Astronomy

A Windy Day on Mars

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Science  21 Jan 2000:
Vol. 287, Issue 5452, pp. 393
DOI: 10.1126/science.287.5452.393c

The solar wind is a plasma with interplanetary magnetic field lines. Earth's intrinsic magnetic field largely shields us from the effects of the solar wind. Mars, however, has no permanent magnetic field; when the solar wind meets Mars, it creates a bow shock, and magnetic field lines compress between the bow shock and ionopause of Mars. This thin layer of plasma is called the magnetic pile-up boundary (MPB).

The Mars Global Surveyor (MGS) spacecraft passes through the MPB daily in its polar orbit. Crider et al. analyzed the electron flux data from one orbit as measured by the electron reflectometer aboard the MGS. They noticed a decrease in the electron flux in the MPB at energies between 40 to 150 electron volts, suggestive of electron impact ionization of neutral H, O, and CO2 above the ionosphere. These data were used to construct a qualitative model incorporating the density of the neutral species, the velocity of the solar wind flow around the planet, and the magnetic field geometry of the interaction. Through electron impact ionization, the incoming solar wind ionizes the neutral species and sweeps them away, thus eroding the martian atmosphere.—LR

Geophys. Res. Lett.27, 45 (2000).

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