Erosion of planetary atmospheres

5.4 Erosion of planetary atmospheres

Even if the solar wind plays no role in the solution to the Faint Young Sun paradox described in Section 5.3, the solar wind may still have had a dramatic effect on planets in the past through the eroding effects of the wind on planetary atmospheres. Solar wind sputtering processes have been proposed as having had important effects on the atmospheres of both Venus and Titan (Chassefière, 1997 ; Lammer et al., 2000 ), but it is the Martian atmosphere that may have been the most dramatically affected by the solar wind. The connection between the Martian atmosphere and the question of whether life once existed on the planet makes the Mars example even more interesting.

Mars apparently had running water on its surface in the distant past, but it is now very dry. Isotopic evidence strongly suggests that the Martian atmosphere was once much thicker and more conducive to the existence of surface water (see Carr, 1996 ; Jakosky and Phillips, 2001 ). What caused the loss of the early Martian atmosphere, and presumably the loss of surface water and habitability as well? Solar wind sputtering is a leading candidate for the cause of these changes (Luhmann et al., 1992 ; Perez de Tejada, 1992 ; Jakosky et al., 1994 ; Kass and Yung, 1995 ; Lundin, 2001 ). Unlike Earth, the Martian atmosphere is not currently protected from the solar wind by a strong magnetosphere. There is evidence that Mars once had a magnetic field, but it disappeared at least 3.9 Gyr ago (Acuña et al., 1999 ). At that point, Mars would have been exposed to a solar wind about 80 times stronger than the current wind according to Figure 15 . The stronger solar wind makes wind erosion an even more likely culprit behind the disappearance of most of the Martian atmosphere. Update

Studies of Martian atmosphere evolution that consider the wind evolution law inferred from the astrospheric measurements, along with modern estimates of the history of solar X-ray and UV fluxes, have already begun (Guinan et al., 2003 ; Lammer et al., 2003 ; Ribas et al., 2005 ). Investigations into how the solar wind may have affected other planets will undoubtedly also be an active area of future research, stimulated in part by the discovery of planets around other stars. Many of these extrasolar planets orbit very close to their stars, which means that they will be exposed to wind fluxes orders of magnitude higher than the Earth or Mars have ever seen. Knowledge of the evolution of solar-like winds is crucial for understanding the atmospheric evolution of these extrasolar planets, and efforts are already underway to try to model wind erosion effects on such planets (Grießmeier et al., 2004 ; Preusse et al., 2005 ).