Explosive volcanism occurred on Mars during its early history (Noachian-Hesperian; ~4.1-3.0 Ga). Because of Mars' cold atmospheric temperatures, water released from explosive eruptions may precipitate as ice or ice-ash aggregates. This process may have supplied ice to equatorial regions, which contain high excess hydrogen and potential buried ice deposits. We simulate explosive volcanic eruptions using the Laboratoire de Météorologie Dynamique Generic Planetary Climate Model and find that up to ~5 meters of ice is delivered to the surface in only one high-magnitude eruptive event. This ice can persist for long periods if preserved by widespread cooling from volcanic sulfuric acid or by burial under dust or pyroclasts. Here we show that over time, explosive eruptions may have served as a recurring mechanism for delivering ice to the equator, explaining elevated ice content at low latitudes independent of obliquity.

Over the last 50 years, orbital missions have collected a wealth of data across the Martian surface, and landers and rovers have visited a handful of specific places to conduct in-depth analyses. As orbital imagery has improved, it has become clear that Mars has significant compositional and geomorphological diversity well beyond that sampled by in-situ missions. Deliberate exploration of end-member terrains with surface assets is critical for furthering our understanding of Martian history. However, designing an architecture that is appropriate for anywhere on the Martian surface is difficult from a practical perspective. This study presents several examples of how the diversity of the Martian surface can be abstracted from orbital data and plots the previous and proposed landing sites in this framework. Starting with a range of proposed landing sites derived from community workshops and reports, we explore the implications of popular science targets on required engineering architecture, including (1) landing site elevation (driving landing architecture), (2) local climate (driving power and thermal architectures), and (3) surface dust environment (driving landing and power architectures).

The Athabasca Valles Flood Lava (AVFL) eruption may have significantly influenced the Late Amazonian environment of Mars by emplacing hot lava and injecting large volumes of water into the atmosphere. Using the Laboratoire de Meteorologie Dynamique Generic Planetary Climate Model, we simulate the dispersal of this water and its climatic effects. Within a day, vapor condenses into cloud layers and precipitates as ice, forming deposits up to-5 m thick around the vent. Clouds exert both warming and cooling influences: latent heat release raises atmospheric temperatures above the vent by up to-80 K and enhances local surface warming by-10 K, while cloud cover blocks radiation, cooling the lower troposphere by up to 30 K. Surface cooling is pronounced, with temperatures decreasing by-70 K due to persistent clouds and high-albedo ice. These processes suggest the AVFL eruption initiated a transient hydrological cycle and ice accumulation in tropical latitudes otherwise unfavorable for long-term stability. Our findings allow us to make predictions about the climate and environmental effects of future eruptions from Cerberus Fossae, a region that may still be volcanically active.

