Narrow range of early habitable Venus scenarios permitted by modeling of oxygen loss and radiogenic argon degassing.

Whether Venus was ever habitable is a key question driving missions to Earth's sister planet in the next decade. Venus today has a dry, O2-poor atmosphere, but recent work has proposed that early Venus may have had liquid water [J. Krissansen-Totton, J. J. Fortney, F. Nimmo, Planet. Sci. J. 2, 216 (2021)] and reflective clouds that could have sustained habitable conditions until 0.7 Ga [J. Yang, G. Boué, D. C. Fabrycky, D. S. Abbot, Astrophys. J. 787, L2 (2014), M. J. Way, A. D. Del Genio, J. Geophys. Res.: Planets 125, e2019JE006276 (2020)]. Water present at the end of a habitable era must since have been lost by photodissociation and H escape, causing buildup of atmospheric oxygen [F. Tian, Earth Planet. Sci. Lett. 432, 126-132 (2015)]. We present a time-dependent model of Venus's atmospheric composition starting from the end of a hypothetical habitable era with surface liquid water. We find that O2 loss to space, oxidation of reduced atmospheric species, oxidation of lava, and oxidation of a surface magma layer formed in a runaway greenhouse climate can remove O2 from up to 500 m global equivalent layer (GEL) (30% of an Earth ocean), unless melts on Venus had a much lower oxygen fugacity than Mid Ocean Ridge melts on Earth, which increases the upper limit twofold. Volcanism is required to supply oxidizable fresh basalt and reduced gases to the atmosphere but also contributes 40Ar. Consistency with Venus's modern atmospheric composition occurs in less than 0.4% of runs, in a narrow parameter range where the reducing power introduced by O2 loss processes can balance O2 introduced by H escape. Our models favor hypothetical habitable eras ending before 3 Ga and very reduced melt oxygen fugacities three log units below the fayalite-magnetite-quartz buffer (fO2< FMQ-3), among other constraints.

Changing spatial distribution of water flow charts major change in Mars's greenhouse effect.

Early Mars had rivers, but the cause of Mars's wet-to-dry transition remains unknown. Past climate on Mars can be probed using the spatial distribution of climate-sensitive landforms. We analyzed global databases of water-worked landforms and identified changes in the spatial distribution of rivers over time. These changes are simply explained by comparison to a simplified meltwater model driven by an ensemble of global climate model simulations, as the result of ≳10 K global cooling, from global average surface temperature [Formula: see text] ≥ 268 K to [Formula: see text] ~ 258 K, due to a weaker greenhouse effect. In other words, river-forming climates on early Mars were warm and wet first, and cold and wet later. Unexpectedly, analysis of the greenhouse effect within our ensemble of global climate model simulations suggests that this shift was primarily driven by waning non-CO2 radiative forcing, and not changes in CO2 radiative forcing.

Warm early Mars surface enabled by high-altitude water ice clouds.

Despite receiving just 30% of the Earth's present-day insolation, Mars had water lakes and rivers early in the planet's history, due to an unknown warming mechanism. A possible explanation for the >102-y-long lake-forming climates is warming by water ice clouds. However, this suggested cloud greenhouse explanation has proved difficult to replicate and has been argued to require unrealistically optically thick clouds at high altitudes. Here, we use a global climate model (GCM) to show that a cloud greenhouse can warm a Mars-like planet to global average annual-mean temperature ([Formula: see text]) ∼265 K, which is warm enough for low-latitude lakes, and stay warm for centuries or longer, but only if the planet has spatially patchy surface water sources. Warm, stable climates involve surface ice (and low clouds) only at locations much colder than the average surface temperature. At locations horizontally distant from these surface cold traps, clouds are found only at high altitudes, which maximizes warming. Radiatively significant clouds persist because ice particles sublimate as they fall, moistening the subcloud layer so that modest updrafts can sustain relatively large amounts of cloud. The resulting climates are arid (area-averaged surface relative humidity ∼25%). In a warm, arid climate, lakes could be fed by groundwater upwelling, or by melting of ice following a cold-to-warm transition. Our results are consistent with the warm and arid climate favored by interpretation of geologic data, and support the cloud greenhouse hypothesis.

Exoplanet secondary atmosphere loss and revival.

