Pluto's Atmosphere Research Articles

Surface Volatile Composition as Evidence for Hydrothermal Processes Lasting Longer in Triton’s Interior than Pluto’s

Ocean worlds, or icy bodies in the outer solar system that have or once had subsurface liquid water oceans, are among the most compelling topics of astrobiology. Typically, confirming the existence of a subsurface ocean requires close spacecraft observations. However, combining our understanding of the chemistry that takes place in a subsurface ocean with our knowledge of the building blocks that formed potential ocean worlds provides an opportunity to identify tracers of endogenic activity in the surface volatiles of Pluto and Triton. We show here that the current composition of the volatiles on the surfaces and in the atmospheres of Pluto and Triton are deficient in carbon, which can only be explained by the loss of CH4 through a combination of aqueous chemistry and atmospheric processes. Furthermore, we find that the relative nitrogen and water abundances are within the range observed in building block analogs, comets, and chondrites. A lower limit for N/Ar in Pluto’s atmosphere also suggests source building blocks that have a cometary or chondritic composition, all pointing to an origin for their nitrogen as NH3 or organics. Triton’s lower abundance of CH4 compared to Pluto, and the detection of CO2 at Triton but not at Pluto points to aqueous chemistry in a subsurface ocean that was more efficient at Triton than Pluto. These results have applications to other large Kuiper Belt objects as well as the assessment of formation locations and times for the four giant planets given future probe measurements of noble gas abundances and isotope ratios.

Triton's photochemical model: Update

Titan, Triton, and Pluto are the Solar System objects with the nitrogen-ethane atmospheres. The progress in Titan's photochemistry, stimulated by the very successful Cassini-Huygens mission, facilitated photochemical interpretation of Pluto's atmosphere that was observed by the New Horizons mission and supporting ground-based measurements. Here is an update of Triton's atmospheric and ionospheric photochemistry that combines the previous photochemical model (Krasnopolsky and Cruikshank 1995, Photochemistry of Triton's atmosphere and ionosphere, J. Geophys. Res. 100, 21,271-21,286) with findings in Titan's and Pluto's photochemistry and progress in chemical kinetics.Our modeling of Triton's photochemistry is restricted to the conditions of the Voyager 2 flyby in 1989, when data on vertical profiles of temperature, haze, and densities of N2, CH4, N, and electrons were obtained using radio- and solar UVS occultations and imaging.Significant differences between the composition and photochemistry of Triton's atmosphere and those of Titan and Pluto are mostly caused by the low CH4 tropospheric abundance of 100 ppm, smaller than those on Titan and Pluto by two orders of magnitude. This methane is strongly depleted photochemically to a ppb level in the upper atmosphere of Triton, while it is enriched by diffusive separation on Titan and Pluto. Therefore atomic species and their chemistry are much more abundant on Triton than on Titan and Pluto. Atomic ions C+ and N+ with slow recombination rates dominate in Triton's ionosphere with electron densities, significantly exceeding those on Titan and Pluto.The proposed model agrees with all observational constraints and presumes thermal escape of H2 + H with a rate of 45 g cm−2 Byr−1, loss of nitrogen of 40 g cm−2 Byr−1 on thermal escape and precipitation of nitriles, and sublimation and loss of methane of 270 g cm−2 Byr−1.C2H6 and HCN are minor species in the photochemical precipitations on Triton, and their detection in Triton's ice may refer to either the primordial composition or ice photochemistry.Seven versions of the model are calculated to study its sensitivity to some basic input parameters. The model can be used for planning future studies of Triton's atmosphere. An analog of the Ion-Neutral Mass Spectrometer onboard Triton's orbiter looks promising.

