Late evolution of the Martian atmosphere.

Details Late evolution of the Martian atmosphere. Late evolution of the Martian atmosphere.

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..232g&link_type=abstract

European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a

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The study of the past evolution of terrestrial planets relies heavily on models and assumptions due to the lack of data and in situ measurements. However, it is necessary to explain some of the features that are observed by recent missions. We show that even with the few data available we can propose a scenario for the evolution of the Martian atmosphere over the last three billion years. We study possible states of the past Martian atmosphere consistent with present observation. Our model is obtained with a back integration of the Martian atmosphere, and takes into account the effects of volcanic degassing, which constitutes an input of volatiles, and atmospheric escape into space. We focus on CO2, the predominant Martian atmospheric gas.The degassing of volcanic CO2 is a direct consequence of the amount of melted material created by the activity of the mantle of the planet. Here, crust production rates from numerical models [1], [2], [3] are taken as input for the mantle degassing. By estimating the volatile contents of the lavas, the amount of volatiles released in the atmosphere is estimated for different scenarios. In the absence of carbonates, atmospheric escape seems to be the principal mechanism for the loss of CO2. Both non-thermal processes (related to the solar activity) and thermal processes are studied and nonthermal processes are incorporated in our modelling of the escape [4] for hydrodynamic escape mostly takes place during the first few hundreds of million years and is a minor effect during the late evolution of the planet, involving mostly very light species. We used measurements from ASPERA and Mars Express and these models to estimate the amount of lost atmosphere. An evolution of the CO2 pressure consistent with its present state is then obtained. A crustal production rate of at least 0.01 to 0.1 km3/year is needed for the atmosphere to be at steady state at present time, which is consistent with low activity as observed today. Moreover, we show that for our scenarios a rapid loss of the primary (and primordial) atmosphere due to atmospheric escape is required in the first 2 Gy. The period of "high" CO2 pressures coincides with the formation of fluvial landforms [5] and we show that the atmosphere at that time was probably able to sustain liquid water on the surface at least for short periods provided the surface temperature is high enough. However our model shows that over the past three billion years it is unlikely Mars could harbour a thick (>1 bar) atmosphere. It also implies that present-day atmosphere has probably a volcanic origin and has been created late in the history of the planet. If the volcanic activity and the degassing are intense enough, our models strongly imply that in most of the cases the atmosphere of Mars is not much older than 1.5 Gy, with some cases where it is as young as 1 Gy rather than some residue from a primordial one that would have been depleted by atmospheric escape over geological times. Our study finally provides us with constraints on the CO2 concentration in the mantle of Mars. Low CO2 compositions seem to be required to obtain the present atmosphere and the CO2 concentration in the Martian mantle probably doesn't exceed 200 to 500ppm, which is consistent with data from Martian meteorites and terrestrial values [6]. We complete our work with the investigation of the melt production and volatile degassing occurring over the history of Mars in the Tharsis Province. We use a 3D numerical model to study the possibility of a single stable plume rising under Tharsis and producing melting consistent with the formation of the Province during the first billion years as well as with late lava flows discovered in this region [7]. References [1] Breuer D. and Spohn T. (2006) Planetary and Space Science 54, 153-169. [2] Manga, M. et al. (2006) American Geophysical Union Fall Meeting 2006, Abstract #P31C-0149. [3] O'Neill, C., et al., (2007) Journal of Geophys. Res., vol. 112. [4] Chassefière, E. et al. (2006) Planetary and Space Science, Volume 55, 343-357. [5] Mangold, N. et al. (2004) Science, vol. 305, 78-81. [6] Trull, T. et al., (1993) Earth and Planetary Science Letters, 118, 43-64. [7] Neukum, G. et al., (2004) Bulletin of the American Astronomical Society, Vol. 36, p.1136. [8] Gillmann, C. et al., submitted to epsl.

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