Last month a paper was published that examined the loss of atmosphere at Mars during different solar wind conditions. The team, including Swedish and UK astrophysicists, used the Advanced Composition Explorer, ACE, satellite to watch the solar wind emerging from the Sun and the Mars Express satellite to examine conditions at Mars. This month, the paper was included in the research highlights of Geophysical Research Letters.
The solar wind is, if you like, the gas in the outer layer of the Sun expanding due to its heat and radiation pressure. You would therefore expect it to flow outward as smoothly as the solar surface, but the solar surface isn’t quite smooth. Activity on the surface of the Sun can lead to Coronal Mass Ejections, CMEs that dump a lot of material into space in a single burst. Additionally, looking at the Sun in x-ray light, something strange can be seen – certain large areas of the surface are dark. These coronal holes are accompanied by a faster solar wind and as the Sun spins round, this wind is directed into the back of the slow solar wind and so pushes it forward. This forms regions of higher solar wind density (corotating interaction regions, or CIRs) and areas of very low density (rarefaction regions). Mars is in the grip of a CIR or CME about fifteen percent of the time, with normal conditions reigning over the other eighty-five percent.
Mars is without a fully fledged global magnetic field and so has less protection than the Earth against the solar wind. Instead of it being deflected far from the main bulk of the atmosphere, the wind comes down and strikes the Martian skies. The solar wind is composed of a plasma and will try and capture the ionised part of the Martian atmosphere and drag it away. Mars Express measures this by looking at the amount of heavy ions (big atoms like oxygen as opposed to small ones like hydrogen) flowing away from the nightside of the planet.
Forty-one ‘events’ were measured by ACE in the study, mostly CIRs, but some CMEs. Mars Express confirmed that thirty-six of them definitely hit the red planet. During the time of the impacts, Mars Express measured an increase in pickup ions (plasma that can be seen to be different from the normal solar wind) of two and a half times normal fluxes. This means that in the fifteen percent of the time that Mars is in the grip of a solar wind pulse, one third of the total annual atmospheric loss occurs.