28 November 2013 08:25

Generations of science fiction fans have grown up thinking of Mars as the home of weird, and rarely friendly, extra-terrestrial creatures. The exotic red planet has almost nothing in common with our own home and is the very definition of an alien world. Or is it? Red Planet Day seems like a good opportunity to reconsider our Martian neighbour.

It may come as something of a surprise, but in many respects Mars is rather similar to the Earth: the radius of its orbit around the Sun is only slightly (approximately 50%) larger than the Earth’s and its diameter is only slightly (approximately 50%) smaller. Like the Earth it is a rocky body, and like the Earth it probably has a metallic core. Compared to the truly exotic gas giants in the outer solar system, Mars looks very similar to our own planet. However, in other important respects, there are stark differences: Mars is a cold, dry world shrouded in a thin atmosphere with surface atmospheric pressure almost two hundred times lower than at Earth. The real surprise is why has Mars has turned out so differently from the Earth?

The reason is partly because of each planet’s connection to the space environment. When the Earth and Mars were formed in the early solar system, both planets comprised a mixture of rocky and metallic material. The denser metals in these early bodies settled in the centres of the planets, forming cores of nickel and iron. In the Earth’s case, enough of the heat of formation (and the heat produced by the decay of radioactive elements) remains to this day for the metallic core to remain molten. Currents flowing in the liquid core give rise to a dynamo action, generating a strong magnetic field. This force field shields our atmosphere from the erosive effect of the solar wind – the stream of sub-atomic electrically charged particles constantly emitted by the Sun. But at Mars, the planet’s smaller size meant that it contained less heat after formation and that the planet cooled at a much faster rate. The molten core would have frozen and solidified about four billion years ago and, without a dynamo at its heart, Mars has not had a strong magnetic field to divert the solar wind flow around the planet. The solar wind has had direct access to the Martian atmosphere and over billions of years this has contributed to the loss of the red planet’s atmosphere to interplanetary space.

On 18 November 2013, NASA’s latest mission to the red planet blasted off from Cape Canaveral in Florida. The Mars Atmosphere and Volatile EvolutioN (MAVEN mission), is NASA’s first mission dedicated to exploring the upper atmosphere of Mars and finding out how this process happened, possibly turning a planet once habitable, to microbial life into the cold and barren desert world we see today. The spacecraft is specifically designed to skim the upper atmosphere and make a series of “deep dip” measurements to measure the rate at which atmospheric volatiles such as water vapour are being stripped away into space. Here at Lancaster we’re interested in the aspects of the mission that also investigate how the remaining pockets of weakly magnetised regions of the crust modulate the interactions between the solar wind and the Martian atmosphere. Decades of space environment research at Earth have revealed how our planet’s strong magnetic shield is essential for the development of sustainability of biological life. Soon, for the first time in human history, we’ll be able to study in detail how in the absence of a planetary magnetic field, the same space environment can drive climate change with catastrophic implications for a planet’s habitability.

What do you think? Share your comments with us below.

Professor Jim Wild teaches on our BSc Physics, Astrophysics and Cosmology programme.


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