That last breath you took? You can thank the Earth's magnetic field for it. Without the field, researchers believe that solar wind (energetic protons and electrons streaming out from the sun) would have stripped much of the air from our planet long ago. For the field, you can thank the giant, molten ball of iron that comprises our planet's core. And for the fact that it's molten — so that it can slowly spin and churn like a lava lamp, creating a magnetic field — you can say a word of gratitude for the relatively large size of Earth. Who knew breathing was so complicated?
Mars, at about one tenth the mass of Earth, wasn't so lucky. Scientists have plenty of evidence that it, too, once had a molten metal core, and hence a magnetic field. However, because Mars is relatively small, its core cooled and is no longer churning and convecting. Without that churning iron core, Mars lost its magnetic field, leading to the loss of its atmosphere. This seems to account for Martian air pressure being less than one percent of what we experience here in Humboldt County, say.
We know Mars once had liquid water, circa four billion years ago. Orbital photographs clearly show ancient dry riverbeds and alluvial fans, while our Curiosity rover has found water-formed clays and ancient water channels. After losing most of its atmosphere, Mars essentially froze. Scientists would love to understand the process better, especially if dreams of "terraforming" the Red Planet ever come to fruition.
Quick primer on terraforming Mars: The idea is to pump vast quantities of greenhouse gases, such as carbon dioxide and methane, into the Martian atmosphere. These would trap heat from the sun, causing the surface to warm up sufficiently to begin melting the permafrost that lies just beneath much of the surface. After raising Mars' temperature by about 5 degrees Celsius, the process would become self-sustaining: the warmer the surface, the more melting would occur, releasing more gases and water vapor into the atmosphere. Liquid water would return, plants would bloom and colonists could walk around in shirtsleeves, with "rebreathers" on their backs to allow their survival in the largely carbon dioxide atmosphere.
Of course, the long-term problem is that solar wind would strip away the newly minted air, which would eventually need replenishment. One way to estimate just how large a problem this might be is to analyze Mars' atmosphere now. Later this year, NASA will launch a sophisticated Mars orbiter designed to do exactly that. The MAVEN, which stands for Mars Atmosphere and Volatile EvolutioN, is designed to analyze the Martian atmosphere and determine just how Mars lost its air and water to space. By the time MAVEN reaches Mars, in the fall of 2014, Curiosity will have given researchers a pretty clear picture of the thin air in Gale Crater. Between the two sets of readings from Curiosity and MAVEN, dreams of terraforming may get a little closer to reality.
Barry Evans (firstname.lastname@example.org) hears the word "Mars" and thinks "terraforming." Not this year, or next, but one day. You'll see.