seasonal activity and morphological changes in martian gullies abstract recent studies of martian dune and non dune gullies have suggested a seasonal control on present day gully activity the timing of current gully activity especially activity involving the formation or modification of channels which commonly have been taken as evidence of fluvial processes has important implications regarding likely gully formation processes and necessary environmental conditions in this study we describe the results of frequent meter scale monitoring of several active gully sites by the high resolution imaging science experiment hirise on the mars reconnaissance orbiter mro the aim is to better assess the scope and nature of current morphological changes and to provide improved constraints on timing of gully activity on both dune and non dune slopes our observations indicate that 1 gully formation on mars is ongoing today and 2 the most significant morphological changes are strongly associated with seasonal frost and defrosting activity observed changes include formation of all major components of typical gully landforms although we have not observed alcove formation in coherent bedrock these results reduce the need to invoke recent climate change or present day groundwater seepage to explain the many martian gullies with pristine appearance

ferrovolcanism iron volcanism on metallic asteroids metallic asteroids the exposed cores of disrupted planetesimals are expected to have been exposed while still molten some would have cooled from the outside in crystallizing a surface crust which would then grow inward because the growing crust is expected to be more dense than the underlying melt this melt will tend to migrate toward the surface whenever it is able compressional stresses produced in the crust while it cools will be relieved locally by thrust faulting which will also provide potential conduits for melt to reach the surface we predict iron volcanism to have occurred on metallic asteroids as they cooled and discuss the implications of this process for both the evolution and the modern appearance of these bodies

long term surface temperature modeling of pluto abstract nasa s new horizons reconnaissance of the pluto system has revealed at high resolution the striking albedo contrasts from polar to equatorial latitudes on pluto as well as the sharpness of boundaries for longitudinal variations these contrasts suggest that pluto must undergo dynamic evolution that drives the redistribution of volatiles using the new horizons results as a template we explore the surface temperature variations driven seasonally on pluto considering multiple timescales these timescales include the current orbit 248 years as well as the timescales for obliquity precession peak to peak amplitude of 23 over 3 million years and regression of the orbital longitude of perihelion 3 7 million years these orbital variations create epochs of extreme seasons where one pole receives a short relatively warm summer and long winter while the other receives a much longer but less intense summer and short winter we use thermal modeling to build upon the long term insolation history model described by earle and binzel 2015 and investigate how these seasons couple with pluto s albedo contrasts to create temperature effects from this study we find that a bright region at the equator once established can become a site for net deposition we see the region informally known as sputnik planitia as an example of this and find it will be able to perpetuate itself as an always available cold trap thus having the potential to survive on million year or substantially longer timescales meanwhile darker low albedo regions near the equator will remain relative warm and generally not attract volatile deposition we argue that the equatorial region is a preservation zone for whatever albedo is seeded there this offers insight as to why the equatorial band of pluto displays the planet s greatest albedo contrasts

novae on venus geology classification and evolution 1 novae are radially fractured centers 100 300 km in diameter and 64 have been identified on venus dense radial fracturing and upraised topography are common and massive amounts of volcanism are seen in some novae are interpreted to be the result of updoming and fracturing of the surface due to interaction of mantle diapirs with the lithosphere and radial fracturing caused by dike emplacement we analyzed the topography and character of deformational and morphological structures of all 64 novae leading to a better understanding of their evolution and relation to diapir dynamics and their age we subdivided novae into four topographic classes 1 upraised 2 annular 3 flat and negative and 4 plateau like novae are located mostly in areas of regional rises and rift zones with a small number in the lowlands about one half the novae studied show association with rifts in some of these the rift troughs surround nova rises and form plateau like novae plateau like novae may predate postdate or form simultaneously with rifts our analysis generally confirms and updates previous models of novae evolution with the following geological factors being important 1 depth of the neutral buoyancy level of the rising mantle diapir 2 rheological characteristics of the overlying lithosphere 3 whether or not the visco plastic material above the diapir undergoes spreading and 4 the nature and influence of regional stress and rift structures these combined factors exert control on the direction of nova evolution and appear to determine the primary characteristics of nova such as topography tectonics and volcanism we found clear evidence for 1 the important role of dike swarm emplacement in the formation of radial patterns of novae and 2 the formation of concentric tectonic features during the relaxation stage of novae novae have multiple evolutionary stages and are long lived structures stratigraphic analysis of all 64 local areas showed a similarity in the sequence of regional geologic units we found that 40 3 of the novae population started to form before emplacement of regional plains with wrinkle ridges and that 11 3 completed their activity before this time 88 7 of the population of novae was active after regional plains formation in contrast to novae coronae activity was greatest before formation of regional plains which may be due to thickening of the lithosphere with time detailed structural analysis shows that novae evolution does not always lead to the formation of corona like features

reorientation and faulting of pluto due to volatile loading within sputnik planitia the location of sputnik planitia on pluto is shown to result from volatiles sequestered within the basin forcing the reorientation of the dwarf planet as supported by the planet wide fault network james keane et al report that the location of sputnik planitia the approximately 1 000 kilometre diameter topographic basin that dominates the surface of pluto is a natural consequence of the infill of volatile ices within the basin and the resulting reorientation true polar wander of the dwarf planet as sputnik planitia slowly loaded with volatiles the pull of tidal torques from charon would have resulted in the eventual alignment of the basin with the pluto charon tidal axis 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 this is one of four papers on the geology of sputnik planitia in this issue of nature in news views amy barr puts these latest contributions into context 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 basin1 2 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 thick1 2 3 4 5 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 pluto6 7 pluto s past present and future orientation is controlled by feedbacks between volatile sublimation and condensation changing insolation conditions and pluto s interior structure

origin of washboard moraines of the des moines lobe spatial analyses with lidar data abstract the des moines lobe dml the largest lobe along the southern margin of the laurentide ice sheet deposited low relief 1 5 m transverse moraine ridges herein called washboard moraines that are most prominent in central iowa their origin as either subglacial or ice marginal features bears on the dynamics of the lobe geomorphic reconstructions of it and interpretations of similar ridges elsewhere data from airborne laser swath mapping provide the first digital elevation models with sufficient spatial resolution to study in detail the geometric attributes of these topographically subtle features spectral analysis of profiles perpendicular to ridge crests indicates that most of them are spaced with statistically significant periodicities with dominant wavelengths of 70 150 m normalizing and stacking these profiles indicate that on average they display no systematic asymmetry locally washboard moraines are intersected by discontinuous longitudinal ridge segments trends of the crests of moraines are scalloped with cusps that point upglacier and are coincident with outwash trains which were overridden by the lobe our interpretation of these data is that the moraines formed subglacially as crevasse squeeze ridges overridden outwash trains likely supported low basal water pressures and resulted in anomalously slow basal slip causing upglacier deflection of crevasses that extended to the bed this crevasse squeeze interpretation reinforces evidence that the dml surged to its maximum position bering glacier as a temperate surge type glacier that deposited crevasse squeeze ridges similar to the washboard moraines of the dml may be a good analog for the lobe

pluto pits and mantles on uplands north and east of sputnik planitia abstract the highlands region north and east of sputnik planitia can be subdivided into seven terrain types based on their physiographic expression the northern rough uplands are characterized by jagged uplands and broad troughs and it may contain a deeply eroded ancient mantle dissected terrain has been interpreted to have been eroded by paleo glaciation the smooth uplands and pits terrain contains broad rolling uplands surrounding complexes of pits some of which contain smooth floors the uplands are mantled by smooth surfaced deposits possibly derived from adjacent pits through low power explosive cryovolcanism or through slow vapor condensation the eroded smooth uplands appear to have originally been smooth uplands and pits terrain modified by small scale sublimation pitting the bright pitted uplands features intricate texturing by reticulate ridges that may have originated by sublimation erosion volatile condensation or both the bladed terrain is characterized by parallel ridges oriented north south and is discussed in a separate paper the dark uplands are mantled with reddish deposits that may be atmospherically deposited tholins their presence has affected long term landform evolution widespread pit complexes occur on most of the terrain units most appear to be associated with tectonic lineations some pits are floored by broad expanses of ices whereas most feature deep conical depressions a few pit complexes are enclosed by elevated rims of uncertain origin

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

landslides on charon abstract we investigated five large landslides identified in the serenity chasma region of charon the identification of these landslides involved a search for these features in images taken by cameras onboard the new horizons spacecraft various landslide properties were analyzed based on their morphologies using a digital terrain model of the region we found that landslides are confined to the walls of the large normal fault scarps that make up serenity chasma based on extensive landslide runout lengths l relative to their drop heights h we classified these features as long runout landslides by analyzing their geometries we estimated the friction coefficients of the landslide material h l to be between 0 15 to 0 31 and the runout efficiencies l h to be between 3 2 and 6 8 we also estimated that the specific energy released during landslide motion ranged from 0 8 to 1 3 kj kg 1 these amounts of energy were too low to have generated significant melt around landslide particles