The surface of Titan is composed of varied geomorphic units indicative of a vivid depositional, erosional and tectonic history. Dominant at Titan's equatorial regions are vast eolian landscapes of dunes, sand sheets, yardangs and wind streaks. These features reveal the action of wind in time and space, which has moved and shaped unconsolidated materials across the surface. Cassini Visual and Infrared Mapping Spectrometer (VIMS) spectra of these materials are most consistent with organics, and particle sizes from Cassini Synthetic Aperture Radar (SAR) vary from dust to cobbles [1]. Most grains appear to be sand-sized, based on the predominance of SAR-dark (smooth at 2 cm) material organized into dune forms at the equator, and models for wind requirements to build dunes on Titan [2]. Dunes cover ~15% of Titan’s surface, or 13 million km2 [3, 4] in the form of linear dunes 1-2 km wide and spaced by 1-4 km [5]. This type and size of dune is dominant in the Arabian and African deserts and is also known as longitudinal [6], referring to the net direction of the transport of sand, which aligns with dune crests [7, 8]. All dunes visible in the Cassini SAR imagery have now been traced down their long axes, revealing regional and global orientations (proxy for wind) and distributions, controlled by obstacles and regional elevations. Over 30,000 dunes have been traced (Fig. 1), and given SAR image coverage, this represents ~40% of all dunes thought to be present based on sand sea locations from the Cassini Imaging Science Subsystem (ISS), meaning we may expect as many as 75,000 dunes to exist on Titan. A total dune length of 1.309 million km has now been calculated from the measured dunes; extrapolated to all of Titan, this length would be 3.273 million km. Using an average dune width of 1 km and corresponding height of 100 m from a 0.1-0.2 dune height:width relationship [9] and isolated measurements [10] we find a total measured dune sand volume of 130,900 km3. If scaled up to all dunes thought to be present, the total volume of sand from dunes alone of 327,250 km3 falls within previous estimates for sand volumes [11, 4, 12]; however, those studies also included sand sheets without detectable dunes. Thus, this new measurement may reveal greater sand volumes from dunes alone than previously thought. This represents a significant material volume that has been moved through wind action, even if from locations close to the (unknown) source. At higher latitudes, several distinct regions of SAR-bright dunes or wind-carved ridges or yardangs are also present (Fig. 2). These can be found in flat plains [13], where they may be inactive dunes, or on the elevated, fluvially carved domes of the midlatitudes, postulated to be volcanic laccoliths [14]. Ash or otherwise derived fine organic grains would be ideally soft yardang materials, comparable to those on Earth, and reveal that winds can also remove substrate on Titan, similar to the action of wind in locations on Mars [15]. Statistical comparisons between possible yardangs support their existence on the domes but are consistent with them being dunes in other locations [16]. The dome yardangs would have resulted from removal of fine-grained materials and loss to the atmosphere as airborne particles and eventually other surfaces – new estimates of the volume of yardang erosion on one small dome, based on yardang heights of ~100 m from shadows, is ~125 km3, and there are a handful of identified other similar yardang regions. Creation of dust from dune sand movement has not yet been estimated (and there are significant uncertainties about material properties) but with yardang processes could yield large volumes that now reside in unknown locations, including interdunes, high latitude plains, lakes and seas. SAR and ISS bright streaks are visible at low to midlatitudes on Titan, similar in orientation to the dunes where they overlap, and found behind obstacles [17, 18]. Their SAR brightness is consistent with a larger particle size, which may be supported by lower density materials, but wind streaks are also often identified with fine particle sizes. This reveals yet another sand and dust location on Titan. Yardangs and wind streaks, along with the ubiquitous equatorial dunes, confirm a general W-E flow of wind, with some N or S deviations that may be reflections of a global undulatory wind pattern at high latitudes. The Dragonfly mission will study dune regions in depth and will reveal particle sizes and compositions, nature of the interdune, level of dune activity and sand (and perhaps dust) movement at the present day, and magnitude and direction of modern winds [19]. These observations will anchor our understanding of eolian processes in the Solar System. [1] Le Gall, A. et al. 2010, Icarus 207, 948-958. [2] Lorenz, R.D. et al. 2006, Science 312, 724-727. [3] Le Gall, A. et al. 2011, Icarus 213, 608-624. [4] Rodriguez, S. et al. 2014, Icarus 230, 168-179. [5] Radebaugh, J. et al. 2008, Icarus 194, 690-703. [6] Courrech du Pont et al. 2024; Earth-Science Reviews, 104772. [7] Lorenz and Radebaugh 2009; Geophysical Research Letters 36. [8] Lucas et al. 2014; Geophysical Research Letters 41, 6093-6100. [9] Lancaster, N. 1995, Geomorphology of Desert Dunes. [10] Neish, C.D. et al. 2010. Icarus 208, 385-394. [11] Arnold, K. 2013, BYU MS Thesis. [12] Lorenz, R.D. et al. 2008, Geophysical Research Letters 35. [13] Lopes, R. et al. 2020. Nature Astronomy 4, 228-233. [14] Schurmeier, L. et al. 2023, Icarus 404, 115664. [15] Kerber, L. et al. 2011. Icarus 216, 212-220. [16] Northrup, D. et al. in progress. [17] Malaska, M.J. et al. 2016. Icarus 270, 183-196. [18] Cohen-Zada, A. et al. 2016. Aeolian Research 20, 108-125. [19] Barnes, J.W. et al. 2021. The Planetary Science Journal 2, 130.Fig. 1. Dunes traced in Shangri-La region of Titan. ISS basemap. Dunes may still be present in large, dark areas, though there is not SAR coverage.Fig. 2. SAR-bright yardangs on an eroded dome. From T64, northern midlatitudes. Arrow shows SAR illumination direction.

