Analytic investigation of climate stability on Titan: sensitivity to volatile inventory

Lorenz, R. D.; McKay, Christopher P.; Lunine, Jonathan I.

doi:10.1016/s0032-0633(99)00038-0

Cited by 51 publications

(45 citation statements)

References 38 publications

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“…Climate stability can be undermined by several previously studied climate instabilities. These include atmospheric collapses (Haberle et al 1994;Read & Lewis 2004), photochemical collapses (Zahnle et al 2008;Lorenz et al 1997), greenhouse runaways (Kasting 1988;Lorenz et al 1999;Sugiyama et al 2002), ice-albedo feedback (Roe & Baker 2010), and ocean thermohaline circulation bistability (Stommel 1961;EPICA Community Members 2006). Climate stability can also be undermined if the sign of the dependence of mean surface weathering rate on mean surface temperature is reversed.…”

Section: Discussionmentioning

confidence: 99%

Climate Instability on Tidally Locked Exoplanets

Kite

Gaidos

Manga

2011

ApJ

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Feedbacks that can destabilize the climates of synchronously rotating rocky planets may arise on planets with strong day-night surface temperature contrasts. Earth-like habitable planets maintain stable surface liquid water over geologic time. This requires equilibrium between the temperature-dependent rate of greenhouse-gas consumption by weathering, and greenhouse-gas resupply by other processes. Detected small-radius exoplanets, and anticipated M-dwarf habitable-zone rocky planets, are expected to be in synchronous rotation (tidally locked). In this paper, we investigate two hypothetical feedbacks that can destabilize climate on planets in synchronous rotation. (1) If small changes in pressure alter the temperature distribution across a planet's surface such that the weathering rate goes up when the pressure goes down, a runaway positive feedback occurs involving increasing weathering rate near the substellar point, decreasing pressure, and increasing substellar surface temperature. We call this feedback enhanced substellar weathering instability (ESWI). (2) When decreases in pressure increase the fraction of surface area above the melting point (through reduced advective cooling of the substellar point), and the corresponding increase in volume of liquid causes net dissolution of the atmosphere, a further decrease in pressure will occur. This substellar dissolution feedback can also cause a runaway climate shift. We use an idealized energy balance model to map out the conditions under which these instabilities may occur. In this simplified model, the weathering runaway can shrink the habitable zone and cause geologically rapid 10 3 -fold atmospheric pressure shifts within the habitable zone. Mars may have undergone a weathering runaway in the past. Substellar dissolution is usually a negative feedback or weak positive feedback on changes in atmospheric pressure. It can only cause runaway changes for small, deep oceans and highly soluble atmospheric gases.Both instabilities are suppressed if the atmosphere has a high radiative efficiency. Our results are most relevant for atmospheres that are thin, have low greenhouse-gas radiative efficiency, and have a principal greenhouse gas that is also the main constituent of the atmosphere. ESWI also requires land near the substellar point, and tectonic resurfacing (volcanism, mountain-building) is needed for large jumps in pressure. These results identify a new pathway by which habitable-zone planets can undergo rapid climate shifts and become uninhabitable.

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Section: Discussionmentioning

confidence: 99%

Climate Instability on Tidally Locked Exoplanets

Kite

Gaidos

Manga

2011

ApJ

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“…Although downwelling thermal radiation from the warm haze affects the heat balance, the high surface temperature requires some sunlight to be absorbed at the surface. Analytic approximations (Lorenz et al 1999b) to the McKay et al numerical radiative-convective model suggest that if Titan's average surface reflectivity is 0.4, a methane profile with a surface relative humidity of about 60% is needed to provide enough greenhouse warming to produce the observed surface temperature. On the other hand, if the average surface reflectivity is only 0.1, more sunlight is absorbed at the surface and hence only a weaker greenhouse effect is needed, with a relative humidity near 40%.…”

Section: Multiple Scatteringmentioning

confidence: 99%

Methane Abundance on Titan, Measured by the Space Telescope Imaging Spectrograph

Lemmon

Smith

Lorenz

2002

Icarus

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Although methane is the dominant absorber in Titan's reflection spectrum, the amount of methane in the atmosphere has only been determined to an order of magnitude. We analyzed spectra from the Space Telescope Imaging Spectrograph, looking at both a bright surface region (700-km radius) and a dark surface region. The difference between the spectra of the two regions is attributed to light that has scattered off the surface, and therefore made a roundtrip through all of Titan's methane. Considering only absorption, the shape of the difference spectrum provides an upper limit on methane abundance of 3.5 km-am. Modeling the multiple scattering in the atmosphere further constrains the methane abundance to 2.63 ± 0.17 km-am. In the absence of supersaturation and with a simplified methane vertical profile, this corresponds to a surface methane-mole fraction near 3.8% and a relative humidity of 0.32. With supersaturation near the tropopause, the surface methane mole fraction could be as low as 3%. c 2002 Elsevier Science (USA)

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“…We consider two extreme primordial Titans as initial conditions: an airless cold Titan and a relatively warm Titan, as proposed previously, with substantial N 2 and CH 4 on its surface 17 .…”

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confidence: 99%

“…A relatively high T surf (~70-85 K) 17 , resulting from greenhouse effects, would have increased the mass of the atmosphere, leading in turn to efficient atmospheric erosion. The present results suggest that efficient atmospheric erosion would have resulted in the loss of most of the pre-existing N 2 during the LHB, and impact-induced N 2 would have become dominant in the aftermath of the LHB (Fig.…”

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confidence: 99%