Alejandro Soto

• Home
• Research
• Publications
• Flight Projects
• Photos
• Contact

Dynamical Paleoclimatology of Mars

For a range of realistic initial CO2 inventories, atmospheric evolution models show an early and rapid collapse of the Martian atmosphere, forming massive ice sheets, and possibly precluding sustained surface liquid water. The extent of this phenomena depends critically on the dynamics of atmospheric collapse. I am investigating the dynamics controlling collapse, including heat transport and ice sheet formation processes, using the MarsWRF GCM.

The evolution of the Martian atmosphere is a fundamental problem in both atmospheric and geological science. Study of the martian surface shows a rich history of chemical and physical modification, suggesting a more active and clement surface environment in the past. A long-standing problem has been reconciling the current thin and cold atmosphere with this presumably thicker and warmer environment. Atmospheric loss to space can erode only limited quantities of CO2 [Luhmann et al., 1992; Kass and Yung, 1995], while the absence of extensive carbonate deposits on the martian surface [Bandfield et al., 2003; Bibring et al., 2005 and references therin] has cast doubt on the role of chemical weathering as a primary loss mechanism. Impact erosion fluxes are still somewhat uncertain following the late heavy bombardment, and though some have argued for the important role of impact erosion [e.g. Melosh and Vickery, 1989; Brain and Jakosky, 1998] the jury is still out on the efficacy of impacts for removing a significant frac-tion of the atmosphere [cf. Melosh and Vickery, 1989 and Newman et al., 1999], and it is not the predominant sink of CO2 after the first billion years [Brain and Jakosky, 1998]. In any case, this problem has spawned efforts to track the evolution of the planetary CO2 budget considering a range of reservoirs and loss processes.

Decrease in the surface pressure through geological time appears to have been thought of as a roughly secular, steady process until the evolution study of Haberle et al. [1994]. Although the dynamics of CO2 ice caps had already been extensively examined in terms of seasonal and orbital forcing [e.g. Leighton and Murray, 1966; Ward, 1974; Ward et al., 1974; Fanale et al., 1982], the timing of (quasi-)permanent CO2 cap formation in Martian history had not been formally addressed before. A major finding of Haberle et al. [1994] was that surprisingly early in planetary history, and for a range of initial total CO2 inventories, the atmosphere would become unable to transport enough heat to the poles to stave off year-round CO2 polar caps. As a consequence of cap formation, the atmosphere would collapse to a vapor pressure, or cap-buffered, state. If Mars were trapped in a collapsed state for most of its planetary history, the amount of time available for physical and chemical weathering would, as a result, be greatly limited. I use the following definition of the term "collapsed" or "collapsed atmospheric state": the presence of at least one permanent CO2 ice cap, and vapor pressure balance between the atmosphere and that cap. It is debatable (or at least marginal) whether the present martian atmosphere is collapsed, since the southern residual ice cap contains such a small amount of CO2 that it cannot significantly buffer the atmosphere.

If collapse indeed occurs, it is of huge importance for understanding the geological history of Mars and needs to be considered as the foundation of a reconstituted geological paradigm. If collapse did not occur, previous paleoclimate evolution models (which justify collapse) are missing important physics and presumably making bad predictions of the history of volatile abundance as a result. In either case, a thorough investigation of collapse dynamics is sorely needed to understand just how widely physical climate dynamics and the geo-logical record of climate are disconnected, and, likewise, the fields of atmospheric science and geology.

The dynamics of collapse needs more detailed study because the prediction of collapse in the extant global-mean climate models (e.g. Haberle et al. [1994]; Manning et al. [2006]) involves representation of an inherently three-dimensional, time varying process, such as heat transport, in terms of a single, globally uniform "constant". This constant is unavoidably the weakest link in any low-order (0-D and 1-D) atmospheric evolution model, though its proper representation is only of critical importance when the atmosphere is near a significant transition, such as the threshold for collapse.

A basic theory of atmospheric heat transport as a function of atmospheric mass and planetary spin/orbital parameters is still lacking for atmospheres. Heralded approaches such as the maximum entropy production formulation [Lorenz et al., 2001; Lorenz, 2001] are insufficient for our purposes. The latitudinal dependence of ice cap formation and of the thermal forcing of the surface also suggest the need for a higher-dimensional study. Tellingly, Haberle et al. [1994] noted nearly 15 years ago that, "Clearly, a more sophisticated model . . . is needed to access the reality of massive early caps." Yet such an approach has still never been attempted. The model needed requires sufficient sophistication to prognostically study at-mospheric heat transport by a range of dynamical processes. The problem thus requires study by a general circulation model, which is the primary technique that I am applying. My research focuses on the thermodynamical, as opposed to chemical or loss-driven, stability and collapse of the Martian paleoclimate with a model capable of prognostically simulating the processes.

Bandfield, J. L., T. D. Glotch, and P. R. Christensen (2003), Spectroscopic identification of carbonate minerals in the martian dust, Science, 301, 1084-1087.

Bibring, J.-P. and 10 authors (2005), Mars surface diversity as revealed by the OMEGA/ Mars Express observations, Science, 307, 1576-1581.

Brain, D. A. and B. M. Jakosky (1998), Atmospheric loss since the onset of the martian geologic record: Combined role of impact erosion and sputtering, J. Geophys. Res., 103(E10), 22,689-22,694.

Fanale, F. P., J. R. Salvail, W. B. Banerdt, and R. S. Saunders (1982), Mars?The rego-lith-atmosphere-cap system and climate change, Icarus, 50, 381-407.

Haberle R. M., D. Tyler, C. P. McKay, and W. L. Davis (1994), A model for the evolu-tion of CO2 on Mars, Icarus, 109, 102-120.

Kass, D. M. and Y. L. Yung (1995), Loss of atmosphere from Mars due to solar wind-induced sputtering, Science, 268, 697.

Lorenz, R. D. (2001); Erratum: ?Titan, Mars and Earth: Entropy production by latitu-dinal heat transport?, Geophys.Res. Lett., 28, 3169-3170.

Lorenz, R. D., J. I. Lunine, P. G. Withers, and C. P. McKay (2001), Titan, Mars and Earth: Entropy production by latitudinal heat transport, Geophys. Res. Lett., 28, 415-418.

Luhmann, J. G., R. E. Johnson and M. H. G. Zhang (1992), Evolutionary impact of sput-tering of the Martian atmosphere by O(+) pickup ions, Geophys. Res. Lett., 19, 2151-2154.

Manning, C. V., C. P. McKay and K. J. Zahnle (2006), Thick and thin models of the evo-lution of carbon dioxide on Mars, Icarus, 180, 38-59.

Newman, W. I., E. M. D. Symbalisty, T. J. Ahrens and E. M. Jones (1999), Impact ero-sion of planetary atmospheres: Some surprising results, Icarus, 138, 224-240.

Richardson, M. I., A. D. Toigo, and C. E. Newman (2007), PlanetWRF: A general pur-pose, local to global numerical model for planetary atmospheric and climate dy-namics, J. Geophys. Res., doi:10.1029/2006JE002825.

Ward, W. R. (1974), Climatic variations on Mars. I. Astronomical theory of insolation, J. Geophys. Res., 79, 3375-3386.

Ward, W. R., B. C. Murray, and M. C. Malin (1974), Climatic variations on Mars. II. Evolution of carbon dioxide atmosphere and polar caps, J. Geophys. Res., 79, 3385-3395.