Introduction: Although Mars is now a cold, dry planet, the geological record shows that, early in its history, Mars not only went through episodes of wet-dry cycling [1], in which Mars could maintain liquid water on its surface for sustained periods of time, but was also volcanically active [2]. As a high-pressure CO2 and H2O atmosphere alone could not induce the required annually-averaged temperatures for liquid water given the brightness of the Sun at the time [3,4], one hypothesis suggested sulphur dioxide (SO2) and hydrogen sulphide (H2S) emitted by active volcanoes on the surface of Mars as a source of greenhouse warming [5]. Later studies then suggested that any greenhouse warming from SO2and H2S would be negated by the cooling effect of sulphuric acid (H2SO4) and elemental polysulphur (S8) clouds that would result from the reaction of SO2and H2S with water vapour in the atmosphere. However, these studies either relied on photochemical models in 1-D [6,7], which neglect spatial variations in cloud formation, or simple parametrisations of sulphur which do not adequately account for the formation timescales of H2SO4and S8 clouds [8]. All of these factors result in major uncertainties in the magnitude and duration of any warming or cooling on early Mars.We therefore wish to investigate, using a 3-D Global Climate Model (GCM), how cycles of emission, reaction, condensation and deposition of sulphur would have affected the radiative balance of Mars, and hence the timescales of any volcanically-induced warming and cooling cycles that took place on Mars. In particular, we wish to observe whether the finite timescales of formation of H2SO4 and S8­ clouds were significant enough to allow for a short period of time just following a volcanic eruption in which SO2 and H2S greenhouse warming could dominate over atmospheric cooling from H2SO4 and S8­ clouds.Method: We present the first implementation of the sulphur cycle on early Mars in a 3-D Global Climate Model (the Generic Planetary Climate Model (PCM) [9]) that takes a number of processes, most notably atmospheric chemistry, into account as shown in Figure 1. We simulate volcanic emission of sulphur according to a point surface flux of SO2, H2S, S2, HCl, CO and H2 as per the thermodynamic constraints on silicate partitioning in the Martian mantle [2], with a more reducing mantle favouring emission of H2S and S2, and a more oxidising mantle favouring SO2. Assuming a background atmosphere of 95% carbon dioxide and variable water vapour [4], we then simulate atmospheric chemistry according to 270 reactions that take odd-hydrogen, sulphur, nitrogen and chlorine chemistry into account [6,10-12], with the end products being H2SO4 (favoured in an oxidising atmosphere) and S­8 (favoured in a reducing atmosphere). These two molecules then condense out of the atmosphere and are deposited onto the ground.Although H2SO4 and H­2O are expected to condense out together, a complex microphysical model involving binary H­2SO4-H2O condensation is difficult to implement due to the lack of knowledge of the density of cloud condensation nuclei in the early Martian atmosphere, as well as the lack of laboratory constraints on microphysical parameters at the low atmospheric temperatures predicted for early Mars. We therefore assume a constant cloud particle radius and ratio of H2SO4 to H2O, and model condensation according to diffusion-limited growth [13] in order to allow for supersaturation of H2SO4 in the atmosphere and thereby delay the onset of the anti-greenhouse effect of H2SO4 clouds as much as possible. Figure 1. (top) a diagram of the major processes included in our model of the sulphur cycle, (bottom) the major photochemical pathways involved in the production of S8 and H2SO4 from outgassed SO2, H2S and S2 based on the redox state of the atmosphere.Results: We confirm the results of [7,8] and find that the amount of greenhouse warming induced by volcanic SO2and H2S emission is both too weak and too short to melt liquid water on the surface of Mars. Although the anti-greenhouse effect from H2SO4 and S8 cloud formation can be delayed by increasing the atmospheric pressure, the increased thermal inertia of the atmosphere also delays the greenhouse effect from SO2 and H2S (Figure 2). A particularly large eruption can even induce runaway cooling of the atmosphere, eventually leading to atmospheric collapse as CO2 is no longer stable in the atmosphere in its gaseous form. We are unable to mitigate this either by changing the oxygen fugacity and water content of the Martian mantle, or the microphysical properties of the binary H2SO4-H2O condensate cloud particles.Figure 2. Simulation of a volcanic outgassing event (at the green cross) starting from three different average surface pressures, (top) surface temperature 7 days after the event and (bottom) increase in temperature relative to scenario where no eruption took place. Acknowledgments: This work was carried out at the Jet Propulsion Laboratory California Institute of Technology under a contract with NASA. We recognize support for this project from NASA grant 20-SSW20-0086.References: [1] Rapin, W. et al. (2023) Nature, 620, 299-302. [2] Gaillard, F. et al. (2013) Space Sci. Rev., 174, 251-300. [3] Forget, F. et al. (2013) Icarus, 222, 81-99. [4] Wordsworth, R. et al. (2015) J. Geophys. Res. Plan., 120, 1201-1219. [5] Yung, Y. L. et al. (1997) Icarus, 130, 222-224. [6] Johnson, S. S. et al. (2009) J. Geophys. Res. Plan., 114, E11011. [7] Tian, F. et al. (2010) EPSL, 295, 412-418. [8] Kerber, L. et al. (2015) Icarus, 261, 133-148 [9] Forget, F. et al. (1999) J. Geophys. Res., 104, 24155-24176. [10] Catling, D. C. et al. (2010) J. Geophys. Res., 115, E00E11. [11] Sholes, S. F. et al. (2017). [12] Stolzenbach, A. et al. (2023) Icarus, 395, 115447. [13] Hu, R. et al. (2012) ApJ, 761, 166.

