Exoplanet sub-Neptunes are nearly as common as stars, but their nature remains uncertain. Sub-Neptunes orbit close to their host stars, and many are apparently shrouded in large and hot hydrogen-dominated atmospheres. Harsh irradiation conditions and the presence of greenhouse gases enhance the melting of the underlying silicate surface and promote the exchange of volatiles between the atmosphere and a magma ocean. Here we show that magma oceans can dissolve ≥2 wt% hydrogen at typical conditions relevant to the interface between the atmosphere and the condensed surface in such planets. The influx of hydrogen into the molten silicate lowers magma density, which alters the sub-Neptune radius-mass relationship (equation of state). Moreover, the hydrogen influx changes the redox state of the magma ocean (which becomes reduced), and generates a massive outflux of oxygen, which reacts with atmospheric hydrogen to form large amounts of water vapor. Thus, the release of oxygen will profoundly change the atmospheric chemistry and bear spectral signatures observable from space telescopes. These atmospheric-composition changes are a testable signature of hidden magma oceans in exoplanets. The uptake of volatiles by the magma implies that the interiors of sub-Neptunes are more volatile-rich than once thought. Moreover, our atomistic simulations suggest that the dissolution and evaporation process is chemically complex, forming hundreds of molecular species in the atmosphere. These magma-buffered atmospheric recipes will need to be accounted for in future studies addressing models of exoplanet atmosphere origins.

An open question in exoplanet science is whether rocky exoplanets extremely close in to their host stars, `lava worlds', can retain significant atmospheres. Thermal emission observations via secondary eclipse can be used to determine whether a rocky exoplanet possesses an atmosphere. It is expected that an atmosphere could increase the planet's albedo and/or redistribute heat away from the dayside, reducing the secondary eclipse depth measured. Recent eclipse observations of lava planets, rocky planets hot enough to have liquid magma surfaces, have suggested the presence of atmospheres. Here, we present a single JWST MIRI/LRS partial secondary eclipse of the lava planet TOI-431b. We measure an eclipse depth of 81 ± 16 ppm, which corresponds to a brightness temperature of 1967^+237_-253 K and a brightness temperature ratio R = 0.81±0.10, being 1.2 σ higher than the value reported by Spitzer. The observed brightness temperature ratio is 1.8σ below that of a zero-albedo, zero heat redistribution bare rock (R=1). Given that magma pools are expected to have low albedos, we find that our results are best explained by the presence of an atmosphere. Future work to measure the exact composition of TOI-431b's atmosphere and improve models of lava planet atmospheres would better constrain the nature and evolution of lava planets.

The basal strata of the orbitally defined Layered Sulfate unit (LSu) in Gale crater, Mars, marks the end of a major mineralogical transition from clay-to sulfate-bearing stratigraphy. This has previously been attributed to a period of significant aridification of Gale's climate. Here, we present ground-based observations by the Curiosity rover of the sedimentary facies and architecture of the basal strata of the LSu, and reconstruct the past depositional environments. The succession predominantly comprises four members of the Mirador formation: the Contigo, Catrimani, Amapari, and Chenapau members. The Contigo member records the migration of simple aeolian dunes. The Catrimani and Chenapau members record a facies characteristic of wind-ripple migration in aeolian sand sheets. Periodic damp sand sheet deposition is inferred through candidate adhesion structures and evaporite crystals. Evidence for periodic surface water occurs as lacustrine deposits (the Amapari member) and an embedded fluvial lens. Near-surface water likely restricted the availability of loose sediment, suppressing dune formation and promoting the accumulation of a thick sandsheet succession that, at similar to 60 m, exceeds known terrestrial equivalents. Deposition under these conditions would have required a continuously rising water table, but the rate of sediment accumulation was likely high enough for a predominantly aeolian environment. Contrary to earlier aridification models for the LSu, this basal succession likely records an overall aeolian environment controlled by a rising water table in Gale crater during the Hesperian.