The next step on the path toward another Earth is to find atmospheres similar to those of Earth and Venus-high-molecular-weight (secondary) atmospheres-on rocky exoplanets. Many rocky exoplanets are born with thick (>10 kbar) H2-dominated atmospheres but subsequently lose their H2; this process has no known Solar System analog. We study the consequences of early loss of a thick H2 atmosphere for subsequent occurrence of a high-molecular-weight atmosphere using a simple model of atmosphere evolution (including atmosphere loss to space, magma ocean crystallization, and volcanic outgassing). We also calculate atmosphere survival for rocky worlds that start with no H2 Our results imply that most rocky exoplanets orbiting closer to their star than the habitable zone that were formed with thick H2-dominated atmospheres lack high-molecular-weight atmospheres today. During early magma ocean crystallization, high-molecular-weight species usually do not form long-lived high-molecular-weight atmospheres; instead, they are lost to space alongside H2 This early volatile depletion also makes it more difficult for later volcanic outgassing to revive the atmosphere. However, atmospheres should persist on worlds that start with abundant volatiles (for example, water worlds). Our results imply that in order to find high-molecular-weight atmospheres on warm exoplanets orbiting M-stars, we should target worlds that formed H2-poor, that have anomalously large radii, or that orbit less active stars.

Persistence of intense, climate-driven runoff late in Mars history.

Mars is dry today, but numerous precipitation-fed paleo-rivers are found across the planet's surface. These rivers' existence is a challenge to models of planetary climate evolution. We report results indicating that, for a given catchment area, rivers on Mars were wider than rivers on Earth today. We use the scale (width and wavelength) of Mars paleo-rivers as a proxy for past runoff production. Using multiple methods, we infer that intense runoff production of >(3-20) kg/m2 per day persisted until <3 billion years (Ga) ago and probably <1 Ga ago, and was globally distributed. Therefore, the intense runoff production inferred from the results of the Mars Science Laboratory rover was not a short-lived or local anomaly. Rather, precipitation-fed runoff production was globally distributed, was intense, and persisted intermittently over >1 Ga. Our improved history of Mars' river runoff places new constraints on the unknown mechanism that caused wet climates on Mars.

Sustained eruptions on Enceladus explained by turbulent dissipation in tiger stripes.

Spacecraft observations suggest that the plumes of Saturn's moon Enceladus draw water from a subsurface ocean, but the sustainability of conduits linking ocean and surface is not understood. Observations show eruptions from "tiger stripe" fissures that are sustained (although tidally modulated) throughout each orbit, and since the 2005 discovery of the plumes. Peak plume flux lags peak tidal extension by ∼1 rad, suggestive of resonance. Here, we show that a model of the tiger stripes as tidally flexed slots that puncture the ice shell can simultaneously explain the persistence of the eruptions through the tidal cycle, the phase lag, and the total power output of the tiger stripe terrain, while suggesting that eruptions are maintained over geological timescales. The delay associated with flushing and refilling of O(1)-m-wide slots with ocean water causes erupted flux to lag tidal forcing and helps to buttress slots against closure, while tidally pumped in-slot flow leads to heating and mechanical disruption that staves off slot freezeout. Much narrower and much wider slots cannot be sustained. In the presence of long-lived slots, the 10(6)-y average power output of the tiger stripes is buffered by a feedback between ice melt-back and subsidence to O(10(10)) W, which is similar to observed power output, suggesting long-term stability. Turbulent dissipation makes testable predictions for the final flybys of Enceladus by Cassini Our model shows how open connections to an ocean can be reconciled with, and sustain, long-lived eruptions. Turbulent dissipation in long-lived slots helps maintain the ocean against freezing, maintains access by future Enceladus missions to ocean materials, and is plausibly the major energy source for tiger stripe activity.

Optical images of an exosolar planet 25 light-years from Earth.

Fomalhaut, a bright star 7.7 parsecs (25 light-years) from Earth, harbors a belt of cold dust with a structure consistent with gravitational sculpting by an orbiting planet. Here, we present optical observations of an exoplanet candidate, Fomalhaut b. Fomalhaut b lies about 119 astronomical units (AU) from the star and 18 AU of the dust belt, matching predictions of its location. Hubble Space Telescope observations separated by 1.73 years reveal counterclockwise orbital motion. Dynamical models of the interaction between the planet and the belt indicate that the planet's mass is at most three times that of Jupiter; a higher mass would lead to gravitational disruption of the belt, matching predictions of its location. The flux detected at 0.8 mum is also consistent with that of a planet with mass no greater than a few times that of Jupiter. The brightness at 0.6 mum and the lack of detection at longer wavelengths suggest that the detected flux may include starlight reflected off a circumplanetary disk, with dimension comparable to the orbits of the Galilean satellites. We also observe variability of unknown origin at 0.6 mum.