We have shown that during the early high-luminosity phase of M dwarfs, terrestrial planets can lose several Earth oceans of water. Oxygen amounts, on the other hand, are slightly higher; planets around all M dwarfs can develop atmospheres with hundreds to thousands of bar of O 2. Perhaps even more interestingly, Figs. NASA's Hubble and Spitzer have detected water vapor, sodium and potassium in the atmosphere of WASP-39 b. Similar to Fig. 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. 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 ). For reference, we plot the XUV fluxes calculated from the equations in Penz and Micela (2008) and Lammer et al. Saturn: Water in frozen and vapour form. We extend this mechanism and demonstrate that hundreds to thousands of bar of abiotic oxygen are possible for planets throughout the HZs of M dwarfs. If there was oxygen on a far away planet, how would we know? While oxygen is critical for life on Earth, an atmosphere with high oxygen levels can be a sign of a planet not suitable for life. In particular, planets that form in situ in the HZs of M dwarfs could be small and dry (Lissauer, 2007; Raymond et al., 2007), while those that migrate from farther out could be unable to shed their thick H/He envelopes if they are more massive than about 1M (Lammer et al., 2014; Luger et al., 2015). (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. Based on measurements of oxygen diffusion in magma by Wendlandt (1991), Gillmann et al. (2011a), Lammer et al. (2009) argued that photolytically produced oxygen on Venus could diffuse to a depth of 1km over 100Myrcertainly not enough to absorb all Venus' oxygen, as this would require an oxidation depth of hundreds of kilometers. Finally, a magma ocean could have removed most or all of the O2 (Hamano et al., 2013), though rigorous quantitative studies of this process are lacking. An official website of the United States government. Scaling this to different ocean masses mocean, we have. 6, for instance: above 0.6M, both the total water loss and the O2 amount change discontinuously as the stellar mass increases, leading to a jagged pattern near the RG limit and high O2 buildup. In the first case, we assume that water vapor is the dominant atmospheric species and that atmospheric escape occurs in the energy-limited regime. Accessibility Given that neither Mars nor Venus possess an active tectonic cycle, it is possible that many exoplanets lack plate tectonics, which could significantly delay the removal of O2 from their atmospheres. FOIA Dark blue corresponds to less than 0.1 TO; dark red corresponds to complete desiccation. Of the planets shown here, only Kepler-62f does not build up any oxygen. Learn about what Earth is made of and where. Next, in Fig. (2007) but is significantly longer than that predicted by the scaling arguments in Lissauer (2007). Water found on a potentially life-friendly alien planet Oxygen on Earth is continuously produced by photosynthesis; in the absence of a steady source, continental weathering, volcanic outgassing of reducing gases, and oxidation of basalt via hydrothermal processes at oceanic ridges would quickly remove most of the atmospheric O2 (Lcuyer and Ricard, 1999). The 23 Moons and Planets With Water in Our Solar System - Popular Mechanics Future work will investigate the mantle cooling process during runaway greenhouses on M dwarf planets. Figure 4 shows the duration of the runaway phase for planets that form at 10Myr, assuming the runaway occurs interior to the RG limit. During this time, the star's shrinking radius and roughly constant effective temperature result in a decrease in its luminosity by 1 or even 2 orders of magnitude. This is because, at lower efficiency, hydrogen escape is slower, and O2 drag is less efficient, leading to a quicker oxygen buildup. As a library, NLM provides access to scientific literature. the contents by NLM or the National Institutes of Health. In order to solve for the individual escape rates, it is convenient to define a reference particle flux , equal to the energy-limited particle escape flux of H in the absence of oxygen (Chassefire, 1996b): By combining (4) and (6), we obtain as in Chassefire (1996b) the true hydrogen particle flux in terms of the reference flux: Inserting this into (3) and doing a little algebra, we obtain an expression for the crossover mass in terms of the reference flux (5): In this derivation, we used mO=16mH as well as XH=2/3 and XO=1/3, assuming that all H and O are photolytically produced and that the dissociation of H2 and O2 is fast enough that both species are atomic close to the base of the flow (Chassefire, 1996b). This is because close to the inner edge of the HZ the ocean is lost quickly and the planet is desiccated early on, when the XUV flux is high and oxygen escape is efficient. Inclusion in an NLM database does not imply endorsement of, or agreement with, Are exoplanets with oxygen atmospheres overrated? - Astronomy Magazine A dense enough envelope could