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However, a convecting magma ocean with a vertical mixing scale of order the mantle thickness (3000km) could effectively remove all the atmospheric oxygen (on the order of several hundred bar) on Venus during its early runaway period. (1998) for solar metallicity. Conceptual image of water-bearing (left) and dry (right) exoplanets with oxygen-rich atmospheres. 1 and and8,8, noting that planets around M dwarfs remain in a runaway for 0.11Gyr, during which time XUV300 for a saturation time of 1Gyr. Diffusion through molecular oxygen is faster, but if the oxygen is photolyzed below the base of the hydrodynamic wind, it is diffusion through atomic oxygen that will bottleneck the escape. Most oxygen in a fledgling planet's atmosphere takes the form of water, or H 2 O. The vertical axis (stellar mass) ranges from late M dwarfs (0.08M) to early K dwarfs (0.9M). (2011b), UV transit observations of EUV-heated expanded thermospheres of Earth-like exoplanets around M-stars: testing atmosphere evolution scenarios, Lammer H., Erkaev N.V., Odert P., Kislyakova K.G., Leitzinger M., and Khodachenko M.L. I. Such a high oxidative power could have strong implications for planetary evolution and habitability, which we discuss in Section 6. This means that the rate of oxygen buildup is constant in time and does not vary with the XUV flux (provided XUV>crit). As we discuss in Section 2.3, this is probably an upper limit to the migration timescale for planets that form beyond the snow line. If that is the case, a potential mechanism for forming a wet, Earth-sized planet in the HZ is early formation beyond the snow line (the region of the circumstellar disk beyond which water and other volatiles are able to condense into ices and serve as building blocks for planets) followed by disk-driven migration into the HZ. 9. Chemical equilibrium between the atmosphere and a magma ocean during a runaway could lead to the buildup of hundreds of bar of CO2 early on (Elkins-Tanton, 2008). The code halts if (1) the age of the system reaches 5Gyr, (2) the planet enters the HZ (corresponding to the end of the runaway greenhouse phase), or (3) the planet is completely desiccated. Let us now consider expression (24), the rate of oxygen buildup in the atmosphere in the energy-limited regime. See Section 3 for a detailed description of our model. Such elevated quantities of O2 are possible throughout the HZs of all M dwarfs, except near the outer edge of those more massive than about 0.5M, where planets are in runaway greenhouses for only a few million years. The XUV luminosity of M dwarfs also evolves with time. Detecting Life's Influence on Planetary Atmospheres Many papers have explored the effects of a runaway greenhouse on Venus, arguing that it may have lost one or more Earth oceans of water as a consequence of an early runaway (Kasting et al., 1984; Kasting, 1988; Chassefire, 1996a, 1996b; Kulikov et al., 2006; Gillmann et al., 2009). Moreover, water is an essential ingredient for both plate tectonics (Mackwell et al., 1998; Moresi and Solomatov, 1998) and the carbonate-silicate cycle, which together regulate CO2 in the Earth's atmosphere. Planets that lose significant amounts of water also undergo extreme surface oxidation. On the other hand, whether these planets develop a magma ocean capable of absorbing most of the O2 is unclear, as they may not have rocky surfaces but instead a thick layer of water and ice extending down to great depths. The HZ of M dwarfs, on the other hand, moves in significantly, even after the formation of terrestrial planets. At each time step, we calculate the HZ limits for 1M and 5M planets from Kopparapu et al. Planets around K dwarfs (M0.6M), on the other hand, lose significant amounts of water only close to the RG limit. High surface temperatures could also inhibit plate tectonics by promoting rapid lithospheric healing and grain growth, increasing the viscosity and erasing weak zones where plate subduction can occur (e.g., Driscoll and Bercovici, 2013). 