Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus

“Venus’ atmosphere has been desiccated of water [Kasting, 1988].”

“This function, presented in the appendix of WK97, is a parameterized fit (third order in T and fourth order in ln[pCO2 ]) to results from a radiative-convective model (Kasting & Ackerman 1986; Kasting 1988, 1991).”

“However, for an initial water endowment of >50 Earth oceans in the HZ of an FGK star, the moist greenhouse cannot cause complete ocean loss (Kasting 1988; Lammer et al. 2009; Luger & Barnes 2015; Zahnle & Catling 2017) (Figure 1).”

“In addition, in both the temperate and moist greenhouse simulations, the upper atmosphere is pushed into a classical moist greenhouse state (Kasting 1988), despite the temperate simulation having relatively dry surface conditions (Figure 5c).”

“In order to investigate these planets might appear to TPF, we have computed syn spectra for planets that are representative of these two cas For our Venus-like planet, we assumed a “moist greenho scenario (Kasting 1988).”

“…not lose its water via a moist greenhouse effect, which would occur if the solar flux at Earth were more than 10% higher than its present value (Kasting 1988) — a moist greenhouse occurs when the stratosphere becomes wet, and H2O is lost through UV dissociation and the subsequent loss of…”

“t i, the distinction between merely retaining an atmosphere vs. the retention of specically water must be kept in mind. While the exact evolutionary history and escape mechanisms are debatable (e.g. Kasting, 1988; Elkins-Tanton, 2013; Hamano et al., 2013), there is little argument that Venus has lost a huge amount of…”

“would have avoided going into a state of runaway greenhouse [e.g., Kasting, 1988 ].”

“A hot CO2 greenhouse can also cause rapid water loss to space [Kasting, 1988] (though Wordsworth and Pierrehumbert [2013] argues against this), or even a steam atmosphere if surface temperatures exceed water’s critical temperature of 647 K.”

“Regardless of the relatively small differences between merger times in these models, they are all completely coalesced by 6.2 Gyr from today.”

“Inside the HZ, the atmospheric water vapor content is a function of the orbital distance: around the present Sun, in the absence of other greenhouse gases and assuming a large ocean at the surface, the partial pressure of H2O would be 1 bar at 0.93 AU and would reach the critical pressure (220 bar) at 0.84 AU ( Kastin…”

“Thereafter, with q unchanging rather than RH, a ‘Planckian’ response–emission at each wavelength increasing with temperature following Planck’s Law–would have allowed equilibration, with hydrogen loss on longer time-scales eventually bringing the planet to its present state (Kasting, 1988).”

“Venus might have experienced the runaway greenhouse process (Ingersoll 1969, or at least moist greenhouse, Kasting 1988), resulting in very little water and a high D/H ratio in the atmosphere.”

“, 1982) or a warm ‘moist greenhouse’ Venus with an oceanic water inventory that was eventually lost (Kasting, 1988).”

“(2) has already154 been used by, e.g., Pavlov et al. (2000); Segura et al. (2003) or Grenfell et al.155 (2007a) and is based on Kasting et al. (1984b) and Kasting (1988).”

“The H escape flux is calculated at 200 km altitude, which is considered, from Kasting and Pollack (1983), as the base of the expansion.”

“An atmosphere’s temperature structure and circulation critically influence a planet’s surface and atmospheric evolution as well as its potential habitability (Kasting 1988; Abe et al. 2011; Yang et al. 2013).”

“The numerical scheme is based on Kasting et al. (1984b) and Kasting (1988) and was improved and described in von Paris (2010) and Kitzmann et al. (2015).”

“Hydrogen escape must be an essential process for evolution of the Venusian atmosphere since the present Venusian atmosphere contains almost no hydrogen [e.g., Kasting and Pollack, 1983; Kasting, 1988].”

“Massive fractionating loss by exospheric escape from a large primordial reservoir is a likely mechanism for the D/H signature seen in the Venus atmosphere today [Donahue and Hodges, 1992; Kasting, 1988; Pepin, 1991; Zahnle and Kasting, 1986].”

“This limit is also known as the runaway greenhouse (Ingersoll, 1969; Kasting, 1988; Pierrehumbert, 2010; Goldblatt and Watson, 2012).”

“In that scenario, the dense atmosphere would provide a strong greenhouse effect, raising the surface temperature enough to cause any oceans to boil away, as is thought to have happened to Venus early in the lifetime of the solar system (e.g., Kasting 1988).”

“However, atmospheric evolution of these planets diverged significantly (Kasting, 1988; Kulikov et al. , 2007; Driscoll and Bercovici, 2013).”

“The latter is the definition favored by Kasting et al. (1993); the former definition is used by Hart (1979). In the present Earth atmosphere water escape is limited by the cold trap at the tropopause which loses its efficiency in a sufficiently warm, moist atmosphere (Kasting et al.”

