Early Mars climate research has well-defined goals (MEPAG
2018
). Achieving these goals requires geologists and climate modelers to coordinate. Coordination is easier if results are expressed in ...terms of well-defined parameters. Key parameters include the following quantitative geologic constraints. (1) Cumulative post-3.4 Ga precipitation-sourced water runoff in some places exceeded
1
km
column. (2) There is no single Early Mars climate problem: the traces of ≥2 river-forming periods are seen. Relative to rivers that formed earlier in Mars history, rivers that formed later in Mars history are found preferentially at lower elevations, and show a stronger dependence on latitude. (3) The duration of the longest individual river-forming climate was
>
(
10
2
–
10
3
)
yr
, based on paleolake hydrology. (4) Peak runoff production was
>
0.1
mm
/
hr
. However, (5) peak runoff production was intermittent, sustained (in a given catchment) for only <10% of the duration of river-forming climates. (6) The cumulative number of wet years during the valley-network-forming period was
>
10
5
yr
. (7) Post-Noachian light-toned, layered sedimentary rocks took
>
10
7
yr
to accumulate. However, (8) an “average” place on Mars saw water for
<
10
7
yr
after the Noachian, suggesting that the river-forming climates were interspersed with long globally-dry intervals. (9) Geologic proxies for Early Mars atmospheric pressure indicate pressure was not less than 0.012 bar but not much more than 1 bar. A truth table of these geologic constraints versus currently published climate models shows that the late persistence of river-forming climates, combined with the long duration of individual lake-forming climates, is a challenge for most models.
Water on Hot Rocky Exoplanets Kite, Edwin S.; Schaefer, Laura
Astrophysical journal. Letters,
03/2021, Letnik:
909, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
Data suggest that most rocky exoplanets with orbital period
p
< 100 days (“hot” rocky exoplanets) formed as gas-rich sub-Neptunes that subsequently lost most of their envelopes, but whether ...these rocky exoplanets still have atmospheres is unknown. We identify a pathway by which 1–1.7
R
⊕
(1–10
M
⊕
) rocky exoplanets with orbital periods of 10–100 days can acquire long-lived 10–2000 bar atmospheres that are H
2
O-dominated, with mean molecular weight >10. These atmospheres form during the planets’ evolution from sub-Neptunes into rocky exoplanets. H
2
O that is made by reduction of iron oxides in the silicate magma is highly soluble in the magma, forming a dissolved reservoir that is protected from loss so long as the H
2
-dominated atmosphere persists. The large size of the dissolved reservoir buffers the H
2
O atmosphere against loss after the H
2
has dispersed. Within our model, a long-lived, water-dominated atmosphere is a common outcome for efficient interaction between a nebula-derived atmosphere (peak atmosphere mass fraction 0.1–0.6 wt%) and oxidized magma (>5 wt% FeO), followed by atmospheric loss. This idea predicts that most rocky planets that have orbital periods of 10–100 days and that have radii within 0.1–0.2
R
⊕
of the lower edge of the radius valley still retain H
2
O atmospheres. This prediction is imminently testable with James Webb Space Telescope and has implications for the interpretation of data for transiting super-Earths.
Exoplanet secondary atmosphere loss and revival Kite, Edwin S.; Barnett, Megan N.
Proceedings of the National Academy of Sciences - PNAS,
08/2020, Letnik:
117, Številka:
31
Journal Article
Recenzirano
Odprti dostop
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–molecularweight 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.
Habitability of Exoplanet Waterworlds Kite, Edwin S.; Ford, Eric B.
