Mars Mission Critics Misunderstand Chemistry

I’m finally back from a series of vacations and ready to pick up regular blogging again. And there’s going to be plenty of good material coming up the rest of the summer. Sunday night/Monday morning NASA’s latest Mars mission will attempt to land. Today’s Inquirer featured this front page story about the science behind the mission. Here’s an excerpt:

The rover, called Curiosity, is huge compared to its predecessors, the toy-size Sojourner and the golf-cart-size Spirit and Opportunity. It carries a host of instruments, including devices that measure radiation, detect carbon, and use lasers to vaporize and analyze minerals.

At the landing site, Gale Crater, orbiting spacecraft have previously revealed layers of sediments possibly laid down by a lake. Inside the crater is Mount Sharp, which promises exposed layers of Martian geologic history.

Scientists chose the site in hopes it could address a profound mystery of the Martian climate. Geologic features suggest water flowed there in the distant past, but nobody knows where the water went or why the climate froze up.

"There was a big event in the history of Mars where it goes from being a warm planet, wet planet, that possibly could have been amenable to life, to one that's harsh and extreme," said John Grotzinger, chief scientist for the overall mission, officially known as the Mars Science Laboratory. Solving that mystery might help scientists better understand and model the climate on Earth.

Water is one substance common to all life on Earth, said Grotzinger, a Philadelphia native and now a professor at the California Institute of Technology. On Earth, he said, life exists just about everywhere that has a source of water, energy, and carbon. One of Curiosity's missions is to determine whether Mars ever had those three components of habitability.

Some of the critics in the comments section say they can’t believe the scientists can be so stupid as to focus on carbon based life as opposed to, say, xenon-based life. Those people not only slept through high-school chemistry, they missed the column last week which featured Andy Knoll of Harvard.

NASA defines life as any self-replicating system that can undergo Darwinian evolution. It's a broad enough denfinition that it allows life without carbon, but there are reasons that carbon is well-suited to the task.

Carbon has a tendency to form chains and rings with other carbon atoms. Carbon is good at forming the backbones of long, complex molecules. Silicon has some similar chemical properties but it can’t make up the same variety of compounds. And xenon – I don’t think so.

Knoll said he reminds his students that other planets have the same periodic table of elements and same rules of chemistry that we do here on Earth. If something is chemically impossible here, it’s chemically impossible on Mars too.
If we define life too loosely, then any rock can be declared a living organism, and the quest becomes uninteresting. Scientists have to have some standards. Read more here.