How Mars lost its atmosphere

Mars, as we know it today, has a thin atmosphere. The volume of the gases, of which the majority is carbon dioxide, in the atmosphere under 1% of that of Earth's. Evidence from the planet's surface suggests however that Mars atmosphere had to have been a lot thicker. This would have created a strong greenhouse effect that would have trapped the light of the Sun.

Thanks in large part to the many missions to Mars, we now know a few things about Mars’ infancy. For instance, we that in the earlier days of the life of the planet (until roughly 4 billion years ago) Mars had a strong magnetic field.

This magnetic field was created in the same manner that Earth’s was, by the convection currents created by the molten metal of the planet’s core. Unlike Earth though, the core of the planet Mars cooled enough that this mechanism was ‘switched off’. When this happened, there was no global magnetic field.

With this magnetic field gone, Mars was not as well protected from solar winds – this is the stream of charged particles that flows out from the Sun.

How did the magnetic field collapse on Mars?

Nobody is certain about how the magnetic field on Mars came to collapse in the first place, but there are a couple of theories on how it happened.

One theory suggests that Mars once had three moons and not the two that we can observe today (Phobos and Deimos). This third moon, according to the theory, would have been much larger than the two remaining moons, which are relatively small.

The theory goes that the third moon would have had to have been around the same size as Io, a Jupiter moon, and it collided with Mars very shortly before the magnetic field collapse.

A second theory is quite similar if a little more plausible. This one suggests that large asteroids colliding with Mars were responsible.

In both theories, the energy released by these massive impacts would have disrupted the flow of heat in the core of the planet. This in turn would cool the core, shutting off the magnetic field and opening the door for the solar winds to ravage the atmosphere.

Nobody is quite sure how our planet sustains its magnetic field but instead of failing it 'flips' every once in a while – which could be a clue as to how it keeps going. Studies have shown that our magnetic field has weakened by as much as 15% in the last 200 years, and while it is unlikely to fail, it will probably reverse 'poles'. While this reversal happens, the field will reduce to near zero but it will kick back into gear again.

Solar winds are certainly devastating to planetary atmospheres, but we don’t have anything to worry about just yet.

Solar wind killed the Red Planet

Solar wind, unimpeded by a global magnetic field, began stripping away the majority of the Mars atmosphere – this happened in just a few hundred million years after the collapse of the magnetic field.

The process of stripping away the atmosphere happened so quickly (quickly for galactic events, that is) because the Sun rotated a lot faster when it was younger.

A faster rotating, younger Sun, had much more intense solar winds and ultraviolet radiation. Due to this, it was surmised from reports generated by NASA's MAVEN (Mars Atmosphere and Volatile Evolution Mission) spacecraft that atmospheric losses were much greater during this earlier period.

It was also surmised by the team on the MAVEN project that microbial life could very well have existed on the surface of Mars before these processes intensified (i.e. before the collapse of the global magnetic field).

As the atmosphere on Mars began to be lost to space, any life that was present on the surface may have been forced underground.

Losing the majority of the planet's atmosphere to space was a major contributor to Mars' transition from wet and warm, to cold and dry as we know it now. In the end, it was because Earth kept its magnetic field that allowed life to develop and thrive here.