Convection in a volatile nitrogen-ice-rich layer drives Pluto's geological vigour

Pluto’s Surface Mapping Using Unsupervised Learning from Near-infrared Observations of LEISA/Ralph

We map the surface of Pluto using an unsupervised machine-learning technique using the near-infrared observations of the LEISA/Ralph instrument on board NASA’s New Horizons spacecraft. The principal-component-reduced Gaussian mixture model was implemented to investigate the geographic distribution of the surface units across the dwarf planet. We also present the likelihood of each surface unit at the image pixel level. Average I/F spectra of each unit were analyzed—in terms of the position and strengths of absorption bands of abundant volatiles such as N2, CH4, and CO and nonvolatile H2O—to connect the unit to surface composition, geology, and geographic location. The distribution of surface units shows a latitudinal pattern with distinct surface compositions of volatiles—consistent with the existing literature. However, previous mapping efforts were based primarily on compositional analysis using spectral indices (indicators) or implementation of complex radiative transfer models, which need (prior) expert knowledge, label data, or optical constants of representative end-members. We prove that an application of unsupervised learning in this instance renders a satisfactory result in mapping the spatial distribution of ice compositions without any prior information or label data. Thus, such an application is specifically advantageous for a planetary surface mapping when label data are poorly constrained or completely unknown, because an understanding of surface material distribution is vital for volatile transport modeling at the planetary scale. We emphasize that the unsupervised learning used in this study has wide applicability and can be expanded to other planetary bodies of the solar system for mapping surface material distribution.

The New Astronomical Frontier of Interstellar Objects

The upcoming commencement of the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will greatly enhance the discovery rate of interstellar objects (ISOs). 'Oumuamua and Borisov were the first two ISOs confirmed in the Solar System, although the first interstellar meteor was detected earlier. We explore the intriguing mass budget of ejected planetesimals implied by the detections of 'Oumuamua and Borisov and explore the expected abundance of ISOs as a function of size in the solar neighborhood. Specifically, we find that a significant fraction of stellar mass must go toward producing ISOs and that ISOs outnumber Solar System objects in the Oort cloud. We consider signatures of ISOs colliding with Earth, the Moon, and neutron stars, as well as the possibility of differentiating ISOs from Solar System objects in stellar occultation surveys, and we show that these methods are observationally feasible. We introduce a test for dynamical anisotropy that is capable of determining the typical ejection speed of ISOs from their parent stars. Finally, we predict a new population of dynamically distinct ISOs originating from stars in the Galactic halo. One of the two branches of the newly established Galileo Project¹ seeks to learn more about the nature of ISOs like 'Oumuamua by performing new searches and designing follow-up observations.

Large-scale cryovolcanic resurfacing on Pluto

The New Horizons spacecraft returned images and compositional data showing that terrains on Pluto span a variety of ages, ranging from relatively ancient, heavily cratered areas to very young surfaces with few-to-no impact craters. One of the regions with very few impact craters is dominated by enormous rises with hummocky flanks. Similar features do not exist anywhere else in the imaged solar system. Here we analyze the geomorphology and composition of the features and conclude this region was resurfaced by cryovolcanic processes, of a type and scale so far unique to Pluto. Creation of this terrain requires multiple eruption sites and a large volume of material (>10⁴ km³) to form what we propose are multiple, several-km-high domes, some of which merge to form more complex planforms. The existence of these massive features suggests Pluto’s interior structure and evolution allows for either enhanced retention of heat or more heat overall than was anticipated before New Horizons, which permitted mobilization of water-ice-rich materials late in Pluto’s history.

Giant icy volcanos (cryovolcanos) on Pluto are unique in the imaged solar system and provide evidence for unexpected, active geology late in Pluto’s history.

Sublimation-driven convection in Sputnik Planitia on Pluto

Sputnik Planitia is a nitrogen-ice-filled basin on Pluto¹. Its polygonal surface patterns² have been previously explained as a result of solid-state convection with either an imposed heat flow³ or a temperature difference within the 10-km-thick ice layer⁴. Neither explanation is satisfactory, because they do not exhibit surface topography with the observed pattern: flat polygons delimited by narrow troughs⁵. Internal heating produces the observed patterns⁶, but the heating source in such a setup remains enigmatic. Here we report the results of modelling the effects of sublimation at the surface. We find that sublimation-driven convection readily produces the observed polygonal structures if we assume a smaller heat flux (~0.3 mW m^-2) at the base of the ice layer than the commonly accepted value of 2-3 mW m^-2 (ref. ⁷). Sustaining this regime with the latter value is also possible, but would require a stronger viscosity contrast (~3,000) than the nominal value (~100) considered in this study.

