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Triton and Pluto are believed to share a common origin, both forming initially in the Kuiper Belt but Triton being later captured by Neptune. Both objects display similar sizes, densities, and atmospheric and surface ice composition, with the presence of volatile ices N2, CH4, and CO. Yet their appearance, including their surface albedo and ice distribution strongly differ. What can explain these different appearances? A first disparity is that Triton is experiencing significant tidal heating due to its orbit around Neptune, with subsequent resurfacing and a relatively flat surface, while Pluto is not tidally activated and displays a pronounced topography. Here we present long-term volatile transport simulations of Pluto and Triton, using the same initial conditions and volatile inventory, but with the known orbit and rotation of each object. The model reproduces, to first order, the observed volatile ice surface distribution on Pluto and Triton. Our results unambiguously demonstrate that obliquity is the main driver of the differences in surface appearance and in climate properties on Pluto and Triton, and give further support to the hypothesis that both objects had a common origin followed by a different dynamical history. [ABSTRACT FROM AUTHOR]

The outer Solar System has many bodies of interest that have continued to captivate the planetary science community, recently Triton, a captured Kuiper belt object (KBO) of Neptune, and Pluto. Limited fly‐by observational data shows evidence that nitrogen is the dominant constituent on these two bodies, potentially also existing on other KBOs as well. Current simulations related to answering fundamental science questions and also to develop future mission science packages and vehicles require accurate, reliable thermodynamic, and mechanical property data of solid and gaseous nitrogen at relevant surface and atmospheric conditions. This paper thus presents an exhaustive review of all available experimental N2 property data dating back to 1887. Each historical study is systematically analyzed and summarized and then the consolidated database is assembled. Comments are made on the validity of data sets, with an emphasis on specific heat capacity at constant pressure and constant volume (CP, Cv), thermal conductivity (κ), volume thermal expansion (αV), density (ρ), equilibrium vapor pressure (Pvap), heat of sublimation (ΔHS), heat of transition (ΔHT), Gruneisen parameter (γG), adiabatic and isothermal compressibility (xS, xT) and moduli of elasticity (C11, C12, C13, C33, C44). Results here can be used directly and immediately to perform new simulations on N2‐based bodies as well as to determine gaps in the consolidated database for future experiments. Plain Language Summary: The outer solar system has been of significant interest to the scientific community recently, particularly objects found in or were from the region of space that surrounds our solar system (the Kuiper belt). Many of the objects such as Neptune's moon Triton, and the dwarf planet Pluto have been shown in many studies to have significant amounts of nitrogen. Since there is interest in these objects, many members in the scientific community will likely conduct more studies for these worlds to answer scientific questions about them. Nitrogen is a primary component and as such, a thorough understanding of both thermal and mechanical properties of both nitrogen ice and nitrogen gas would be valuable to have for these studies. Because of this, a thorough search for measured nitrogen ice and gas property data has been carried out reviewing papers made recently to papers published more than a century ago. The data for each property was compiled with comments made to the validity to any questionable data that were not in agreement with data from other authors. The results here can be used directly in any new studies on nitrogen‐based worlds as well as determine any missing gaps in the property data as functions of temperature that could be measurable regions of interest for future property experiments. Key Points: An exhaustive literature review is conducted for cryogenic nitrogen property data relevant to outer solar system bodiesSeventy sources dating back to 1887 were collected and surveyedThe new consolidated database consists of 1,521 nitrogen property data points [ABSTRACT FROM AUTHOR]

Zusammenfassung Die Eismonde von Uranus, Neptun und Pluto wurden bislang nur von zwei Raumsonden erforscht: Voyager 2 und New Horizons. Ihre Aufnahmen zeigen eine große Vielfalt an Oberflächenformationen. Diese Eismonde weisen Ähnlichkeiten mit denen von Jupiter und Saturn auf. So gibt es offensichtlich auch im äußersten Bereich des Sonnensystems Kryovulkanismus. Auch Dehnungsstrukturen und mögliche Auswirkungen von planetaren Kollisionen sind erkennbar. Mehr als 50 Jahre nach Voyager 2 gibt es Pläne für eine neue Mission ins Uranus‐ und Neptunsystem. Noch weitgehend unerforscht sind die Zwergplaneten und kleineren Körper im Kuiper‐Gürtel. Der Vorbeiflug an Pluto und Charon im Juli 2015 war ein erster Blick in diese noch sehr fremde Welt. Entwürfe für weitere Missionen in den Kuiper‐Gürtel, zumindest seitens der NASA, existieren bereits auf dem Reißbrett für die Zeit nach 2040. [ABSTRACT FROM AUTHOR]