“The albedo for a cloudless steam atmosphere, based on 1D models and 3D global circulation model calculations lies between [0.15, 0.40] (Kasting 1988; Goldblatt et al. 2013; Leconte et al. 2013; Pluriel et al. 2019).”

“Below the tropopause the temperature profile is determined by a wet adiabatic lapse rate,b sed on Kasting (1988) and Ingersoll (1969), using the relative humidity profile of Manabe & Wetherald (1967).”

“In addition, for planets hotter than Earth, volatiles can be lost via atmospheric escape (the so-called moist greenhouse; Kasting 1988).”

“…humidities are near 75% at the surface and 50% above 700 mb. One-dimensional studies typically assume fully saturated atmospheres and thus they artificially amplify the strength of the water vapor greenhouse [Kasting, 1988; Kasting et al., 1993; Goldblatt et al., 2013; Kopparapu et al., 2013].”

“On the other hand, models considering a CO2–H2O atmosphere suggest the possibility of water ocean condensation on Venus at the end of a rapid (less than a few Myr) magma ocean stage, especially if clouds or Venus rotation are considered (e.g. Kasting 1988; Way et al. 2016; Salvador et al. 2017).”

“This concept is essentially analogous to the “moist greenhouse” of Kasting [37], except that the moist greenhouse is the specific case in which enhanced water vapor photolysis can desiccate a planet with an Earth-like water inventory within the age of the solar system (as discussed earlier).”

“However, under a regime of increasing solar luminosity, which is estimated to be 118% of the present-day value at 1.75 Gyr, we consider an irreversible runaway greenhouse scenario to be more likely (Kasting, 1988; Goldblatt and Watson, 2012).”

“Thereafter, the thermal radiation outgoing to space is fixed to 280–300 Wm−2, which is called the radiation limit or runaway greenhouse limit of steam atmospheres (Kasting 1988; Abe and Matsui 1988; Kopparapu et al. 2013; Goldblatt et al. 2013).”

“The dry and moist convective layers are constructed using Kasting (1988) formulation of the lapse rate, and as done in Marcq et al. (2017).”

“I follow a similar technique as that in previous studies (e.g. Kasting, 1988; Ramirez et al., 2014b; Ramirez and Kasting, 2017) to gauge the radiative effects of cloud forcing.”

“Hart (1979) provided estimates for the Solar System and later models of runaway greenhouse for Venus were incorporated into the calculations by Pollack (1971) and Kasting (1988).”

“One should also note that for the surface temperatures of∼500 K, which are expected during the “Mush” stage (Lebrun et al., 2013), according to Kasting (1988) one can also expect water vapor mixing ratios at the mesopause level n ar to 1.”

“Since the conditions on the terrestrial planets and especially the amount of CO2 and H2O in the atmosphere could have varied over geological history, the greenhouse effect could have played a more important role in the past (Kasting 1988).”

“At this level of stellar flux, the stratospheric water vapor concentration, ≥ 3, 000 ppmv, is high enough that H2O photo-dissociation and subsequent H escape to space becomes significant, which is known as the moist greenhouse state (Kasting 1988; Kasting et al. 1993).”

“greenhouse’ process can destabilize the climate on the basis of simple one-dimensional model calculations, Kasting [35] found that on an Earth-like planet around the Sun, oceans would completely vapourize below a threshold around 0.”

“The thin solid line corresponds to mean surface temperatures above 40◦C, which is the lower edge of the moist runaway greenhouse zone (Kasting 1988; Pierrehumbert 1995)—our model does not include fluxes of latent heat or the greenhouse effect of water vapor, so the position of this line is notional.”

“Finally, rising temperatures cause rapid ocean evaporation (Kasting 1988; Bounama et al. 2001; Vásquez et al. 2010), effectively sterilising most of the biosphere (see Fig.”

“Climate model calculations by Kasting (1988), summarized in Figure 2, indicate that this would be more than sufficient to drive all of Earth's water into its atmosphere and create a "runaway greenhouse."”

“A planet with oceans has an upper limit of planetary radiation (Kasting, 1988; Nakajima et al., 1992; Kodama et al., 2015).”

“This assumption has been used extensively in the models from Jim Kasting’s group (Kasting et al., 1984; Kasting and Ackerman, 1986; Kasting, 1987, 1988; Kasting et al., 1993; Pavlov et al., 2000, 2003; Kasting and Howard, 2006; Haqq-Misra et al., 2008), which, together with parametrisations and…”

“Validity close to the critical point of water vapor (Pc = 22.06 MPa, Tc = 647.1 K) is not guaranteed, however [Kasting, 1988; Ingersoll, 1969; Abe and Matsui, 1988].”

“This qualitative behavior was also reproduced by older random band or gray models [Nakajima et al., 1992; Kasting, 1988], albeit with larger values for NL on the order of 300 W/m2 or more.”