Infrared complex refractive index of N-containing astrophysical ices free of water processed by cosmic-ray simulated in laboratory

Several nitrogen-containing molecules have been unambiguously identified in the Solar System and in the Interstellar Medium. It is believed that such a rich inventory of species is a result of the energetic processing of astrophysical ices during the interaction with ionizing radiation. An intrinsic parameter of matter, the complex refractive index, stores all the "chemical memory" triggered by energetic processing, and therefore might be used to probe ice observations in the infrared. In this study, four N-containing ices have been condensed in an ultra-high vacuum chamber and processed by heavy ions (O and Ni) with energies between 0.2 and 15.7 MeV at the Grand Accélérateur National d'Ions Lourds (GANIL), in Caen, France. All chemical changes were monitored in situ by Infrared Absorption Spectroscopy. The complex refractive index was calculated directly from the absorbance spectrum, by using the Lambert-Beer and Kramers-Kroning relations, and the values are available in an online database: https://www1.univap.br/gaa/nkabs-database/data.htm. As a result, other than the database, it was observed that non-polar ices are more destroyed by sputtering than polar ones. Such destruction and chemical evolution lead to variation in the IR albedo of samples addressed in this paper.

Impact craters on Pluto and Charon indicate a deficit of small Kuiper belt objects

The flyby of Pluto and Charon by the New Horizons spacecraft provided high-resolution images of cratered surfaces embedded in the Kuiper belt, an extensive region of bodies orbiting beyond Neptune. Impact craters on Pluto and Charon were formed by collisions with other Kuiper belt objects (KBOs) with diameters from ~40 kilometers to ~300 meters, smaller than most KBOs observed directly by telescopes. We find a relative paucity of small craters ≲13 kilometers in diameter, which cannot be explained solely by geological resurfacing. This implies a deficit of small KBOs (≲1 to 2 kilometers in diameter). Some surfaces on Pluto and Charon are likely ≳4 billion years old, thus their crater records provide information on the size-frequency distribution of KBOs in the early Solar System.

Dunes on Pluto

The surface of Pluto is more geologically diverse and dynamic than had been expected, but the role of its tenuous atmosphere in shaping the landscape remains unclear. We describe observations from the New Horizons spacecraft of regularly spaced, linear ridges whose morphology, distribution, and orientation are consistent with being transverse dunes. These are located close to mountainous regions and are orthogonal to nearby wind streaks. We demonstrate that the wavelength of the dunes (~0.4 to 1 kilometer) is best explained by the deposition of sand-sized (~200 to ~300 micrometer) particles of methane ice in moderate winds (<10 meters per second). The undisturbed morphology of the dunes, and relationships with the underlying convective glacial ice, imply that the dunes have formed in the very recent geological past.

Untangling the methane chemistry in interstellar and solar system ices toward ionizing radiation: a combined infrared and reflectron time-of-flight analysis

Pure methane (CH₄/CD₄) ices were exposed to three ionizing radiation sources at 5.5 K under ultrahigh vacuum conditions to compare the complex hydrocarbon spectrum produced across several interstellar environments. These irradiation sources consisted of energetic electrons to simulate secondary electrons formed in the track of galactic cosmic rays (GCRs), Lyman α (10.2 eV; 121.6 nm) photons simulated the internal VUV field in a dense cloud, and broadband (112.7-169.8 nm; 11.0-7.3 eV) photons which mimic the interstellar ultra-violet field. The in situ chemical evolution of the ices was monitored via Fourier transform infrared spectroscopy (FTIR) and during heating via mass spectrometry utilizing a quadrupole mass spectrometer with an electron impact ionization source (EI-QMS) and a reflectron time-of-flight mass spectrometer with a photoionization source (PI-ReTOF-MS). The FTIR analysis detected six small hydrocarbon products from the three different irradiation sources: propane [C₃H₈(C₃D₈)], ethane [C₂H₆(C₂D₆)], the ethyl radical [C₂H₅(C₂D₅)], ethylene [C₂H₄(C₂D₄)], acetylene [C₂H₂(C₂D₂)], and the methyl radical [CH₃(CD₃)]. The sensitive PI-ReTOF-MS analysis identified a complex array of products with different products being detected between experiments with general formulae: C_nH_2n+2 (n = 4-8), C_nH_2n (n = 3-9), C_nH_2n-2 (n = 3-9), C_nH_2n-4 (n = 4-9), and C_nH_2n-6 (n = 6-7) from electron irradiation and C_nH_2n+2 (n = 4-8), C_nH_2n (n = 3-10), C_nH_2n-2 (n = 3-11), C_nH_2n-4 (n = 4-11), C_nH_2n-6 (n = 5-11), and C_nH_2n-8 (n = 6-11) from broadband photolysis and Lyman α photolysis. These experiments show that even the simplest hydrocarbon can produce important complex hydrocarbons such as C₃H₄ and C₄H₆ isomers. Distinct isomers from these groups have been shown to be important reactants in the synthesis of polycyclic aromatic hydrocarbons like indene (C₉H₈) and naphthalene (C₁₀H₈) under interstellar conditions.