effectively shield the surface and prevent the dissociation of H2O, provided the water settles below the base of the hydrodynamic wind. We thus emphasize that the removal of a few hundred bar of O2 during/after the runaway greenhouse period is entirely plausible but highly dependent on properties of the star-planet system. Energy-Limited Escape: 1M, 1 TO. Astrobiology 15, 119143. The expression above is valid provided mcmO; otherwise, FO=0. . It wasn't until 2.5 billion years ago that we started to see clear signs of oxygen accumulation in our atmosphere, and it wasn't until maybe somewhere in the last 540 million years that we see atmospheric oxygen become similar to today. This is also the case for loss at the diffusion limit, since the escape flux scales with the surface gravity of the planet. 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. Uranus: Frozen water. (2014). Before we present our results for M dwarf planets, we briefly examine water loss and O2 buildup on Venus. Assuming an XUV saturation fraction f0=103, a saturation timescale of 0.1Gyr, a formation time of 50Myr, an initial water content of 1 TO (Raymond et al., 2006), and a runaway greenhouse interior to the RG limit, we find that Venus is completely desiccated in both the energy-limited and diffusion-limited escape regimes, accumulating an equivalent O2 pressure between 120 bar (energy-limited) and 240 bar (diffusion-limited). As discussed in Section 2.5, this could be the case for water worlds or for planets with a pre-oxidized surface/interior, inefficient outgassing of reducing compounds, inefficient resurfacing processes, and so on. Given the high XUV fluxes of M dwarfs early on, it is important to consider the case where the oxygen escapes along with the hydrogen. (1998), Evolutionary models for solar metallicity low-mass stars: mass-magnitude relationships and color-magnitude diagrams, Habitable planets around white and brown dwarfs: the perils of a cooling primary, Barnes R., Raymond S.N., Jackson B., and Greenberg R. (2008), Tides and the evolution of planetary habitability, Barnes R., Mullins K., Goldblatt C., Meadows V.S., Kasting J.F., and Heller R. (2013), Tidal Venuses: triggering a climate catastrophe via tidal heating, Borucki W.J., Agol E., Fressin F., Kaltenegger L., Rowe J., Isaacson H., Fischer D., Batalha N., Lissauer J.J., Marcy G.W., Fabrycky D., Dsert J.M., Bryson S.T., Barclay T., Bastien F., Boss A., Brugamyer E., Buchhave L.A., Burke C., Caldwell D.A., Carter J., Charbonneau D., Crepp J.R., Christensen-Dalsgaard J., Christiansen J.L., Ciardi D., Cochran W.D., DeVore E., Doyle L., Dupree A.K., Endl M., Everett M.E., Ford E.B., Fortney J., Gautier T.N., III, Geary J.C., Gould A., Haas M., Henze C., Howard A.W., Howell S.B., Huber D., Jenkins J.M., Kjeldsen H., Kolbl R., Kolodziejczak J., Latham D.W., Lee B.L., Lopez E., Mullally F., Orosz J.A., Prsa A., Quintana E.V., Sanchis-Ojeda R., Sasselov D., Seader S., Shporer A., Steffen J.H., Still M., Tenenbaum P., Thompson S.E., Torres G., Twicken J.D., Welsh W.F., and Winn J.N. This may prevent the onset of a carbonate-silicate cycle on these planets, making them unable to remove atmospheric CO2 and maintaining permanently high surface temperatures. We further build on the results of Wordsworth and Pierrehumbert (2013), who showed that significant water loss can occur for planets near the inner edge of the HZ of M dwarfs; however, those authors considered a constant stellar luminosity and thus did not account for the early runaway greenhouse state, which we show can result in water loss rates that are orders of magnitude higher. Therefore, regardless of whether the escape of hydrogen is energy-limited or diffusion-limited, oxygen buildup will occur at its diffusion limit. Whether this oxygen remains in these planets' atmospheres past the early runaway phase depends on the efficiency of their surface sinks. By expressing EL as a function of , we show in Appendix B that the rate at which oxygen accumulates in the atmosphere/at the surface is completely independent of the XUV flux above the critical value given in (9). This applies even at early times, when the XUV flux is very high; the escape of even a small amount of oxygen in the energy-limited regime tends to slow down the rate of ocean loss, given that a large fraction of the XUV energy goes into driving the escape of the heavier species. Thus, for planets that build up significant amounts of oxygen in their atmospheres, the H particle escape flux is given by the smaller of (7) and (13), the O particle escape flux is zero, and the rate at which the ocean is lost is 9 times the H escape rate. Oxygen is soluble in seawater, saturating at about 8mL/L (3.8104 mol/L) at 0C and 35 salinity (Levitus, 1982). Duration of the runaway greenhouse for planets that formed at 10Myr with abundant surface water. (2014), Strong dependence of the inner edge of the habitable zone on planetary rotation rate, Aeronomy of extra-solar giant planets at small orbital distances, Zahnle