In situ and orbital observations have suggested that Jezero crater (field site for the Mars 2020 Perseverance rover) once hosted a paleolake fed by two inlet valleys. An outlet valley, Pliva Vallis, is present on the eastern rim of the crater and raises the question of whether the lake system operated as an open basin or as a closed basin system with one or more overflow events. To tackle this uncertainty, we present a detailed morphological study of Pliva Vallis, using digital elevation models and imagery. The atypical morphology of the valley, including reincised fluvial layered deposits, a perched valley, and bedrock incision terraces, led us to interpret that Pliva Vallis was formed by discontinuous and episodic flows rather than from a steady outlet river. These observations allowed us to infer at least four main breach episodes and propose a new scenario for the evolution of the lake system over time. We give a minimum estimate of the duration of these overflow events using a 0-D model, simulating a valley formation by breach erosion and lake overflow. Modeling results suggest that each flood event causing a part of the incision of the outlet valley would not have lasted for more than a few weeks, and some may have lasted only a few days. These time scales are consistent with our morphological interpretation that the outlet valley was carved by discontinuous and temporary flows. In this scenario, Jezero lake was predominantly a closed basin, spilling over episodically in at least four breach events.

Ancient Mars had surface liquid water and a dense carbon dioxide (CO2)-rich atmosphere. Such an atmosphere would interact with crustal rocks, potentially leaving a mineralogical record of its presence. We analyzed the composition of an 89-meter stratigraphic section of Gale crater, Mars, using data collected by the Curiosity rover. An iron carbonate mineral, siderite, occurs in abundances of 4.8 to 10.5 weight %, colocated with highly water-soluble salts. We infer that the siderite formed in water-limited conditions, driven by water-rock reactions and evaporation. Comparison with orbital data indicates that similar strata (deposited globally) sequestered the equivalent of 2.6 to 36 millibar of atmospheric CO2. The presence of iron oxyhydroxides in these deposits indicates that a partially closed carbon cycle on ancient Mars returned some previously sequestered CO2 to the atmosphere.

We present observations of the 1.35 ± 0.07 Earth radius planet L 98-59 c, collected using Wide Field Camera 3 on the Hubble Space Telescope (HST). L 98-59 is a nearby (10.6 pc), bright ( H = 7.4 mag) M3V star that harbors three small, transiting planets. As one of the closest known transiting multi-planet systems, L 98-59 offers one of the best opportunities to probe and compare the atmospheres of rocky planets that formed in the same stellar environment. We measured the transmission spectrum of L 98-59 c, and the extracted spectrum showed marginal evidence (2.1 σ ) for wavelength-dependent transit depth variations that could indicate the presence of an atmosphere. We forward-modeled possible atmospheric compositions of the planet based on the transmission spectrum. Although L 98-59 was previously thought to be a fairly quiet star, we have seen evidence for stellar activity, and therefore we assessed a scenario where the source of the signal originates with inhomogeneities on the stellar surface. We also see a correlation between transits of L 98-59 c and L 98-59 b collected 12.5 hr apart, which is suggestive (but at <2 σ confidence) of a contaminating component from the star impacting the exoplanet spectrum. While intriguing, our results are inconclusive and additional data are needed to verify any atmospheric signal. Fortunately, additional data have been collected from both the HST and James Webb Space Telescope. Should this result be confirmed with additional data, L 98-59 c would be the first planet smaller than 2 Earth radii with a detected atmosphere.