N2 accretion, metamorphism of organic nitrogen, or both processes likely contributed to the origin of Pluto's N2

Molecular nitrogen (N2) plays a profound role in supporting processes on the surface and in the atmosphere of Pluto, yet the origin of Pluto's N2 remains a mystery. However, this may begin to change as the 14N/15N ratio of N2 was recently estimated based on a non-detection of HC15N in Pluto's atmosphere, while accounting for 14N/15N fractionation between HCN and N2 using a photochemical model. Here, I show that, if this latter step of translating isotope ratios is adequately understood, then the derived 14N/15N ratio represents the first distinguishing constraint on the origin of Pluto's N2. One notable finding of the present study is that isotopic fractionation between atmospheric N2 and N2-rich ices on the surface of Pluto does not appear to be significant. I infer a lower limit of ∼197 for the 14N/15N ratio of the dominant (solid) reservoir of N2 on Pluto; i.e., mostly contained in Sputnik Planitia. From this lower limit, an endmember ammonia source of Pluto's N2 can be ruled out. I perform N isotope mixing calculations that enable quantitative understanding of the relationships between contributions by primordial N2, NH3, and nitrogen originally sourced in organic materials (Norg) to Pluto's observed N2 inventory. These calculations also address how uncertainties in the isotopic composition of Norg and the history of atmospheric escape affect the allowed ranges of primordial N2, NH3, and Norg contributions. While present uncertainties are substantial, I find that a contribution by primordial N2, Norg, or both is implied, and the sum of their contributions should be at least ∼45%. Hence, it is likely that Pluto formed from building blocks that were cold enough to trap N2 (e.g., <30 K), or Pluto has a thermally processed and dynamic interior that supports generation of N2 from Norg (at temperatures above ∼350 °C) and N2 transport to the surface. Furthermore, the lower limit on 14N/15N suggests that NH3 has been a less significant contributor to the origin of N2 on Pluto than on Titan, which is indicative of a key difference in the origin and evolution of these worlds. Recommendations are given for future work that can continue to advance and contextualize understanding of the origin of Pluto's N2. A new mission that can determine the origin of N2 on Neptune's moon Triton should be a priority. Such a mission would offer an unprecedented opportunity for comparison of volatile origins on large Kuiper belt objects (past or present) with distinct histories.

Scaling laws for planetary sediment transport from DEM-RANS numerical simulations

We use an established discrete element method (DEM) Reynolds-averaged Navier–Stokes (RANS)-based numerical model to simulate non-suspended sediment transport across conditions encompassing almost seven orders of magnitude in the particle–fluid density ratio$s$, ranging from subaqueous transport ($s=2.65$) to aeolian transport in the highly rarefied atmosphere of Pluto ($s=10^7$), whereas previous DEM-based sediment transport studies did not exceed terrestrial aeolian conditions ($s\approx 2000$). Guided by these simulations and by experiments, we semi-empirically derive simple scaling laws for the cessation threshold and rate of equilibrium aeolian transport, both exhibiting a rather unusual$s^{1/3}$-dependence. They constitute a simple means to make predictions of aeolian processes across a large range of planetary conditions. The derivation consists of a first-principle-based proof of the statement that, under relatively mild assumptions, the cessation threshold physics is controlled by only one dimensionless control parameter, rather than two expected from dimensional analysis. Crucially, unlike existing models, this proof does not resort to coarse-graining the particle phase of the aeolian transport layer above the bed surface. From the pool of existing models, only that by Pähtzet al.(J. Geophys. Res.: Earth, vol. 126, 2021, e2020JF005859) is somewhat consistent with the combined numerical and experimental data. It captures the scaling of the cessation threshold and the$s^{1/3}$-dependence of the transport rate, but fails to capture the latter's superimposed grain size dependence. This hints at a lack of understanding of the transport rate physics and calls for future studies on this issue.