K., Pollack J.B., and Kasting J.F. However, the escape of hydrogen depends on the availability of hydrogen atoms at the base of the flow. However, due to the high surface temperature on a runaway planet, the thermal structure follows a dry adiabat throughout most of the troposphere (e.g., Kasting, 1988), along which the vapor pressure is lower than the saturation vapor pressure and thus water cannot condense. The extent of the convective zone increases with decreasing stellar mass; below 0.35M, M dwarfs are fully convective (Chabrier and Baraffe, 1997), resulting in larger values of (see, e.g., Pizzolato et al., 2000). Gillmann et al. However, interior to the critical distance, complete desiccation occurs at progressively earlier times. Dark blue corresponds to insignificant O2 buildup; dark red corresponds to 200 bar of oxygen. While this process should not directly affect water loss to spacesince several tens of bar of water vapor remain in the atmosphere during the magma ocean phase (Matsui and Abe, 1986; Zahnle et al., 1988; Hamano et al., 2013)it may prevent the accumulation of atmospheric oxygen. 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 obtain the following from (28) and (24): In Fig. Heating of the upper atmosphere by XUV radiation can then drive a hydrodynamic wind that carries the hydrogen (and potentially some of the oxygen) to space, leading to the irreversible loss of a planet's surface water, oxidation of the surface, and possible accumulation of oxygen in the atmosphere. The duration of the pre-main sequence (PMS) phase is inversely proportional to the mass of the star; while a star like the Sun reaches the MS in 50Myr (Baraffe et al., 1998), M dwarfs can take several hundred millions of years to fully contract and reach the MS (Reid and Hawley, 2005). If, on the other hand, we assume the more optimistic HZ boundary, such that a runaway greenhouse occurs only once Venus is interior to the RV limit, Venus loses a maximum of 0.5 TO and builds up a maximum of 120 bar of O2 in both regimes. Put another way, in order for oxygen to be retained after photolysis, it must diffuse out of the hydrodynamic flow that is dragging it away, and the rate at which it can do so is equal to the diffusion limit. High-energy irradiances (11700 ), Ricker G.R., Latham D.W., Vanderspek R.K., Ennico K.A., Bakos G., Brown T.M., Burgasser A.J., Charbonneau D., Clampin M., Deming L.D., Doty J.P., Dunham E.W., Elliot J.L., Holman M.J., Ida S., Jenkins J.M., Jernigan J.G., Kawai N., Laughlin G.P., Lissauer J.J., Martel F., Sasselov D.D., Schingler R.H., Seager S., Torres G., Udry S., Villasenor J.N., Winn J.N., and Worden S.P. This could happen in the case of planets with vigorous resurfacing processes or convecting magma oceans (see Section 2.5.3 and Section 3 for a discussion). Nevertheless, we see from these figures that the O2 pressures are a factor of 23 times larger on a super-Earth than on an Earth-mass planet, implying substantially more O2 buildup. This is due to the fact that stars more massive than about 0.6M switch from convective to radiative energy transport toward the end of their contraction phase, during which time their effective temperatures rise, leading to a temporary increase in L just before reaching the MS (see, e.g., Reid and Hawley, 2005). In the classical picture, hydrogen escapes to space while oxygen interacts with the surface, oxidizing the rocks. (2013), A dynamically packed planetary system around GJ 667C with three super-Earths in its habitable zone, Ballard S., Charbonneau D., Fressin F., Torres G., Irwin J., Desert J.-M., Newton E., Mann A.W., Ciardi D.R., Crepp J.R., Henze C.E., Bryson S.T., Howell S.B., Horch E.P., Everett M.E., and Shporer A. Total amount of water lost and amount of oxygen absorbed at the surface for a 1M planet formed at 10Myr with 1 TO of surface water, assuming the planet is in a runaway interior to the RG limit, the oxygen is instantaneously absorbed by the surface, and the escape is energy-limited. The dashed vertical line corresponds to the 10Myr formation time we assume in our model. 6, where the final oxygen pressure is not a monotonic function of either M or the position in the HZ, peaking close to the outer edge of the HZ in some cases. If a 100 kilometer-deep ocean existed below the Europan ice shell, it. The surface is then unable to cool effectively, and the temperature increases to 1500K, leading to the complete evaporation of the planet's oceans and effectively sterilizing the surface (for a review, see Pierrehumbert, 2010; Goldblatt and Watson, 2012). As a consequence, these planets are likely to be in a runaway greenhouse provided they have sufficient surface water (see, e.g., Kasting, 1988; Kopparapu et al., 2013). In Fig. B1 we plot , , and ocean as a function of XUV for two planet masses and two XUV absorption efficiencies. While the final O2 equivalent pressure is a complex function of the stellar/planetary mass and the semimajor axis, our results in the energy-limited regime can be