Evidence of paleo-rivers, fans, deltas, lakes, and channel networks across Mars has prompted much debate about what climate conditions would permit the formation of these surface water derived features. Pediments, gently sloping erosional surfaces of low relief developed in bedrock, have also been identified on Mars. On Earth, these erosional landforms, often thought to be created by overland flow and shallow channelized runoff, are typically capped by fluvial sediments, and thus in exceptionally arid regions, pediments are interpreted to record past wet periods. Here we document the Greenheugh pediment in Gale crater, exploiting the observational capability of the Curiosity rover. The absence of a fluvial cap suggests that the pediment was likely cut by wind erosion, not fluvial processes. The pediment was then buried by an aeolian deposit (Stimson sandstone) that mantled the lower footslopes of Aeolis Mons (informally known as Mt. Sharp). This burial terminated active wind erosion, preserving the pediment surface (as an angular unconformity). Groundwater was present prior-to, during, and shortly after Stimson deposition, perhaps contributing to lithification and certainly to early diagenesis. Post lithification, wind erosion first cut canyons in the northern most footslopes (north of Vera Rubin ridge). Unlithified gravels were deposited in these canyons, likely due to runoff from Mt. Sharp. Boulder-rich fluvial and debris flow deposits built a > 70 m thick sequence (Gediz Vallis ridge) on the southern Greenheugh pediment. Continued wind erosion left elevated patches of gravel on the northern footslopes, and exposure age dating shows that erosion essentially ceased before 1 Ga (but possibly much earlier). Erosion to the south led to emergence of Vera Rubin ridge, retreat of the Greenheugh pediment, and the formation of Glen Torridon valley. Hence, this footslope environment of Mt. Sharp records climate-driven periods of wind erosion, aeolian deposition (and groundwater activity), surface runoff and sediment deposition, followed by further significant wind erosion that declined to present very slow rates. This likely occurred during the late Hesperian and possibly into the Amazonian.

We report first results from the JWST Rocky Worlds Director's Discretionary Time program. Two secondary eclipses of the terrestrial exoplanet GJ 3929b were recently observed using MIRI photometric imaging at 15 mu m. We present a reduction of these data using the updated SPARTA pipeline. We also refine the planet mass, radius, and predicted time of secondary eclipse using a new sector of TESS data and new, high-precision radial velocities from the MAROON-X spectrograph. For the two JWST observations, we recover secondary eclipse depths of 177( -45+4)7 ppm and 143( -35)(+34) ppm at times consistent with a nearly circular orbit, as expected from the radial velocity data. A joint fit of the two visits yields a dayside brightness temperature T-p,T-dayside = 782 +/- 79 K for GJ 3929b, which is consistent with the maximum brightness temperature T-max=737 +/- 14 K for a bare black rock (i.e., assuming zero Bond albedo and no heat redistribution). These results rule out CO2-rich atmospheres thicker than 100 mbar at >3 sigma, suggesting that GJ 3929b has lost any significant secondary atmosphere. The radial velocity data also indicate two additional nontransiting planets in the system: a previously identified planet in a 15.0 days orbit and a newly identified planet candidate in a 6.1 days orbit.

Early Mars was habitable, at least intermittently, but major questions remain about how much water flowed and for how long. The paleoclimate evolution of Mars is captured by the stratigraphic record in Gale crater (Milliken et al. 2010). Climbing through mostly aeolian deposits reflecting arid conditions within Gale crater, the Mars Science Laboratory Curiosity rover encountered wave-rippled lake sediments of the basin-spanning Amapari Marker Band (AMB) that have very high metal enrichments (Fe, Mn, Zn). What caused the association between relatively wet primary depositional environment, and metal enrichment? Tentative, but reasonable extrapolation of rover metal data across the AMB suggests an excess Fe mass of 0.2 Gt. Transporting this Fe likely required similar to 10,000 km(3) of water flow, much more than the volume of the lake, across >10(3) yr. Deposition of the Fe could be due to a redox or pH front within or just beneath the lake. One possible basin-scale synthesis involves a climate excursion consisting of initial cooling then subsequent warming: initial cooling permits wind scour in Gale basin and ice build-up on Gale's rim, while subsequent melting fills the lake and mobilizes Fe. Alternatively, the data can be explained by water-table fluctuations. In either case, the metal enrichment likely contributed to the hardness of these rocks, aiding wave-ripple preservation.