Passive degassing, or volatile release from persistent, non-eruptive volcanic activity, may have contributed significantly to the Martian ice budget over the planet's history by providing a pathway for volatile species, such as water vapor, to outgas from the interior into the atmosphere and accumulate on the surface. Passive degassing from Martian volcanoes can result in ice accumulations in unusual locations (e.g., outside of the typical cold traps like the Tharsis Rise and polar ice caps). Such locally derived water could cover the volcanic source in an ice-rich veneer that may later serve as a source of ice for subsequent interactions between impactors or lava. Using the Laboratoire de M & eacute;t & eacute;orologie Dynamique Generic Planetary Climate Model, we model the spatial distribution of ice that results from passive degassing from five major Martian volcanic centers, Cerberus Fossae, Apollinaris Mons, Elysium Mons, Hadriacus Mons, and Pityusa Patera, and assess the sensitivity of these results to a range of volcanic, atmospheric, orbital, environmental, and numerical parameters. Previous studies have placed estimates on the amount of water released from volcanic outgassing (e.g., Carr, 1987; Craddock and Greeley, 2009; Grott et al., 2011; Carr and Head, 2015), but this is the first study to track the water released from specific volcanoes and to determine the spatial distribution of resulting ice deposits. We find that volcanic variables, such as mass flux and duration of degassing, primarily drive the thickness of volcanogenic ice fields -- higher mass fluxes create the thickest ice deposits (up to similar to 210 mm around the volcanic center for a mass flux 10(6) kg s(-1)) and durations of >6 months are required for ice to reach near-global distributions. Ice accumulation around the degassing source is maximized if the latitude of passive degassing occurs near the south pole (i.e., Pityusa Patera) due to its exposure to circulation patterns that promote ice deposition in that region. Changes in the season, dust visible optical depth, and eccentricity can also impact the thickness of ice, with ice accumulation around the volcanic source being maximized in the spring, with higher dust visible optical depths, and when the eccentricity is low, due to more favorable temperatures and atmospheric circulation. Variances in the obliquity can also influence ice fields by modulating which latitudes receive more surface ice accumulation. The amount of activated cloud condensation nuclei (CCN) in the atmosphere has a lesser impact on the thickness and spatial distribution of ice fields than other parameters, with lower amounts of CCN leading to narrower, but slightly thicker deposits compared to higher amounts of CCN. The mean surface pressure and longitude of perihelion both affect the timing of ice fields. The surface pressure influences how long water lingers in the atmosphere before depositing to the surface, while the longitude of perihelion controls the time of year in which the thickest ice deposits accumulate around the degassing source. Ice fields are highly sensitive to the model resolution. Higher model resolutions allow for a more detailed representation of physical processes, such as cloud formation and ice deposition, while lower resolution model runs provide a more approximate guide to the representation of these processes and may underestimate surface ice thicknesses around the degassing source. The thickest deposits of ice form around the source of passive degassing, but later migrates and restabilizes in cold traps after degassing has ceased, unless it is protected against sublimation by the deposition of dust or pyroclastic material. Assuming the ice is protected, recurrent episodes of passive degassing could have facilitated the gradual accumulation of ice deposits several meters thick over time.

