His argument is based on two themes: that Dr. Gilbert Levin -- the creator of the Viking Landers' "labeled release experiment for detecting Martian microbes -- still thinks (unlike most scientists) that his instrument did detect viable Martian microbes growing inside it; and that the chances that such microbes might be dangerous to Earth are really quite high.

To quote DiGregorio: "One of the most common misconceptions about bringing back living microorganisms from Mars is that because 'Martian bugs' would have evolved independently of 'Earth bugs', they will be harmless to life on Earth.

This theory assumes that Earth life has had millions of years of competitive [evolutionary] struggle... [which] is what makes microbes effective enough to cause harm, and that a Martian microbe would not 'know where to begin' and that any feeble attempts at 'learning' how life on Earth 'operates' in order to take advantage of it, would be 'fended off' by immune systems with millions of years' worth of head start...

"However, the alarming truth is that Martian microbial life would not have to 'learn' how Earth life 'operates' at all. All that is required is that Martian microbes find Earth life to be a good source of food, and then have the capacity to harvest it... All higher life on this planet has evolved in the context of the microbial life that exists here.

That is, it evolved to cope with the latter's constant incursions... It is unlikely that terrestrial animals and plants could withstand attacks of alien microbial life which it has not evolved to cope with.

"All any Martian microbe has to do is perceive Earth life as a source of food, and then have the capacity to 'harvest' it. For example, if Martian microbes had amino acids similar to terrestrial life and liked some of the same compounds -- that would be good enough to make us attractive to them. Earth microbes might in turn find the Martian microbes a good food source, but by no means does that imply they would wipe it out. When the entire biosphere hangs in the balance, it is adventuristic in the extreme to bring Martian microbes directly to the surface of the Earth as is now planned."

The Terra of Fightback
This is simply not correct. The whole point about the immune systems of most Earth organisms (including us) is that they have evolved to adapt very quickly to an attack by an unfamiliar organism, and to quickly develop new chemical antibodies specifically tailored to attack that microbe's biochemistry.

Without this ability, after all, we would literally be rotting alive, as our bodies were constantly invaded by unfamiliar species of Earth bacteria, which we regularly encounter on a new basis -- all of whom are eager to "harvest us as a source of food", and promptly do just that as soon as we die and our immune systems shut down.

(Indeed, I wonder whether this may be one of the reasons why complex multicellular organisms evolved in the first place -- they can develop more complex immune systems, in which a single immune cell which discovers a successful antibody against an invading germ can quickly spread that ability to the rest of the body.)

The simple and dramatic fact is that your body is the site of a constant and dramatic war against new types of invading enemies -- and that war is almost always entirely successful, or we wouldn't be around.

Our immune system is a kind of chemical military computer capable of adapting quickly to attacks by a huge range of enemies. The few species of bacteria that can successfully slip around it and establish themselves in large numbers in our bodies are those which have themselves stumbled across, by evolutionary chance, some complex stratagem for doing so -- a stratagem that is specifically tailored to deal with the operating details of the human immune system (and often involves a specialized attack on some aspect of the immune system itself).

As for viruses, they have evolved in an even more specialized way to commandeer part of a particular cell's genetic sequence in order to reproduce themselves -- most of them can infect only one species of living thing, and virtually none of them infect more than a few.

Any Martian microbes would be biological "rubes" on Earth, almost certain to be obliterated by Earth organisms' immune systems with which they would be unprepared to cope.

But the risk of a Martian disease harmful to humans -- while incredibly small -- is not quite zero, because a small number of Earth diseases (such as the botulism germ) secrete, by sheer chance, substances so toxic to humans that even though the germs are swiftly killed off by our immune systems, they can still do fatal damage to the body during the brief time before that occurs. (This is also what Michael Crichton's fictional "Andromeda Strain" did.)

And what about the risk that an invading Martian microbe might find some niche to its liking in Earth's incredibly varied ecosystem and establish itself as a serious ecological pest?

Again, the chance that it would be a disease bacterium specifically adapted to thrive in the body of any species of Earth organism is virtually impossible; but it still might establish itself as a new species capable of competing with Earth organisms for some food source and thus crowding out some native Earth species -- as has certainly happened often enough on Earth itself, when foreign newcomers found some new country greatly to their taste and crowded out native species to the point of extermination.

Microbes, unlike more complex organisms, are so unspecialized in their food sources that it is extremely unlikely that any Earth organisms could be starved into extinction by competition from some invading Martian germ -- but the possibility does exist, and it is distinctly more likely than the chance of a harmful Martian disease.

