Firstly there is a good deal of apprehension among the general public that samples returned from other worlds such as Mars - just might - contain alien germs capable of turning into a worldwide plague, or at least wreaking havoc with the Earth's natural environment.

Beside this fear of "back contamination", there is also a fear of "forward contamination" - the possibility that spacecraft might contaminate the worlds they land on with Earth microbes, destroying scientifically priceless alien lifeforms before we even have a chance to study them.

As yet there has been little public fuss over this - but the fuss is bound to grow in coming years, especially when it sinks into the general public's awareness that NASA wants to drop cans of Martian soil into the Utah desert sometime in the next decade or two.

Consider the level of public unease over the Earth flyby of Cassini in August 1999, with its 23.4 kg of plutonium. Moreover, while the scientific community itself is a good deal less worried about back contamination, most scientists feel that there is at least a small element of risk that must be taken into account.

The questions of "back contamination" of Earth by alien germs, and "forward contamination" of other worlds by Earth, are somewhat separate issues amd I plan to write about the back-contamination question in a later article.

In this article, the forward contamination problem will be the central focus and how we can avoid, or at least minimize the risk of, accidentally wiping out life on other worlds? Without doubt this problem will have a central impact on any future plans to land humans on Mars.

Earth Invades Mars

When the U.S. sent the Viking probes to land on Mars in 1976, it was already thought that the surface of Mars was savagely inhospitable to Earth microbes for three reasons:

• Mars' atmospheric pressure is so low that water actually cannot exist on its surface in liquid form without boiling into vapor;
• that same wispy atmosphere is totally useless at blocking out solar ultraviolet light capable of sterilizing Mars' surface;
• and all of Mars' surface most of the time (and most of its surface all the time) is well below freezing, and it's hard to see how any living organism in the universe can survive without liquid water.

Given that destroying any Martian life forms before they could be studied would be an enormous scientific tragedy, NASA - in accord with the requirements of international agreement - went to considerable effort and expense to make the Viking landers sterile - including baking them for 40 hours at 112 deg C. - increasing the cost of the already expensive mission by 10 percent.

Our reward for all this effort was that the Vikings ended up discovering that Mars' surface was even more ferocious than we had thought: in addition to all those other obstacles, it turned out that the upper layers of Mars' soil are laced with oxidant chemicals that have a powerful antiseptic effect - apparently produced by solar UV light.

Therefore, in 1984 - and again in 1994 - the international Committee on Space Research (COSPAR) decided to relax the sterilization requirements for future Mars landers.

They concluded that there was virtually no chance of any Earth microbes being able to reproduce and spread across the Martian surface from the landing site of any contaminated spacecraft - but there was still a real danger of "importation of terrestrial organic contaminants, alive or dead, in amounts sufficient to compromise the search for evidence of past or present life on Mars itself."

That is, a lander contaminated with either Earth microbes or their dead remains could easily befoul the very samples of Martian material it was examining for the tiny traces of either present-day Martian microbes or the "chemical fossils" left by ancient ones, that might yet exist on Mars.

Therefore COSPAR concluded that any future Mars landers equipped with life-detection experiments should still be strenuously sterilized, like the Vikings - but for all other Mars landers, it would be sufficient to take less extensive measures, such as assembling them in "clean rooms" and wiping down all their parts with alcohol, to minimize their load of germ contaminants.

This is, in fact, the strategy the U.S. followed with the Mars Pathfinder lander, and which it, and all other nations, intend to follow with their future Mars landers.

But obviously, when we are dealing with spacecraft designed to collect samples of the Martian surface and return them to Earth, with the main scientific purpose being to inspect them for signs of present or past life - as the U.S. intends to start doing in 2003 - more thorough sterilization measures are necessary.

The current plan is to sterilize these spacecraft in a "patchy" way - that is, to thoroughly sterilize every part and component of the spacecraft that has any serious chance of coming into contact with the Martian soil and rock samples from the moment they are collected to the time they are landed on Earth in a sealed capsule, while still using the less strenuous (and far cheaper) cleaning measures for the other parts of the spacecraft.

Watery Tales From Mars

It's now clear, though, that there are two more factors complicating this strategy for Martian exploration.

First, until recently it was the belief of most scientists that the chances of life still existing somewhere on Mars today were minuscule; and that, if they did exist, Martian microbes could only exist in a few spatially isolated underground "oases": small pockets under the polar caps or around a few areas where Mars may still have volcanic activity, where liquid water could still exist.

Even if one of these isolated areas was contaminated by Earth, the Earthly microbes couldn't possibly spread to any other isolated oases.

In the past few years, however, a growing number of planetologists have come to believe that there is one quite large region on (or rather in) Mars where living microbes can still exist - namely, kilometers underneath its surface.

Mars, contrary to common belief, has not lost most of its original supply of water; many scientists believe that much of it still exists underground, including a buried layer of permafrost - varying in thickness from perhaps two kilometers at the equator to several times that at the poles - which scientists have begun to call the Martian "cryosphere".

Underneath that, there is a region - again, several kilometers thick - where Mars' internal heat is still sufficient to keep the water liquid, and there are still enough pores in the rock for a significant amount of liquid water to be stored.

In recent years, it has also become clear that similar very deeply buried regions on Earth contain a surprisingly large supply of living bacteria, depending for their energy largely on volcanically produced mineral deposits (and perhaps even on chemicals in the basalt rock itself, though this is still controversial).

Well, if microbes on Earth could colonize such a region, then microbes on Mars - during the period of several hundred million to a billion years in which Mars is thought to have been friendly to life - could very well evolve species that could do so.