Impacts of Organic Ice Condensation on the Optical Properties of Haze on Pluto

The flyby of the New Horizons spacecraft in 2015 July revealed an unexpected cold atmosphere of Pluto and confirmed the existence of its atmospheric haze. The observed and simulated vertical profiles of chemical species and microphysical processes suggest that the haze particles in Pluto’s middle and lower atmosphere may contain organic ice condensation. Such organic ice components can potentially affect Pluto’s haze chemistry and optical properties, as well as its energy budget. This study investigates the influence of the ice components on the scattering properties of Pluto’s haze by comparing New Horizons observations and simulated particle scattering properties. Comprehensive tests are performed for various haze particle parameters, including their size, chemical component, ice content, and morphology. Scattering properties of these ice-bearing haze particles are calculated by a discrete dipole approximation method and compared to multispectral observations obtained by four New Horizons instruments in spectral regions ranging from the ultraviolet to the near-infrared. The results indicate that the inclusion of the organic ice component leads to higher ratios of backscattering in the visible to extinction in the ultraviolet and provides better agreement with observations compared to monodispersed homogeneous aggregates. But it alone is not sufficient to explain the observed forward scattering values in the visible and near-infrared. Therefore, other scattering sources and/or mechanisms are still required to explain the full set of scattering observations. Further observations, as well as laboratory measurements and numerical tests, are anticipated to improve our understanding of the morphology and ice content of Pluto’s haze.

Optical constants of Pluto aerosol analogues from UV to near-IR

Photochemical aerosols were detected as high as 350 km of altitude in Pluto's atmosphere during the New Horizons fly-by. These aerosols are thought to affect Pluto's climate, by acting as cooling agents, and the colours of Pluto's surface, in particular in the dark regions named Cthulhu and Krun and at the North Pole. Pluto atmospheric and surface models have so far used the optical constants of Titan aerosol analogues (tholins), whereas their chemical composition is known to differ from that of Pluto aerosol analogues.In order to provide a new set of optical constants for Pluto tholins, we synthesized analogues of Pluto's aerosols and determined with spectroscopic ellipsometry their optical constants from 270 to 2100 nm. Three types of samples were produced from N2:CH4:CO gas mixtures differing in their CH4:N2 mixing ratio, representative of different altitudes in Pluto's current atmosphere or different seasons or epochs of Pluto.Our analysis shows a strong absorption by Pluto tholins in the UV and visible spectral ranges, with k index of a few 10−1 at 270 nm, in agreement with N- and O-bearing organic molecules. Pluto tholins are less absorbent in the near-IR than in the UV–Vis wavelength range, with k of a few 10−3 between 600 and 2100 nm. Our comparative study highlights the dependency of n and k indices to the CH4:N2 mixing ratio. Aerosols formed at different altitudes in Pluto's atmosphere or during different seasons or epochs of Pluto will therefore affect the budget of Pluto radiative transfer differently.The optical constants presented in this study were tested with a Pluto surface model and with a model of light scattering. The surface modelling results highlight the suitability of these optical constants to reproduce Pluto compositional observations in the visible spectral range by MVIC and LEISA. The atmospheric modelling results conclude that Pluto tholins absorb 5 to 10 times less than Titan tholins at 500 nm, and this lower absorption is consistent with Alice observations of Pluto's haze.

Geologically Diverse Pluto and Charon: Implications for the Dwarf Planets of the Kuiper Belt

Pluto and Charon are strikingly diverse in their range of geologies, surface compositions, and crater retention ages. This is despite the two having similar densities and presumed bulk compositions. Much of Pluto's surface reflects surface-atmosphere interactions and the mobilization of volatile ices by insolation. Abundant evidence, including past and present N2 ice glacial activity, implies that Pluto has undergone substantial climate evolution. An ancient impact basin contains a massive, convectively overturning N2 ice reservoir, whose position and surrounding tectonics suggest a subsurface ocean. Aligned blades of methane ice hundreds of meters tall, found only at high altitude, likely cover much of Pluto's low latitudes and may be a consequence of obliquity variation–driven volatile migration. Multikilometer-high possible cryovolcanic constructs and apparent fissure eruptions indicate relatively late endogenic activity on Pluto. Pluto's range of surface ages is extreme, whereas Charon's surface, while old, displays a large resurfaced plain and a globally engirdling extensional tectonic network attesting to earlier endogenic vigor. ▪ The vast N2 ice sheet Sputnik Planitia controls Pluto's atmosphere and climate, comparable in importance with the role of Greenland and Antarctica on the climate of Earth. ▪ Spectacular evidence for erosion such as now-unoccupied glacial valley networks implies a vigorous early climate, and more widespread N2 ice glaciation, on Pluto. ▪ Geological activity on both bodies requires or required sustained internal heat release and suggests a past (Charon) or present (Pluto) ammoniated, subsurface ocean. ▪ The varieties of geologic experience witnessed on Pluto and Charon should play out among the many and varied dwarf planets of the Kuiper belt.