AbstractThe Mars Reconnaissance Orbiter and its Context Camera (CTX) have acquired more than 100,000 separate panchromatic images that capture nearly the entire surface of Mars at ∼5–6 m/pixel. This paper describes a data processing workflow used to generate the first contiguous global mosaic of CTX data, which represents a large improvement in spatial resolution over existing 100 m/pixel contiguous global mosaics. We describe the overarching strategy for the mosaic's construction, which was to maximize the scientific utility of a continuous mosaic that is 5.7 trillion pixels in size. The pipeline used for data processing prioritized traceability and reproducibility of the final mosaic, such that the provenance of all pixels is reported, equipping scientists with information to differentiate mosaic artifacts from surface landforms and to incorporate critical image metadata into their analyses. The CTX data set synthesized into a global CTX mosaic facilitates ready analysis and provides a new capability in transitioning global studies of Mars from high‐resolution investigations of individual images to systematic studies of the entire Martian surface at outcrop‐resolving quality without regard to image boundaries.

Gullies on Mars resemble water-carved channels on Earth, but they are mostly at elevations where liquid water is not expected under current climate conditions. It has been suggested that sublimation of carbon dioxide ice alone could have formed Martian gullies. We used a general circulation model to show that the highest-elevation Martian gullies coincide with the boundary of terrain that experienced pressures above the triple point of water when Mars' rotational axis tilt reached 35°. Those conditions have occurred repeatedly over the past several million years, most recently ~630,000 years ago. Surface water ice, if present at these locations, could have melted when temperatures rose >273 kelvin. We propose a dual gully formation scenario that is driven by melting of water ice followed by carbon dioxide ice sublimation.

Natural pits on the Moon expose deep cross-sections of the lunar maria, enabling direct investigation of the Moon's volcanic history and providing potential access to subsurface lava tubes. The Moon Diver mission concept seeks to explore the Mare Tranquillitatis pit, which exposes the largest wall of bedrock of the mare pits (similar to 65 m). The concept is enabled by two innovative capabilities: pinpoint landing near the pit and robotic access to its nearvertical wall with an instrument package to examine the elemental chemistry, mineralogy, and morphology of these bedrock layers. Pinpoint landing uses closed-loop guidance with terrain-relative navigation (TRN), which was advanced by Perseverance landing on Mars, to deliver the lander within a 100-m ellipse. The Axel robotic explorer, which remains tethered to the lander, would egress onto the lunar surface and traverse the relatively flat terrain to the pit's funnel entrance. The lander, which is the data link to Earth, also serves as an anchor and provides power and communication to the rover through its tether. The rover is a novel two-wheeled platform with a trailing boom and a spool that pays out the tether as the rover traverses toward the pit. The 300-m long tether is well margined for the rover to scale the pit wall. The rover carries a surface preparation tool and three additional instrument types: (a) three high-resolution cameras for acquiring context images of the near and far walls with the near-wall pair in a stereoscopic configuration, (b) an alpha-particle-X-ray spectrometer (APXS) for elemental composition, and (c) a multi-spectral microscopic imager (MMI) that uses controlled lighting for mineralogy. The surface-preparation tool removes dust and patina from the rock wall by grinding a small area. This tool, the MMI, and the APXS would be deployed from an instrument bay inside the wheel wells. The rover would independently point each instrument at the same target on the wall with millimeter repeatability. Landing shortly after sunrise, the surface mission timeline is just shy of a lunar daytime (14 Earth days). Beyond the primary mission, the rover would be capable of descending from the overhang and peering into the void that may open to a large cave or lava tube. Lunar pits provide an exciting new target for exploration using innovative robotic capabilities that have been tested with integrated science instruments at multiple terrestrial analogue sites including a pit with basaltic layers in Arizona.