The Mars Science Laboratory (MSL) Curiosity rover continues to ascend Aeolis Mons in Gale crater, Mars, with the goal of characterising formerly habitable palaeoenvironments. Since September 2022, Curiosity has been traversing Gediz Vallis, a ~9-km long canyon incising into sulfate-bearing, sedimentary rocks on the northern margins of Aeolis Mons. Along the Gediz Vallis floor is the upper Gediz Vallis Ridge (uGVR), a quasi-sinuous, ~1.5 km long, ~80-100 m wide, ~5-30 m high, ridge. Upslope, uGVR is clearly set within an erosional channel, which disappears downslope. Near the Gediz Vallis outlet, uGVR transitions into the broader, lower GVR, recently interpreted by Bryk et al. (2023, AGU Fall Meeting) as a degraded alluvial fan. Since entering Gediz Vallis, Curiosity has undertaken an extensive long-distance imaging campaign of the eastern uGVR flank, acquiring multiple Mastcam and ChemCam Long Distance Remote Micro Imager (LD-RMI) mosaics. Additionally, in August 2023, Curiosity approached the ridge margins and conducted an in-situ investigation (“Region B”). A major objective of Curiosity’s uGVR campaign is to the determine the primary depositional conditions and palaeoenvironment of the ridge, which may record evidence for late-stage surface water flow in Gale. Thus far, most uGVR exposures observed are formed of loosely consolidated, very poorly sorted, decimeter to meter-scale blocks. Most blocks are dark in tone and some are partially embedded within a finer-grained, matrix-like material. The blocks themselves are reworked, lithified, sedimentary rocks and many display a diversity of internal planar and/or cross-stratification, although others appear more massive. Where visible, bedding within the blocks is typically mm to cm in thickness. We used the Pro3D software package to measure mean diameter size of 70 blocks at Region B from stereo Mastcam images: 0.15±0.11 m; although we note that the largest block (~ 7 m diameter) occurs elsewhere. We also note that many blocks are fractured into multiple pieces, potentially due to post-depositional weathering processes. There is no obvious source bedrock from within the ridge that the blocks could be eroding from, consistent with the blocks being transported clasts, rather than erosional lag originating from in-situ bedrock. Generally, the clasts are very angular to sub-rounded, suggesting relatively limited transport and a local source bedrock. The large clast size (cobble, boulder) and very poor sorting is consistent with deposition by debris flows: gravity-driven flows in which clasts are supported by a cohesive muddy matrix. The confinement of the uGVR to a channel argues against a completely unconfined flow, such as a drier landslide, forming the deposit. Post-depositional erosion has likely winnowed much of the finer grained fraction of the deposit. The sedimentary structures within the clasts (e.g., asymptotic cross-stratification) are similar to those in the Stimson formation, an aeolian sandstone, pointing to a potential upslope extension of Stimson, consistent with recent long-distance observations, though multiple sources are also possible. If our interpretation is correct, this would demonstrate that uGVR is providing access to lithologies transported from higher up Aeolis Mons.

The “cosmic shoreline,” a semi-empirical relation that separates airless worlds from worlds with atmospheres as proposed by K. J. Zahnle & D. C. Catling, is now guiding large-scale JWST surveys aimed at detecting rocky exoplanet atmospheres. We expand upon this framework by revisiting the shoreline using existing hydrodynamic escape models applied to Earth-like, Venus-like, and steam atmospheres for rocky exoplanets, and we estimate energy-limited escape rates for CH _4 atmospheres. We determine the critical instellation required for atmospheric retention by calculating time-integrated atmospheric mass loss. Our analysis introduces a new metric for target selection in the Rocky Worlds Director’s Discretionary Time and refines expectations for rocky planet atmosphere searches. Exploring initial volatile inventory ranging from 0.01% to 1% of planetary mass, we find that its variation prevents the definition of a unique clear-cut shoreline, though nonlinear escape physics can reduce this sensitivity to initial conditions. Additionally, uncertain distributions of high-energy stellar evolution and planet age further blur the critical instellations for atmospheric retention, yielding broad shorelines. Hydrodynamic escape models find atmospheric retention is markedly more favorable for higher-mass planets orbiting higher-mass stars, with carbon-rich atmospheres remaining plausible for 55 Cancri e despite its extreme instellation. We caution that our estimates are sensitive to processes with poorly understood dynamics, such as atomic line cooling. Finally, we illustrate how density measurements can be used to statistically test the existence of the cosmic shorelines, emphasizing the need for more precise mass and radius measurements.

