Mars has been a favorite place for speculations about life for centuries, right through the canal controversy. In the 20th century, things got more complicated when it became clear that the Martian atmosphere was much too thin to support Earth-style respiration (and, by such expedients as carrying telescopes to the summit of Mt. Whitney, astronomers learned there was no measurable water vapor in the Martian atmosphere). The environmental conditions are formidable - atmospheric pressure less than 1% of the Earth's, too little to warm the surface through a greenhouse effect, and this almost entirely carbon dioxide. The diurnal temperature swings are huge, though most parts of the planet don't get above the freezing point of water. Most of the solar ultraviolet radiation reaches the surface unimpeded by the tiny amount of ozone in the atmosphere. When people describe Mars as the most hospitable of the other planets, this might be more accurately translated as "not trying to kill you as hard as the others".

Still, experience on Earth has taught us that life is very hardy and can survive in the most unlikely niches, such as undersea volcanic vents and boiling volcanic hot springs (not to mention bathrooms, as anyone can attest who's tried disinfecting one). Any place with remotely hospitable conditions is worth examining, especially with the kind of evidence seen on Mars for a friendlier past. The Viking missions were designed with searching for life as their major task, and maybe their biggest lesson was that we didn't know enough about chemistry or biology 30 years ago to ask the right questions.

A new thread came into the discussion with the realization that we already have Martian samples in the lab. A few meteorites have been found to have chemistry matching the Martian surface (and trapped gas bubbles matching the Martian atmosphere). Amid much controversy, and a presidential opening statement, a group of NASA and university scientists concluded in 1997 that one such rock, found in the Alan Hills of Antarctica and known as ALH84001, showed chemical and fossil evidence of Martian life from 3.5 billion years ago. The chemical evidence is a bit abstruse, involving carbonates (minerals deposited only in water, to my mind the most important result), and magnetite, which is still known in similar crystals on Earth only when deposited by bacteria, appearing in tiny globules about 0.25 mm across (right). What grabbed most people's attention, though, were the electron micrographs showing tiny structures that look remarkably like microorganisms or microfossils. These are smaller than it was thought free-living terrestrial organisms could be, an objection that has shrunk with the study of nano- (or nanno-)bacteria. Still, despite the best practice in analysis and the pristine Antarctic environment, there's always lingering suspicion of terrestrial contamination. To the extent that there is a consensus among astrobiologists, it seems to be that the evidence for biological origin of these structures is not as compelling as originally thought.

In the meantime, terrestrial biology has learned much about organisms in extreme environments on our own world. Hot springs, the deep ocean, sulfur-rich caves, the Antarctic, all have shown hitherto unsuspected organisms, often very tiny and related only distantly to the larger life forms we usually encounter. These findings make it more plausible that life could exist in exotic places (whether it could originate there or not), and improve the likely odds that any life starting out on a warm and wet Mars could survive the gradual freeze-drying of the planet.

An obvious next step would be a proper analysis of samples of Martian soil and rock (in case anything's hiding in the pores). This would require picking up the rock from a known place on Mars and returning it to Earth. (For a while, NASA and the Centers for Disease Control were arguing about who was better equipped to deal with such analysis, and I kind of liked the vision of a Martian sample re-entering toward downtown Atlanta.) Plans were moving along, when the whole scheme was stalled by the failures of Mars Climate Orbiter and the Polar Lander in 1999. The scheme required four launches from three countries to all work over a five-year period, anyway, with a European probe finally picking two grapefruit-sized canisters out of Mars orbit for a return to the Utah desert. You can read about the revised plans at NASA's Mars Exploration site.

Should we worry about the Andromeda Strain? What if we return Martian organisms that we have no defenses against? Not only is it unlikely that any response could cross not just species, but biospheres, easily, but the fact that there are Martian meteorites known on Earth says that some such organisms have made the trip in the past - and we're still here. Unless we're the Martians, which would explain a lot of political headlines these days.

Sample return is still a priority, so maybe in a decade or so we'll see something happen. Of course, sending scientists there would still be much more flexible and productive, but there seem to be no plans that would get this done anytime soon. With our current technology the process would be expensive and slow - launch something the size of Mir on a three-year round trip. For this to work, food would need to be grown on board, and water recycled very efficiently - it wouldn't pay to think too deeply about where that last sip came from, not that it does in a modern city either. The psychological problems are also profound- as one veteran of Mir said, a long-duration space mission presents all the ingredients for murder.

By now it's pretty clear that any life on Mars would be in the form of microorganisms - strange and exotic bacteria, more or less. If it's not big enough or smart enough to say "Take me to your leader?", why would we care? Mostly because having two worlds to examine is a lot different than one. Do all these life forms have the same chemistry? If so, does that mean both started out ono the same world and somebody hitch-hiked off on a meteor, or that there really is only a single chemical basis for life possible? And going further, if life could appear on two worlds in one system, the odds start to look better for finding it elsewhere - the buried oceans of Europa, and worlds in the planetary systems that have been found around many sunlike stars in the last few years. These are the ones that we might really be interested in meeting.

Last changes: August 2003