Cryovolcanic flooding in Viking Terra on Pluto

A prominent fossa trough (Uncama Fossa) and adjacent 28-km diameter impact crater (Hardie) in Pluto's Viking Terra, as seen in the high-resolution images from the New Horizons spacecraft, show morphological evidence of in-filling with a material of uniform texture and red-brown color. A linear fissure parallel to the trough may be the source of a fountaining event yielding a cryoclastic deposit having the same composition and color properties as is found in the trough and crater. Spectral maps of this region with the New Horizons LEISA instrument reveal the spectral signature of H₂O ice in these structures and in distributed patches in the adjacent terrain in Viking Terra. A detailed statistical analysis of the spectral maps shows that the colored H₂O ice filling material also carries the 2.2-μm signature of an ammoniated component that may be an ammonia hydrate (NH₃nH₂O) or an ammoniated salt. This paper advances the view that the crater and fossa trough have been flooded by a cryolava debouched from Pluto's interior along fault lines in the trough and in the floor of the impact crater. The now frozen cryolava consisted of liquid H₂O infused with the red-brown pigment presumed to be a tholin, and one or more ammoniated compounds. Although the abundances of the pigment and ammoniated compounds entrained in, or possibly covering, the H₂O ice are unknown, the strong spectral bands of the H₂O ice are clearly visible. In consideration of the factors in Pluto's space environment that are known to destroy ammonia and ammonia-water mixtures, the age of the exposure is of order ≤10⁹ years. Ammoniated salts may be more robust, and laboratory investigations of these compounds are needed

Cryovolcanic flooding in Viking Terra on Pluto

A prominent fossa trough (Uncama Fossa) and adjacent 28-km diameter impact crater (Hardie) in Pluto's Viking Terra, as seen in the high-resolution images from the New Horizons spacecraft, show morphological evidence of in-filling with a material of uniform texture and red-brown color. A linear fissure parallel to the trough may be the source of a fountaining event yielding a cryoclastic deposit having the same composition and color properties as is found in the trough and crater. Spectral maps of this region with the New Horizons LEISA instrument reveal the spectral signature of H₂O ice in these structures and in distributed patches in the adjacent terrain in Viking Terra. A detailed statistical analysis of the spectral maps shows that the colored H₂O ice filling material also carries the 2.2-μm signature of an ammoniated component that may be an ammonia hydrate (NH₃nH₂O) or an ammoniated salt. This paper advances the view that the crater and fossa trough have been flooded by a cryolava debouched from Pluto's interior along fault lines in the trough and in the floor of the impact crater. The now frozen cryolava consisted of liquid H₂O infused with the red-brown pigment presumed to be a tholin, and one or more ammoniated compounds. Although the abundances of the pigment and ammoniated compounds entrained in, or possibly covering, the H₂O ice are unknown, the strong spectral bands of the H₂O ice are clearly visible. In consideration of the factors in Pluto's space environment that are known to destroy ammonia and ammonia-water mixtures, the age of the exposure is of order ≤10⁹ years. Ammoniated salts may be more robust, and laboratory investigations of these compounds are needed

The nature and origin of Charon's smooth plains

Charon displays extensive plains that cover the equatorial area and south to the terminator on the sub-Pluto hemisphere observed by New Horizons. We hypothesize that these plains are a result of Charon's global extension and early subsurface ocean yielding a large cryoflow that completely resurfaced this area leaving the plains and other features that we observe today. The cryoflow consisted of ammonia-rich material, and could have resurfaced this area either by cryovolcanic effusion similar to lunar maria emplacement or a mechanism similar to magmatic stoping where lithospheric blocks foundered. Geological observations, modeling of possible flow rheology, and an analysis of rille orientations support these hypotheses

New Frontiers-class Uranus Orbiter: Exploring the feasibility of achieving multidisciplinary science with a mid-scale mission

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Mercury’s Lobate Scarps Reveal that Polygonal Impact Craters Form on Contractional Structures

Analysis of polygonal impact craters (PICs) can be used to investigate the presence and orientations of subtle and/or buried faults and fractures across the solar system that may otherwise be unobservable in spacecraft images. Although this technique has been vetted for the analysis of extensional structures, no previous work has investigated if PICs also form on contractional thrust faults. This determination, which we investigated in this work, is critical for accurate tectonic setting interpretations from PICs. Mercury shows an abundance of thrust-fault-related landforms, making it an ideal laboratory to perform this investigation. In this work, we found that Mercury’s thrust faults, and their overlying folds and fractures, cause some complex craters ∼20 km or larger to form PICs. However, in most cases, craters form as circular impact craters on these structures. When PIC straight rim segments do form, they parallel the lobate scarp thrust faults and fold hinges. Some PICs likely formed as a result of an impact’s interaction with the thrust fault itself, while others may have interacted with fold hinge joints. The parallel relationship between PICs and shortening structures is consistent with the well-established relationship between PICs and extensional structures. Therefore, in addition to extensional fractures, contractional features should also be taken into consideration when utilizing PICs to interpret tectonic settings on bodies across the solar system