Global climate model occultation lightcurves tested by August 2018 ground-based stellar occultation

Pluto's atmospheric profiles (temperature and pressure) have been studied for decades from stellar occultation lightcurves. In this paper, we look at recent Pluto Global Climate Model (GCM) results (3D temperature, pressure, and density fields) from Bertrand et al. (2020) and use the results to generate model observer's plane intensity fields (OPIF) and lightcurves by using a Fourier optics scheme to model light passing through Pluto's atmosphere (Young, 2012). This approach can accommodate arbitrary atmospheric structures and 3D distributions of haze. We compared the GCM model lightcurves with the lightcurves observed during the 15-AUG-2018 Pluto stellar occultation. We find that the climate scenario which best reproduces the observed data includes a N2 ice mid latitude band in the southern hemisphere. We have also studied different haze and P/T ratio profiles: the haze effectively reduces the central flash strength, and a lower P/T ratio both reduces the central flash strength and incurs anomalies in the shoulders of the central flash.

LORRI observations of waves in Pluto's atmosphere

Observations during the New Horizons (NH) spacecraft flyby of Pluto in July 2015 revealed that Pluto's atmosphere supports an extensive circumplanetary haze with embedded layers, suggesting several possible microphysical and/or dynamical excitation processes. The purpose of this paper is to build upon existing observations and analyses of Pluto's atmosphere—specifically of the complex haze layer structures—to identify wave structure in Pluto's atmosphere. Here three NH/Long Range Reconnaissance Imager (LORRI) image sequences from the flyby at high phase angles (148°–169°) and three different resolutions (0.093 km/pix, 0.96 km/pix, and 3.86 km/pix) are analyzed. Several haze layer characteristics were extracted, namely—slope, amplitude, waveform, and the associated power spectral densities (PSDs); and their variations with local geography. These are then explored in the context of possible wave types in Pluto's atmosphere, such as tidal and orographically driven inertia-gravity (buoyancy) waves. PSD peaks at 8–10 km and 18–22 km vertical wavelength are found in NH images, which is consistent with the perturbations seen in Earth-based stellar occultations of Pluto's atmosphere. The 8–10 km signals are localized to low-latitudes and equatorial regions and the 18–22 km signals are more globally distributed. Haze layer background relative amplitudes were found to be around 0.01–0.04. Slopes of layers were found to be correlated with the emergence and disappearance of a 25 km layer around 30°N. An amplitude increase of oscillations below 30 km altitude exists in the high-resolution image sequence. These findings indicate the possibility of waves in Pluto's atmosphere and motivate further studies of wave dynamics combining NH data with state-of-the-art models of Pluto's atmosphere. These results are important because they can provide strong constraints to models and to the type of waves that can be present in Pluto's atmosphere.

Chemical composition of Pluto aerosol analogues

Photochemical aerosols were observed in Pluto atmosphere during the New Horizons flyby on July 14th, 2015, as several thin haze layers extending at >350 km of altitude. This flyby has raised numerous questions on the aerosols formation processes and their impact on Pluto radiative transfer and climate. In order to gain a better understanding, we synthesized Pluto aerosol analogues in a room-temperature dusty plasma experiment and inferred their chemical composition from infrared spectroscopy, elemental composition analysis and very high-resolution mass spectrometry (ESI+/Orbitrap device). Three types of samples were synthesized at 0.9 ± 0.1 mbar, called P400, P600 and PCO-free. The samples P400 and P600 were produced from gas mixtures mimicking Pluto atmosphere at around 400 and 600 km of altitude, respectively, in order to determine if CH4 mixing ratio has an influence on the chemical composition of the aerosols. The sample PCO-free was produced from a gas mixture similar to the one forming the sample P400, but without carbon monoxide, in order to identify the impact of CO.Our study shows that the molecules constituting samples P400 and P600 are very rich in nitrogen atoms (up to 45% in mass of N elements) and, compared to the molecules constituting the PCO-free sample, a significant incorporation of oxygen atoms was detected. Moreover, our results on the variation of CH4 mixing ratio demonstrate that different ratios lead to different reactivity between N2, CH4 and CO. In particular, more nitrogen and oxygen atoms are detected in the bulk composition of the analogues P400. Due to the presence of nitrogenated and oxygenated molecules in the analogues of Pluto aerosols, we suggest that these aerosols will have an impact on Pluto radiative transfer, and thus on climate, that will differ from predictions based on the optical constants of Titan aerosol analogues.

