Stability of hydroxylated minerals on Mars: A study on the
effects of exposure to ultraviolet radiation
Albert S. Yen
Jet Propulsion Laboratory, California Institute of Technology, Pasadena,
California
Bruce Murray and George R. Rossman
Division of Geological and Planetary Sciences, California Institute of Technology,
Pasadena, California 91125-2500
Frank J. Grunthaner
Jet Propulsion Laboratory, California Institute of Technology, Pasadena,
California
Abstract
The density and composition of the Martian atmosphere allow solar ultraviolet
photons with wavelengths as short as 190 nm to reach the surface. We investigate
the hypothesis that this UV radiation is capable of inducing the release of
water from iron oxyhydroxide minerals resulting in the formation of oxide
phases. These experiments, which utilize a quadrupole mass spectrometer to
monitor the water vapor pressure above mineral samples during cyclic exposure
to ultraviolet radiation, offer 5 to 6 orders of magnitude greater sensitivity
than previous attempts to establish and quantify this process. We find no
evidence that UV photons are capable of liberating OH from the crystal lattice
of minerals, and we set a minimum ultraviolet radiation-induced dehydroxylation
time of I 108 years for removal of this structural OH from mineral
particles at the Martian surface. The overturning timescales for surface fines
are likely to be shorter than this lower limit for exposure time. Thus we
conclude that UV-stimulated dehydroxylation is not a significant process at
the Martian surface and that iron oxyhydroxides, if formed during an earlier
water-rich environment, should still be found on Mars today. The lack of clear
evidence for iron oxyhydroxides at the Martian surface further suggests that
Mars' surface was never warm and wet for a long enough period of time for
Earth-like weathering to have occurred.
Journal of Geophysical Research 104, 27,031-27,041