convection in a volatile nitrogen ice rich layer drives pluto s geological vigour the volatile ice filled basin informally named sputnik planum is central to pluto s geological activity this ice layer is organized into cells or polygons and it is now shown that convective overturn in a several kilometre thick layer of solid nitrogen can explain both the presence of the cells and their great width nasa s new horizons spacecraft has revealed fascinating details of the surface of pluto including a vast ice filled basin known as sputnik planum which is central to pluto s geological activity much of the surface of sputnik planum consisting mostly of nitrogen ice is divided into irregular polygons that are tens of kilometres in diameter and whose centres rise tens of metres above their sides two papers in this issue of nature analyse new horizons images of this polygonal terrain both conclude that it is continually being resurfaced by convection but arrive at contrasting models for the process alexander trowbridge et al report a parameterized convection model in which the nitrogen ice is vigorously convecting ten or more kilometres thick and about a million years old william mckinnon et al from the new horizons team show that sluggish lid convective overturn in a several kilometre thick layer of solid nitrogen can explain both the presence of the cells and their great width the vast deep volatile ice filled basin informally named sputnik planum is central to pluto s vigorous geological activity1 2 composed of molecular nitrogen methane and carbon monoxide ices3 but dominated by nitrogen ice this layer is organized into cells or polygons typically about 10 to 40 kilometres across that resemble the surface manifestation of solid state convection1 2 here we report on the basis of available rheological measurements4 that solid layers of nitrogen ice with a thickness in excess of about one kilometre should undergo convection for estimated present day heat flow conditions on pluto more importantly we show numerically that convective overturn in a several kilometre thick layer of solid nitrogen can explain the great lateral width of the cells the temperature dependence of nitrogen ice viscosity implies that the ice layer convects in the so called sluggish lid regime5 a unique convective mode not previously definitively observed in the solar system average surface horizontal velocities of a few centimetres a year imply surface transport or renewal times of about 500 000 years well under the ten million year upper limit crater retention age for sputnik planum2 similar convective surface renewal may also occur on other dwarf planets in the kuiper belt which may help to explain the high albedos shown by some of these bodies

solid state ice volcanism on the satellites of uranus voyager images of the uranian satellites ariel and miranda show flow features with morphologies indicating that ice has been extruded to the satellites9 surfaces in the solid state these images provide the first observational evidence for solid state ice volcanism in the solar system topographic profiles have been measured across a number of flow features on ariel with a simple model of extrusion spreading and cooling of a viscous flow the initial viscosity of the flow material is found to have been no more than about 10 16 poise far lower than expected for h 2 o ice at the ambient surface temperatures in the uranian system sharply reduced viscosities may have resulted from incorporation of ices like nh 3 or ch 4 in the uranian satellites

onset and migration of spiral troughs on mars revealed by orbital radar the northern polar cap of mars containing enough water to cover the entire planet to a depth of several metres features two major landforms that stand above all others these are the enormous canyon chasma boreale and a series of spiral troughs the processes leading to their formation have remained unclear now two papers in this issue present detailed histories of both systems john holt and colleagues use penetrating radar imagery from mars reconnaissance orbiter s radar sounder to show that depositional processes rather than a catastrophic event formed the chasma boreale isaac smith and john holt use mars reconnaissance orbiter data to rule out erosional cutting of polar ice as the cause of the central troughs and instead conclude that they too are largely depositional having migrated polewards and upwards in elevation in the past two million years a pinwheel array of deep troughs has been one of the most perplexing features of the north polar layered deposits on mars many ideas have been put forward about how it formed but there is as yet no consensus here penetrating radar has been used to rule out erosional cutting as a mechanism for the formation of the array instead it is concluded that the troughs are largely depositional in origin and have migrated to the poles and upwards in elevation over the past two million years or so the landscape of the north polar layered deposits of mars npld is dominated by a pinwheel array of enigmatic spiral troughs1 the troughs have intrigued planetary scientists since the mariner 9 spacecraft returned the first close up image in 1972 but conclusive evidence of their origin has remained elusive debate continues regarding all aspects of the troughs including the possibility that they have migrated2 3 4 5 their age in relation to the current npld surface6 and whether they are fundamentally erosional6 7 or constructional2 4 features the troughs are probably related to climatic processes2 8 yet the nature of this relationship has remained a mystery previous data characterizing only the exposed nlpd surface were insufficient to test these hypotheses here we show that the central spiral troughs initiated after deposition of three quarters of the npld quickly reached a stable morphology and migrated approximately 65 kilometres poleward and 600 metres in altitude over the past two million years or so our radar stratigraphy rules out hypotheses of erosional incision post dating deposition6 7 9 10 and instead largely validates an early hypothesis for constructional trough migration2 3 4 5 with wind transport and atmospheric deposition as dominant processes these results provide hard constraints for palaeo climate models and a new context for evaluating imagery spectral data and now radar sounding data the better to understand the link between orbital parameters and climate the role of climate in shaping the polar ice of mars and eventually the age of the polar deposits themselves8 11 12 13

the ch4 cycles on pluto over seasonal and astronomical timescales abstract 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 flux

a search for temporal changes on pluto and charon abstract a search for temporal changes on pluto and charon was motivated by 1 the discovery of young surfaces in the pluto system that imply ongoing or recent geologic activity 2 the detection of active plumes on triton during the voyager 2 flyby and 3 the abundant and detailed information that observing geologic processes in action provides about the processes a thorough search for temporal changes using new horizons images was completed images that covered the same region were blinked and manually inspected for any differences in appearance the search included full disk images such that all illuminated regions of both bodies were investigated and higher resolution images such that parts of the encounter hemispheres were investigated at finer spatial scales changes of appearance between different images were observed but in all cases were attributed to variability of the imaging parameters especially geometry or artifacts no differences of appearance that are strongly indicative of a temporal change were found on the surface or in the atmosphere of either pluto or charon limits on temporal changes as a function of spatial scale and temporal interval during the new horizons encounter are determined the longest time interval constraint is one pluto charon rotation period 6 4 earth days contrast reversal and high phase bright features that change in appearance with solar phase angle are identified the change of appearance of these features is most likely due to the change in phase angle rather than a temporal change had active plumes analogous to the plumes discovered on triton been present on the encounter hemispheres of either pluto or charon they would have been detected the absence of active plumes may be due to temporal variability i e plumes do occur but none were active on the encounter hemispheres during the epoch of the new horizons encounter or because plumes do not occur several dark streak features that may be deposits from past plumes are identified

climate zones on pluto and charon abstract we give an explanatory description of the unusual climate zones on pluto that arise from its high obliquity mean 115 and high amplitude 12 of obliquity oscillation over a 2 8 million year period the zones we describe have astronomically defined boundaries and do not incorporate atmospheric circulation for such a high mean obliquity the lines of tropics greatest latitudes where the sun can be overhead cycle closer to each pole than does each arctic circle which in turn cycle nearly to the equator as a consequence in an astronomical context pluto is more predominantly tropical than arctic up to 97 of pluto s surface area can experience overhead sun when the obliquity cycle is at its minimum of 103 at this same obliquity phase most recently occurring 0 8 myr ago 78 of pluto s surface experienced prolonged intervals without sunlight or arctic winter and corresponding arctic summer the intersection of these climate zones implies that a very broad range of pluto s latitudes spanning 13 77 in each hemisphere 75 of the total surface area are both tropical and arctic while some possible correlations to these climate zones are suggested by comparison with published maps of pluto and charon yielded by the new horizons mission in this work we present a non physical descriptive analysis only for example the planet wide dark equatorial band presented by stern et al 2015 science 350 292 299 corresponds to pluto s permanent diurnal zone in this zone spanning latitudes within 13 of the equator day night cycles occur each pluto rotation 6 4 days such that neither arctic winter nor arctic summer has been experienced in this zone for at least 20 million years the stability of this and other climate zones may extend over several gyr temperature modeling shows that the continuity of diurnal cycles in this region may be the key factor enabling a long term stability for the high albedo contrast between tombaugh regio adjacent to the dark cthulhu regio earle et al 2017 icarus special issue submitted all names are informal charon s synchronous alignment with pluto dictates that both bodies in the binary pair have the same climate zone structure but any effects on charon s morphology may be limited if volatile transport there is minimal or absent cold trapped methane rich volatiles on top of its water ice surface may be responsible for forming charon s dark red north polar cap grundy et al 2016b and we note the most concentrated area of this feature resides almost entirely within the permanent polar zone above 77 latitude where the sun never reaches the overhead point and arctic seasons have been most consistently experienced over at least tens of millions of years pluto is not alone among bodies in the kuiper belt and uranian satellites in having high obliquities overlapping tropical and arctic zones and latitude bands that remain in a continuous diurnal cycle over long terms

new horizons at pluto the new horizons mission has revealed pluto and its moon charon to be geologically active worlds the familiar yet exotic landforms suggest that geologic processes operate similarly across the solar system even in its cold outer reaches

mobile lid convection beneath enceladus south polar terrain 1 enceladus south polar region has a large heat flux 55 110 mw m 2 that is spatially associated with cryovolcanic and tectonic activity tidal dissipation and vigorous convection in the underlying ice shell are possible sources of heat however prior predictions of the heat flux carried by stagnant lid convection range from fconv 15 to 30 mw m 2 too low to explain the observed heat flux the high heat flux and increased cryovolcanic and tectonic activity suggest that near surface ice in the region has become rheologically and mechanically weakened enough to permit convective plumes to reach close to the surface if the yield strength of enceladus lithosphere is less than 1 10 kpa convection may instead occur in the mobile lid regime which is characterized by large heat fluxes and large horizontal velocities in the near surface ice i show that model ice shells with effective surface viscosities between 1016 and 1017 pa s and basal viscosities between 1013 and 1015 pa s have convective heat fluxes comparable to that observed by composite infrared spectrometer if this style of convection is occurring the south polar terrain should be spreading horizontally with v 1 10 mm a 1 where a is years and should be resurfaced in 0 1 10 ma on the basis of cassini imaging data the south polar terrain is 0 5 ma old consistent with the mobile lid hypothesis maxwell viscoelastic tidal dissipation in such ice shells is not capable of generating enough heat to balance convective heat transport however tidal heat may also be generated in the near surface along faults as suggested by nimmo et al 2007 and or viscous dissipation within the ice shell may occur by other processes not accounted for by the canonical maxwell dissipation model