7, the corresponding energy-limited case. (2010), Transiting Exoplanet Survey Satellite (TESS) [abstract 450.06], Robertson P., Mahadevan S., Endl M., and Roy A. Water vapor and dust are also part of Earth 's atmosphere. For simplicity, we use a saturation time of 0.1Gyr for K dwarfs (M>0.6M) and 1Gyr for M dwarfs. The terrestrial planets in the Solar System are thought to have formed in situ between 10 and 100Myr after the formation of the Sun (Chambers, 2004; Kleine et al., 2009; Raymond et al., 2013). 14), the final O2 pressure begins to decrease. Note that while M dwarfs dim monotonically during their PMS contraction phase, stars of type K and earlier display a bump in their luminosity prior to 100Myr. It is important to note that since we have been plotting the O2 equivalent pressure rather than the actual amount, we must take care in comparing it between planets of different masses. For XUV fluxes below crit, the oxygen escape rate is zero, and the rate of buildup in the atmosphere scales linearly with the flux; one must calculate this directly from (24). Careers, Unable to load your collection due to an error. Above about 0.8M, the duration is negligible, except in the vicinity of the RG boundary; this is the case for planets around solar-type stars. We live at the bottom of an invisible ocean called the atmosphere, a layer of gases surrounding our planet. IIINaked T Tauri stars associated with the Taurus-Auriga complex, Watson A.J., Donahue T.M., and Walker J.C.G. Even after a planet leaves the runaway greenhouse, diffusion of water vapor into the stratosphere and H escape can still be significant; for instance, Wordsworth and Pierrehumbert (2014) show that about 28% of a TO can be lost from an N2-poor Earth in the HZ of a solar-type star over 4 Gyr. While Fig. Which Planet Has the Most Oxygen? 10, planets build up between 100 and 1000 bar of O2 in their atmospheres throughout most of the HZ of M dwarfs. Thus, planets that lose 1 TO of water build up 270=240 bar of O2 in their atmospheres. Solid lines correspond to the model adopted in this paper, where fluxes are calculated using the stellar evolution model of Baraffe et al. We have shown that during the early high-luminosity phase of M dwarfs, terrestrial planets can lose several Earth oceans of water. In fact, any Earth-mass planet that removes oxygen at a rate slower than 5 bar/Myr (25 bar/Myr for a super-Earth) will build up O2 in its atmosphere during the runaway phase and likely transition to diffusion-limited escape. 14, where we plot a cross section along M=0.4M in Fig. Studies by Raymond et al. This is due to the fact that the pressure at the emission level of a saturated atmosphere scales with surface gravity; at a higher pressure, the temperature of the emission level is higher and the planet is able to cool more effectively, delaying the runaway state (Pierrehumbert, 2010). Above 0.2M, planets close to the outer edge of the HZ retain some of their water due to the shorter runaway phase; however, even planets in the center of the HZ of high-mass M dwarfs (M0.4M) are completely desiccated. Is there oxygen on any other planet? We assumed that terrestrial planets form with abundant surface water. (2011), Ocean-like water in the Jupiter-family comet 103P/Hartley 2, Stellar evolution in early phases of gravitational contraction, Hedelt P., von Paris P., Godolt M., Gebauer S., Grenfell J.L., Rauer H., Schreier F., Selsis F., and Trautmann T. (2013), Spectral features of Earth-like planets and their detectability at different orbital distances around F, G, and K-type stars, Runaway greenhouse effect on exomoons due to irradiation from hot, young giant planets, Howell S.B., Sobeck C., Haas M., Still M., Barclay T., Mullally F., Troeltzsch J., Aigrain S., Bryson S.T., Caldwell D., Chaplin W.J., Cochran W.D., Huber D., Marcy G.W., Miglio A., Najita J.R., Smith M., Twicken J.D., and Fortney J.J. (2014), The K2 mission: characterization and early results, Publications of the Astronomical Society of the Pacific, The escape of light gases from planetary atmospheres, Hunten D.M., Pepin R.O., and Walker J.C.G. VDOM DHTML tml>. (2013), XUV-exposed, non-hydrostatic hydrogen-rich upper atmospheres of terrestrial planets. The atmospheres and interiors of the four giant planets -- Jupiter, Saturn, Uranus and Neptune -- are thought to contain enormous quantities of the wet stuff, and their moons and rings have substantial water ice. We therefore set the