Both atmospheres of Pluto and Neptune's largest satellite Triton have cold surfaces with surface gravitational accelerations and atmospheric surface pressures of comparable magnitude. To study their atmospheres we have updated Zhu et al. (2014) model for Pluto's atmosphere by adopting Voigt line profiles in the radiation module with the latest spectral database and extended the model to Triton's atmosphere by including additional parameterized heating due to the magnetospheric electron transport and energy deposition. The CH 4 mixing ratio profiles play central roles in differentiating the atmospheres of Pluto and Triton. On Pluto the surface CH 4 mole fraction is in the range of 0.3-0.8%, sufficiently high to ensure that it is well mixed in the lower atmosphere and not subject to photochemical destruction. Near the exobase CH 4 attains comparable density to N 2 due to gravitational diffusive separation and escapes at 500 times the N 2 rate (= 1 × 10 23 N 2 s − 1 ). In Triton's atmosphere, the surface CH 4 mole fraction is on the order of 0.015%, sufficiently low to ensure that it is photochemically destroyed irreversibly in the lower atmosphere and that N 2 remains the major species, even at the exobase. With solar EUV power only, Triton's upper thermosphere is too cold and magnetospheric heating, approximately comparable to the solar EUV power, is needed to bring the N 2 tangential column number density in the 500–800 km range up to values derived from the Voyager 2 UVS observations (Broadfoot et al. , 1989). Due to their cold exobase temperatures relative to the gravitational potential energy wells that N 2 resides in, atmospheric escape from Triton and Pluto is not dominated by N 2 Jeans escape but by CH 4 from Pluto and H, C, N and H 2 from Triton. The atmospheric thermal structure near the exobase is sensitive to the atmospheric escape rate only when it is significantly greater than 2 × 10 27 amu s −1 , above which enhanced Jeans escape and larger radial velocity adiabatically cools the atmosphere to a lower temperature. Finally we suggest that Pluto's thermosphere is a cold ∼ 70 K due to ablation of H 2 O molecules from the influx of dust grains detected by New Horizons Student Dust Counter. [ABSTRACT FROM AUTHOR]

We investigate the origins of Triton’s deformed and young surface. Assuming Triton was captured early in Solar System history, the bulk of the energy released during capture will have been lost, and cannot be responsible for its present-day activity. Radiogenic heating is sufficient to maintain a long-lived ocean beneath a conductive ice shell, but insufficient to cause convective deformation and yielding at the surface. However, Triton’s high inclination likely causes a significant (≈0.7°) obliquity, resulting in large heat fluxes due to tidal dissipation in any subsurface ocean. For a 300 km thick ice shell, the estimated ocean heat production rate (≈0.3 TW) is capable of producing surface yielding and mobile-lid convection. Requiring convection places an upper bound on the ice shell viscosity, while the requirement for yielding imposes a lower bound. Both bounds can be satisfied with an ocean temperature ≈240 K for our nominal temperature-viscosity relationship, suggesting the presence of an antifreeze such as NH 3 . In our view, Triton’s geological activity is driven by obliquity tides, which arise because of its inclination. In contrast, Pluto is unlikely to be experiencing significant tidal heating. While Pluto may have experienced ancient tectonic deformation, we do not anticipate seeing the kind of young, deformed surfaces seen at Triton. [ABSTRACT FROM AUTHOR]