formation of the sputnik planum basin and the thickness of pluto s subsurface ocean we simulate the formation of the large elliptical impact basin associated with pluto s sputnik planum sp informal name the location of sp suggests that it represents a large positive mass anomaly to find the conditions necessary for sp to have a positive mass anomaly we consider impacts into targets with a range of thermal states and ocean thicknesses assuming the basin evolves to its current day configuration we calculate the mass and gravity anomalies associated with sp we find that sp can only achieve a large positive mass anomaly if pluto has a more than 100 km thick salty ocean this conclusion may help us better understand the composition and thermal evolution of pluto furthermore our work supports the hypothesis that sp basin has an impact origin

thickness constraints on the icy shells of the galilean satellites from a comparison of crater shapes a thin outer ice shell on jupiter s large moon europa would imply easy exchange between the surface and any organic or biotic material in its putative subsurface ocean1 2 3 4 the thickness of the outer ice shell is poorly constrained however with model dependent estimates ranging from a few kilometres5 6 to ten or more kilometres7 here i present measurements of depths of impact craters on europa ganymede and callisto that reveal two anomalous transitions in crater shape with diameter the first transition is probably related to temperature dependent ductility of the crust at shallow depths 7 8 km on europa the second transition is attributed to the influence of subsurface oceans on all three satellites3 8 9 which constrains europa s icy shell to be at least 19 km thick the icy lithospheres of ganymede and callisto are equally ice rich but europa s icy shell has a thermal structure about 0 25 0 5 times the thicknesses of ganymede s or callisto s shells depending on epoch the appearances of the craters on europa are inconsistent with thin ice shell models1 and indicate that exchange of oceanic and surface material could be difficult

shield volcano topography and the rheology of lava flows on io stereo and photoclinometry derived topography of shield like volcanoes on io indicate little relief 3 km and very low slopes 0 2 to 0 6 several shield volcanoes appear to be associated with broad rises of 1 to 3 km but only 5 shield volcanoes have been identified with steep flank slopes between 4 and 10 these steep slopes are restricted to within 20 30 km of the summit but where discernable most of the lava flows observed on these edifices occur on the outer flanks where slopes are less than a degree despite their abundance ionian shield volcanoes are among the flattest in the solar system the steepest volcanoes on io are most comparable to large venusian shield volcanoes using simplistic bingham rheologies we estimate the viscosity and yield strengths of ionian lavas yield strengths are estimated at 101 102 pa lower than most basaltic lavas viscosity estimates range from 103 to 105 pa s although these are probably upper limits actual values may have been as low as 100 pa s viscosity is sensitive to flow velocity which is poorly known on io the best constraint on flow velocity comes from observations of the 1997 pillan eruption which bracket the eruptive phase to 132 day maximum and more probably less than 50 days low slopes long run out distances and our estimated rheologic properties are consistent with but not proof of a low silica low viscosity high temperature composition for ionian lavas supporting arguments for low silica lava compositions such as basalt or komatiite we cannot eliminate sulfur on rheologic grounds however

fjord insertion into continental margins driven by topographic steering of ice fjords line mountainous continental margins where icesheets and glaciers once stood a two dimensional model simulation suggests that fjords can be eroded within one million years primarily in response to topographic ice steering and erosion from ice discharge subsequent glaciers that form on these landscapes are smaller and exhibit greater responses to climate change

pluto s global surface composition through pixel by pixel hapke modeling of new horizons ralph leisa data abstract on july 14th 2015 nasa s new horizons mission gave us an unprecedented detailed view of the pluto system the complex compositional diversity of pluto s encounter hemisphere was revealed by the ralph leisa infrared spectrometer on board of new horizons we present compositional maps of pluto defining the spatial distribution of the abundance and textural properties of the volatiles methane and nitrogen ices and non volatiles water ice and tholin these results are obtained by applying a pixel by pixel hapke radiative transfer model to the leisa scans our analysis focuses mainly on the large scale latitudinal variations of methane and nitrogen ices and aims at setting observational constraints to volatile transport models specifically we find three latitudinal bands the first enriched in methane extends from the pole to 55 n the second dominated by nitrogen continues south to 35 n and the third composed again mainly of methane reaches 20 n we demonstrate that the distribution of volatiles across these surface units can be explained by differences in insolation over the past few decades the latitudinal pattern is broken by sputnik planitia a large reservoir of volatiles with nitrogen playing the most important role the physical properties of methane and nitrogen in this region are suggestive of the presence of a cold trap or possible volatile stratification furthermore our modeling results point to a possible sublimation transport of nitrogen from the northwest edge of sputnik planitia toward the south

numerical modeling of glacial erosion and headwall processes in alpine valleys abstract how rock relief is developed by glacial erosion in alpine landscapes is a key question in understanding the controls on mountain range topography any generation of relief by glaciers at the highest elevations in mountain ranges can affect the distribution of precipitation in alpine landscapes creating feedbacks between glacier mass balance and glacial erosion relief production at high elevation mountain crests is addressed using a numerical model that incorporates glacier dynamics and subglacial erosion as well as relevant hillslope processes blowing snow from a valley bounding plateau and avalanching onto the accumulation zone of the glacier is included in the model furthermore temperature dependent frost cracking is used as a proxy for headwall backwearing the parameterization of subglacial erosion produces about 500 m of erosion over the 400 ka of each numerical simulation in steady and varying climate scenarios valley floors flatten as headwalls increase in height and retreat headward over time headwall slopes increase in simulations in which quarrying is explicitly incorporated rates of quarrying are high near the base of the headwall increasing headwall length and slope as a result of valley floor lowering high rates of subglacial erosion and production of headwall relief also result when elastic flexural uplift occurs in response to mass unloading small overdeepenings form at the base of the headwall in steady climate simulations when the glacier is small at the end of the model run while the final profiles are relatively insensitive to the erosion rule used quarrying is most effective near the head of the glacier whereas rates of abrasion reflect the instantaneous pattern of integrated ice discharge or ice flux in simulations in which cirques are formed the equilibrium line altitude is hundreds of meters above the cirque floor however the time averaged horizontal position of the equilibrium line corresponds well with the downvalley position of the cirques

evolution of lumpy glacial landscapes at 10 100 m scales glaciated landscapes are lumpy sporting rocky bedforms with abraded upvalley faces and sharp quarried downvalley edges using models that operate at two time scales i address glacial landform development at these scales in order to explore the dependence of bed topography and erosion rate on glacier and rock properties sliding at the bed a prerequisite for both abrasion and quarrying is strongly modulated by water pressures short time scale models designed to capture the transient hydro sliding system beneath a temperate glacier quantify the temporal and spatial pattern of effective stress on the glacier bed the bed is subjected to repetitive stressing focused on the edges of bed steps the frequency and magnitude of stress events increasing with ice thickness the numerical model of bed evolution at longer time scales incorporates both abrasion and quarrying of fracture bound blocks the probability of quarrying of a block by a particular stress event is taken to depend inversely upon both block size and the depth of the niche in which it sits upglacier migrating rock bedforms with 10 100 m wavelengths inevitably emerge reflecting efficient quarrying of blocks from downvalley facing steps in the bed the relative importance of abrasion and quarrying and bed roughness are controlled by fracture spacing and major steps in the valley floor are attributable to transitions in fracture spacing

methane distribution on pluto as mapped by the new horizons ralph mvic instrument abstract the data returned from nasa s new horizons spacecraft have given us an unprecedented detailed look at the pluto system new horizons ralph mvic multispectral visible imaging camera is composed of 7 independent ccd arrays on a single substrate among these are a red channel 540 700 nm near infrared channel 780 975 nm and narrow band methane channel 860 910 nm by comparing the relative reflectance of these channels we are able to produce high resolution methane equivalent width based on the 890 nm absorption band and spectral slope maps of pluto s surface from these maps we can then quantitatively study the relationships between methane distribution redness and other parameters like latitude and elevation we find pluto s surface to show a great diversity of terrains particularly in the equatorial region between 30 n and 30 s latitude methane equivalent width also shows some dependence on elevation while spectral slope shows very little

a post new horizons global climate model of pluto including the n 2 ch 4 and co cycles abstract we have built a new 3d global climate model gcm to simulate pluto as observed by new horizons in 2015 all key processes are parametrized on the basis of theoretical equations including atmospheric dynamics and transport turbulence radiative transfer molecular conduction as well as phases changes for n 2 ch 2 and co pluto s climate and ice cycles are found to be very sensitive to model parameters and initial states nevertheless a reference simulation is designed by running a fast reduced version of the gcm with simplified atmospheric transport for 40 000 earth years to initialize the surface ice distribution and sub surface temperatures from which a 28 earth year full gcm simulation is performed assuming a topographic depression in a sputnik planum sp like crater on the anti charon hemisphere a realistic pluto is obtained with most n 2 and co ices accumulated in the crater methane frost covering both hemispheres except for the equatorial regions and a surface pressure near 1 1 pa in 2015 with an increase between 1988 and 2015 as reported from stellar occultations temperature profiles are in qualitative agreement with the observations in particular a cold atmospheric layer is obtained in the lowest kilometers above sputnik planum as observed by new horizons s rex experiment it is shown to result from the combined effect of the topographic depression and n 2 daytime sublimation in the reference simulation with surface n 2 ice exclusively present in sputnik planum the global circulation is only forced by radiative heating gradients and remains relatively weak surface winds are locally induced by topography slopes and by n 2 condensation and sublimation around sputnik planum however the circulation can be more intense depending on the exact distribution of surface n 2 frost this is illustrated in an alternative simulation with n 2 condensing in the south polar regions and n 2 frost covering latitudes between 35 n and 48 n a global condensation flow is then created inducing strong surface winds everywhere a prograde jet in the southern high latitudes and an equatorial superrotation likely forced by barotropic instabilities in the southern jet using realistic parameters the gcm predict atmospheric concentrations of co and ch 4 in good agreement with the observations n 2 and co do not condense in the atmosphere but ch 4 ice clouds can form during daytime at low altitude near the regions covered by n 2 ice assuming that nucleation is efficient enough this global climate model can be used to study many aspects of the pluto environment for instance organic hazes are included in the gcm and analysed in a companion paper bertrand and forget icarus this issue