hydrogen escape flux to the minimum of the energy-limited escape flux (7) and the diffusion-limited escape flux (13). (2007), Roche lobe effects on the atmospheric loss from hot Jupiters., Erkaev N.V., Lammer H., Odert P., Kulikov Y.N., Kislyakova K.G., Khodachenko M.L., Gdel M., Hanslmeier A., and Biernat H. (2013), XUV-exposed, non-hydrostatic hydrogen-rich upper atmospheres of terrestrial planets. It belongs to a class of planets called "hot Jupiters." Astronomers previously discovered that the upper . With several upcoming missions capable of detecting potential Earth analogues around low-mass stars, including TESS, K2, and PLATO (Ricker et al., 2010; Howell et al., 2014; Rauer et al., 2014), it is imperative that we understand the processes that govern whether a planet in the HZ is in fact habitable. FREQUENTLY ASKED QUESTIONS Is there life on other planets? For K dwarfs (0.6MM0.9M), only planets that form in the first 10Myr experience a significant change in the location of the HZ. Moreover, the amount of oxygen that builds up is substantially greater than in Fig. (1998), High-temperature deformation of dry diabase with application to tectonics on Venus, Evolution of an impact-induced atmosphere and magma ocean on the accreting Earth, Misra A., Meadows V., Claire M., and Crisp D. (2014), Using dimers to measure biosignatures and atmospheric pressure for terrestrial exoplanets, Morbidelli A., Chambers J., Lunine J.I., Petit J.M., Robert F., Valsecchi G.B., and Cyr K.E. Vocabulary. In the previous section we assumed that the rate of oxygen removal at the surface was much higher than the rate at which oxygen was produced. Far to the right of the HZ, planets are never in an insolation-induced runaway greenhouse. By Pallab Ghosh Science correspondent, BBC News Astronomers have for the first time discovered water in the atmosphere of a planet orbiting within the habitable zone of a distant star. Inclusion in an NLM database does not imply endorsement of, or agreement with, (2011) showed that X-ray emission from low-mass stars is saturated for values of the Rossby number RoProt/0.1, where Prot is the stellar rotation period and is the convective turnover time. Classical calculations of the HZ boundaries (Kasting, 1988; Kopparapu et al., 2013, 2014) rely on one-dimensional models tailored to reproduce conditions on Earth and are unable to capture changes in the atmospheric circulation and cloud formation mechanisms that occur as a planet begins to warm. During the contraction phase following their formation, these stars can be 1 or even 2 orders of magnitude more luminous than when they reach the MS. HD 209458b is only 4.3 million miles from its Sun-like star, completing an orbit in less than 4 days. Under the plane-parallel approximation, the atmospheric pressure scales as Mp/. Note the large fraction of the HZ in the lower left portion of the left panel where planets are completely desiccated. We find that for M0.3M, nearly all Earth-mass planets in the HZ lose at least 1 TO, though tens of TO are typically lost for M0.15M. NASA is planning to make water and oxygen on the Moon and Mars by 2020 | Extremetech Home > Extreme NASA is planning to make water and oxygen on the Moon and Mars by 2020 NASA is forging. Gillmann et al. Initially, therefore, the rate of buildup is the same as in the energy-limited case. Scientists have discovered an exoplanet located 90 light-years from Earth with an atmosphere that could contain water clouds. Program Home. Evolution of the flux received by planets that are at the inner edge of the theoretical HZ at 5Gyr for different stellar masses (blue: 0.1M, green: 0.5M, red: 0.7M, cyan: 1.0M). Are exoplanets with oxygen atmospheres overrated? - Astronomy Magazine 6 (1 TO) but assuming that the escape of hydrogen is diffusion-limited. The higher XUV flux early on results in more oxygen escape and less buildup. However, interior to the critical distance, complete desiccation occurs at progressively earlier times. Planets throughout most of the HZ of M dwarfs now lose at least 1 TO of water; those close to the RG limit and around low-mass M dwarfs lose close to 10 TO. According to Henry's law, the solubility is proportional to the partial pressure of the gas in equilibrium with the