Abstract: We use a statistical–thermodynamic model to investigate the formation and composition of noble-gas-rich clathrates on Pluto’s surface. By considering an atmospheric composition close to that of today’s Pluto and a broad range of surface pressures, we find that Ar, Kr and Xe can be efficiently trapped in clathrates if they formed at the surface, in a way similar to what has been proposed for Titan. The formation on Pluto of clathrates rich in noble gases could then induce a strong decrease in their atmospheric abundances relative to their initial values. A clathrate thickness of order of a few centimeters globally averaged on the planet is enough to trap all Ar, Kr and Xe if these noble gases were in protosolar proportions in Pluto’s early atmosphere. Because atmospheric escape over an extended period of time (millions of years) should lead to a noble gas abundance that either remains constant or increases with time, we find that a potential depletion of Ar, Kr and Xe in the atmosphere would best be explained by their trapping in clathrates. A key observational test is the measurement of Ar since the Alice UV spectrometer aboard the New Horizons spacecraft will be sensitive enough to detect its abundance ∼10 times smaller than in the case considered here. [Copyright &y& Elsevier]

Abstract: We present a 3D general circulation model of Pluto and Triton’s atmospheres, which uses radiative–conductive–convective forcing. In both the Pluto and Triton models, an easterly (prograde) jet is present at the equator with a maximum magnitude of 10–12ms−1 and 4ms−1, respectively. Neither atmosphere shows any significant overturning circulation in the meridional and vertical directions. Rather, it is horizontal motions (mean circulation and transient waves) that transport heat meridionally at a magnitude of 1 and 3×107 W at Pluto’s autumn equinox and winter solstice, respectively (seasons referenced to the Northern Hemisphere). The meridional and dayside–nightside temperature contrast is small (⩽5K). We find that the lack of vertical motion can be explained on Pluto by the strong temperature inversion in the lower atmosphere. The height of the Voyager 2 plumes on Triton can be explained by the dynamical properties of the lower atmosphere alone (i.e., strong wind shear) and does not require a thermally defined troposphere (i.e., temperature decreasing with height at the surface underlying a region of temperature increasing with height). The model results are compared with Pluto stellar occultation light curve data from 1988, 2002, 2006, and 2007 and Triton light curve data from 1997. [Copyright &y& Elsevier]

Abstract: We report photochemical studies of thin cryogenic ice films composed of N2, CH4 and CO in ratios analogous to those on the surfaces of Neptune’s largest satellite, Triton, and on Pluto. Experiments were performed using a hydrogen discharge lamp, which provides an intense source of ultraviolet light to simulate the sunlight-induced photochemistry on these icy bodies. Characterization via infrared spectroscopy showed that C2H6 and C2H2, and HCO are formed by the dissociation of CH4 into H, CH2 and CH3 and the subsequent reaction of these radicals within the ice. Other radical species, such as C2, , CN, and CNN, are observed in the visible and ultraviolet regions of the spectrum. These species imply a rich chemistry based on formation of radicals from methane and their subsequent reaction with the N2 matrix. We discuss the implications of the formation of these radicals for the chemical evolution of Triton and Pluto. Ultimately, this work suggests that , CN, HCO, and CNN may be found in significant quantities on the surfaces of Triton and Pluto and that new observations of these objects in the appropriate wavelength regions are warranted. [Copyright &y& Elsevier]

Abstract: We present here a search for solid ethane, C2H6, on the surfaces of Pluto and Triton, based on near-infrared spectral observations in the H and K bands (1.4–2.45μm) using the Very Large Telescope (VLT) and the United Kingdom Infrared Telescope (UKIRT). We model each surface using a radiative transfer model based on Hapke theory (Hapke, B. [1993]. Theory of Reflectance and Emittance Spectroscopy. Cambridge University Press, Cambridge, UK) with three basic models: without ethane, with pure ethane, and with ethane diluted in nitrogen. On Pluto we detect weak features near 2.27, 2.405, 2.457, and 2.461μm that match the strongest features of pure ethane. An additional feature seen at 2.317μm is shifted to longer wavelengths than ethane by at least 0.002μm. The strength of the features seen in the models suggests that pure ethane is limited to no more than a few percent of the surface of Pluto. On Triton, features in the H band could potentially be explained by ethane diluted in , however, the lack of corresponding features in the K band makes this unlikely (also noted by Quirico et al. (Quirico, E., Doute, S., Schmitt, B., de Bergh, C., Cruikshank, D.P., Owen, T.C., Geballe, T.R., Roush, T.L. [1999]. Icarus 139, 159–178)). While Cruikshank et al. (Cruikshank, D.P., Mason, R.E., Dalle Ore, C.M., Bernstein, M.P., Quirico, E., Mastrapa, R.M., Emery, J.P., Owen, T.C. [2006]. Bull. Am. Astron. Soc. 38, 518) find that the 2.406-μm feature on Triton could not be completely due to 13CO, our models show that it could not be accounted for entirely by ethane either. The multiple origin of this feature complicates constraints on the contribution of ethane for both bodies. [Copyright &y& Elsevier]