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

recent tectonic activity on pluto driven by phase changes in the ice shell the new horizons spacecraft has found evidence for geologic activity on the surface of pluto including extensional tectonic deformation of its water ice bedrock see moore et al 2016 one mechanism that could drive extensional tectonic activity is global surface expansion due to the partial freezing of an ocean we use updated physical properties for pluto and simulate its thermal evolution to understand the survival of a possible subsurface ocean for thermal conductivities of rock less than 3 w m 1 k 1 an ocean forms and at least partially freezes leading to recent extensional stresses in the ice shell in scenarios where the ocean freezes and the ice shell is thicker than 260 km ice ii forms and causes global volume contraction since there is no evidence for recent compressional tectonic features we argue that ice ii has not formed and that pluto s ocean has likely survived to present day

geology before pluto pre encounter considerations the cameras of new horizons will provide robust data sets that should be imminently amenable to geological analysis of the pluto system s landscapes in this paper we begin with a brief discussion of the planned observations by the new horizons cameras that will bear most directly on geological interpretability then we broadly review the major geological processes that could potentially operate on the surfaces of pluto and its major moon charon we first survey exogenic processes i e those for which energy for surface modification is supplied externally to the planetary surface impact cratering sedimentary processes including volatile migration and the work of wind we conclude with an assessment of the prospects for endogenic activity in the form of tectonics and cryovolcanism

physical state and distribution of materials at the surface of pluto from new horizons leisa imaging spectrometer abstract from earth based observations pluto is known to be the host of n2 ch4 and co ices and also a dark red material very limited spatial distribution information is available from rotational visible and near infrared spectral curves obtained from hemispheric measurements in july 2015 the new horizons spacecraft reached pluto and its satellite system and recorded a large set of data the leisa spectro imager of the ralph instruments are dedicated to the study of the composition and physical state of the materials composing the surface in this paper we report a study of the distribution and physical state of the ices and non ice materials on pluto s illuminated surface and their mode and degree of mixing principal component analysis as well as various specific spectral indicators and correlation plots are used on the first set of 2 high resolution spectro images from the leisa instrument covering the whole illuminated face of pluto at the time of the new horizons encounter qualitative distribution maps have been obtained for the 4 main condensed molecules n2 ch4 co h2o as well as for the visible dark red material based on specific spectral indicators using either the strength or the position of absorption bands these 4 molecules are found to indicate the presence of 3 different types of ices n2 rich ch4 co ices ch4 rich co n2 ices and h2o ice the mixing lines between these ices and with the dark red material are studied using scatter plots between the various spectral indicators ch4 is mixed at the molecular level with n2 most probably also with co thus forming a ternary molecular mixture that follows its phase diagram with low solubility limits the occurrence of a n2 rich ch4 rich ices mixing line associated with a progressive decrease of the co ch4 ratio tells us that a fractionation sublimation sequence transforms one type of ice to the other forming either a n2 rich ch4 rich binary mixture at the surface or an upper ch4 rich ice crust that may hide the n2 rich ice below the strong ch4 rich h2o mixing line witnesses the subsequent sublimation of the ch4 rich ice lag left behind by the n2 co sublimation n spring summer or a direct condensation of ch4 ice on the cold h2o ice s autumn the weak mixing line between ch4 containing ices and the dark red material and the very sharp spatial transitions between these ices and this non volatile material are probably due to thermal incompatibility finally the occurrence of a h2o ice red material mixing line advocates for a spatial mixing of the red material covering h2o ice with possibly a small amount intimately mixed in water ice from this analysis of the different materials distribution and their relative mixing lines h2o ice appears to be the substratum on which other ices condense or non volatile organic material is deposited from the atmosphere n2 rich ices seem to evolve to ch4 dominated ices possibly still containing traces of co and n2 as n2 and co sublimate away the spatial distribution of these materials is very complex the high spatial definition of all these composition maps as well as those at even higher resolution that will be soon available will allow us to compare them with pluto s geologic features observed by lorri panchromatic and mvic multispectral imagers to better understand the geophysical processes in action at the surface of this astonishingly active frozen world

present and past glaciation on pluto abstract modern n2 ice glaciers flow from highlands to the east of the 750 1400 km2 lowland of sputnik planum sp and merge with the ices of similar composition on sp we explore the possibility that glaciation may be fed by n2 sublimation from sp followed by redeposition on the highlands the uplands to the northeast north and west of sp have been erosionally sculpted into a variety of dissected terrains that feature linear depressions valleys locally in dendritic networks we interpret these dissected terrains to have been carved by n2 glaciers formerly covering the uplands depositional glacial landforms moraines eskers outwash have not been identified however n2 glaciation would have a different erosional manifestation because the substrate porous water ice and ch4 rich mantles probably has lower density than n2 and also because of the lack of freeze thaw weathering if sufficiently thick 1 4 km n2 glaciers might have experienced basal melting past flow of n2 glaciers from the highlands into sp may have detached and transported the prominent mountainous water ice mountains along the western border of sp

a large sedimentary basin in the terra sirenum region of the southern highlands of mars abstract different lines of evidence point to hydrological cycling in the martian past the extent duration and magnitude of this cycling remains unclear as well as the magnitude of aqueous processes on the surface here we provide geomorphic and mineralogic evidence of a large inter crater sedimentary basin located in the terra sirenum region which was once covered by a body of liquid water with an areal extent of at least 30 000 km 2 and a depth of approximately 200 m the topographic basin which is modified by a number of large impact craters is partly controlled by ancient impact and tectonic structures as a result of evaporation of the large body of water salt flats formed in the lowest topographic reaches of the basin hydrated phyllosilicates occur in close proximity to the salt flats and in the ejecta and rim materials of small impact craters with stratigraphic relations that suggest that they underlie the evaporite deposits crater statistics place the maximum age of aqueous activity during the late noachian epoch the relatively pristine mineral deposits in the basin have a high potential to yield information of the geochemistry and water activity during the ancient noachian period when conditions were seemingly more conducive to life

a new stereo topographic map of io implications for geology from global to local scales we use voyager and galileo stereo pairs to construct the most complete stereo digital elevation model dem of io assembled to date controlled using galileo limb profiles given the difficulty of applying these two techniques to io due to its anomalous surface albedo properties we have experimented extensively with the relevant procedures in order to generate what we consider to be the most reliable dems our final stereo dem covers 75 of the globe and we have identified a partial system of longitudinally arranged alternating basins and swells that correlates well to the distribution of mountain and volcano concentrations we consider the correlation of swells to volcano concentrations and basins to mountain concentrations to imply a heat flow distribution across io that is consistent with the asthenospheric tidal heating model of tackley et al 2001 the stereo dem reveals topographic signatures of regional scale features including loki patera ra patera and the tvashtar paterae complex in addition to previously unrecognized features including an 1000 km diameter depression and a 2000 km long topographic arc comprising mountainous and layered plains material

cold glaciers erode and deposit evidence from allan hills antarctica here we report previously undescribed features of erosion and deposition by a cold polar glacier a recent study challenged the assumption that cold glaciers neither slide nor abrade their beds but no geological evidence was offered the features we describe include abrasion marks subglacial deposits glaciotectonically deformed substrate isolated blocks ice cored debris mounds and boulder trains all products of a recent cold ice advance and retreat mapping these features elsewhere in antarctica will document recent shifts in the east antarctic ice sheet margin providing new insight on regional mass balance changes

basins fractures and volcanoes global cartography and topography of pluto from new horizons abstract the 2015 new horizons flyby has produced the first high resolution maps of morphology and topography of pluto and charon the most distant objects so mapped global integrated mosaics of pluto were produced using both lorri framing camera and mvic line scan camera data showing the best resolution data obtained for all areas of the illuminated surface 78 of the body a unique feature of the pluto imaging data set is the observation of terrains illuminated only by light scattered from atmospheric haze allowing us to map terrains in the southern hemisphere that would otherwise have been in darkness mvic 4 color data were combined with the panchromatic map to produce full color global maps digital elevation models dems over 42 of pluto were produced using combinations of mvic hemispheric scans and lorri mosaics from which slopes at scales of 1 km can be determined pluto can be divided into regions each with distinct topographic signatures corresponding with major physiographic terrain types large areas of pluto are comprised of low relief moderately cratered plains units deeply pitted and glaciated plains east of sputnik planitia are elevated 0 7 km the most dominant topographic feature on pluto is the 1200 by 2000 km wide depression enclosing the bright sputnik planitia ice sheet the surface of which is 2 5 to 3 5 km deep relative to the rim and 2 km deep relative to the mean radius the partial ring of steep sided massifs several of which are more than 5 km high along the western margins of sputnik planitia produce some of the locally highest and steepest relief on pluto with slopes of 40 50 the second major topographic feature is a complex eroded ridge trough system 300 400 km wide and at least 3200 km long extending north to south along the 155 meridian this enormous structure has several kilometers of relief it may predate the large impact event forming the basin though some post sputnik planitia deformation is evident the large depressed partially walled plain hyecho palus lies due southwest of sputnik planitia near the center of hyecho palus lie the circular constructional edifices wright and piccard montes wright mons rises 4 5 km above these plains with a central depression 4 5 km deep whereas piccard mons best observed in haze light rises 5 5 km above the plains but has a bowl shaped central depression 5 5 km below the plains for a total relief of up to 11 km the greatest observed on pluto both of these features are interpreted as constructional volcanic in nature additional prominent topographic features include a 2 3 km high and 600 km wide dome centered on the illuminated iau pole and the amoeboidal plateaus of bladed terrains in the equatorial region which rise 2 5 km above local terrains and are the highest standing geologic units on the encounter hemisphere the mean elevations in the integrated dem for the two radio occultation areas are consistent with the 5 6 km difference in elevation as determined independently by the radio experiment and a limb profile near the egress point confirms the presence of elevated bladed terrains in that area local relief of 3 5 km at massifs troughs and pits supports conclusions that the icy shell of pluto is relatively rigid numerous examples of topographic control of ice or frost deposition occur across pluto including the distinct coloration of the polar dome the elevated terrains of eastern tombaugh regio and along the ridge trough system where ridge tops and fossae rims are covered in different ices than at lower elevations the topographic hypsogram of pluto s encounter hemisphere is strongly bimodal due to the large sputnik planitia depression otherwise the topographic signature of pluto is controlled by deviations from the otherwise dominant low plains including elevated bladed terrain plateaus and the depressed volcanic province including wright and piccard montes

