05 January 2012

Why Mars Lost its Air

Last year NASA approved a new orbiting satellite mission to Mars, called Mars Atmosphere and Volatile Evolution, or MAVEN for short. There is growing evidence that Mars once had water on its surface - and water cannot exist if there is no atmosphere, because it would evaporate quickly. There are unmistakable rivulet marks on rocky Martian slopes, evidence of sedimentary layering in giant Martian basins, minerals detected by various means that can only form in the presence of water - all discovered by orbiters and semi-automated rovers like Sojourner, Spirit, and Opportunity.

If Mars once had an atmosphere, then where did it go? More important: WHY did it go? Planetary scientists speculate that Mars lost its atmosphere when it lost its magnetic field.


Most people have no idea how important our Earth's magnetic field is. If you've seen videos of the Northern Lights, you have seen our planet's magnetic field at work: it traps the huge CME (Coronal Mass Ejection) blasts that come out of the Sun during the more active times of the Solar Sunspot cycle. The magnetic field bends these highly energetic charged solar particles until they come into the atmosphere near the magnetic poles.Those beautiful shimmering curtains of light you see are highly energetic particles literally tearing up the upper atmosphere.

Dr. Robert Brown was my adviser at Berkeley when I was a young student there. He was a great guy, and never hesitated to stop whatever he was doing to answer the physics questions I always had. His own experiments as a physicist concerned sending balloons into the troposphere to collect some of these energetic solar particles. They were so energetic, he told me, that he had actually captured iron nuclei with so much energy that all the electrons had been stripped off! That sort of stuff does terrible things to soft tissue - it's a burn that goes far beyond a simple sunburn. It is hard ionic radiation, and it kills.

Without our planet's magnetic field, the latitudes humans live at would be constantly blasted by CME's. Without the atmosphere, those deadly high-energy particles would blast right down to the ground, killing all life, sterilizing the planet's surface like the Moon.

So how are the magnetic field and the atmosphere related? If the magnetic field is turned off, the CME's would not be deflected, but would scour our atmosphere unimpeded and blast it off the face of the planet into interstellar space, instead of just nibbling at the atmosphere around the poles. Point a blow-torch at something and what happens? You've got the picture. That's what scientists believe happened to Mars sometimes in its ancient past: as its smaller core cooled, its magnetic field died, and the solar wind and CME's stripped the atmosphere away.

Let's go a little deeper - literally. If the interior of the planet cooled, convective movement of the hot liquid material inside would slow and stop - and with it, the magnetic field created by that conductive fluid dynamo. Interestingly, our Earth's magnetic field might or might not have something to do with radiation. Heavy radioactive minerals, that sank to the mantle and core of the Earth during planetary formation, will still decay. The heat has to go somewhere, and geomagnetic specialists believe this sets up convection like what you see in your kitchen: heat rises because hotter materials expand and have lower density.

Much if not most of the original heat in the inner Earth probably comes from gravitational collapse during the formation of the primordial Earth, however. There is also compositional convection thought to be going on in the core itself as it cools: metallic iron plates out onto the solid mostly-iron inner core, leaving lighter materials to float up towards the mantle. The Earth's magnetic field is a complex thing made up of a dipolar field and a secular field: a big, steady north-south magnetic dipole, and something added on top of this that "drifts" the north magnetic pole westward about 0.2 degrees each year.

An interesting aside from my days as a high-pressure solid-state physicist: rocks that are resistive at room temperature generally become highly conductive and plastic when heated and pressurized (even diamond), and the moving conductor that results is what generates the Earth's magnetic dynamo.

Perhaps we can also look farther out. Gene Shoemaker, my great friend and the brilliant father of Astrogeology, felt very strongly that life exists on Earth also in large part because of Jupiter. Yes, Jupiter. As the largest planet by far, it has a huge gravity field - as proven by the impact of Comet Shoemaker-Levy 9 in 1994. The dark blots that SL9 caused in Jupiter's banded atmosphere were as large as the planet Earth! Jupiter and the Sun drag comets coming in from the Oort Cloud to themselves, and away from the inner worlds - the Sun by itself would just draw more bombardment to the inner solar system if it was by itself. Gene actually thought that the Drake Equation - which is an estimate of the probability of life outside of the Solar System - should be modified to include a factor he called Fj - the Jupiter factor. Like a big brother on the playground, Jupiter protects the Earth from the Oort bullies.

So we are alive because of heat, perhaps some of it due to radioactive decay - and a Big Brother planet orbiting outside of the inner solar system.


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