Radon-222 ad polonium-210 in the martian atmosphere : a new insight into the exchange of volatiles and the dust cycle
Titre du congrès :LPSC 2007 38th Lunar and Planetary Science Conference
Ville du congrès :League City
Date du congrès :12/03/2007
Radon-222 and polonium-210, two radioactive isotopes from the uranium-238 series, have been measured at the surface of the Moon for decades,
from Explorer 35 to Lunar Prospector, in an attempt to trace today’s lunar outgassing activity. On Mars, however, these isotopes (alpha emitters) had
never been measured up to now, probably owing to the short range of alpha particles in the martian atmosphere (a few meters), while these particles can be detected from orbit on the Moon, and the radon-222 mapped accordingly.
Sabroux et al. have stressed the relevance of measuring radon-222 at the surface of Mars, in particular to estimate the water abundance of the first meters of the subsurface, the emanation of radon being a steep function of the water content in the 0-10wt% range, a likely range in the mid-latitudes regions of Mars according to neutron data . Other issues of interest that can be addressed by measuring radon and its progeny (the short-lived 218Po, 214Po and the longlived 210Po) include the exchange of volatiles between the regolith and the atmosphere, the dynamics of the planetary boundary layer, the detection of active “hot spots”, the atmospheric aerosol cycle and the estimation of the 238U/232Th ratio, provided radon-220 and its progeny are also measured.
An unanticipated by-product of the MER mission was to provide the first evidence of polonium-210 at the surface of Mars, proving in the same time that
radon-222 is present in the martian atmosphere (which was of course expected, but never assessed). This discovery was made possible by the alpha detector of Opportunity’s Alpha Particles X-rays Spectrometer (APXS), whose primary goal was to measure the chemical composition of rocks, soil and dust particles by Rutherford backscattering of alpha particles emitted by a 244Cm source. In the next section, we briefly describe the main steps leading to this detection. We then discuss what the inferred activity level could imply in terms of radon flux on a global scale. We finally compare these results with lunar and terrestrial figures, and draw some preliminary conclusions.