CO2–SO2 clathrate hydrate formation on early Mars

Abstract

It is generally agreed that a dense CO 2-dominant atmosphere was necessary in order to keep early Mars warm and wet. However, current models have not been able to produce surface temperature higher than the freezing point of water. Most sulfate minerals discovered on Mars are dated no earlier than the Hesperian, despite likely much stronger volcanic activities and more substantial release of sulfur-bearing gases into Martian atmosphere during the Noachian. Here we show, using a 1-D radiative-convective-photochemical model, that clathrate formation during the Noachian would have buffered the atmospheric CO 2 pressure of early Mars at ~2 bar and maintained a global average surface temperature ~230 K. Because clathrates trap SO 2 more favorably than CO 2 , all volcanically outgassed sulfur would have been trapped in Noachian Mars cryosphere, preventing a significant formation of sulfate minerals during the Noachian and inhibiting carbonates from forming at the surface in acidic water resulting from the local melting of the SO 2-rich cryosphere. The massive formation of sulfate minerals at the surface of Mars during the Hesperian could be the consequence of a drop of the CO 2 pressure below a 2-bar threshold value at the late Noachian-Hesperian transition, which would have released sulfur gases into the atmosphere from both the Noachian sulfur-rich cryosphere and still active Tharsis volcanism. A lower value of the pressure threshold, down to ~0.5 bar, could have been sufficient to maintain middle and high latitude regions below the clathrate formation temperature during the Noachian and to make the trapping of SO 2 in clathrates efficient. Our hypothesis could allow to explain the formation of chaotic terrains and outflow channels, and the occurrence of episodic warm episodes facilitated by the release of SO 2 to the atmosphere. These episodes could explain the formation of valley networks and the degradation of impact craters, but remain to be confirmed by further modeling.

CO2-SO2clathrate hydrate formation on early Mars

“…Based on the previous result, we have shown (Chassefière et al, 2013) that the formation of CO 2 -SO 2 clathrates at Noachian and Hesperian times could have played an important role in controlling and stabilizing the level of volcanic sulfur in the atmosphere, as well as the level of atmospheric CO 2 for earliest times. If the CO 2 pressure exceeded a threshold of 2 bar due to an efficient Noachian volcanism, the CO 2 in excess of 2 bar could have been stored in the cryosphere under the form of CO 2 clathrates.…”

Mars: a small terrestrial planet

“…While current variations of climate were thought to be related mainly to astronomical parameters (eccentricity, obliquity), recent observations from the SHARAD radar have shown the presence of a buried deposit of CO 2 ice within the south-polar layered deposits which could have enhanced the total atmosphere pressure by almost a factor of two in recent high obliquity phases (Phillips et al 2011). In addition, clathrates/hydrates are theoretically stable at depth and could contribute to the carbon cycle (Prieto-Ballesteros et al 2006;Chastain and Chevrier 2007;Gainey and Elwood Madden 2012;Mousis et al 2013;Chassefière et al 2013), including the sporadic presence of methane in the atmosphere. These examples show how the poor knowledge of the deep cryosphere is a limitation to current understanding of the current carbon cycle.…”

“…Such a reservoir could contribute significantly to the volatile budget. Buried methane (in clathrate or other geological reservoirs) remains central in questions related to its detection in the atmosphere (Lyons et al 2005;Atreya et al 2007;Chassefière et al 2013). Whether the origin of Mars' methane is biotic or not, its presence will play a central role in future science and exploration efforts, and is also of relevance to the carbon cycle.…”

Martian zeolites as a source of atmospheric methane

“…The origin of the martian methane (CH 4 ) is still poorly understood. Despite the fact that the presence of CH 4 ppbv was evidenced during a timespan of ∼6 months (see Table 1 Mousis et al 2013Mousis et al , 2015. However, because clathrates are more likely thermodynamically stable in the martian subsurface and at depths depending on the soil's porosity (Mousis et al 2013), their existence has never be proven by remote or in situ observations.…”

“…Despite the fact that the presence of CH 4 ppbv was evidenced during a timespan of ∼6 months (see Table 1 Mousis et al 2013Mousis et al , 2015. However, because clathrates are more likely thermodynamically stable in the martian subsurface and at depths depending on the soil's porosity (Mousis et al 2013), their existence has never be proven by remote or in situ observations. Interestingly, it has been recently proposed that halite or regolith could also sequestrate CH 4 on the martian surface (Fries et al 2015;Hu et al 2015), but these mechanisms still need to be thoroughly investigated.…”