Pluto's Beating Heart Regulates the Atmospheric Circulation: Results From High‐Resolution and Multiyear Numerical Climate Simulations

AbstractPluto's atmosphere is mainly nitrogen and is in solid‐gas equilibrium with the surface nitrogen ice. As a result, the global nitrogen ice distribution and the induced nitrogen condensation‐sublimation flows strongly control the atmospheric circulation. It is therefore essential for Global Climate Models to accurately account for the global nitrogen ice distribution in order to realistically simulate Pluto's atmosphere. Here we present a set of new numerical simulations of Pluto's atmosphere in 2015 performed with a Global Climate Model using a 50‐km horizontal resolution (3.75° 2.5°) and taking into account the latest topography and ice distribution data, as observed by the New Horizons spacecraft. In order to analyze the seasonal evolution of Pluto's atmosphere dynamics, we also performed simulations at coarser resolution (11.25° 7.5°) but covering three Pluto years. The model predicts a near‐surface western boundary current inside the Sputnik Planitia basin in 2015, which is consistent with the dark wind streaks observed in this region. We find that this atmospheric current could explain the differences in ice composition and color observed in the northwestern regions of Sputnik Planitia, by significantly impacting the nitrogen ice sublimation rate in these regions through processes possibly involving conductive heat flux from the atmosphere, transport of dark materials by the winds, and surface albedo positive feedbacks. In addition, we find that this current controls Pluto's general atmospheric circulation, which is dominated by a retrorotation, independently of the nitrogen ice distribution outside Sputnik Planitia. This exotic circulation regime could explain many of the geological features and longitudinal asymmetries in ice distribution observed all over Pluto's surface.

Haze in Pluto's atmosphere: Results from SOFIA and ground-based observations of the 2015 June 29 Pluto occultation

On UT 29 June 2015, the occultation by Pluto of a bright star (r′ = 11.9) was observed from the Stratospheric Observatory for Infrared Astronomy (SOFIA) and several ground-based stations in New Zealand and Australia. Pre-event astrometry allowed for an in-flight update to the SOFIA team with the result that SOFIA was deep within the central flash zone (~22 km from center). Analysis of the combined data leads to the result that Pluto's middle atmosphere is essentially unchanged from 2011 and 2013 (Person et al. 2013; Bosh et al. 2015); there has been no significant expansion or contraction of the atmosphere. Additionally, our multi-wavelength observations allow us to conclude that a haze component in the atmosphere is required to reproduce the light curves obtained. This haze scenario has implications for understanding the photochemistry of Pluto's atmosphere.

Unistellar eVscopes: Smart, portable, and easy-to-use telescopes for exploration, interactive learning, and citizen astronomy

Unistellar has created the Enhanced Vision Telescope (eVscope), a compact telescope that amplifies light so users can see hundreds of nebulae and galaxies directly through its electronics eyepiece. It can also pinpoint and identify objects in the sky, making amateur astronomy fun and more accessible to the public. Thanks to its sensitivity and accuracy, the eVscope is a powerful tool able to generate data that can be used by scientists to search for transient events like supernovae, near-Earth asteroids, and comets. This constellation of small, & smart telescopes could also enhance and supplement data generated by the small number of large telescopes operating today. Unistellar initiated a partnership with the SETI Institute to identify and implement scientific applications for a network of eVscopes. We summarize in this article the technology behind the eVscope and its real-time data processing (Enhanced Vision, Automatic Field Detection), then show several applications accessible to future users, including asteroid occultations, the detection of the atmosphere of Pluto and observations of near-Earth asteroids. The Unistellar network has the potential to make citizen astronomy a popular reality by offering all users (new or experienced) a tool to explore the night sky with a powerful and reliable instrument while they contribute to scientific investigations.