Neptune's moon Triton shares many similarities with Pluto, including volatile cycles of N 2 , CH 4 and CO, and represents a benchmark case for the study of surface-atmosphere interactions on volatile-rich Kuiper Belt objects. The observations of Pluto by New Horizons acquired during the 2015 flyby and their analysis with volatile transport models (VTMs) shed light on how volatile sublimation-condensation cycles control the climate and shape the surface of such objects. Within the context of New Horizons observations as well as recent Earth-based observations of Triton, we adapt a Plutonian VTM to Triton, and test its ability to simulate its volatile cycles, thereby aiding our understanding of its climate. Here we present numerical VTM simulations exploring the volatile cycles of N 2 , CH 4 and CO on Triton over long-term and seasonal timescales (cap extent, surface temperatures, surface pressure, sublimation rates) for varying model parameters (including the surface ice reservoir, albedo, thermal inertia, and the internal heat flux). We explore what scenarios and model parameters allow for a best match of the available observations. In particular, our set of observational constraints include Voyager 2 observations (surface pressure and cap extent), ground-based near-infrared (0.8–2.4 μm) disk-integrated spectra (the relative surface area of volatile vs. non-volatile ice) and the evolution of surface pressure as retrieved from stellar occultations. Our results show that Triton's poles act as cold traps for volatile ices and favor the formation of polar caps extending to lower latitudes through glacial flow or through the formation of thinner seasonal deposits. As previously evidenced by other VTMs, North-South asymmetries in surface properties can favor the development of one cap over the other. Our best-case simulations are obtained for a bedrock surface albedo of 0.6–0.7, a global reservoir of N 2 ice thicker than 200 m, and a bedrock thermal inertia larger than 500 SI (or smaller but with a large internal heat flux). The large N 2 ice reservoir implies a permanent N 2 southern cap (several 100 m thick) extending to the equatorial regions with higher amounts of volatile ice at the south pole, which is not inconsistent with Voyager 2 images but does not fit well with observed full-disk near-infrared spectra. Our results also suggest that a small permanent polar cap exists in the northern (currently winter) hemisphere if the internal heat flux remains relatively low (e.g. radiogenic, < 3 mW m−2). A non-permanent northern polar cap was only obtained in some of our simulations with high internal heat flux (30 mW m−2). The northern cap will possibly extend to 30°N in the next decade, thus becoming visible by Earth-based telescopes. On the basis of our model results, we also discuss the composition of several surface units seen by Voyager 2 in 1989, including the bright equatorial fringe and dark surface patches. Finally, we provide predictions for the evolution of ice distribution, surface pressure and CO and CH 4 atmospheric mixing ratios in the next decades. According to our model, the surface pressure should slowly decrease but remain larger than 0.5 Pa by 2060. We also model the thermal lightcurves of Triton for different climate scenarios in 2022, which serve as predictions for future James Webb Space Telescope observations. • Results suggest large global N 2 reservoir (>200 m) and thermal inertia (>500 SI or smaller if internal heat flux >30 mW m−2). • Results suggest a permanent N 2 southern cap (100 m to 1.5 km thick) extending to the equator with max. thickness at the pole. • Results suggest a small permanent polar cap should exist in the northern hemisphere, and could extend to 30°N by 2030. • Modeled surface pressures are consistent with a moderate increase of pressure during the 1990-2000 period. • JWST/MIRI will help in discriminating climate scenarios and in providing constraints on thermal/energetic surface properties. [ABSTRACT FROM AUTHOR]

