The students were perplexed. They had just heard the latest updates on the Curiosity rover, a complex piece of space hardware that is slowly rolling its way up a layer cake of rock known as Mount Sharp. Its mission? To decipher Mars' wetter past.
"OK," said the students. "But what if during this slow-motion ascent, Curiosity passes a rock riddled with tiny bits of protoplasm. Would it pounce on the biggest discovery ever made on the Red Planet? Would it recognize life?"
Maybe not. The rover is on the hunt for interesting mineralogy, not biology. If it encounters life, Curiosity could remain as oblivious as a French waiter.
Well then, what's the hang-up, here? The "water found on Mars" story is as perennial as Christmas. NASA doesn't need to tell us that again. So why not cut out the seemingly endless stream of robotic middlemen, and just send hardware that will search not for new data on geology or hydrology, but for life, big or small? Isn't it time to cut to the chase?
That's what the students wanted to know.
Well, I suggested that there were three possible reasons for the space agency's apparent slow motion. I laid them out as follows:
1. NASA has already found life on Mars. The agency is hiding lander and orbital photos showing artifacts such as pyramids or other structures that indicate not just biology, but biology of a high order. However, NASA doesn't want to tell you this, because -- after all -- you would just spin around on the ground if you heard this disturbing news, and society would implode.
Right. Except that this conspiracy theory is nuttier than a bowl of cashews, not least because it ignores the fact that nothing -- nothing -- would increase NASA's funding and prestige more quickly than to announce it had found extraterrestrial life.
2. NASA has been spooked. In 1975, it sent two Viking landers to Mars, the most sophisticated, and most expensive spacecraft it had ever built. They sampled and sniffed the Red Planet's dusty dirt, looking for microbial metabolism. They didn't find it (although one member of the experiment team isn't so sure.)
NASA had gambled big, but walked away from the table empty-handed. It was a sobering experience.
Then in 1993, the agency's SETI program -- a bold initiative to find signals from intelligent life far beyond the solar system -- was canceled in a myopic move by the U.S. Senate. Efforts to look for life were getting a bad rap, and once burned, NASA was twice shy. Or perhaps more accurately, it was twice burned and quadruply shy.
There's no doubt that this is an appealing explanation for the agency's current reluctance to make a forthright search for Red Planet biology. But appeal doesn't make it true.
3. The third possibility is that neither conspiracy nor failure account for NASA's caution. The real explanation is simply a display of good sense: NASA is doing the right thing.
To begin with, in the nearly four decades since Viking, we've learned that the martian surface is as hostile to life as hot lye. It's bone dry, and stung by strong ultraviolet from the Sun. If there's biology on Mars, it's almost certainly sequestered in aquifers hundreds of feet or more below the surface. That's a hard place to investigate with our current crop of robot explorers.
In addition, if you're going to build a machine that can recognize biology, you have to deal with the "pornography problem." How to you know it when you see it? Or sample it? The Viking landers were designed to find life that's analogous to terrestrial biology. That's a pretty specific filter, a bit like sending someone with a butterfly net into the Canadian arctic. It might not be the right tool to snag a sample of the local wildlife.
Michael Meyer, the lead scientist for NASA's Mars Exploration Program, points out that looking for extant life is incredibly restrictive. "It's focused on the here and now, a thin slice of time in which the conditions for life are apparently not so favorable. If you look for extinct life, you have all of martian geologic history to search."
In other words, NASA's step-by-step reconnaissance can identify places where life might once have existed -- even life that died out billions of years ago. And those places could be both more plentiful and more accessible than the regions where life might be around today.
Meyer adds "Looking for extant life is akin to a treasure hunt. Each time you look, the answer is yes or no, and your knowledge gain is either incrementally disappointing or, maybe, eureka!"
So NASA's approach may seem tentative, but the agency is being smart about playing the odds.
The students understood all of this, but with the exuberance that is the hallmark of youth, many still wanted to immediately go for the brass ring, to make the long bet. And of course, it's hard not to smile, be sympathetic, and secretly envision a world in which caution is occasionally thrown to the winds.