Elysium Planitia includes several outflow channels that were likely carved by aqueous erosion and subsequently infilled by younger lava flows, making Elysium Planitia the youngest volcanic terrain on Mars. Studying this region is critical for constraining the recent hydrological and thermal evolution of the planet. Here, we investigate the lava flow areas, thicknesses, and volumes in Elysium Planitia using Context (CTX) camera images in combination with SHAllow RADar (SHARAD) sounder data. Compiling 1,777 reflectors over an area of 9,126,790 km2 allows us to reconstruct the subsurface landscape evolution over time. Our findings show that Elysium Planitia is composed of material from about 40 episodes of effusive volcanic activity. We report volumes for individual eruptions of 4,000 +/- 1,600 km3 infilling Athabasca Valles, 12,200 +/- 2,500 km3 in Marte Vallis, and 16,000 +/- 4,000 km3 in Rahway Valles for the major flow units and volumes as small as 100 +/- 50 km3 in Cerberus Plains. The surface morphologies and inferred dielectric properties of lobe interfaces suggests that the regions consists of basaltic lava. The region also experienced multiple aqueous flooding events. Although, we found evidence of past lava-water interactions, present-day ground-ice (if present) is likely limited to local patches. Further, the pre-eruption landscape reveals that the aqueously carved Marte Vallis is more areal extensive, but shallower than previously suggested, with a likely paleo-flow direction from northwest to southeast. The channel is most likely sourced from a segment in the northwestern portion of Cerberus Fossae, and is now buried by multiple Late Amazonian lavas with the same lava flow direction. Elysium Planitia on Mars has a fascinating history of water and lava flows that shaped its landscape. It is the youngest volcanic terrain on the planet, and studying it helps us to better understand Mars' past as well as recent hydrological and volcanic history. We examined this region by using spacecraft images and radar data to constrain areas, thicknesses, and volumes. An area almost as large as Europe was investigated. The study revealed the products of more than 40 volcanic events, with one of the largest flows infilling Athabasca Valles with a volume of 4,000 km3. The surface appearance and material properties suggest that Elysium Planitia is composed of basalt, the most common type of lava on Earth. The area also experienced several large floods of water, and there is evidence that lava and water interacted in the past. However, while there could be ice in the ground today, it likely occurs in small patches. The study also provides new insights into the Marte Vallis outflow channel. It seems to be larger, but not as deep as previously thought, with water flowing from northwest to southeast and fed from a fissure in the northwest. Marte Vallis was later covered by several lava layers. We performed detailed surface and subsurface mapping of the entire Elysium Planitia region to constrain lava areas, thicknesses, and volumesElysium Planitia is composed of the products of about 40 effusive eruptions including large flood lava flows and lava shieldsResults indicate that there is no singular direction in dike propagation

This repository contains: Shapefiles of irregular polygonal ridge networks within mapping extent. MOLA elevation and slope data extracted over mapped ridge networks.

The longest distance traveled to date by a Mars rover is 45 kilometers (km), driven by Opportunity in 14 Earth years. The primary mission for Perseverance is expected to cover up to about 20 km in about 2 Earth years. In contrast, the recent Intrepid lunar rover mission concept study envisions driving about 1,800 km in 4 Earth years, benefitting from lower gravity, greater solar power availability, simpler concepts of operation, and much shorter communication latency with Earth. This raises questions of what fundamentally limits rover range on Mars and what new mission concepts might be possible if Mars rover range could be increased substantially. Assuming a few key technology advances, this paper will present a model for energy-limited driving range for a solar-powered rover in the 100 to 200 kilogram (kg) size range that shows that order of magnitude increases in driving range should be possible over goals for Perseverance. These technologies are (1) high-speed, heaterless mobility actuators, (2) avionics with much lower size, weight, and power (SWaP) and much greater onboard computing capability, and (3) more advanced algorithms for onboard autonomy that reduce the frequency of required interactions with human operators, or “ground-in-the-loop” (GITL) cycles. We will discuss three new classes of Mars rover missions that could be enabled by such capability. These are (1) more thorough exploration of individual geologic type localities, which requires total range on the order of 100 kilometers (km), (2) visiting two or more type localities in a single mission, which requires total range on the order of at least several hundred km, and (3) exploring the north polar layered deposits, where driving only about 10 km in one Mars summer might enable sampling roughly a million years of ice layers.

We provide the first solar system wide compendium of speleogenic processes and products. An examination of 15 solar system bodies revealed that six cave‐forming processes occur beyond Earth including volcanic (cryo and magmatic), fracturing (tectonic and impact melt), dissolution, sublimation, suffusion, and landslides. Although no caves (i.e., confirmed entrances with associated linear passages) have been confirmed, 3,545 SAPs (subsurface access points) have been identified on 11 planetary bodies and the potential for speleogenic processes (and thus SAPs) was observed on an additional four planetary bodies. The bulk of our knowledge on extraterrestrial SAPs is based on global databases for the Moon and Mars, which are bodies for which high‐resolution imagery and other data are available. To further characterize most of the features beyond the Moon and Mars, acquisition (preferably global coverage) and subsequent analysis of high‐resolution imagery will be required. The next few decades hold considerable promise for further identifying and characterizing caves across the solar system.