A photochemical model of Pluto's atmosphere and ionosphere

The model for Titan by Krasnopolsky (2014, Icarus 236, 83-91) has been adjusted to Pluto's conditions during the New Horizons flyby. The model includes 419 reactions of 83 neutrals and 33 positive ions plus 10 reactions of CO+ and HCO+ that were missing for Titan because of the low CO abundance. The model has 289 altitude steps up to the exobase at 1600 km with thermal escape for neutrals and diffusion velocities for ions as the upper boundary conditions. The model involves condensation on the haze and the surface with sticking coefficient γ = 0.002 for hydrocarbons, H2O, and CO2, γ = 0.01 for nitriles except HC3N with γ = 0.1. The photochemistry is driven by the solar EUV and UV radiation, the interplanetary Lyman-alpha emission, and the galactic cosmic rays. The adopted eddy diffusion K = 3 × 104 cm2 s−1 facilitates transport of C2H4 and C2H6, their condensation on the surface, and does not require the revision of the laboratory data on saturated vapor densities of C2H4 and C2H6 by orders of magnitude to fit the New Horizons observations. The CH4 homopause is at 110 km for this K, and the CH4 vertical profile is mostly controlled by molecular diffusion and agrees with the NH observations. Productions and losses of major hydrocarbons, nitriles, oxygen species, and hydrogen are briefly discussed. The daytime ionosphere is predicted with a maximum electron density of 800 cm−3 at 750 km. The most abundant ions are HCNH+ and C9H11+ above and below 600 km, respectively. Chemical effects of the ion reactions on some neutral species are significant. The predicted ion densities are measurable by an analog of the Cassini ion-neutral mass spectrometer. Evolution of the atmosphere includes thermal escape of H2 + H, CH4 + CH3, and N2 + N (92, 203, and 1 g cm−2 Byr−1, respectively), condensation of hydrocarbons and nitriles (306 and 101 g cm−2 Byr−1, respectively), and polymerization (30 g cm−2 Byr−1). (Annually mean values are halves of those.) The composition and photochemistry of Pluto during the New Horizons flyby are very different from those of Triton during the Voyager 2 flyby and controlled by the methane mole fraction near the surface. Seasonal variations of the atmospheric methane should cover both states of the atmospheres on both Pluto and Triton.

New Horizons Observations of the Atmosphere of Pluto

New Horizons data provide a snapshot of the current state of Pluto's atmosphere. Winds are slow and mostly controlled by sublimation of surface ices. Molecular nitrogen is the primary constituent below 1,800 km, while methane and carbon monoxide are important minor species. Photolysis of these gases leads to a thin haze that encompasses Pluto from the surface up to &gt;500-km altitude and is important in heating and cooling the atmosphere. A cold (∼70 K) upper atmosphere curtails the escape of Pluto's molecular nitrogen to space, although there is substantial escape of methane (∼5 × 1025 molecules s−1), coincidentally about equal to its loss by photochemistry. It is unknown if the current atmosphere is representative of its long-term average state. From the inferred rapid rate of haze settling, it seems that Pluto's atmosphere must occasionally undergo collapse to allow time for radiation processing of the colorless haze material into the dark deposits found on the surface. ▪ This article outlines what has been gleaned about Pluto's atmosphere in the years since the New Horizons flyby. ▪ Pluto's atmosphere is most similar to Titan's—with the photochemistry of supervolatile nitrogen and hydrocarbons resulting in a kind of factory for cold haze production. ▪ Much has been learned about Pluto's atmosphere, but many new questions have arisen, and these will likely remain unanswered until there is a follow-up mission—no doubt a long time from now.

