The Goldilocks Zone

Panorama of Martian landscape around the Phoenix Mars Lander

In the comments thread of this post on Vjack's Atheist Revolution site, a Christian commenter going by the name of Thomas wrote:

Now on a different note, this always amazed me. Science tells us that if the earth was a small amount space closer to the sun, our atmosphere would disintegrate and we would burn up. If the earth were a small amount of space farther from the sun, we would all freeze and die. What keeps us the EXACT distance from the sun so that life will continue on our planet? Is it something we as humans can take credit for?

I replied to Thomas that he was spouting the "Goldilocks Zone" argument, which is another way of saying that our Earth is "fine tuned" for life. After all, is it just a coincidence that our planet is in just the right spot for life? Of course, that assumes that we are at the "EXACT distance."

Yesterday, NASA announced that "[l]aboratory tests aboard NASA's Phoenix Mars Lander have identified water in a soil sample. The lander's robotic arm delivered the sample Wednesday to an instrument that identifies vapors produced by the heating of samples." There is now considerable evidence that there were once "vast lakes, flowing rivers and deltas on early Mars, all of which were potential habitats for microbes."

From the mission objective for the Phoenix Mars Lander:

"Recent discoveries have shown that life can exist in the most extreme conditions. Indeed, it is possible that bacterial spores can lie dormant in bitterly cold, dry, and airless conditions for millions of years and become activated once conditions become favorable. Such dormant microbial colonies may exist in the Martian arctic, where due to the periodic wobbling of the planet, liquid water may exist for brief periods about every 100,000 years making the soil environment habitable.

Phoenix will assess the habitability of the Martian northern environment by using sophisticated chemical experiments to assess the soil's composition of life-giving elements such as carbon, nitrogen, phosphorus, and hydrogen. Identified by chemical analysis, Phoenix will also look at reduction-oxidation (redox) molecular pairs that may determine whether the potential chemical energy of the soil can sustain life, as well as other soil properties critical to determine habitability such as pH and saltiness."

So, Thomas is wrong. If the Earth was a small amount of space farther from the sun, we would not necessarily all freeze and die. If Mars had periods where parts of its surface were covered with liquid water, then our Earth could have been 100 million miles from the sun instead of 93 million miles, and our planet should still have been warm enough to possess liquid water. Then again, even at its current distance from the sun, there are some scientists who theorize that some 700 million years ago, our Earth was entirely encased in ice. This theory has been dubbed "Snowball Earth." As this article shows, while there is counter-evidence that the entire planet was not covered in ice, even critics of the "Snowball Earth" theory acknowledge that the Earth during the time period in question experienced more glaciation than at any other time in the planet's existence.

Of course, under such conditions, the human race would probably freeze to death. But life itself need not. There are organisms that seem to thrive in environments and temperatures hostile to human life. We call these organisms, appropriately enough, extremophiles. As Doctor Penelope Boston writes in the article I just linked to, "On Earth, we have environments ranging from the superheated waters of submarine volcanic vents to the ultra-dry bitter cold of the Antarctic Dry Valleys. We find organisms living in caves dripping with sulfuric acid and others thriving in intensely alkaline solutions. We find creatures happily existing in saturated salt solutions, enduring megadoses of ionizing radiation, or deriving their food and energy sources from unpromising inorganic materials like manganese, iron, and sulfur compounds."

Will we find evidence that such organisms currently or once did exist on Mars? Who knows? NASA reports that it is extending the mission of the Mars Phoenix Lander to September 30, 2008. In the weeks after that date, the data collected from the probe will continue to be analyzed. I, for one, eagerly await what new discoveries await us.