Astrophysical journal/The Astrophysical journal,
09/2018, Letnik:
864, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Many habitable zone (HZ) exoplanets are expected to form with water mass fractions higher than that of the Earth. For rocky exoplanets with 10-1000× Earth's H2O but without H2, we model the multi-Gyr ...evolution of ocean temperature and chemistry, taking into account C partitioning, high-pressure ice phases, and atmosphere-lithosphere exchange. Within our model, for Sun-like stars, we find that: (1) the duration of habitable surface water is strongly affected by ocean chemistry; (2) possible ocean pH spans a wide range; (3) surprisingly, many waterworlds retain habitable surface water for >1 Gyr, and (contrary to previous claims) this longevity does not necessarily involve geochemical cycling. The key to this cycle-independent planetary habitability is that C exchange between the convecting mantle and the water ocean is curtailed by seafloor pressure on waterworlds, so the planet is stuck with the ocean mass and ocean cations that it acquires during the first 1% of its history. In our model, the sum of positive charges leached from the planetary crust by early water-rock interactions is-coincidentally-often within an order of magnitude of the early-acquired atmosphere+ocean inorganic C inventory overlaps. As a result, pCO2 is frequently in the "sweet spot" (0.2-20 bar) for which the range of semimajor axis that permits surface liquid water is about as wide as it can be. Because the width of the HZ in the semimajor axis defines (for Sun-like stars) the maximum possible time span of surface habitability, this effect allows for Gyr of habitability as the star brightens. We illustrate our findings by using the output of an ensemble of N-body simulations as input to our waterworld evolution code. Thus (for the first time in an end-to-end calculation) we show that chance variation of initial conditions, with no need for geochemical cycling, can yield multi-Gyr surface habitability on waterworlds.
ABSTRACT We provide estimates of atmospheric pressure and surface composition on short-period, rocky exoplanets with dayside magma pools and silicate-vapor atmospheres. Atmospheric pressure tends ...toward vapor-pressure equilibrium with surface magma, and magma-surface composition is set by the competing effects of fractional vaporization and surface-interior exchange. We use basic models to show how surface-interior exchange is controlled by the planet's temperature, mass, and initial composition. We assume that mantle rock undergoes bulk melting to form the magma pool, and that winds flow radially away from the substellar point. With these assumptions, we find that: (1) atmosphere-interior exchange is fast when the planet's bulk-silicate FeO concentration is low, and slow when the planet's bulk-silicate FeO concentration is high; (2) magma pools are compositionally well mixed for substellar temperatures 2400 K, but compositionally variegated and rapidly variable for substellar temperatures 2400 K; (3) currents within the magma pool tend to cool the top of the solid mantle ("tectonic refrigeration"); (4) contrary to earlier work, many magma planets have time-variable surface compositions.
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 10⁶-y average power output of the tiger stripes is buffered by a feedback between ice melt-back and subsidence to O(1010) 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.
Planets with 2 R⊕ < R < 3 R⊕ and orbital period <100 days are abundant; these sub-Neptune exoplanets are not well understood. For example, Kepler sub-Neptunes are likely to have deep magma oceans in ...contact with their atmospheres, but little is known about the effect of the magma on the atmosphere. Here we study this effect using a basic model, assuming that volatiles equilibrate with magma at T ∼ 3000 K. For our Fe-Mg-Si-O-H model system, we find that chemical reactions between the magma and the atmosphere and dissolution of volatiles into the magma are both important. Thus, magma matters. For H, most moles go into the magma, so the mass target for both H2 accretion and H2 loss models is weightier than is usually assumed. The known span of magma oxidation states can produce sub-Neptunes that have identical radius but with total volatile masses varying by 20-fold. Thus, planet radius is a proxy for atmospheric composition but not for total volatile content. This redox diversity degeneracy can be broken by measurements of atmosphere mean molecular weight. We emphasize H2 supply by nebula gas, but also consider solid-derived H2O. We find that adding H2O to Fe probably cannot make enough H2 to explain sub-Neptune radii because >103 km thick outgassed atmospheres have high mean molecular weight. The hypothesis of magma-atmosphere equilibration links observables such as atmosphere H2O/H2 ratio to magma FeO content and planet formation processes. Our model's accuracy is limited by the lack of experiments (lab and/or numerical) that are specific to sub-Neptunes; we advocate for such experiments.