on convection in ice i shells of outer solar system bodies with detailed application to callisto it has been argued that the dominant non newtonian creep mechanisms of water ice make the ice shell above callisto s ocean and by inference all radiogenically heated ice i shells in the outer solar system stable against solid state convective overturn conductive heat transport and internal melting oceans are therefore predicted to be or have been widespread among midsize and larger icy satellites and kuiper belt objects alternatively at low stresses where non newtonian viscosities can be arbitrarily large convective instabilities may arise in the diffusional creep regime for arbitrarily small temperature perturbations for callisto ice viscosities are low enough that convection is expected over most of geologic time above the internal liquid layer for plausible ice grain sizes a few mm the alternative for early callisto a conducting shell over a very deep ocean 450 km is not compatible with callisto s present partially differentiated state moreover if convection is occurring today the stagnant lid would be quite thick 100 km and compatible with the lack of active geology nevertheless callisto s steady state heat flow may have fallen below the convective minimum for its ice i shell late in solar system history in this case convection ends the ice shell melts back at its base and the internal ocean widens considerably the presence of such an ocean of order 200 km thick is compatible with callisto s moment of inertia but its formation would have caused an 0 25 radial expansion the tectonic effects of such a late slow expansion are not observed so convection likely persists in callisto possibly subcritically ganymede due to its greater size rock fraction and full differentiation has a substantially higher heat flow than callisto and has not reached this tectonic end state titan if differentiated and triton should be more similar to ganymede in this regard pluto like callisto may be near the tipping point for convective shutdown but uncertainties in its size and rock fraction prevent a more definitive assessment

modeling cryogenic mud volcanism on pluto abstract subsurface ice cryofluids and ice interactions with ammonia for cryovolcanic materials give the potential ingredients for the formation of mud volcanoes on pluto in order to constrain the possible morphology from images acquired by the new horizons spacecraft we have modeled the formation of mud volcanoes isostatic layers of ice overlain by denser material from a series of possible solid liquid mud compositions methane and ammonia as well as accounting for the full range of eruption variables e g mud viscosity conduit diameter and eruption duration we categorize them into nine specific scenarios based on the composition of the top ice layer and mud composition from these scenarios the maximum height of the potential mud volcano can be calculated we find that ammonia mud sources produce smaller edifice heights and wider flow profiles compared to methane dominant mud cryovolcanoes assuming certain preliminary parameters we also propose the evolution and development of the edifice of wright mons in as little as 3 8 million years given a 10 wt ammonia subsurface mud composition if characterized on pluto s surface mud volcanoes would provide an interesting insight on the subsurface dynamics of solids and cryofluids on pluto

the geology of pluto and charon through the eyes of new horizons nasa s new horizons spacecraft has revealed the complex geology of pluto and charon pluto s encounter hemisphere shows ongoing surface geological activity centered on a vast basin containing a thick layer of volatile ices that appears to be involved in convection and advection with a crater retention age no greater than 10 million years surrounding terrains show active glacial flow apparent transport and rotation of large buoyant water ice crustal blocks and pitting the latter likely caused by sublimation erosion and or collapse more enigmatic features include tall mounds with central depressions that are conceivably cryovolcanic and ridges with complex bladed textures pluto also has ancient cratered terrains up to 4 billion years old that are extensionally faulted and extensively mantled and perhaps eroded by glacial or other processes charon does not appear to be currently active but experienced major extensional tectonism and resurfacing probably cryovolcanic nearly 4 billion years ago impact crater populations on pluto and charon are not consistent with the steepest impactor size frequency distributions proposed for the kuiper belt

formation of ganymede s grooved terrain by convection driven resurfacing the heat flux and strain rate inferred for grooved terrain formation on ganymede can be produced in a convecting ice shell 10 100 km thick with weak near surface ice smooth linear grooves may have formed by convection driven lithospheric spreading and long wavelength compressional folds may form atop convective downwellings and would possibly be detectable with mapping from esa s upcoming jupiter icy moon explorer mission

three regimes of mantle convection with non newtonian viscosity and stagnant lid convection on the terrestrial planets numerical simulations of convection with strongly temperature dependent viscosity suggest that non newtonian viscosity convection dislocation creep passes through three convective regimes similar to those observed for newtonian viscosity convection diffusion creep the small viscosity contrast regime the transitional regime and the stagnant lid regime for realistic viscosity contrasts mantle convection is in the stagnant lid regime characterized by formation of a very viscous slowly creeping lid on top of an actively convecting mantle this explains the tectonic style observed on the terrestrial planets and the moon on the other hand this eliminates the possibility that the plates on earth could be mobile due to non newtonian viscosity the nature of the mobility of lithospheric plates on earth has yet to be explained

impact basin relaxation on rhea and iapetus and relation to past heat flow abstract evidence for relaxation of impact crater topography has been observed on many icy satellites including those of saturn and the magnitude of relaxation can be related to past heat flow e g moore j m schenk p m bruesch l s asphaug e mckinnon w b 2004 icarus 171 421 443 dombard a j mckinnon w b 2006 j geophys res 111 e01001 http dx doi org 10 1029 2005je002445 we use new global digital elevation models of the surfaces of rhea and iapetus generated from cassini data to obtain crater depth diameter data for both satellites and topographic profiles of large basins on each in addition to the factor of three lower amplitude of global topography on rhea compared to iapetus we show that basins on iapetus 100 km in diameter show little relaxation compared to similar sized basins on rhea because of the similar gravities of rhea and iapetus we show that iapetus basin morphologies can be used to represent the initial unrelaxed morphologies of the rhea basins and we use topographic profiles taken across selected basins to model heat flow on both satellites we find that iapetus has only experienced radiogenic heat flow since formation whereas rhea must have experienced heat flow reaching a few tens of mw m 2 although this heat flow need only be sustained for as little as several million years in order to achieve the observed relaxation magnitudes rhea experienced a different thermal history from iapetus which we consider to be primarily related to their different formation mechanisms and locations within the saturnian system a recent model for the formation of saturn s mid sized icy satellites interior to and including rhea charnoz s et al 2011 icarus 216 535 550 describes how rhea s orbit would have expanded outwards after its accretion from a giant primordial ring which would have instigated early heating through rapid despinning and tidal interaction with saturn and other satellites rhea s basins would therefore be required to have formed within the first few tens of myr of rhea s formation in order to relax due of this heating and if so may provide an important anchor point for saturn system chronology none of these heating mechanisms are viable for iapetus in its isolated position far from saturn and as such it has remained dynamically inert since formation confirming conclusions based on thermal modeling of iapetus interior rapid and complete relaxation and subsequent erosion by bombardment of a first generation of large basins on rhea is regarded as an explanation for the lower counts of large basins on rhea relative to iapetus and the overall lower amplitude of topography on rhea compared to iapetus

thermal convection as a possible mechanism for the origin of polygonal structures on pluto s surface high resolution pictures of pluto s surface obtained by the new horizons spacecraft revealed among other surface features a large nitrogen ice glacier informally named sputnik planitia the surface of this glacier is separated into a network of polygonal cells with a wavelength of 20 40 km this network is similar to the convective patterns obtained under certain conditions by laboratory experiments suggesting that it is the surface expression of thermal convection here we investigate the surface planform obtained for different convective systems in 3d cartesian geometry with different modes of heating and rheologies we find that bottom heated systems as assumed by previous studies do not produce surface planforms consistent with the observed pattern alternatively for a certain range of rayleigh roberts number rah a volumetrically heated system produces a surface planform similar to this pattern we then combine scaling laws with values of rah within its possible range to establish relationships between the critical parameters of sputnik planitia in particular our calculations indicate that the glacier thickness and the surface heat flux are in the ranges 2 10 km and 0 1 10 mw m 2 respectively however a difficulty is to identify a proper source of internal heating we propose that the long term variations of surface temperature caused by variations in pluto s orbit over millions of year produces secular cooling equivalent to internal heating we find that this source of heating is sufficient to trigger thermal convection but additional investigations are needed to determine under which conditions it can produce surface patterns similar to those of sputnik planitia