Penitentes as the origin of the bladed terrain of Tartarus Dorsa on Pluto

Penitentes are snow and ice features formed by erosion that, on Earth, are characterized by bowl-shaped depressions several tens of centimetres across, whose edges grade into spires up to several metres tall. Penitentes have been suggested as an explanation for anomalous radar data on Europa, but until now no penitentes have been identified conclusively on planetary bodies other than Earth. Regular ridges with spacings of 3,000 to 5,000 metres and depths of about 500 metres with morphologies that resemble penitentes have been observed by the New Horizons spacecraft in the Tartarus Dorsa region of Pluto (220°-250° E, 0°-20° N). Here we report simulations, based upon a recent model representing conditions on Pluto, in which deepening penitentes reproduce both the tri-modal (north-south, east-west and northeast-southwest) orientation and the spacing of the ridges of this bladed terrain. At present, these penitentes deepen by approximately one centimetre per orbital cycle and grow only during periods of relatively high atmospheric pressure, suggesting a formation timescale of several tens of millions of years, consistent with crater ages. This timescale implies that the penitentes formed from initial topographic variations of no more than a few tens of metres, consistent with Pluto's youngest terrains.

Reorientation and faulting of Pluto due to volatile loading within Sputnik Planitia

Pluto is an astoundingly diverse, geologically dynamic world. The dominant feature is Sputnik Planitia-a tear-drop-shaped topographic depression approximately 1,000 kilometres in diameter possibly representing an ancient impact basin. The interior of Sputnik Planitia is characterized by a smooth, craterless plain three to four kilometres beneath the surrounding rugged uplands, and represents the surface of a massive unit of actively convecting volatile ices (N₂, CH₄ and CO) several kilometres thick. This large feature is very near the Pluto-Charon tidal axis. Here we report that the location of Sputnik Planitia is the natural consequence of the sequestration of volatile ices within the basin and the resulting reorientation (true polar wander) of Pluto. Loading of volatile ices within a basin the size of Sputnik Planitia can substantially alter Pluto's inertia tensor, resulting in a reorientation of the dwarf planet of around 60 degrees with respect to the rotational and tidal axes. The combination of this reorientation, loading and global expansion due to the freezing of a possible subsurface ocean generates stresses within the planet's lithosphere, resulting in a global network of extensional faults that closely replicate the observed fault networks on Pluto. Sputnik Planitia probably formed northwest of its present location, and was loaded with volatiles over million-year timescales as a result of volatile transport cycles on Pluto. Pluto's past, present and future orientation is controlled by feedbacks between volatile sublimation and condensation, changing insolation conditions and Pluto's interior structure.

The rapid formation of Sputnik Planitia early in Pluto's history

Pluto's Sputnik Planitia is a bright, roughly circular feature that resembles a polar ice cap. It is approximately 1,000 kilometres across and is centred on a latitude of 25 degrees north and a longitude of 175 degrees, almost directly opposite the side of Pluto that always faces Charon as a result of tidal locking. One explanation for its location includes the formation of a basin in a giant impact, with subsequent upwelling of a dense interior ocean. Once the basin was established, ice would naturally have accumulated there. Then, provided that the basin was a positive gravity anomaly (with or without the ocean), true polar wander could have moved the feature towards the Pluto-Charon tidal axis, on the far side of Pluto from Charon. Here we report modelling that shows that ice quickly accumulates on Pluto near latitudes of 30 degrees north and south, even in the absence of a basin, because, averaged over its orbital period, those are Pluto's coldest regions. Within a million years of Charon's formation, ice deposits on Pluto concentrate into a single cap centred near a latitude of 30 degrees, owing to the runaway albedo effect. This accumulation of ice causes a positive gravity signature that locks, as Pluto's rotation slows, to a longitude directly opposite Charon. Once locked, Charon raises a permanent tidal bulge on Pluto, which greatly enhances the gravity signature of the ice cap. Meanwhile, the weight of the ice in Sputnik Planitia causes the crust under it to slump, creating its own basin (as has happened on Earth in Greenland). Even if the feature is now a modest negative gravity anomaly, it remains locked in place because of the permanent tidal bulge raised by Charon. Any movement of the feature away from 30 degrees latitude is countered by the preferential recondensation of ices near the coldest extremities of the cap. Therefore, our modelling suggests that Sputnik Planitia formed shortly after Charon did and has been stable, albeit gradually losing volume, over the age of the Solar System.