• In 2004, a spacecraft that has been hovering 1.6 million kilometers from Earth for three years will land in Utah with a sample of the solar wind.

• In January 2006, two more spacecraft will return with tiny samples of material from a comet and an asteroid - and a few years later, another may return small samples of material from both the moons of Mars.

• And in 2008, the biggest one: the U.S. (in partnership with France) plans to return one kilogram of rock and soil from the surface of Mars, in order to carry out incredibly detailed analyses in the hope that it may contain evidence that the planet once possessed life.

• The current plan is to return similar samples from different parts of Mars at four-year intervals, perhaps including some drilled up from dozens, or even hundreds, of meters below the surface - which, some scientists think might even include "extant" (still living) Martian microbes that have survived in Mars' deeply buried liquid water table.

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 plans to start dropping cans of Mars soil into the Utah desert in nine years.

Consider the recent level of public unease over the Earth flyby of Cassini, 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.

Scientists are now eager to start drilling hundreds, or even thousands, of meters into the Martian surface to look for such critters - there has even been some tentative planning as to how this might possibly be done with unmanned spacecraft!

But if this whole huge region is hospitable for Martian microbes, then the accidental contamination of one local site by Earth germs - some of which are very likely to find the subsurface Martian environment hospitable - would be free to spread around much of Mars; extremely slowly, but unstoppably.

All such drilling operations - manned or unmanned - will have to include extreme precautions.

Man: The Inherent Polluter

The other - more obvious - problem is what to do about manned expeditions to Mars.

It is, of course, flatly impossible to sterilize a manned ship - every time the airlock opens it will puff out clouds of Earth germs onto the Martian landscape; the sewage and garbage will be rife with them no matter how carefully it is processed; and even the most airtight spacesuit spews air containing thousands of germs per minute out of its seams.

Every time a manned expedition lands on Mars, it will quickly contaminate the very Martian material it is trying to examine for evidence of life.

The same two problems I've mentioned before - local contamination of surface sites and the very samples taken from them, and the possibility of more widespread contamination of the underground Martian water table - will be far more severe for manned landings than for unmanned ones.

So severe, that the Space Studies Board of the National Academy of Sciences has said that "It is... critical that a major effort be made to determine whether there are places in local Martian environments, such as active hydrothermal areas, where life might plausibly survive, and to more closely examine these areas robotically, before contamination by humans occurs."

And it made this statement in 1992, before it was apparent that underground Martian life might perhaps be widespread.

It is possible that when humans first travel to Mars, international agreement may require that it be a long time before they actually land there. And that initially the astronauts stay in orbit around Mars or on one of the Martian moons, operating complex robotic exploration equipment by remote control.

This will avoid the problem of the very long radio signal time delay from four minutes up to a half hour that makes exploring Mars by remote control from Earth so difficult.

Carefully sterilized vehicles could rocket samples from the Martian surface up to labs on such manned orbiting scientific stations, or all the way back to Earth. For a long time, Mars may be humanity's biggest nature preserve.

Onward To Europa

Then there is the second world in the Solar System where it is thought that there is a serious chance of past or present life: Jupiter's moon Europa, which may have a liquid-water ocean underneath its kilometers-thick ice crust that could perhaps be capable of sustaining bacterial life even today - and which certainly had such an ocean in its warmer early days, so that the remains of ancient Europan life may be frozen into the ice in large amounts even if no liquid water exists on Europa today.

The forward-contamination problem is less serious on Europa because that world is so effectively sealed by its ice crust, and because Jupiter's intense radiation belts will sterilize anything on Europa's actual surface in a matter of minutes.

We can dig around in that solidly frozen ice to a very substantial degree without fear of contaminating anything beyond the immediate ice we are touching - and it will be a very long time (if ever) before men land on Europa.

But if our early spacecraft confirm that a subsurface ocean still exists, our later plans call for unmanned vehicles to melt all the way through that thick ice crust into the underlying ocean to investigate it - and we will, once again, have to be extremely careful to make sure that such vehicles are sterilized, since Earth germs would spread even more quickly and inevitably in an ocean than they would in the Martian water table.

NASA plans to rehearse such sterilization procedures during the next few years when it (in collaboration with Russia) begins drilling into "Lake Vostok" - a huge lake of liquid water as big as Lake Ontario, recently discovered four kilometers under the ice of Antarctica, which it is eager to investigate for signs of ancient isolated species of bacteria, and which also makes an excellent testbed for the kinds of subterranean ice probes that will be used later on Europa.

And we will still have to take some precautions even when we are digging samples out of Europa's frozen upper ice layers. There is a strong suspicion that subsurface eruptions of liquid water into Europa's upper ice from below still occur occasionally; and, if so, they could spread local pockets of frozen hibernating Earth germs to other areas of the Europan ice crust, where they could later revive and multiply when gifted again with liquid water.

For all these reasons, the National Academy of Sciences already has a team carrying out a study of the best ways to avoid contaminating Europa, which is due to be completed next year.

The same considerations apply to any other ice-covered moons in the outer Solar System, such as Ganymede and Callisto, where a subsurface liquid-water ocean may exist - although the chances that any other such worlds could ever have supported life is remote because of their lack of the proper chemical energy sources for such microbes.

If multicellular life - any life more complex than microbes - existed on Mars or Europa, our ethical responsibility to avoid contaminating those two worlds would obviously be tremendously increased.

But - for reasons I hope to detail in a later article - there is virtually no chance that non-microscopic life exists on either world today. Although Dr. Christopher McKay has given reasons for thinking there is a small chance that it might have had time to evolve on early Mars before becoming extinct as the planet lost its supplies of air and surface liquid water.

Nevertheless, we do have responsibilities. And, as I've said, I have a suspicion that the discovery of present or past life on Mars - the one thing that could make a manned scientific expedition to that planet urgent - would, ironically, also act to substantially delay the date on which humans actually first set foot on Mars.

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