liquid; given a partial pressure of 0.21 bar of O2 on Earth, this corresponds to roughly 0.015 bar of dissolved O2 per bar of atmospheric O2 (for a planet with 1 TO of water). NASA's science, technology and mission management office for the exploration of exoplanets. If life exists on other planets or moons it may be chemically . (2001), Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth, Structure and evolution of low-mass stars, Planetary accretion in the inner Solar System, Hydrodynamic escape of hydrogen from a hot water-rich atmosphere: the case of Venus, Hydrodynamic escape of oxygen from primitive atmospheres: applications to the cases of Venus and Mars, NOTE: Loss of water on the young Venus: the effect of a strong primitive solar wind, Chassefire E., Wieler R., Marty B., and Leblanc F. (2012), The evolution of Venus: present state of knowledge and future exploration, Cook B.A., Williams P.K.G., and Berger E. (2014), Trends in ultracool dwarf magnetism. This could significantly compromise their habitability. Close to the inner edge of the lowest-mass M dwarfs, planets with larger initial water inventories can lose several tens of TO. Oxidation of rocks at the seafloor could further deplete the atmospheric O2, but this would require efficient mixing of the oxygen to great depths, which may be difficult for planets with deep oceans. Both oxygen and nitrogen are produced by life, and . Abe Y., Abe-Ouchi A., Sleep N.H., and Zahnle K.J. (2007), Planetary radii across five orders of magnitude in mass and stellar insolation: application to transits, Fossati L., Bisikalo D., Lammer H., Shustov B., and Sachkov M. (2014), Major prospects of exoplanet astronomy with the World Space ObservatoryUltraViolet mission, Gillmann C., Chassefire E., and Lognonn P. (2009), A consistent picture of early hydrodynamic escape of Venus atmosphere explaining present Ne and Ar isotopic ratios and low oxygen atmospheric content, The runaway greenhouse: implications for future climate change, geoengineering and planetary atmospheres, Gomes R., Levison H.F., Tsiganis K., and Morbidelli A. Note that once a planet's water is depleted, it is technically no longer in a runaway greenhouse, since the atmospheric infrared windows will open up and the surface will cool. Noting that. 9 and and1010 for initial surface water contents of 1 and 10 TO, respectively. (2013), Exoplanet characterization by proxy: a transiting 2.15, Baraffe I., Chabrier G., Allard F., and Hauschildt P.H. Evolution of the position of the inner edge of the empirical HZ (RV limit) as a function of time. Note also that, unlike in the previous figures, the oxygen amount is a monotonic function of the position in the HZ; because no oxygen escapes, planets closer to the inner edge build up more O2. Perhaps counter-intuitively, we find that both the amount of water lost and the final oxygen pressure scale with planet mass; super-Earths tend to lose substantially more water and develop more massive O2 atmospheres than Earth-mass planets. (2005), Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets, Emergence of two types of terrestrial planet on solidification of magma ocean, Hartogh P., Lis D.C., Bockele-Morvan D., de Val-Borro M., Biver N., Kppers M., Emprechtinger M., Bergin E.A., Crovisier J., Rengel M., Moreno R., Szutowicz S., and Blake G.A. However, we only considered water loss and O2 buildup during the runaway greenhouse state. (2001) showed that the modern rate of (oxidized) Fe3+ subduction is equivalent to a removal rate of 5.01011 to 1.91012 mol O2/year, or 312 bar/Gyr; volcanic outgassing contributes an extra 15 bar/Gyr (Catling, 2014). Combining expressions (17), (19), and (21), we have, with EL given by (2). Our calculations are therefore conservative in this sense. Furthermore, Hamano et al. The code then updates stellar and planetary parameters and loops. Astrobiology 15, 119143. In this paper, we define 1 TO1.391024 g (270 bar) of H2O, the total amount of water in Earth's oceans (Kasting, 1988; Kulikov et al., 2006). Another issue is the possibility that planets forming beyond the snow line could develop substantial hydrogen/helium envelopes before they migrate into the HZ. In the classical picture, hydrogen escapes to space while oxygen interacts