The CH4 cycles on Pluto over seasonal and astronomical timescales

Pluto's surface is covered in numerous CH4 ice deposits, that vary in texture and brightness, as revealed by the New Horizons spacecraft as it flew by Pluto in July 2015. These observations suggest that CH4 on Pluto has a complex history, involving reservoirs of different composition, thickness and stability controlled by volatile processes occurring on different timescales. In order to interpret these observations, we use a Pluto volatile transport model able to simulate the cycles of N2 and CH4 ices over millions of years. By assuming fixed solid mixing ratios, we explore how changes in surface albedos, emissivities and thermal inertias impact volatile transport. This work is therefore a direct and natural continuation of the work by Bertrand et al. (2018), which only explored the N2 cycles. Results show that bright CH4 deposits can create cold traps for N2 ice outside Sputnik Planitia, leading to a strong coupling between the N2 and CH4 cycles. Depending on the assumed albedo for CH4 ice, the model predicts CH4 ice accumulation (1) at the same equatorial latitudes where the Bladed Terrain Deposits are observed, supporting the idea that these CH4-rich deposits are massive and perennial, or (2) at mid-latitudes (25°− 70°), forming a thick mantle which is consistent with New Horizons observations. In our simulations, both CH4 ice reservoirs are not in an equilibrium state and either one can dominate the other over long timescales, depending on the assumptions made for the CH4 albedo. This suggests that long-term volatile transport exists between the observed reservoirs. The model also reproduces the formation of N2 deposits at mid-latitudes and in the equatorial depressions surrounding the Bladed Terrain Deposits, as observed by New Horizons. At the poles, only seasonal CH4 and N2 deposits are obtained in Pluto's current orbital configuration. Finally, we show that Pluto's atmosphere always contained, over the last astronomical cycles, enough gaseous CH4 to absorb most of the incoming Lyman-α flux.

Future of Planetary Atmospheric, Surface, and Interior Science Using Radio and Laser Links

AbstractScientific studies using spacecraft radio links, which are shared with communications and navigation functions, have been carried out on almost every solar system exploration mission in the past five decades. They have led to numerous scientific discoveries as well as technological advances. These radio science experiments have investigated the interior structure of every planet in the solar system and several of their satellites, the Moon, and several comets and asteroids and sounded the atmospheres of every planet in the solar system except Mercury, and the atmospheres of Pluto and several large satellites, the Io Plasma Torus, Saturn's and Uranus' rings, and the solar corona. Future experiments at Mercury, the Jovian system, asteroid Psyche, and other targets are currently in various planning and development phases. Over the next 30 years, significant advances in radio and laser link science technologies, including one order of magnitude improvement achievable in range rate and range accuracy and advanced calibration techniques, could enable many additional scientific breakthroughs. Future exploration concepts focus on applications of small spacecraft and entry probes and can include constellations for studies of atmospheric dynamics, interior structures, and surface properties. Selected science‐enabling technologies specific to small spacecraft instrumentation on future missions are under study. Examples include field tests of radio scattering to determine surface properties, constellations for high‐resolution spatial and temporal atmospheric sounding, small science‐quality software‐defined transponders, miniature ultrastable oscillators, and advanced radio‐metric calibrations at the Deep Space Network.

Ongoing resurfacing of KBO Eris by volatile transport in local, collisional, sublimation atmosphere regime

Kuiper belt object (KBO) Eris is exceptionally bright with a greater visible geometric albedo than any other known KBO. Its infrared reflectance spectrum is dominated by methane, which should form tholins that darken the surface on timescales much shorter than the age of the Solar System. Thus one or more ongoing processes probably maintain its brightness. Eris is predicted to have a primarily nitrogen atmosphere that is in vapor pressure equilibrium with nitrogen-ice and is collisional (not ballistic). Eris’s eccentric orbit is expected to result in two atmospheric regimes: (1) a period near perihelion when the atmosphere is global (analogous to the atmospheres of Mars, Triton, and Pluto) and (2) a period near aphelion when only a local atmosphere exists near the warmest region (analogous to the atmosphere of Io). A numerical model developed to simulate Eris’s thermal and volatile evolution in the local atmosphere regime is presented. The model conserves energy, mass, and momentum while maintaining vapor pressure equilibrium. It is adaptable to other local, collisional, sublimation atmospheres, which in addition to Io and Eris, may occur on several volatile-bearing KBOs. The model was applied for a limiting case where Eris is fixed at aphelion and has an initial nitrogen-ice mass everywhere equal to the precipitable column of nitrogen in Pluto's atmosphere during the New Horizons encounter (the resultant mass if the Pluto atmosphere collapsed uniformly onto the surface). The model results indicate that (1) transport of nitrogen in the local, collisional, sublimation atmosphere regime is significant, (2) changes of Eris’s albedo or color from nitrogen transport may be observable, and (3) uniform collapse of a global, nitrogen atmosphere likely cannot explain Eris’s anomalous albedo in the present epoch. Seasonal volatile transport remains a plausible hypothesis to explain Eris’s anomalous albedo and geologic processes that renew Pluto’s brightest surfaces, such as convection and glaciation, may also be operating on Eris.