Surface modification on Jupiter's volcanically active moon, Io, has to date been attributed almost exclusively to lava emplacement and volcanic plume deposits. Here we demonstrate that wind-blown transport of sediment may also be altering the Ionian surface. Specifically, shallow subsurface interactions between lava and Io's widespread sulfur dioxide (SO$_2$) frost can produce localized sublimation vapor flows with sufficient gas densities to enable particle saltation. We calculate anticipated outgassing velocities from lava-SO$_2$ frost interactions, and compare these to the saltation thresholds predicted when accounting for the tenuous nature of the sublimated vapor. We find that saltation may occur if frost temperatures surpass 155 K. Finally we make the first measurements of the dimensions of linear features in images from the Galileo probe, previously termed "ridges", which demonstrate certain similarities to dunes on other planetary bodies. Io joins a growing list of bodies with tenuous and transient atmospheres where aeolian sediment transport may be an important control on the landscape.

<p><strong>Abstract</strong></p> <p>Wind-dominated landscapes can be found on all Solar System bodies with atmospheres. These landscapes record the point of interaction of the atmosphere with the solid surface in a narrow region that also correlates with where field geologists interact with a planetary body. The Altiplano-Puna of Argentina is a high altitude, hyperarid landscape with young and easily eroded deposits and high wind activity. These factors make the region an exceptional analogue for Mars and other planetary regions where wind erosion is dominant, which includes portions of Titan, Venus and Pluto. Our field studies inform our understanding of how wind affects planetary surfaces.</p> <p><img src="" alt="" /></p> <p>Figure 1. Yardangs of the Puna, carved into ash, surrounded by gravels. With young cinder cone.</p> <p><strong>Introduction</strong></p> <p>Wind erosion of planetary bodies yields a variety of landforms, including yardangs (wind-carved ridges), bedrock ridges, sastrugi (ridges formed in hardened snow), with many of these features covered or surrounded by gravels or sands (Fig. 1, ) [1, 2, 3].</p> <p>These landforms are also found on Mars [4, 5], Titan [6] and Venus. The regular spacing seen in many of these wind-eroded landforms is postulated to have arisen naturally out of wind-sediment relationships possibly controlled by bedrock hardness [7] and sediment supply [8] more than pre-existing fracturing or river channels, though both of those have also been implicated [9].</p> <p>Our ongoing field project in the Puna of Argentina involves field operations over 5-7 days using observations and simple instrumentation. Our methods are designed to measure the effects of current and past winds, rock properties, and erosive power of sands and gravels on yardang and bedrock ridge morphologies. We investigate the proposition that the simple action of wind on solid surfaces can lead to the complex, self-organized forms observed where wind is dominant and surfaces are erodible.</p> <p><strong>The Puna Field Site and Methods</strong></p> <p>The Puna desert is dominated by young deposits of volcanic ash [10], evaporites and basalt cinder cones. The high elevation means air density is lower about ~1/3 compared to sea level. On Mars, the lower air density leads to large and high dust devils, and similarly large dust devils have been observed during our field campaigns in the Puna [11]. The ash deposits are widespread, and often contain up to km-scale (mega) yardangs. The youngest and softest ignimbrite, called the Campo de Piedra Pomez (CPP) [10], contains abundant medium-sized (meter-scale), mesoyardangs. We visited this region in 2015, 2018 and 2019 (Fig. 1).</p> <p><img src="" alt="" width="601" height="297" /></p> <p>Fig. 2. Smoke released near a lone yardang, revealing sideways (secondary) winds.</p> <p>We examined field relationships such as orientations of yardangs, rock hardness and layering. We observed differences in rock colors related to intrinsic composition as well as weathering over time. We recorded relative matrix/pumice hardnesses and subsequent variations in erosional properties. We measured locations and sizes of wind indicators such as dedos (protrusions protected by harder lithics) and scours. We observed the action of wind on a single yardang over the course of a 7-day field excursion with an array of small instruments including Kestrel anemometers, simple smoke candles and a camera (Fig. 2), and a tuft net [12]. We deployed two DJI Mavic drones across the main CPP field to obtain a 3 cm DEM (Fig. 3), which also reveals parameters such as length, orientation, and spacing.</p> <p><strong>Key Observations</strong></p> <p>Winds over the time we observed the lone yardang (Fig. 3) in 2019 were dominantly from a direction oblique to the yardang long axis orientation and previously observed primary winds. This secondary wind may impact yardang size and shape. Preliminary results from study of the drone DEM obtained in 2019 reveal a 1:1 width/spacing relationship, similar to other studies. This is ascribed to the steady operation of wind and erosive agents over time on a relatively uniform substrate. A similar ratio was found using a DEM for a portion of the Medusae Fossae Formation (MFF) on Mars.</p> <p>A dark orange coating on the leeward sides of the yardangs [13] (Figs. 2 and 3) is not presently being eroded. They are likely older surfaces reflective of the slopes of the wind-eroded surfaces at the onset of yardang emergence.</p> <p><img src="" alt="" width="599" height="303" /></p> <p>Fig. 3. Drone view of CPP yardang field (DEM images obtained from higher altitude). Person for scale.</p> <p><strong>Summary and Conclusions</strong></p> <p>Field studies in the Puna have revealed that unique landforms emerge when wind is dominant. Regularly spaced features such as yardangs or ridges emerge out of the interaction between wind and bedrock. Sizes and spacings of features likely reflect rock hardness properties. The utility of simple, reconnaissance-style field campaigns is evident in the knowledge gained through our studies of the Puna.</p> <p><strong>References</strong></p> <p>[1] Blackwelder (1934), Yardangs. <em>GSA Bulletin</em> 45.</p> <p>[2] Ward (1979), Yardangs on Mars. JGR 8147-8166.</p> <p>[3] de Silva et al. (2013) Gravel-mantled megaripples of the Puna, <em>GSA Bulletin 125</em>.</p> <p>[4] Greeley & Iverson (1985), Threshold speeds on Venus, LPSC.</p> <p>[5] Kerber et al. (2011), Origin of Medusa Fossae Formation Mars, <em>Icarus 216</em>.</p> <p>[6] Paillou et al. (2016), Radar scattering of linear dunes and mega-yardangs on Titan, <em>Icarus</em> 270.</p> <p>[7] de Silva S. et al. (2010), Yardangs in terrestrial ignimbrites, <em>PSS 58</em>.</p> <p>[8] Pelletier et al. (2018), Yardang development <em>JGR 123</em>.</p> <p>[9] Dong et al. (2012), Yardangs in the Kumtaugh, <em>Geomorphology 139</em>.</p> <p>[10] Baez et al. (2020), Puna flow dynamics, <em>Bull. Volc</em>. 82.</p> <p>[11] Lorenz & Radebaugh (2016), Dust devils in thin air, <em>GRL 43</em>.</p> <p>[12] Kerber et al., in prog, Puna Yardang Observations.</p> <p>[13] Aulinas et al., (2015), Rock Varnish in Dusty Regions, <em>ESP 40</em>.</p>