Determining which rocky exoplanets have atmospheres, and why, is a key goal for the James Webb Space Telescope. So far, emission observations of individual rocky exoplanets orbiting M stars (M-Earths) have not provided definitive evidence for atmospheres. Here, we synthesize emission data for M-Earths and find a trend in measured brightness temperatures (ratioed to its theoretical maximum value) as a function of instellation. However, the statistical evidence of this trend is dependent on the choice of stellar model, and we consider its identification tentative. We show that this trend can be explained by either the onset of thin/tenuous (<1 bar) CO2-rich atmospheres on colder worlds, or a population of bare rocks with stronger space weathering and/or coarser regolith on closer-in worlds. Such grain coarsening may be caused by sintering near the melting point of rock or frequent volcanic resurfacing. Furthermore, we highlight considerations when testing rocky planet hypotheses at the population level, including the choice of instrument, stellar modeling, and how brightness temperatures are derived. We also find that fresh (unweathered) fine-grained surfaces can serve as a false positive to the detection of moderate atmospheric heat redistribution through eclipse observations. However, we argue that such surfaces are unlikely given the ubiquity of space weathering in the Solar System, the low albedo of Solar System airless bodies, and the high stellar wind environments of M-Earths. Emission data from a larger sample of M-Earths will be able to confirm or reject this tentative trend and diagnose its cause through spectral characterization.

Symmetrical wave ripples identified with NASA’s Curiosity rover in ancient lake deposits at Gale crater provide a key paleoclimate constraint for early Mars: At the time of ripple formation, climate conditions must have supported ice-free liquid water on the surface of Mars. These features are the most definitive examples of wave ripples on another planet. The ripples occur in two stratigraphic intervals within the orbitally defined Layered Sulfate Unit: a thin but laterally extensive unit at the base of the Amapari member of the Mirador formation, and a sandstone lens within the Contigo member of the Mirador formation. In both locations, the ripples have an average wavelength of ~4.5 centimeters. Internal laminae and ripple morphology show an architecture common in wave-influenced environments where wind-generated surface gravity waves mobilize bottom sediment in oscillatory flows. Their presence suggests formation in a shallow-water (<2 meters) setting that was open to the atmosphere, which requires atmospheric conditions that allow stable surface water.

Carbonate-silicate weathering feedback is thought to stabilize Earth's climate on geologic timescales. If climate warms, faster mineral dissolution and increased rainfall speed up weathering, increasing CO2 drawdown and opposing the initial warming. Limits to where this feedback might operate on terrestrial exoplanets with N2-O2-CO2-H2O atmospheres are used to define the 'habitable zone'-the range of orbits around a star where liquid water can be stable on a planet's surface. However, the impacts on long-term habitability of randomly varying volcanic outgassing, tectonic collisions, and tectonic parameters (e.g., number of continental plates, size of plates, plate velocity) remain poorly understood. In this work, we present an idealized and broadly-applicable quasi-2D model of the long-term climate stability of abiotic Earth-twins. The model tracks atmospheric CO2 as 'disks' collide, promoting uplift and supplying new weatherable minerals through erosion. Without resupply, soils become less weatherable and the feedback's strength wanes, making a planet susceptible to catastrophic warming events or hard snowballs where the surface becomes frozen over. We find that tectonic uplift spurred by continental collisions cannot be the sole supplier of weatherable minerals within our model framework, as such climates either become uninhabitably hot (for complex life) as soils become leached of weatherable minerals or experience extreme swings in temperature over short timescales. This conclusion is strengthened when taking into account the destabilizing effects of outgassing variability and increasing stellar luminosity. In addition to frequent collisions, other resupply mechanisms for weatherable minerals, such as wind-driven dust transport, glacial erosion, and/or seafloor weathering, are likely required for long-term stability on Earth-like terrestrial exoplanets.