(And, again, what if -- by sheer long-shot bad luck -- some Martian microbe that happens to secrete a poison to which Earth organisms have not yet evolved a defense should thrive on Earth?)

In short, while I am convinced that DiGregorio has tremendously exaggerated the risks that alien organisms might present to Earth, I also think that some other writers -- including Robert Zubrin in his book "The Case for Mars" -- are much too blithely sanguine about the risk.

Recently, however, another argument has been put forth to suggest that anyone who worries about a microbial Invasion From Mars is a Chicken Little -- namely, that if it was going to happen it would have happened already.

Almost all scientists now accept that some rocks blasted completely off Mars' surface into solar orbit by giant meteoroid impacts have eventually wandered to Earth and crashed here -- and, quoting Zubrin:

"Despite the fact that in general each such meteorite must wander through space for millions of years before reaching earth, it is the opinion of experts that neither this period traveling through hard vacuum, nor the trauma associated with either the initial ejection from Mars or entry at Earth, would have been sufficient to sterilize these objects if they had contained bacterial spores. It has been estimated that these Martian rocks continue to rain down upon the Earth at a rate of about 500 kilograms per year.

Leave Earth Fast
So, if you're scared of Martian germs, your best bet is to leave Earth fast, because when it comes to Martian biological warfare projectiles, this planet is smack in the middle of torpedo alley."

DiGregorio completely ignores this fact -- and if (as he and Dr. Levin think), Mars' surface is actually quite rich in living microbes, then most species of them may have ben transplanted to Earth already without disastrous effects.

But how certain is Zubrin's conclusion? We don't know for sure just how much Martian material does hit Earth each year, but there seems to be a consensus that it is several hundred kilograms per year.

And Dr. Brett Gladman -- generally regarded as the leading expert in just how many Martian rocks do reach Earth each year and how long it takes them to get here -- believes that about one one-thousandth of this Martian material (that is, several hundred grams per year) reaches Earth in less than 10,000 years, while 1/100 of that arrives in less than a century.

As for the survival of bacterial spores embedded within pores of such rocks, the main limiting factor would seem to be their exposure to high-energy radiation.

Even within huge chunks of rock meters side, such spores would be exposed to about 5 rads per year of extremely penetrating cosmic rays -- and within smaller rocks, they would be exposed to several dozen rads per year of solar protons and X-rays.

The spores of a few species of bacteria can survive total doses of hundreds of thousands -- or even several millions -- of rads; but most species can only endure at most a few tens of thousands of rads, and many are killed by far less.

Moreover, since most surviving species of Martian bacteria would probably survive underground, there's no particular reason to think that they would have evolved great radiation resistance.

This means that very probably only one kilogram or so of Martian rock capable of carrying still-viable Martian spores hits Earth each year -- and most of that, of course, is unlikely to actually carry such spores.

The result is that it is plausible to believe that living species of Martian microbes may occasionally be transplanted to Earth -- indeed, there's a growing suspicion that ancient Mars may actually have been a more hospitable place for life to first evolve than ancient Earth, and that we ourselves may well be the distant descendants of one-celled immigrant Martians -- but it seems very unlikely that new species of Martian germs are transplanted to Earth more than once every few hundred thousand years, at absolute most.

And, as some scientists have pointed out, we have no way of knowing that at least a few of these occasional new arrivals haven't produced ecological problems that humans would consider serious if they had been around at the time.

Deep Danger Below
Finally, another factor has been pointed out. To quote Drs. Norman Sleep and Kevin Zahnle: "Rocks, amber, salt, ice, petroeum, and water are routinely extracted from environments that have been out of contact with surface organisms for millions of years, without any attempt at quarantine.

No quarantine is done on samples from the deep sea, including those from hydrothermal vents" -- although such environments could quite possibly contain microbes which, if released into our own modern environment, would represent a new ecological threat, or perhaps even (unlike Mars germs) a new human disease threat.

Such threats are considered so unlikely that it isn't worth going to much trouble to prevent them -- and that judgment, so far, has been right. But it should still be kept in mind that someday we may encounter a counterexample.

In short, this "natural cross-planet contamination" by meteorites once again indicates that the dangers of bringing back Martian samples are enormously less than DiGregorio thinks -- but Zubrin is also wrong to cavalierly assume that it completely eliminates any such dangers.

