Solar UV irradiation conditions on the surface of Mars
Solar UV Irradiation Conditions on the Surface of Mars
ABSTRACT
The UV radiation environment on planetary surfaces and within atmospheres is of importance in a wide range of scientific disciplines. Solar UV radiation is a driving force of chemical and organic evolution and serves also as a constraint in biological evolution. In this work we modeled the transmission of present and early solar UV radiation from 200 to 400 nm through the present-day and early (3.5 Gyr ago) Martian atmosphere for a variety of possible cases, including dust loading, observed and modeled 03 concentrations. The UV stress on microorganisms and/or molecules essential for life was estimated by using DNA damaging effects (specifically bacteriophage T7 killing and uracil dimerization) for various irradiation conditions on the present and ancient Martian surface. Our study suggests that the UV irradiance on the early Martian surface 3.5 Gyr ago may have been comparable with that of present-day Earth, and though the current Martian UV environment is still quite severe from a biological viewpoint, we show that substantial protection can still be afforded under dust and ice.
INTRODUCTION
Studies of the UV environment of the Archean era on early Earth (2.5-3.8 Gyr ago) suggest that the atmosphere was essentially anoxic (1-4) resulting in an O3 column abundance insufficient for protecting the surface in the UV-B (280-315 nm) and the UV-C (200-280 nm) ranges. Because solar radiation in the UV-B and UV-C range penetrated through to the surface on early Earth, associated biological consequences may be expected. Recently Cockell and Homeck (1) investigated the history of the UV radiation of Earth via theoretical and space-based observations and found that an effective O^sub 3^ column was established at an O2 fraction of about 5 x 10^sup -3^ of the present atmospheric level.
From 1996 to 2000 the European Space Agency (ESA) began preparation of a Planetary Exploration Initiative, now called AURORA, to analyze future biological research beyond Low Earth Orbit (LEO) and recommended the development of exobiology exploration packages to be placed on the Martian surface (5). It is expected that in future, biological research in LEO and on other planets will play a major role in the study of the origin of life. Although the UV climate of early Earth is still somewhat speculative, the study of the biological effects of UV radiation on Mars will not only help us understand the potential fate of biological evolution on Mars or the fate of organisms transferred to Mars on spacecraft but could also be applied to understanding the potential UV stress on microorganisms and molecules essential for life on the surface of early Earth if the ancient Earth lacked a protective O^sub 3^ column abundance.
The present 7 mbar pressure CO2 atmosphere is a result of strong atmospheric escape to space (6-10) and surface weathering processes (11-13). Ancient fluvial networks on the surface suggest that Mars was warmer and wetter billions of years ago, implying a higher atmospheric surface pressure. Surface features resembling massive outflow channels provide evidence that the Martian crust contained the equivalent of a planet-wide reservoir of H2O several hundred meters deep (e.g. [14,15]).
Future Mars missions like ESA's Mars Express with the Beagle 2 lander, the planned Exobiology Multi-User Facility, Mars Netlander and other spacecraft are searching for indirect and direct evidence of traces of past and present microbial life-forms, or at least for organic molecules in the Martian surface. Mars appears to have no life on its dry surface today, but by analogy with Earth, life or precursors of life may have originated on the planet early in its history.
Beagle 2 is a 30 kg lander for Mars optimized for exobiology, that will be launched in 2003 as part of ESA's Mars Express mission (16). One of the instrument packages is a suite of sensors designed to monitor the local meteorological and radiation environment, to aid in the determination of whether life did, or still can exist on the Martian surface. One of these sensors is a UV sensor array, consisting of six photodiodes selectively monitoring different bandpasses between 200 and 400 nm. In situ measurements of the UV environment at the Martian surface will therefore be possible for the first time (17).
Nucleic acids are prominent targets for UV photons, and damage to these structures can serve as a good indicator of the hazard for living systems/molecules due to solar radiation. Nucleic acids of various conformations, their specific components or their complexes with proteins can modify the quality and/or quantity of UV photoproducts. Thus using different nucleic acid targets, indirect information can be obtained on the radiation environment of different planets at different periods during their history.
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