Tethys’s Heat Fluxes Varied with Time in the Ithaca Chasma and Telemus Basin Region

We investigated how lithospheric heat fluxes varied temporally and spatially on the Saturnian moon Tethys, focusing on the region of Ithaca Chasma that overprints Telemus Impact Basin. Our results, derived from flexure associated with Ithaca, indicate elastic thicknesses of 4.1 ± 0.3 km to 6.4 ± 0.4 km and heat fluxes ranging from 12 to 39 mW m ^−2 assuming a nonporous pure H _2 O ice lithosphere. Our results for Ithaca’s south limb are similar to previous estimates within the north limb, indicating consistent heat fluxes across a large spatial extent in this area. However, our estimates are lower than those for the older Telemus Basin (>60 mW m ^−2 ), revealing evidence that Tethys experienced a substantial temporal variation in heat fluxes in this region. Heat fluxes reflected by Ithaca are similar to previous estimates for Tethys’s two youngest impact basins, Melanthius and Odysseus, suggesting that Ithaca may also be relatively young. If Tethys’s lithosphere is porous, then our heat flux estimates for Ithaca Chasma drop to 12–38 mW m ^−2 , 11–35 mW m ^−2 , and 10–33 mW m ^−2 for 5%, 15%, and 25% porosities, respectively. If Tethys’s lithosphere includes ∼10% NH _3 -hydrates, then the estimates are 5–16 mW m ^−2 , 5–15 mW m ^−2 , 4–14 mW m ^−2 , and 4–13 mW m ^−2 for 0%, 5%, 15%, and 25% porosities, respectively. Although we find that some ground-based reflectance spectra hint at 2.2 μ m bands that may result from NH _3 -bearing species, the detected features are weak and may not result from surface constituents. Consequently, our heat flux estimates that assume a pure H _2 O ice lithosphere are likely more accurate

Titania's Heat Fluxes Revealed by Messina Chasmata

Messina Chasmata is a relatively young tectonic structure on Titania based on cross-cutting relationships, although an absolute age has not been estimated. We investigated lithospheric flexure bounding Messina and found that the terrain along both rims reflects Titania’s thermal properties. We estimate Titania’s heat fluxes to have been 5–12 mW m ^−2 in this region, assuming that the lithosphere is composed of pure H _2 O ice without porosity. These estimates are lower if lithospheric porosity and/or NH _3 –H _2 O are also present. If Messina is ancient, forming as a result of freeze expansion, then the reflected heat fluxes are consistent with radiogenic heating. However, if Messina is instead young, then an additional heat source is required. In this scenario, perhaps tidal heating associated with the past three-body resonance shared between Titania, Ariel, and Umbriel generated this heat. However, this scenario is unlikely because the amount of tidal heating produced on Titania would have been minimal. Titania’s heat fluxes are notably lower than estimates for Miranda or Ariel, and future work is needed to investigate Umbriel and Oberon to gain a fuller understanding of Uranian moon thermal and orbital histories. Additionally, further constraints on Titania’s more ancient heat fluxes could be obtained by investigating relatively older features, such as some viscously relaxed impact craters

Evidence for Nitrogen-bearing Species on Umbriel: Sourced from a Subsurface Ocean, Undifferentiated Crust, or Impactors?

Near-infrared spectra of Umbriel and the other classical Uranian moons exhibit 2.2 μ m absorption bands that could result from ammonia (NH _3 ) bearing species, possibly exposed in the geologically recent past. However, Umbriel has an ancient surface with minimal evidence for recent endogenic activity, raising the possibility that more refractory species are present, and/or that NH _3 is retained over long timescales. We analyzed 33 spectra of Umbriel to investigate its 2.2 μ m band, along with three other absorption features we identified near 2.14, 2.22, and 2.24 μ m. We assessed the subobserver longitudinal distributions of these four bands, finding that they are present across Umbriel and may be spatially associated with geologic features such as craters and large basins. We compared the bands to 15 candidate constituents. We found that Umbriel’s 2.14 μ m and 2.22 μ m bands are most consistent with the spectral signature of organics, its 2.24 μ m band is best matched by NH _3 ice, and its 2.2 μ m band is consistent with the signatures of NH _3 –H _2 O mixtures, aluminum-bearing phyllosilicates, and sodium-bearing carbonates. However, some of these candidate constituents do not match Umbriel’s spectral properties in other wavelength regions, highlighting the gaps in our understanding of the Uranian moons’ surface compositions. Umbriel’s 2.14 μ m band may alternatively result from a 2 _ν _3 overtone mode of CO _2 ice. If present on Umbriel, these candidate constituents could have formed in contact with an internal ocean and were subsequently exposed during Umbriel’s early history. Alternatively, these constituents might have originated in an undifferentiated crust or were delivered by impactors