Interplanetary dust delivery of water to the atmospheres of Pluto and Triton

Context. Both Pluto and Triton possess thin, N2-dominated atmospheres controlled by sublimation of surface ices. Aims. We aim to constrain the influx and ablation of interplanetary dust grains into the atmospheres of both Pluto and Triton in order to estimate the rate at which oxygen-bearing species are introduced into both atmospheres. Methods. We use (i) an interplanetary dust dynamics model to calculate the flux and velocity distributions of interplanetary dust grains relevant for both Pluto and Triton and (ii) a model for the ablation of interplanetary dust grains in the atmospheres of both Pluto and Triton. We sum the individual ablation profiles over the incoming mass and velocity distributions of interplanetary dust grains in order to determine the vertical structure and net deposition of water to both atmospheres. Results. Our results show that &lt;2% of silicate grains ablate at either Pluto or Triton while approximately 75% and &gt;99% of water ice grains ablate at Pluto and Triton, respectively. From ice grains, we calculate net water influxes to Pluto and Triton of ~3.8 kg day−1 (8.5 × 103 H2O cm−2 s−1) and ~370 kg day−1 (6.2 × 105 H2O cm−2 s−1), respectively. The significant difference in total water deposition between Pluto and Triton is due to the presence of Triton within Neptune’s gravity well, which both enhances interplanetary dust particle (IDP) fluxes due to gravitational focusing and accelerates grains before entry into Triton’s atmosphere, thereby causing more efficient ablation. Conclusions. We conclude that water deposition from dust ablation plays only a minor role at Pluto due to its relatively low flux. At Triton, water deposition from IDPs is more significant and may play a role in the alteration of atmospheric and ionospheric chemistry. We also suggest that meteoric smoke and smaller, unablated grains may serve as condensation nuclei for the formation of hazes at both worlds.

Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto

The presence of N2 in the surface environment of Pluto is critical in creating Pluto's richness of features and processes. Here, we propose that the nitrogen atoms in the N2 observed on Pluto were accreted in that chemical form during the formation of Pluto. We use New Horizons data and models to estimate the amounts of N2 in the following exterior reservoirs: atmosphere, escape, photochemistry, and surface. The total exterior inventory is deduced to be dominated by a glacial sheet of N2-rich ices at Sputnik Planitia, or by atmospheric escape if past rates of escape were much faster than at present. Pluto's atmosphere is a negligible reservoir of N2, and photochemical destruction of N2 may also be of little consequence. Estimates are made of the amount of N2 accreted by Pluto based on cometary and solar compositions. It is found that the cometary model can account for the amount of N2 in Sputnik Planitia, while the solar model can provide a large initial inventory of N2 that would make prodigious atmospheric escape possible. These consistencies can be considered preliminary evidence in support of a primordial origin of Pluto's N2. However, both models predict accreted ratios of CO/N2 that are much higher than that in Pluto's atmosphere. Possible processes to explain “missing CO” that are given quantitative support here are fractional crystallization from the atmosphere resulting in CO burial at the surface, and aqueous destruction reactions of CO subject to metastable thermodynamic equilibrium in the subsurface. The plausibility of primordial N2 as the primary source of Pluto's nitrogen (vs. NH3 or organic N) can be tested more rigorously using future constraints on the 14 N/15 N ratio in N2 and the 36Ar/N2 ratio.