The diverse populations of icy bodies of the outer Solar System (OSS) give critical information on the composition and structure of the solar nebula and the early phases of planet formation. The two principal repositories of icy bodies are the Kuiper belt or disk, and the Oort Cloud, both of which are the source regions of the comets. Nearly 1000 individual Kuiper belt objects have been discovered; their dynamical distribution is a clue to the early outward migration and gravitational scattering power of Neptune. Pluto is perhaps the largest Kuiper belt object. Pluto is distinguished by its large satellite, a variable atmosphere, and a surface composed of several ices and probable organic solid materials that give it color. Triton is probably a former member of the Kuiper belt population, suggested by its retrograde orbit as a satellite of Neptune. Like Pluto, Triton has a variable atmosphere, compositionally diverse icy surface, and an organic atmospheric haze. Centaur objects appear to come from the Kuiper belt and occupy temporary orbits in the planetary zone; the compositional similarity of one well studied Centaur (5145 Pholus) to comets is notable. New discoveries continue apace, as observational surveys reveal new objects and refined observing techniques yield more physical information about specific bodies. [ABSTRACT FROM AUTHOR]

Infrared spectra and radiation chemical behavior of N2-dominated ices relevant to the surfaces of Triton and Pluto are presented. This is the first systematic IR study of proton-irradiated N2-rich ices containing CH4 and CO. Experiments at 12 K show that HCN, HNC, and diazomethane (CH2N2) form in the solid phase, along with several radicals. NH3 is also identified in irradiated N2 + CH4 and N2 + CH4 + CO. We show that HCN and HNC are made in irradiated binary ice mixtures having initial N2/CH4 ratios from 100 to 4, and in three-component mixtures have an initial N2/(CH4 + CO) ratio of 50. HCN and HNC are not detected in N2-dominated ices when CH4 is replaced with C2H6, C2H2, or CH3OH.The intrinsic band strengths of HCN and HNC are measured and used to calculate G(HCN) and G(HNC) in irradiated N2 + CH4 and N2 + CH4 + CO ices. In addition, the HNC/HCN ratio is calculated to be ∼1 in both icy mixtures. These radiolysis results reveal, for the first time, solid-phase synthesis of both HCN and HNC in N2-rich ices containing CH4.We examine the evolution of spectral features due to acid–base reactions (acids such as HCN, HNC, and HNCO and a base, NH3) triggered by warming irradiated ices from 12 K to 30–35 K. We identify anions (OCN−, CN−, and N3−) in ices warmed to 35 K. These ions are expected to form and survive on the surfaces of Triton and Pluto. Our results have astrobiological implications since many of these products (HCN, HNC, HNCO, NH3, NH4OCN, and NH4CN) are involved in the syntheses of biomolecules such as amino acids and polypeptides. [Copyright &y& Elsevier]

The topography of Neptune's large icy moon Triton could reveal important clues to its internal evolution, but has been difficult to determine. New global digital color maps for Triton have been produced as well as topographic data for <40% of the surface using stereogrammetry and photoclinometry. Triton is most likely a captured Kuiper Belt dwarf planet, similar though slightly larger in size and density to Pluto, and a likely ocean moon that exhibited plume activity during Voyager 2′s visit in 1989. No surface features or regional deviations of greater than ±1 km amplitude are found. Volatile ices in the southern terrains may take the form of extended lobate deposits 300–500 km across as well as dispersed bright materials that appear to embay local topography. Limb hazes may correlate with these deposits, indicating possible surface–atmosphere exchange. Triton's topography contrasts with high relief up to 6 km observed by New Horizons on Pluto. Low relief of (cryo)volcanic features on Triton contrasts with high-standing massifs on Pluto, implying different viscosity materials. Solid-state convection occurs on both and at similar horizontal scales but in very different materials. Triton's low relief is consistent with evolution of an ice shell subjected to high heat flow levels and may strengthen the case of an internal ocean on this active body. [ABSTRACT FROM AUTHOR]