with the surface, oxidizing the rocks. The horizontal lines at the bottom of the plot indicate the times during which planets are in a runaway greenhouse. Hunten et al. This process should have led to the production of several hundred bar of O2, a large fraction of which may have been deposited in the atmosphere (e.g., Gillmann et al., 2009). where the negative sign indicates a downward flux. Our present work could also strengthen the results of Wordsworth and Pierrehumbert (2014) and Tian et al. For solar-type stars, Ribas et al. We report the amount of oxygen retained by the planet (either in the atmosphere or absorbed by the surface) as an equivalent pressure in bar, which we define to be the surface pressure of the oxygen if it were the only gas in the atmosphere. (1994), A deep imaging survey of the Pleiades with ROSAT, Strom K.M., Strom S.E., Edwards S., Cabrit S., and Skrutskie M.F. (2013). Most planets in the HZ of M dwarfs are completely desiccated; conversely, those close to the outer edge of high-mass M dwarfs and throughout most of the HZ of K dwarfs lose little or no water. Exoplanet Exploration Program. While the ratio of the oxygen to hydrogen mass escape rates in (21) approaches a maximum value of 8 for 1, the difference between the oxygen production and escape rates remains constant. Does life exist outside of the solar system? We only presented figures where the runaway greenhouse occurs interior to the RG limit. Close to the outer edge, the runaway greenhouse phase is shorter, resulting in weaker ocean loss and similarly low O2 amounts. Dark blue corresponds to insignificant O2 buildup; dark red corresponds to 200 bar of oxygen. Compare Figs. This is consistent with observational studies of late M dwarfs; in particular, West et al. (2013) found that, while Earth's magma ocean lasted for 1.5Myr, it may have lasted as long as 10Myr on Venus due to the blanketing effect of a runaway greenhouse. If the hydrogen particle flux is greater than this limit, oxygen must escape. (2000), Source regions and timescales for the delivery of water to the Earth, Mantle convection with a brittle lithosphere: thoughts on the global tectonic styles of the Earth and Venus, Muirhead P.S., Johnson J.A., Apps K., Carter J.A., Morton T.D., Fabrycky D.C., Pineda J.S., Bottom M., Rojas-Ayala B., Schlawin E., Hamren K., Covey K.R., Crepp J.R., Stassun K.G., Pepper J., Hebb L., Kirby E.N., Howard A.W., Isaacson H.T., Marcy G.W., Levitan D., Diaz-Santos T., Armus L., and Lloyd J.P. (2012), Characterizing the Cool KOIs. This result is no coincidence; it implies that oxygen is retained at its diffusion limit. This perhaps counter-intuitive result stems from the fact that the crossover mass (8) scales inversely with the surface gravity; on super-Earths, the escape of oxygen is greatly suppressed, leading to faster buildup at the surface. Therefore, for each stellar mass, there exists a certain distance at which the O2 pressure peaks. Strong tidal heating could also potentially extend the magma ocean phase and drive rapid resurfacing, which could lead to efficient oxygen removal from the atmosphere. Since terrestrial M dwarf planets probably form within 10Myr after the formation of the parent star, many planets currently in the HZs of M dwarfs were not in the HZs when they formed. The diffusion limit is, from (13). (2005) has data for solar-type stars and is approximately equal to the saturation time for the Sun; it is also roughly the formation time for Earth. Since terrestrial planets with surface oceans are likely to enter the runaway phase somewhere in between these limits, the two runs should roughly bracket the actual evolution. For late M dwarfs, tidal evolution can be quite strong in the HZ (Barnes et al., 2008; Luger et al., 2015), meaning that planets on circular orbits in the HZ today may have started outside the HZ on eccentric orbits. 