RADIATION AND PARTICLE EXPOSURE OF THE MARTIAN PALEOATMOSPHERE: IMPLICATIONS FOR THE LOSS OF WATER

摘要

Multi-wavelength studies of solar like G-type stars at several stages of their main sequence evolution indicate that our Sun may have also undergone a highly active phase in its particle and radiation evolution 3.0-4.5 Gyr ago. Detailed observations of such Sun-like stars by the ROSAT and ASCA X-ray satellites as shown in Fig. 1 indicate that the X-ray luminosity (see Fig. 1) and solar wind density of the early Sun may have been several hundred times higher than today. Studies of isotope anomalies in planetary atmospheres and meteorites suggest also that our early Sun underwent a very active phase after its origin that included continuous flare events and had a particle and energy irradiance several hundred times stronger than today. The early Martian atmosphere proves to be a very efficient shield for X-rays: with the exception of very hard X-rays (1 < 20 A) which penetrate deeper into the atmosphere, the longer wavelength is absorbed in the atmospheric upper layers. This high altitude absorption results in an energy deposition at low-density levels that further enhance the thermal and nonthermal escape processes already known to be driven by the high EUV radiation and solar wind particle interaction. We discuss also the consequences of hydrodynamic escape of water and pick up ion processes from early Mars.