Nearly half a century ago, two papers postulated the likelihood of lunar lava tube caves using mathematical models. Today, armed with an array of orbiting and fly-by satellites and survey instrumentation, we have now acquired cave data across our solar system-including the identification of potential cave entrances on the Moon, Mars, and at least nine other planetary bodies. These discoveries gave rise to the study of planetary caves. To help advance this field, we leveraged the expertise of an interdisciplinary group to identify a strategy to explore caves beyond Earth. Focusing primarily on astrobiology, the cave environment, geology, robotics, instrumentation, and human exploration, our goal was to produce a framework to guide this subdiscipline through at least the next decade. To do this, we first assembled a list of 198 science and engineering questions. Then, through a series of social surveys, 114 scientists and engineers winnowed down the list to the top 53 highest priority questions. This exercise resulted in identifying emerging and crucial research areas that require robust development to ultimately support a robotic mission to a planetary cave-principally the Moon and/or Mars. With the necessary financial investment and institutional support, the research and technological development required to achieve these necessary advancements over the next decade are attainable. Subsequently, we will be positioned to robotically examine lunar caves and search for evidence of life within Martian caves; in turn, this will set the stage for human exploration and potential habitation of both the lunar and Martian subsurface.

Introduction: Yardangs are wind-carved linear ridges that are found selectively on Earth and extensively on Mars and other planets [1-4]. The history of the morphologic development of yardangs is not well known, but is thought to depend on the action of wind on the surface and the material properties of the substrate being acted upon [5-8]. In the Puna high plateau of Argentina there are ignimbrite deposits, many of which have been eroded into yardang fleets. Yardangs fleets are composed of roughly tear-drop shaped, evenly-spaced ridges with noses that to face into the dominant wind, that together resemble a fleet of boats sailing. A prominent fleet, known as Campo de la Piedra Pomez (CCP), was studied during the December 2018 and 2019 field seasons to more fully understand their morphology and formation (Fig. 1) [6-10].