1Astronomy Department, University of Washington, Seattle, Washington. (2010), The Lick-Carnegie exoplanet survey: a 3.1, Thermal escape in the hydrodynamic regime: reconsideration of Parker's isentropic theory based on results of kinetic simulations, Walter F.M., Brown A., Mathieu R.D., Myers P.C., and Vrba F.J. (1988), X-ray sources in regions of star formation. Moreover, planets around M dwarfs are subject to an array of processes that could negatively impact their habitability. Dependence of the oxygen escape parameter (top), the rate of oxygen buildup (center), and the ocean loss rate ocean (bottom) on the XUV flux for a 1M Earth (left) and a 5M super-Earth (right) in the energy-limited regime. The 23 Moons and Planets With Water in Our Solar System - Popular Mechanics M dwarfs are extremely active (Reid and Hawley, 2005; Scalo et al., 2007), emitting large fractions of their luminosity in the X-ray and extreme ultraviolet (jointly referred to as XUV, corresponding to wavelengths of roughly 11000 ). That is, why does Earth have an atmosphere? (1998), but for the lowest-mass M dwarfs the study of Penz and Micela (2008) is likely to significantly overestimate the XUV flux. where XUV is the XUV flux, Mp is the mass of the planet, Rp is the planet radius, RXUV is the radius where the bulk of the energy is deposited (which, for simplicity, we take to be equal to Rp), XUV is the XUV absorption efficiency, and Ktide is a tidal correction term of order unity. 12, we assume that the planet mass is 5M, the oxygen remains in the atmosphere, and the escape is diffusion-limited. During terrestrial planet formation, giant impacts can deliver enough energy to partially or completely melt a planet's mantle; as a consequence, many of the terrestrial bodies in the Solar System may have experienced a magma ocean phase (Matsui and Abe, 1986; Zahnle et al., 1988; Elkins-Tanton, 2008, 2011, 2012; Elkins-Tanton and Seager, 2008; Hamano et al., 2013; Lammer, 2013; Lebrun et al., 2013). Diffusion-Limited Escape, 1M, 1 TO. Such prolonged runaway greenhouses can lead to planetary evolution divergent from that of Earth. The dot corresponds to the earliest time for which the study of Ribas et al. A tidal runaway could lead to a longer period of water loss and oxygen buildup; this will be investigated in future work. The fate of a given planet strongly depends on the extreme ultraviolet flux, the duration of the runaway regime, the initial water content, and the rate at which oxygen is absorbed by the surface. We thus note that, in general, water worlds may take longer to remove a given amount of O2 from their atmospheres. The inner edge is the runaway greenhouse (RG) limit, interior to which a planet is unable to cool sufficiently to prevent the complete evaporation of its surface water. In Fig. Uranus: Frozen water. Without a mechanism to remove atmospheric CO2, desiccated planets may build up dense CO2 atmospheres and maintain high surface temperatures even after the end of the greenhouse phase, much like Venus. (2011) suggests that a large fraction of Earth's water may have been delivered by comets, possibly a result of scattering by the giant planets prior to and during the Late Heavy Bombardment (Gomes et al., 2005). While early on these planets receive stellar fluxes several times that received by Earth, during the later stages of the runaway the lower stellar flux could lead to the solidification of most of their mantles, potentially allowing for the buildup of O2 in the atmosphere. However, it is likely that their saturation timescale is much longer. Left: Location of the RV limit versus stellar age for stars between 0.08M and 1M. In short, our atmosphere is here because of gravity. (Note that modern data shows trace amounts of oxygen and water vapor.) The young planet Mars would have had enough water to cover its entire surface in a liquid layer about 140-meters deep. The thick curves indicate the flux evolution for different stellar masses (blue: a late M dwarf; green: an early M dwarf; red: a K dwarf; cyan: a solar-type G dwarf). In Fig. The HZ of M dwarfs, on the other hand, moves monotonically inward for up to 1Gyr. The blue curve corresponds to the amount of water lost, and the red curve corresponds to the final oxygen pressure. Evidence points to water on other planets in our solar system. The energy-limited mass loss rate (2) is an upper limit to the thermal escape rate from a planetary atmosphere, as it assumes that all the available XUV energy goes into driving the escape (after accounting for an efficiency factor, XUV). In any event, the negligible O2 content of the venusian