Scientists have said that if Yellowstone wasn't a National Park it might be a Superfund site. The place is dotted with hundreds of scalding, pools, with the ph of battery acid and bright with the colors of cadmium, arsenic and other toxic metals. But toxic is in the eye of the beholder. Deadly for humans, sure, but now we know there's a whole section of the tree of life that does just fine here. In fact, the so-called archeans that live in the pools may represent the part of the tree with the deepest roots.

This is where I've spent the last few days – a perfect place for appreciating the diversity of life on the eve of NASA's most ambitious mission to Mars, to arrive on the night of August 5 or morning of the 6th, depending on your time zone.

Yellowstone's microbial communities and other so-called "extremophiles" have revolutionized the way scientists think about the possibility for life on Mars. Life doesn't necessarily need moderate conditions or sunlight. Living things do need some source of energy, but life is flexible – chemical reactions can supply the energy at the base of a food chain.

For years people assumed the bright colors in the hot springs were the work of minerals. It wasn't until the later part of the 20th century that the biology community came around to the realization that the steaming pools were teeming with life. The rings of color come from different types of microbes that colonize different regions, with the darker-ones generally congregating in the less extreme heat.

Next week's mission will put a new rover on Mars – a vehicle named Curiosity. It will explore an ancient crater for signs that liquid water flowed during a long-ago time when the climate was warmer. It will test for carbon in the rocks and sniff the atmosphere for methane – signs that life might have originated on this now desolate, frigid planet.

All life on Earth appears to be related, using DNA and RNA molecules to pass down assembly instructions and other information. We share stretches of this genetic code with bacteria, yeast and amoebas. But if life originated on Mars, it might use a completely different way of storing and transmitting information. NASA defines life as any self-replicating system that undergoes Darwinian evolution.

One of the big challenges for science is being able to recognize signs of past life and not over- react to false alarms. In 1996, NASA made the explosive announcement that a team of researchers had found signs of fossil bacteria in a meteorite that fell from Mars. Tiny forms looked to the NASA team like bacteria, and organic compounds called polycyclic hydrocarbons resembled similar compounds made in living things here. Soon after the announcement, biologists quickly pointed out that the shapes and the compounds could be explained by processes that involved no life.

One of those skeptical voices was paleontologist John Grotzinger, then at MIT and now at Caltech. NASA apparently doesn't hold a grudge against naysayers since he's now chief scientist on the upcoming mission. He's from Philadelphia, so I'll be writing more about him.
The Mars mission will be the subject of this week's column to be posted Sunday night or Monday. Stay tuned.