“The volatile budget of Earth is thought to consist of several bars of N2 and tens to hundreds of bars of CO2 (e.g., Turekian & Clark 1975; Kasting 1988; McKay & Stoker 1989; Lundin & Barabash 2004; Goldblatt et al. 2009).”

“Alternatively, Venus might have been in the hot climate mode (i.e., the moist greenhouse condition), resulting in a loss of water to space (e.g., Kasting 1988).”

“90 AU around the Sun, wider than the traditional one proposed by Kasting et al. (1993), where the limits in the intermediate case, for the Solar System, go from 0.84 to 1.67 AU in the Solar System. For life systems to evolve, the planet should be in the habitable zone continuously during a certain time. To analyze the…”

“If liquid water existed on early Venus (e.g., Kasting et al. 1984; Kasting 1988; Abe et al. 2011; Way et al. 2016), silicate weathering may have served as a sink for atmospheric CO2 (e.g., Kasting et al. 1984).”

“The parameter mf is a free parameter, except that, as discussed above, Kasting (1988) argues that the solar flux cannot have been more than 10% higher at any point in the past since Earth has not lost its oceans due to a runaway moist greenhouse atmosphere.”

“Moist greenhouse atmospheres emerge when the stratospheric H2Ov mixing ratio are sufficiently high, i.e., 3×10−3 mol mol−1 such that water-loss via diffusion-limited escape could occur at a geologically significant rate in the thermosphere (Kasting 1988).”

“Our calculations are performed in a similar manner to Kasting (1988) and Kasting et al. (1993), but cover a smaller range of temperatures, as shown in Fig.”

“However, during a runaway greenhouse, the surface temperature exceeds the temperature of the critical point of water (647 K) and the surface oceans fully evaporate, leading to an upper atmosphere that is dominated by water vapor (e.g., Kasting 1988).”

“The radiation limit of the steam atmospheres has been extensively investigated, and its value is approximately 280 to 310 Wm−2 (Abe & Matsui 1988; Kasting 1988; Nakajima et al. 1992; Kopparapu et al. 2013; Goldblatt et al. 2013).”

“s (g = 3.73 m/s2) as compared to 2Earth (g e = 9.81 m/s ) means that clouds (and precipitation) can be supported at lower pressures, assuming that vertical velocities on both planets are equal (e.g., Kasting, 1988; Ramirez, 2017). Thus, we predict that the height h q on a warm early Mars corresponds to a pressure leve…”

“Such hot atmospheres are typically well-mixed up to large heights (Abe and Matsui, 1988; Kasting, 1988; Lupu et al., 2014).”

“However, Kasting et al. (1984) and Kasting (1988) suggested that a slightly subcritical Venus, with hot oceans and a wet stratosphere existed before this time.”

“However, HDO has transitions where H2O does not mostly between 2500 and 3000 cm , a region that is unimportant for emission from early Earth and Mars, but one which may be important for a runaway greenhouse, such as that which occurred early in Venus’ history [Ingersol, 1969; Kasting, 1988].”

“If the felsic nature of venusian tesserae is confirmed in further studies, this may have important implications for geochemical environments in early history of Venus, indirectly supporting a hypothesis of water-rich early Venus (e.g. Kasting et al., 1984; Kasting, 1988; Grinspoon and Bullock, 2003).”

“Runaway greenhouse planets like early Venus would be suspect because of their position near or inside the inner edge of the habitable zone (Kasting 1988; Kopparapu et al. 2013; Kane et al. 2014; Way et al. 2016).”

“5, the thermal radiation from the top of the atmosphere decreases along with decrease in the surface temperature or increase in the atmospheric mass, but finally approaches ∼ 280 W/m2 (Abe and Matsui 1988; Kasting 1988; Goldblatt et al. 2013; Kopparapu et al. 2013).”

“It could be caught in a moist greenhouse state with surface temperatures up to the critical point of water, that is, 647 K if an Earth-like inventory of water and surface H2O pressure are assumed (Kasting 1988).”

“Extending the work of Pollack (1971), Kasting (1988), and Grinspoon and Bullock (2003), Way et al. (2016) concluded that the clouds and atmospheric dynamics of slowly rotating worlds (such as Venus) would mitigate atmospheric temperatures and water loss, and an ocean could have lasted several…”

“Earlier calculations regarding the inner boundary of the HZ (Kasting 1988; Kasting et al. 1993) assumed instead a saturated troposphere, i.e. a relative humidity of unity.”

“…as a result of a fundamental limit on infrared emission to space by an Earth-like planet, the “runaway greenhouse,” or the loss of a planet’s water through photodissociation and hydrodynamic escape due to high surface temperatures and a moist stratosphere, a “moist greenhouse” (Kasting 1988).”