thermal evolution of pluto and implications for surface tectonics and a subsurface ocean abstract determining whether or not pluto possesses or once possessed a subsurface ocean is crucial to understanding its astrobiological potential in this study we use a 3d convection model to investigate pluto s thermal and spin evolution and the present day observational consequences of different evolutionary pathways we test the sensitivity of our model results to different initial temperature profiles initial spin periods silicate potassium concentrations and ice reference viscosities the ice reference viscosity is the primary factor controlling whether or not an ocean develops and whether that ocean survives to the present day in most of our models present day pluto consists of a convective ice shell without an ocean however if the reference viscosity is higher than 5 10 15 pa s the shell will be conductive and an ocean should be present for the nominal potassium concentration the present day ocean and conductive shell thickness are both about 165 km in conductive cases an ocean will be present unless the potassium content of the silicate mantle is less than 10 of its nominal value if pluto never developed an ocean predominantly extensional surface tectonics should result and a fossil rotational bulge will be present for the cases which possess or once possessed an ocean no fossil bulge should exist a present day ocean implies that compressional surface stresses should dominate perhaps with minor recent extension an ocean that formed and then re froze should result in a roughly equal balance between older compressional and younger extensional features these predictions may be tested by the new horizons mission

pluto s ocean is capped and insulated by gas hydrates many icy solar system bodies possess subsurface oceans on pluto sputnik planitia s location near the equator suggests the presence of a subsurface ocean and a locally thinned ice shell to maintain an ocean pluto needs to retain heat inside on the other hand to maintain large variations in its thickness pluto s ice shell needs to be cold here we show by thermal evolution and viscous relaxation calculations that the presence of a thin layer of clathrate hydrates gas hydrates at the base of the ice shell can explain both the long term survival of the ocean and the maintenance of shell thickness contrasts clathrate hydrates act as a thermal insulator preventing the ocean from completely freezing while keeping the ice shell cold and immobile the most likely clathrate guest gas is methane derived from precursor bodies and or cracking of organic materials in the hot rocky core nitrogen molecules initially contained and or produced later in the core would probably not be trapped as clathrate hydrates instead supplying the nitrogen rich surface and atmosphere the formation of a thin clathrate hydrate layer cap to a subsurface ocean may be an important generic mechanism to maintain long lived subsurface oceans in relatively large but minimally heated icy satellites and kuiper belt objects pluto s subsurface ocean and thickness variation in its ice shell may be maintained by a layer of methane clathrates forming an insulating cap to the ocean according to calculations of thermal evolution and viscous relaxation

development of sediment landform associations at cold glacier margins dry valleys antarctica hambrey m j fitzsimons s j 2010 development of sediment landform associations at cold glacier margins dry valleys antarctica sedimentology 57 3 857 882 michael hambrey is corresponding author sponsorship marsden fund of new zealand university of otago research committee nerc grant no ner a s 2000 01256

the formation of charon s red poles from seasonally cold trapped volatiles a unique feature of pluto s large satellite charon is its dark red northern polar cap similar colours on pluto s surface have been attributed to tholin like organic macromolecules produced by energetic radiation processing of hydrocarbons the polar location on charon implicates the temperature extremes that result from charon s high obliquity and long seasons in the production of this material the escape of pluto s atmosphere provides a potential feedstock for a complex chemistry gas from pluto that is transiently cold trapped and processed at charon s winter pole was proposed as an explanation for the dark coloration on the basis of an image of charon s northern hemisphere but not modelled quantitatively here we report images of the southern hemisphere illuminated by pluto shine and also images taken during the approach phase that show the northern polar cap over a range of longitudes we model the surface thermal environment on charon and the supply and temporary cold trapping of material escaping from pluto as well as the photolytic processing of this material into more complex and less volatile molecules while cold trapped the model results are consistent with the proposed mechanism for producing the observed colour pattern on charon

convective instability in europa s floating ice shell models of the tidally heated floating ice shell proposed for the jovian satellite europa generally find shell thicknesses less than 30 km past parameterized convection models indicated that such shells are stable against convective overturn which otherwise ostensibly leads to freezing of the ocean underneath here i apply the temperature dependent viscosity convection scaling developed by solomatov and coworkers to the europan ice shell the temperature dependent properties of ice are linearized about 260 k as any convective interior should be close to this temperature with the colder ice forming an essentially passive stagnant lid ice shells 10 km thick are found to be unstable to convection at their base for melting point viscosities of 1013 pa s as linearized by tidal stresses if the ice deforms by superplastic creep but such low viscosities require small grain sizes 1 mm this requirement may be met if grain sizes observed in terrestrial polar glaciers can be strain rate scaled to europa regardless convection at the base of the ice shell if initiated may not freeze the ocean because of tidal heating a stagnant lid regime ice shell is much more dissipative than a conductive shell of the same thickness such a shell should thin not thicken and the potential exists for further thermal instabilities and runaways

the minimum confidence limit for diameters in crater counts abstract size frequency distribution i e sfd of impact craters are frequently used to study relative and absolute model ages for planetary surfaces it is well accepted that crater statistics are incomplete and crater recognition is inaccurate at diameter ranges approaching a few pixels of the images used but there is no benchmark about the minimum confidence limit for diameter dmin here we resolve this problem by comparing the sfd of each studied crater population using a same image dataset that has been down sampled by different folds a systematical study of several different aged lunar crater populations using various image datasets found that crater statistics can be regarded as completed and crater rim identification can be regarded as accurate at diameters larger than about 10 pixels of the base images integrating other potential difficulties in crater counts e g illumination conditions personal biases topography roughness size of counting areas would indicate that 10 pixels should be considered as a conservative suggestion using 10 pixels as dmin could substantially enhance the reliability of crater counts especially for prevalent high resolution regional geological studies that employ populations of small craters

the down stress transition from cluster to cone fabrics in experimentally deformed ice abstract during plastic deformation of polycrystalline ice 1h ice crystals become crystallographically aligned due to dislocation glide primarily on the basal slip system such crystallographic preferred orientation cpo introduces a viscous anisotropy in ice and thus strongly influences the kinematics of the flow of glaciers and ice sheets two key mechanisms exert different controls on cpo in axial compression recrystallization dominated by lattice rotation yields a cluster of c axes parallel to compression and recrystallization dominated by grain boundary migration gbm yields a cone shaped distribution of c axes with the cone axis parallel to compression the transition between these dominant mechanisms of cpo formation has not been well quantified in this study we explore how this transition varies with stress ice deformation experiments were conducted using a high pressure gas medium apparatus to prevent fracturing of samples at relatively high stresses samples were deformed in uniaxial compression at a temperature of 10 c and a confining pressure of 10 mpa fabricated ice samples with starting average grain sizes of either 0 23 mm or 0 63 mm were each deformed to an axial strain of 0 2 at a nominally constant strain rate in the range 1 2 10 6 to 2 4 10 4 s 1 yielding flow stresses of 1 17 to 4 31 mpa high quality electron backscatter diffraction reveal the grain size shape subgrain structure and cpos formed at different stresses all deformed samples have strong non random cpos with c axes concentrated in cones the cone angle and cpo strength are observed to decrease with increasing stress as stress increases the fraction of grains with highly curved or lobate grain boundaries decreases and the fraction of polygonal grains with straight grain boundaries increases based on these observations we propose that a transition in the dominant mechanism of cpo formation occurs with increasing stress from gbm which consumes grains with low schmid factors at low stress to lattice rotation caused by slip primarily on the basal slip system which causes c axes to rotate to become parallel to the shortening direction at high stress mapping out the transition from cluster rotation dominated to cone gbm dominated cpos as a function of stress this study and temperature future studies allows for a robust extrapolation to and a fundamental understanding of the cpos formed at glaciological stresses and temperatures

global drainage patterns and the origins of topographic relief on earth mars and titan rivers have eroded the topography of mars titan and earth creating diverse landscapes however the dominant processes that generated topography on titan and to some extent on early mars are not well known we analyzed drainage patterns on all three bodies and found that large drainages which record interactions between deformation and erosional modification conform much better to long wavelength topography on titan and mars than on earth we use a numerical landscape evolution model to demonstrate that short wavelength deformation causes drainage directions to diverge from long wavelength topography as observed on earth we attribute the observed differences to ancient long wavelength topography on mars recent or ongoing generation of long wavelength relief on titan and the creation of short wavelength relief by plate tectonics on earth

mode of formation of ablation hollows controlled by dirt content of snow on observe que des figures de fonte de neige sont importantes en presence d impuretes non solubles mais qu elles sont moins importantes pour une forte concentration d impuretes qui migrent dans les crevasses la reduction de l albedo a un faible effet sur l ablation

primordial n2 provides a cosmochemical explanation for the existence of sputnik planitia pluto abstract 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

ocean worlds in the outer solar system many outer solar system bodies are thought to harbor liquid water oceans beneath their ice shells this article first reviews how such oceans are detected we then discuss how they are maintained when they formed and what the oceans likely characteristics are we focus in particular on europa ganymede callisto titan and enceladus bodies for which there is direct evidence of subsurface oceans we also consider candidate ocean worlds such as pluto and triton

features of glacial valley profiles simply explained 1 glacial occupation of alpine valleys results in a distinct signature in the long valley profile including steepening of the profile in the headwaters flattening at lower elevations and a step in the profile at the convergence of headwater tributaries we present analytic results for glacial erosion patterns by making the following assumptions 1 the initial profile is linear 2 the width of the valley is uniform 3 the annual mass balance varies linearly with elevation 4 the glacier at any time is quasi steady 5 the erosion rate is proportional to ice discharge per unit valley width and 6 glacial erosion rates far exceed fluvial erosion rates a steady glacier under these conditions would erode a parabolic divot in the longitudinal valley profile with its maximum depth coinciding with the down valley position of the equilibrium line altitude ela the calculated flattening of the valley floor down valley of the ela and the steepening of it up valley captures the essence of the glacial signature when a reasonable probability distribution of elas is allowed the predicted erosion peaks at 30 40 of the down valley distance to the glacial limit and the pattern merges smoothly with the steeper fluvial profile downstream of the glacial limit profiles of mass balance that are capped at a maximum value produce shorter glaciers and a slight asymmetry in the expected erosion pattern only when we mimic the tributaries of glacial headwaters by specifying a valley width distribution do we obtain the upper step observed in many valley profiles

