Life, Unbounded

Life, Unbounded

Discussion and news about planets, exoplanets, and astrobiology

Summer Astrobiology Roundup #1: How Mars Lost Its Swagger


Young Mars, old Mars? (Original image: NASA)









Summer on our northern hemisphere, and human life slows a little, for some. Science carries on though, and there have been a number of fascinating discoveries and studies hitting the wires in recent weeks.

#1 How Mars lost its swagger...and atmosphere.

Sometimes it's good to reconfirm a suspicion. The Curiosity rover's Sample Analysis at Mars (SAM) laboratory has been sniffing for the isotopic composition of the tenuous martian air. Earlier measurements of the proportions of isotopes like carbon-12 and carbon-13, oxygen 16, 17, and 18, or hydrogen and deuterium, have shown an enhancement in the heavier atomic nuclei compared to what would be expected from the primordial composition of the planet. It's a very particular signature, seen even in tiny 'bubbles' of atmosphere trapped in martian meteorites collected here on Earth. Now SAM's mass spectrometer and laser spectrometer have both confirmed a 'heavy'-skewed isotopic ratio in the atmosphere.

The most plausible physical cause is that over the past 4.6 billion years Mars has been shedding the lighter isotopes into space - leaving the heavier behind. Lighter atoms are more likely to escape from the top of the atmosphere - where particles stop bumping into each other so often, and effectively zoom around on ballistic trajectories. The less mass an atom or molecule has, the greater the chances it has of moving with escape velocity, and being boosted, or 'sputtered', off to space by solar wind protons or other radiation.

In fact, it looks like the majority of the loss had probably already taken place by 4 billion years ago, just 600 million years or so after Mars formed. Young Mars could have therefore had a thick atmosphere and a robust greenhouse effect to keep liquid water flowing on its surface - which would be consistent with a wealth of other evidence for a wet and warm history.

It's possible that some of this quick atmospheric loss was due to the lack of, or early demise of, a strong planetary magnetic field. There's evidence that this kind of magnetism used to exist, but it seems that it may have vanished when the planet was only some 500 million years old.

By contrast, here on Earth, our long-lived field helps stave off the erosive effects of solar particle radiation and lets us maintain our nice thick atmosphere. Why didn't Mars keep its magnetic field? We don't know, but there has been speculation that something switched off its internal planetary dynamo - possibly even an asteroid impact or two.

These new measurements also help set the stage for NASA's next Mars mission, the Mars Atmosphere and Volatile Evolution Mission (MAVEN), due to launch sometime in late November or early December 2013, and arrive at Mars in September 2014. MAVEN's task is to measure today's atmospheric loss on Mars. It will also dip to an altitude of just 80 miles to directly sample the composition of the upper atmosphere, and monitor the interaction of solar radiation with the martian environment.

The information MAVEN gathers will help us understand Mars today, but more importantly how Mars has evolved over the past several billion years. It will also represent a critical piece of 'ground truth' as we continue to study smaller and smaller exoplanets, many of which may be more like Mars than they are like the Earth.

The views expressed are those of the author and are not necessarily those of Scientific American.

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