By Dr. Robert Thorson

A good atmosphere is easy to take for granted. This was my first thought when I read the joyous NASA press announcement that Mars “once had conditions suited for ancient life.” Mars lost its opportunity for life because its atmosphere changed too much. But what about planet Earth? Though we’re not in danger of losing our life-giving atmosphere, the question is: Will it be a good life?

Imagine a beautiful lake with an in-flowing stream carrying suspended sediment. That was once the base of Gale Crater on Mars. Imagine the water of that lake clearing as mud settled to the shallow bottom, the mud hardening into mudstone and the rock later exposed as outcrop.

Now fast-forward to last month, when that outcrop was drilled by NASA’s rover, Curiosity, a scientific research laboratory on wheels. Guided from Earth by mission controllers, this ultra-sophisticated geological robot wheeled into position, drilled down 2.5 inches, sampled the subsurface rock and transferred the dusty cuttings to onboard chemical and mineral analyzers.

For me, the most astonishing laboratory result was the abundance of real clay: the proverbial raw material of biblical creation, the dust from which Adam was made. The drilled Martian sample contained more than 20 percent clay, which is typical for lake mud on Earth. Its chemistry and mineralogy provide clear evidence of a very habitable place for microbial life: not too salty; not too acidic; not too oxidizing; and with all the chemical elements needed for life — carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus. Also in that sample was a chemical “food” used by many types of microbes on Earth: sulfide minerals that release energy when in the presence of water.

In short, the conditions for microbial life on Mars were ideal at a time when microbial life was thriving on Earth. Yet, in Curiosity’s final analytical step, no evidence for Martian life was detected by its gas chromatograph mass spectrometer, the prime tool in its search for organic compounds.

This raises two questions: Had those conditions remained at the drilling site would complex life have emerged on Mars? More urgently, was life present elsewhere on the Red Planet before its atmosphere changed too much? The answers await further work on this exciting mission, and those to follow.

I suggest converting this exciting story from Mars into a cautionary tale about Earth’s atmosphere changing too much. Lest you think our atmosphere is stable, consider the fact that Earth is now enjoying its fourth chemically distinct planetary atmosphere.

The first one didn’t last long. Composed mainly of hydrogen and helium, it dispersed to space and was blown away by the solar wind. Its second atmosphere was composed of volcanic gases vented as the earth melted and differentiated into layers: mainly water vapor with smaller amounts of carbon dioxide, sulfur dioxide, nitrogen and ammonia. Eventually, Earth cooled enough for the water vapor to condense, rain out of the sky and fill the oceans.

The resulting third atmosphere was mainly nitrogen, a chemically inert gas making up the bulk of our atmosphere today. It was left behind when the carbon dioxide dissolved into the water and precipitated as limestone, and when the ammonia was broken up by UV radiation and its hydrogen lost to space. Earth’s fourth atmosphere — the oxygenated one we live in today — is fairly recent, having been present for only the final fifth of Earth history. Animals could not exist without it.

Ozone, which shields the land from the sun’s powerful ultraviolet radiation, could not exist without it. This fourth atmosphere has changed significantly within Earth’s last eon. The levels of oxygen, ozone, carbon dioxide and acidity have fluctuated significantly, both above and below present levels. At the present, conditions are just right for us.

The question is: Will we change it too much to suit our needs? The fate of Earth’s atmosphere is in our hands.