origin of mountains on io by thrust faulting and large scale mass movements voyager stereoimages of euboea montes io indicate that this mountain formed when a large crustal block was uplifted 10 5 kilometers and tilted by approximately 6 degrees uplift triggered a massive slope failure on the northwest flank forming one of the largest debris aprons in the solar system this slope failure probably involved relatively unconsolidated layers totaling approximately 2 kilometers in thickness overlying a rigid crust or lithosphere at least 11 kilometers thick mountain formation on io may involve localized deep rooted thrust faulting and block rotation due to compression at depth induced during vertical recycling of io s crust

glacial hydrology and erosion patterns a mechanism for carving glacial valleys article i nfo article history most glacial erosion models assume that erosion rates are proportional to ice sliding velocity while recent studies have shown that water plays a major role in modulating sliding velocities the impact it might have on erosion rates is still unclear here we incorporate subglacial hydrology into a glacial erosion model that is based on a sliding rule our results explicitly highlight that adding subglacial hydrology has profound impacts on the temporal and spatial patterns of glacial erosion first it suggests that erosion might mainly occur during melting seasons when subglacial water pressure is large and effective pressure is low i e before the channels fully develop and reduce the water pressure at the ice bedrock interface which promotes sliding and erosion second the distribution of erosion is bimodal with a peak at the equilibrium line altitude and as we demonstrate enhanced erosion at much lower altitudes within the ablation area where water due to melting abounds this has important implications because it explains why glacial erosion can simultaneously

geology and topography of ra patera io in the voyager era prelude to eruption voyager era stereo images are used to map the geology and topography of ra patera a major active volcanic center and possible site of sulfur eruptions on io the summit of ra patera reaches only 1 km above the surrounding plains pre voyager era lava flows occur on slopes of 0 1 0 3 comparable to the lunar mare these flows were emplaced at either low viscosities high eruption rates or both a 600 km long ridged mountain unit rising to 8 km near carancho patera forms a 60 by 90 km wide plateau 0 5 km high 50 km east of ra patera the new lava flows observed by galileo flowed around the southern edge of this plateau

extension of glacier de saint sorlin french alps and equilibrium line altitude during the little ice age glacier de saint sorlin french alps left terminal moraines at 1 3 2 9 and 3 7 km ahead of the present terminus according to proxy data and to historical maps these were formed in the 19th 18th and 17th centuries respectively a plateau at 2700 2625 m was then surrounded by ice but never became an accumulation area this fact shows that the equilibrium line altitude ela on the glacier never dropped below 2300 m the following simple models apply sufficiently to yield reliable estimations of past ela 1 a uniform and constant vertical gradient of the mass balance down to the terminus and 2 a plane bed with a slope of 8 5 and a uniform width then in a steady situation the accumulation area ratio is 1 2 compared to the mean for 1956 72 at the onset of the little ice age the balances were higher by 3 75 m ice a 1 and the ela was 400 m lower correlations between 1956 72 balances and meteorological data suggest that during the melting season the 0 c isotherm was about 800 m lower while the winter precipitation at low altitudes did not change these correlations may have been different in the past but an equal lowering of the ela and of the 0 c isotherm as assumed by several authors seems excluded

convection driven compaction as a possible origin of enceladus s long wavelength topography 1 the long wavelength surface topography of enceladus shows depressions about 1 km in depth and 102 km wide one possible cause of this topography is spatially variable amounts of compaction of an initially porous ice shell driven by spatial variations in heat flux here we show that the heat flux variations associated with convection in the shell can quantitatively match the observed features we develop a simple model of viscous compaction that includes the effect of porosity on thermal conductivity and find that an initial shell porosity of at least 20 25 is required to develop the observed topography over 1 ga this mechanism produces topographic depressions not rises above convective upwellings and does not generate detectable gravity anomalies unlike transient dynamic topography it can potentially leave a permanent record of ancient convective processes in the shallow lithospheres of icy satellites

origin of the pluto charon system constraints from the new horizons flyby abstract new horizon s accurate determination of the sizes and densities of pluto and charon now permit precise internal models of both bodies to be constructed assuming differentiated rock ice structures we find that pluto is close to 2 3 solar composition anhydrous rock by mass and charon 3 5 solar composition anhydrous rock by mass pluto and charon are closer to each other in density than to other large 1000 km diameter kuiper belt bodies despite this we show that neither the possible presence of an ocean under pluto s water ice shell and no ocean within charon nor enhanced porosity at depth in charon s icy crust compared with that of pluto are sufficient to make pluto and charon s rock mass fractions match all four small satellites styx nix kerberos hydra appear much icier in comparison with either pluto or charon in terms of a giant impact origin both these inferences are most consistent with the relatively slow collision of partly differentiated precursor bodies canup astrophys j 141 35 2011 this is in turn consistent with dynamical conditions in the ancestral kuiper belt but implies that the impact precursors themselves accreted relatively late and slowly to limit 26 al and accretional heating the iciness of the small satellites is not consistent with direct formation of the pluto charon system from a streaming instability in the solar nebula followed by prompt collapse of gravitationally bound pebble piles a proposed formation mechanism for kuiper belt binaries nesvorn et al astron j 140 785 793 2010 growth of pluto scale bodies by accretion of pebbles in the ancestral kuiper belt is not ruled out however and may be needed to prevent the precursor bodies from fully differentiating due to buried accretional heat prior to the charon forming impact

reorientation of sputnik planitia implies a subsurface ocean on pluto to explain the position of the sputnik planitia basin on pluto the feature would need to have formed via impact and pluto would need to have a subsurface ocean the present location of sputnik planitia the prominent deep icy basin on pluto is close to one of the longitudes of the dwarf planet s tidal axis by analogy with other large basins in the solar system it is thought to be an impact feature although reorientation arising from tidal and rotational torques can explain the present day location of the basin it requires the feature to be a positive gravity anomaly despite its negative topography francis nimmo et al argue that if sputnik planitia formed via impact and if pluto possesses a subsurface ocean a positive gravity anomaly would naturally result because of shell thinning and ocean uplift followed by later modest nitrogen deposition this is one of four papers on the geology of sputnik planitia in this issue of nature in news views amy barr puts these latest contributions into context the deep nitrogen covered basin on pluto informally named sputnik planitia is located very close to the longitude of pluto s tidal axis1 and may be an impact feature2 by analogy with other large basins in the solar system3 4 reorientation5 6 7 of sputnik planitia arising from tidal and rotational torques can explain the basin s present day location but requires the feature to be a positive gravity anomaly7 despite its negative topography here we argue that if sputnik planitia did indeed form as a result of an impact and if pluto possesses a subsurface ocean the required positive gravity anomaly would naturally result because of shell thinning and ocean uplift followed by later modest nitrogen deposition without a subsurface ocean a positive gravity anomaly requires an implausibly thick nitrogen layer exceeding 40 kilometres to prolong the lifetime of such a subsurface ocean to the present day8 and to maintain ocean uplift a rigid conductive water ice shell is required because nitrogen deposition is latitude dependent9 nitrogen loading and reorientation may have exhibited complex feedbacks7

an upper bound on pluto s heat flux from a lack of flexural response of its normal faults abstract the topography of rifts on icy bodies can be used to probe their internal properties uplifted and curved rift flanks occur when the elastic thickness te of the crust is low indicative of high heat flow out of the body stereo topography of pluto shows no evidence of rift flank uplift associated with large extensional graben to the west of sputnik planitia modeling the amount of topographic deflection expected from varying elastic thicknesses yields a conservative lower bound on te of 8 km at the time of deformation the maximum implied heat flux since graben formation is 66 85 mwm 2 this upper bound is consistent with the predicted paleo heat fluxes from radioactive decay 6 mwm 2 but likely only marginally consistent with nearby apparently viscously relaxed impact craters additionally we also analyze the shear stresses on these faults and find that pluto s faults only need to support low stresses in the range of 100 300 kpa to explain their dimensions

geological mapping of sputnik planitia on pluto abstract the geology and stratigraphy of the feature on pluto informally named sputnik planitia is documented through geologic mapping at 1 2 000 000 scale all units that have been mapped are presently being affected to some degree by the action of flowing n2 ice the n2 ice plains of sputnik planitia display no impact craters and are undergoing constant resurfacing via convection glacial flow and sublimation condensation of atmospheric n2 onto the surface to form a bright mantle has occurred across broad swathes of sputnik planitia and appears to be partly controlled by pluto s obliquity cycles the action of n2 ice has been instrumental in affecting uplands terrain surrounding sputnik planitia and has played a key role in the disruption of sputnik planitia s western margin to form chains of blocky mountain ranges as well in the extensive erosion by glacial flow of the uplands to the east of sputnik planitia