Mars' wet‐to‐dry transition is a major environmental catastrophe, yet the spatial pattern, tempo, and cause of drying are poorly constrained. We built a globally distributed database of constraints ...on Mars late‐stage paleolake size relative to catchment area (aridity index (AI)), and found evidence for climate zonation as Mars was drying out. Aridity increased over time in southern midlatitude highlands, where lakes became proportionally as small as in modern Nevada. Meanwhile, intermittently wetter climates persisted in equatorial and northern‐midlatitude lowlands. This is consistent with a change in Mars' greenhouse effect that left highlands too cold for liquid water except during a brief melt season, or alternatively with a fall in Mars' groundwater table. The data are consistent with a switch of unknown cause in the dependence of AI on elevation, from high‐and‐wet early on, to high‐and‐dry later. These results sharpen our view of Mars' climate as surface conditions became increasingly stressing for life.
Plain Language Summary
Mars' surface was habitable in the past but is sterile today. Mars had multiple lake‐forming eras as the planet dried out, but so far, there has been no globally distributed survey of the size of late‐stage lakes, and the evaporation/precipitation ratio (aridity index (AI)) of the climates that formed them. This is key input/test data for models of Mars' past climate and climate evolution. We built a globally distributed database of AI constraints for late‐stage river‐forming climates on Mars. On average, late‐stage lake‐forming climates had a higher aridity than early‐stage river‐forming climates. Drying‐out was spatially heterogenous, with a “high‐and‐dry” pattern. This apparently contrasts with a “high‐and‐wet” pattern seen for early‐stage river‐forming climates. The reasons for this apparent switch are unknown.
Key Points
Aridity on Mars increased over time, but intermittently wetter climates persisted in lowlands
Consistent with a change in Mars' greenhouse effect that left highlands too cold for liquid water except during a brief melt season
Data are consistent with switch, of unknown cause, in dependence of aridity index on elevation: high‐and‐wet early on, high‐and‐dry later
Habitable rocky planets around M dwarfs that have H2-dominated atmospheres, if they exist, would permit characterizing habitable exoplanets with detailed spectroscopy using JWST, owing to their ...extended atmospheres and small stars. However, the H2-dominated atmospheres that are consistent with habitable conditions cannot be too massive, and a moderate-sized H2-dominated atmosphere will lose mass to irradiation-driven atmospheric escape on rocky planets around M dwarfs. We evaluate volcanic outgassing and serpentinization as two potential ways to supply H2 and form a steady-state H2-dominated atmosphere. For rocky planets of 1–7 M⊕ and early-, mid-, and late M-type dwarfs, the expected volcanic outgassing rates from a reduced mantle fall short of the escape rates by > ∼ 1 order of magnitude, and a generous upper limit of the serpentinization rate is still less than the escape rate by a factor of a few. Special mechanisms that may sustain the steady-state H2-dominated atmosphere include direct interaction between liquid water and mantle, heat-pipe volcanism from a reduced mantle, and hydrodynamic escape slowed down by efficient upper-atmospheric cooling. It is thus unlikely to find moderate-size, H2-dominated atmospheres on rocky planets of M dwarfs that would support habitable environments.
The upcoming launch of the James Webb Space Telescope means that we will soon have the capability to characterize the atmospheres of rocky exoplanets. However, it is still unknown whether such ...planets orbiting close to M dwarf stars can retain their atmospheres, or whether high-energy irradiation from the star will strip the gaseous envelopes from these objects. We present a new method to detect an atmosphere on a synchronously rotating rocky exoplanet around a K/M dwarf, by using thermal emission during secondary eclipse to infer a high dayside albedo that could only be explained by bright clouds. Based on calculations for plausible surface conditions, we conclude that a high albedo could be unambiguously interpreted as a signal of an atmosphere for planets with substellar temperatures of Tsub = 410-1250 K. This range corresponds to equilibrium temperatures of Teq = 300-880 K. We compare the inferred albedos of eight possible planet surface compositions to cloud albedo calculations. We determine that a layer of clouds with optical depths greater than τ = 0.5-7, would have high enough albedos to be distinguishable from a bare rock surface. This method of detecting an atmosphere on a rocky planet is complementary to existing methods for detecting atmospheres, because it provides a way to detect atmospheres with pressures below 1 bar (e.g., Mars), which are too tenuous to transport significant heat but thick enough to host high-albedo clouds.
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