atmosphere today implies that the O2 must have been removed either by surface sinks or other escape mechanisms. In other words, in order for the oxygen to accumulate in the atmosphere, it must diffuse out of the hydrodynamic flow that is attempting to carry it away, and the rate at which it can do so is capped at the diffusion limit. Same as Fig. While the magma ocean phase does not prevent water loss to space, it could suppress the buildup of atmospheric O2. The empirical HZ at 1Gyr is shaded in blue for reference. Whether or not this applies to M dwarfs is unclear. These figures are broadly consistent with previous estimates (Kasting et al., 1984; Kasting, 1988; Chassefire, 1996a, 1996b; Kulikov et al., 2006; Gillmann et al., 2009), though the large uncertainty regarding the initial water content precludes an accurate determination of the amount of O2 retained by the planet. where XO is the oxygen mixing ratio and mO is the mass of an oxygen atom. . Extreme ultraviolet (EUV)-powered escape of hydrogen-rich protoatmospheres, Lammer H., Eybl V., Kislyakova K.G., Weingrill J., Holmstrm M., Khodachenko M.L., Kulikov Y.N., Reiners A., Leitzinger M., Odert P., Xiang Gr M., Dorner B., Gdel M., and Hanslmeier A. Though they're invisible to the naked eye, they produce more oxygen than the largest redwoods. (2013) pointed out, this could lead to the complete desiccation of a planet's mantle, potentially terminating tectonics and resulting in permanently dry surface conditions. For >0, the oxygen buildup rate is constant at 5 bar/Myr for the Earth and 25 bar/Myr for the super-Earth. Before we present our results for M dwarf planets, we briefly examine water loss and O2 buildup on Venus. We considered two limiting cases regarding the escape: energy-limited and diffusion-limited. (2006), High-resolution simulations of the final assembly of Earth-like planets I. Terrestrial accretion and dynamics, Raymond S.N., Scalo J., and Meadows V.S. Mars: Ice, trace amounts of vapour, possibly some liquid water underground. The particle escape flux is determined by inserting (8) into (4): where we define the oxygen escape parameter as. As for planets that form in situ in the HZ, 10Myr is roughly equal to the formation time around a 0.3M star in the simulations of Raymond et al. The constant rate of O2 buildup is a straightforward consequence of mass fractionation during hydrodynamic escape. The rate of ocean loss is still linear in XUV but increases more slowly. We pointed out in Section 2.3 that because of the high luminosities of M dwarfs early on, planets in the HZ could have magma oceans for extended periods of time following their formation. Calculations were performed assuming a mass of 5M, an initial water content of 10 TO, and diffusion-limited escape. Earth itself is thought to have been in an impact-induced runaway greenhouse for a few million years (Zahnle et al., 1988; Hamano et al., 2013), yet it is habitable today. Researchers may have found a way that NASA's James Webb Space Telescope can quickly identify nearby planets that could be promising for our search for life, as well as worlds that are uninhabitable because their oceans have vaporized. Perhaps even more interestingly, Figs. We therefore calculate loss rates in the diffusion limit as described in Section 2.4.3. In Section 3, we describe our model, followed by our results in Sections 4 and 5. Of these, the planets Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune have significant atmospheres. Of the planets shown here, only Kepler-62f does not build up any oxygen. Same as Fig. Both hydrogen and oxygen escape in proportions controlled by the stellar XUV flux. Because hydrogen escapes preferentially over oxygen, large quantities of O2 also build up. Once again, water loss amounts are smaller, but oxygen amounts greater than 2000 bar are now possible around the lowest-mass M dwarfs. Otherwise, the code calculates H and O loss rates and O2 buildup rates as outlined above. (1987) studied mass fractionation during hydrodynamic escape, demonstrating that an escaping species can efficiently drag a heavier species along with it provided the mass of the latter is smaller than the crossover mass mc, equal to, in the case of a background flow of atomic hydrogen.