breaking up is hard to do global cartography and topography of pluto s mid sized icy moon charon from new horizons abstract the 2015 new horizons flyby through the pluto system produced the first high resolution topographic maps of pluto and charon the most distant objects so mapped global integrated mosaics of the illuminated surface of pluto s large icy moon charon have been produced using both framing camera and line scan camera data including four color images at up to 1 47 km pixel scales showing the best resolution data at all areas of the surface digital elevation models dems with vertical precisions of up to 0 1 km were constructed for 40 of charon using stereo imagery local radii estimates for the surface were also determined from the cartographic control network solution for the lorri framing camera data which validate the stereo solutions charon is moderately cratered the largest of which is 250 km across and 6 km deep charon has a topographic range over the observed hemisphere from lowest to highest of 19 km the largest topographic amplitude of any mid sized icy body including ceres other than iapetus unlike saturn s icy moons whose topographic signature is dominated by global relaxation of topography and subsequent impact cratering large scale tectonics and regional resurfacing dominate charon s topography most of charon s encounter hemisphere north of the equator oz terra is broken into large polygonal blocks by a network of wide troughs with typically 3 6 km relief the deepest of these occur near the illuminated pole and are up to 13 km deep with respect to the global mean radius the deepest known surfaces on charon the edge of this terrain is defined by large tilted blocks sloping 5 or so the crests of which rise to 5 or 6 km above charon mean the highest known points on charon the southern resurfaced plains vulcan planitia consist of rolling plains locally fractured and pitted that are depressed 1 km below the mean elevation of the disrupted northern terrains of oz terra that comprise much of the northern hemisphere but 2 2 5 km below the surfaces of the blocks themselves these plains roll downward gently to the south with a topographic range of 5 km the outer margins of vulcan planitia along the boundary with oz terra form a 2 3 km deep trough suggesting viscous flow along the outer margins isolated massifs 2 4 km high also flanked by annular moats lie within the planitia itself the plains may be formed from volcanic resurfacing of cryogenic fluids but the tilted blocks along the outer margins and the isolated and tilted massifs within vulcan planitia also suggest that much of charon has been broken into large blocks some of which have been rotated and some of which have foundered into charon s upper mantle now exposed as vulcan planitia a history that may be most similar to the disrupted terrains of ariel

the pluto system initial results from its exploration by new horizons the pluto system was recently explored by nasa s new horizons spacecraft making closest approach on 14 july 2015 pluto s surface displays diverse landforms terrain ages albedos colors and composition gradients evidence is found for a water ice crust geologically young surface units surface ice convection wind streaks volatile transport and glacial flow pluto s atmosphere is highly extended with trace hydrocarbons a global haze layer and a surface pressure near 10 microbars pluto s diverse surface geology and long term activity raise fundamental questions about how small planets remain active many billions of years after formation pluto s large moon charon displays tectonics and evidence for a heterogeneous crustal composition its north pole displays puzzling dark terrain small satellites hydra and nix have higher albedos than expected

pressurized oceans and the eruption of liquid water on europa and enceladus 1 it is difficult for liquid water to erupt onto the surface of icy satellites such as europa and enceladus because liquid water is more dense than ice if the ice shell thickens the volume expansion of ice upon freezing increases pressure in the subsurface ocean the excess pressure is determined by a balance between compression of ocean water and elastic expansion of the ice shell we show that on europa the freezing of 1 10 km of ice generates tangential stresses that exceed the tensile strength of ice excess pressure however is insufficient for liquid water to erupt to the surface within smaller icy satellites such as enceladus ocean pressure can become large enough to cause an eruption of large amounts of liquid water

cold based mountain glaciers on mars western arsia mons surface environmental conditions on mars are currently extremely cold and hyperarid most equivalent to polar deserts on earth coupling newly acquired mars data with fieldbased observations regarding the flow surface morphology and depositional history of polar glaciers in antarctica we show that the multiple facies of an extensive fan shaped deposit on the western flanks of arsia mons volcano are consistent with deposition from cold based mountain glaciers an outer ridged facies that consists of multiple laterally extensive arcuate and parallel ridges resting without disturbance on both well preserved lava flows and an impact crater is interpreted as drop moraines formed at the margin of an ablating and predominantly receding cold based glacier a knobby facies that consists of equidimensional knobs each to several kilometers in diameter is inward of the ridges this facies is interpreted as a sublimation till derived from in situ downwasting of ashrich glacier ice a third facies comprising distinctive convex outward lobes with concentric parallel ridges and aspect ratios elongated downslope likely represents rock glacier deposits some of which may still be underlain by a core of glacier ice taken together these surficial deposits show that the western flank of arsia mons was occupied by an extensive mountain glacial system accumulating on and emerging from the upper slopes of the volcano and spreading downslope to form a piedmont like fan

topography and geology of uranian mid sized icy satellites in comparison with saturnian and plutonian satellites newly processed global imaging and topographic mapping of uranus s five major satellites reveal differences and similarities to mid sized satellites at saturn and pluto three modes of internal hea

pre glacial topography of the european alps we present a topographic reconstruction of the european alps prior to quaternary glaciation based on the assumption that this topography was a steady state fluvial landscape eroding according to a stream power law but with spatially variable channel steepness we find high pre glacial channel steepness in the western alps and on the harder rocks of the basement and metamorphic nappes while lower values are often associated with weaker alpine flysch deposits the reconstructed pre glacial river network also provides an estimate of distribution and magnitude of glacial erosion and associated isostatic adjustment suggesting an increase of valley scale topographic relief and decrease of mean elevation throughout glaciation

the vanishing cryovolcanoes of ceres ahuna mons is a 4 km tall mountain on ceres interpreted as a geologically young cryovolcanic dome other possible cryovolcanic features are more ambiguous implying that cryovolcanism is only a recent phenomenon or that other cryovolcanic structures have been modified beyond easy identification we test the hypothesis that cerean cryovolcanic domes viscously relax precluding ancient domes from recognition we use numerical models to predict flow velocities of ahuna mons to be 10 500 m myr depending upon assumptions about ice content rheology grain size and thermal parameters slower flow rates in this range are sufficiently fast to induce extensive relaxation of cryovolcanic structures over 108 109 years but gradual enough for ahuna mons to remain identifiable today positive topographic features including a tholus underlying ahuna mons may represent relaxed cryovolcanic structures a composition for ahuna mons of 40 ice explains the observed distribution of cryovolcanic structures because viscous relaxation renders old cryovolcanoes unrecognizable

pluto s haze as a surface material pluto s atmospheric haze settles out rapidly compared with geological timescales it needs to be accounted for as a surface material distinct from pluto s icy bedrock and from the volatile ices that migrate via sublimation and condensation on seasonal timescales this paper explores how a steady supply of atmospheric haze might affect three distinct provinces on pluto we pose the question of why they each look so different from one another if the same haze material is settling out onto all of them cthulhu is a more ancient region with comparatively little present day geological activity where the haze appears to simply accumulate over time sputnik planitia is a very active region where glacial convection as well as sublimation and condensation rapidly refresh the surface hiding recently deposited haze from view lowell regio is a region of intermediate age featuring very distinct coloration from the rest of pluto using a simple model haze particle as a colorant we are not able to match the colors in both lowell regio and cthulhu to account for their distinct colors we propose that after arrival at pluto s surface haze particles may be less inert than might be supposed from the low surface temperatures they must either interact with local materials and environments to produce distinct products in different regions or else the supply of haze must be non uniform in time and or location such that different products are delivered to different places

minimal erosion of arctic alpine topography during late quaternary glaciation alpine topography in europe has been shaped by recent glaciations cosmogenic dating of summits in svalbard suggest that arctic alpine topography is a million years old and that subsequent glaciations have preserved rather than eroded the landscape the alpine topography observed in many mountainous regions is thought to have formed during repeated glaciations of the quaternary period1 2 before this time landscapes had much less relief1 2 3 however the spatial patterns and rates of quaternary exhumation at high latitudes where cold based glaciers may protect rather than erode landscapes are not fully quantified here we determine the exposure and burial histories of rock samples from eight summits of steep alpine peaks in northwestern svalbard 79 5 n using analyses of 10be and 26al concentrations4 5 we find that the summits have been preserved for at least the past one million years the antiquity of svalbard s alpine landscape is supported by the preservation of sediments older than one million years along a fjord valley6 which suggests that both mountain summits and low elevation landscapes experienced very low erosion rates over the past million years our findings support the establishment of northwestern svalbard s alpine topography during the early quaternary we suggest that as the quaternary ice age progressed glacial erosion in the arctic became inefficient and confined to ice streams and high relief alpine landscapes were preserved by minimally erosive glacier armour

radio occultation measurements of pluto s neutral atmosphere with new horizons abstract on 14 july 2015 new horizons performed a radio occultation ro that sounded pluto s atmosphere down to the surface the sensitivity of the measurements was enhanced by a unique configuration of ground equipment and spacecraft instrumentation signals were transmitted simultaneously by four antennas of the nasa deep space network each radiating 20 kw at a wavelength of 4 2 cm the polarization was right circular for one pair of signals and left circular for the other pair new horizons received the four signals and separated them by polarization for processing by two independent receivers each referenced to a different ultra stable oscillator the two data streams were digitized filtered and stored on the spacecraft for later transmission to earth the results reported here are the first to utilize the complete set of observations we calibrated each signal to remove effects not associated with pluto s atmosphere including the limb diffraction pattern we then applied a specialized method of analysis to retrieve profiles of number density pressure and temperature from the combined phase measurements occultation entry sounded the atmosphere at sunset at 193 5 e 17 0 s on the southeast margin of an ice filled basin known informally as sputnik planitia sp occultation exit occurred at sunrise at 15 7 e 15 1 n near the center of the charon facing hemisphere above 1215 km radius 25 km altitude there is no discernible difference between the measurements at entry and exit and the ro profiles are consistent with results derived from ground based stellar occultation measurements at lower altitudes the ro measurements reveal horizontal variations in atmospheric structure that had not been observed previously and they are the first to reach the ground the entry profile has a strong temperature inversion that ends 3 5 km above the surface and the temperature in the cold boundary layer beneath the inversion is nearly constant 38 9 2 1 k and close to the saturation temperature of n 2 the exit profile has a much weaker inversion that extends all the way to the ground where the air temperature is 51 6 3 8 k three factors appear to be responsible for the presence of a cold boundary layer in the entry profile forget et al 2017 a substantial diurnal cycle of sublimation and condensation of n 2 ice in sp the local time of the ro observation and confinement within sp by the surrounding topography and katabatic winds we have also determined the surface pressure and the local radius at both entry and exit the best pressure reference is the mean value 11 5 0 7 microbar at 1189 9 0 2 km