detections, if any, will be due to an HCN or poly-H2CO based ice species. In having evidence for these additional non-water ice phases from the NH flyby spectral mapping of its surface, MU69 appears akin to the most distant Sun (Briceno et al. 2001, Thrane et al. 2006, in an optically thick mid-plane. It was thus able to initially incorporate hypervolatile ices at a local T < 20 K, but that these were quickly lost from the body within the first 1-10 Myr due to the combined action of short-lived radionuclide decay and surface warming upon PPD mid-place clearing. By contrast, Pluto was able to retain its original primordial volatiles via gravitational capture into an exobase. Finally, the advent of the uniquely These two tables present the best-fit parameters compiled/used by  and Fray & Schmitt (2009) to describe the vapor pressure Psat versus temperature T behavior of ices found in solar system bodies. We reproduce them here both to record the values used in making the vapor pressure versus temperature curves used in our manuscript's analysis and as a service to the reader in making their own future analyses.
The 2-parameter fits of  Table 1) assume ice Heats of Vaporization (DHvap's) that are independent of temperature and use data collected at/near room temperature, and so are most accurate at/near room temperature and less so at low cryogenic temperatures. These parameters are most familiar to the cometary community of researchers at the time of this writing. The more extensive compendium of laboratory measurements compounded by Fray & Schmitt (2009 ; Table 2), produced by an extensive search & comparison of the existing literature for laboratory data, expands the list of ices studied to encompass most of the materials expected in outer solar system icy bodies. It also includes the results of heat of vaporization and vapor pressure measurements at temperatures ranging from 10 to 300 K for the candidate ices, without assuming that DHvap is constant with Temperature. The resulting polynomial fits of ln(Psat) in 1/T (i..e, ln(Psat) = A0 + ∑ Ai/T i )were tabulated and compared to those of other works (like Huebner 2006) in their work with good results. These parameters, along with the subset used by , are most familiar to the KBO/TNO community of researchers.