understood in fairly simple terms by considering the mass loss rates (22)(24) derived in Appendix A. Conceptual image of water-bearing (left) and dry (right) exoplanets with oxygen-rich atmospheres. The horizontal axis (semimajor axis) spans the HZ, which is bounded on the left and right by the RV and EM limits (solid lines); from left to right, the RG and MG limits are indicated by the dashed lines. Water on Mars: The Story So Far | News | Astrobiology It is important to note that not all of a planet's water may be at its surface (or in its atmosphere), particularly at early times. Over 10 oceans may be lost close to the inner edge of the HZ, particularly for super-Earths, whose hydrogen diffusion limit is higher, and for planets with efficient O2 absorption processes, as these could prevent atmospheric O2 from building up, resulting in an escape rate that approaches the energy-limited rate. 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. This behavior is rooted in the non-monotonic luminosity evolution of K dwarfs prior to 100Myr. Not only might these planets have highly reducing surfaces, but outgassing of reduced compounds could also lead to the quick removal of atmospheric O2. This means that the rate of oxygen buildup is constant in time and does not vary with the XUV flux (provided XUV>crit). Anglada-Escud G., Tuomi M., Gerlach E., Barnes R., Heller R., Jenkins J.S., Wende S., Vogt S.S., Butler R.P., Reiners A., and Jones H.R.A. KOI 961: a small star with large proper motion and three small planets, Planetary evaporation by UV and X-ray radiation: basic hydrodynamics, X-ray induced mass loss effects on exoplanets orbiting dM stars, Pizzolato N., Maggio A., Micela G., Sciortino S., Ventura P., and D'Antona F. (2000), Determination of convective turnover times in young stars, Stellar Clusters and Associations: Convection, Rotation, and Dynamos, Quintana E.V., Barclay T., Raymond S.N., Rowe J.F., Bolmont E., Caldwell D.A., Howell S.B., Kane S.R., Huber D., Crepp J.R., Lissauer J.J., Ciardi D.R., Coughlin J.L., Everett M.E., Henze C.E., Horch E., Isaacson H., Ford E.B., Adams F.C., Still M., Hunter R.C., Quarles B., and Selsis F. 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(2014), No universal minimum-mass extrasolar nebula: evidence against, Raymond S.N., Quinn T., and Lunine J.I. Recent studies such as those of Abe et al. In the panels on the right, we see that oxygen buildup is larger than in the energy-limited case; recall that these panels now represent the equivalent pressure of oxygen in the atmosphere at the end of the runaway phase. The large weathering rates discussed above assume continental coverage similar to Earth's. For stars more massive than about 0.8M, the HZ moves inward during the PMS phase and then outward due to the steady increase of Lbol of these stars once they are on the MS. (2013), XUV-exposed, non-hydrostatic hydrogen-rich upper atmospheres of terrestrial planets. Furthermore, Hamano et al. Given that over 40% of M dwarfs (the lowest-mass stars, spanning the range 0.08MM0.6M) are expected to harbor an Earth-sized planet in the HZ (Kopparapu, 2013), the detailed spectroscopic characterization of all future detections may be very difficult. 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. Moreover, diffusion-limited escape of hydrogen can occur even on a planet that is not in a runaway state. Complete desiccation now occurs only around low-mass M dwarfs, particularly close to the RG limit. Kulikov et al. Mars: Ice, trace amounts of vapour, possibly some liquid water underground. However, several TO are still lost, particularly around low-mass M dwarfs and close to the inner edge of the HZ. (2014), Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581, Scalo J., Kaltenegger L., Segura A.G., Fridlund M., Ribas I., Kulikov Y.N., Grenfell J.L., Rauer H., Odert P., Leitzinger M., Selsis F., Khodachenko M.L., Eiroa C., Kasting J., and Lammer H. (2007), M stars as targets for terrestrial exoplanet searches and biosignature detection, Synthetic spectra of simulated terrestrial atmospheres containing possible biomarker gases, Schopf J.W., Kudryavtsev A.B., Czaja A.D., and Tripathi A.B. 14, where we plot a cross section along M=0.4M in Fig. In reality, a planet with an Earth-like redox state is likely to start out in the energy-limited regime and transition to the diffusion-limited regime as its surface sinks get overwhelmed; our results for water loss and O2 buildup should therefore bracket these processes on many M dwarf planets. (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. Earth is the only known planet to have bodies of liquid water on its surface. It's bigger than Mercury and Pluto. which approaches EL in the limit 1 (H+O escape) and 9 EL in the limit 0 (H only escape). (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. Same as Fig. 7), both water loss and oxygen amounts are significantly higher. Which planets have water? - Space Centre
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