To quote Dr. Donald DeVincenzi: "By contrast, a Mars Sample return mission will return a 'fresh', protected sample that will traverse the interplanetary distance in about one year, an instantaneous time in geologic terms."

So, even if today's Mars is unexpectedly rich in living microbes, the risk from our returning samples is extremely small -- but not nonexistent. What should we do in response?

Orbital Containment
One obvious suggestion is not to recover the returning Mars sample return capsule by parachuting it directly to the Utah desert, but instead to brake it into Earth orbit and make preliminary studies of it on a lab that is attached to the International Space Station -- sending it to Earth only after those studies have indicated either that there are no still-living organisms on it, or that those organisms are not dangerous.

This idea was studied in detail by NASA back in 1981 under the name "Project Antaeus" (after the evil giant whom Hercules killed by strangling him while holding his feet off the ground so that he couldn't draw new strength by touching the earth), and it is DiGregorio's choice. But it has problems, as many analysts have pointed out -- including DeVincenzi, the lead author of the Project Antaeus report.

The main problem, of course, is simply the added cost -- constructing an entire new module of the Space Station as a facility to carry out complex quarantine and biological test procedures would be very expensive.

But there are others. For one thing, there would be no hope of conducting more than relatively superficial tests on Mars samples in such a small facility to detect any dangers -- it would obviously be impossible to check out Martian microbes for more than a very tiny fraction of the ecological problems they could conceivably cause on Earth, and even studies to see if they might cause a human disease would have to be very cursory.

Moreover, if some sign of danger did turn up, we would be left with the task of trying to make sure the contaminated lab module (and probably the entire Space Station) never reentered Earth's atmosphere with its dangerous passengers -- and that would be very expensive indeed; we'd have to regularly reboost it back into higher Earth orbit for an indefinite period.

It can still be argued, though, that even a very cursory initial inspection of returned Martian samples on the Station for any obvious signs of danger might be worthwhile -- and if such signs did turn up, we would then (as Dr. Levin points out) be free to decide whether the danger was great enough for it to be worthwhile going to the expense of keeping the Station permanently in orbit, or not.

There's no doubt, though, that Earth-based facilities could conduct a tremendously more thorough examination of Martian samples both for any biological dangers, and for scientific purposes.

And we already have a large number of facilities -- both in the U.S. and elsewhere -- which are routinely used today to contain germs which we know for a certainty are extremely dangerous either to humans (such as the Ebola and Marburg viruses) or to other important species on this planet. The Centers for Disease Control, to name just one example, do exactly this in major cities without setting off a public panic.

Any one such facility could be expanded to store and scientifically study returned extraterrestrial samples without prohibitive expense.

It seems to me -- as to DiGregorio -- that the point in the whole process of returning and analyzing Mars samples that we should worry about the most is the actual landing of the sample return capsule on Earth.

Murphy's Law
As the failure of Mars Climate Orbiter has painfully reminded us, Murphy's Law is alive and well when it comes to carrying out any kind of maneuver in space.

It is extremely easy to visualize the possibility that returning capsules containing Martian samples might enter the atmosphere incorrectly, partially burn up, rupture and release their cargo; or that they could crash as a result of parachute failure; or that they could simply be lost in the ocean or somewhere else where they could eventually corrode open and release their contents.

All these things have happened many times before during the Space Age. Once the capsule is actually picked up by a recovery team, the main danger of a Mars sample return mission is over.

For this reason, I wonder about a possible intermediate course of action. If the sample return capsule is braked into a moderately high Earth orbit by a retrorocket (say, 500 kilometers up), it would be a long time before there would be any danger of its reentering the Earth's atmosphere naturally -- more than enough time to launch a Space Shuttle to rendezvous with the capsule in orbit, retrieve it and load it into a heavy armored cask in the Shuttle cargo bay which could be durable enough to survive virtually any accident the Shuttle might suffer during its landing attempt.

Such a cask, even at its heaviest, would still make up only a very small part of the Shuttle's total payload weight; and the same Shuttle mission that carried it could carry out a good many other assignments in orbit first, thus tremendously cutting down on the cost of the retrieval operation itself.

And even if such a Shuttle retrieval mission were substantially delayed (as we have now become used to on Shuttles), the sample return capsule could be relied on to wait in orbit for years.

The main problem with this scheme would be the mass of the retrorocket needed to slow down the returning capsule by 12 to 14,000 km per hour as it returned to Earth in order to insert it into low Earth orbit -- but such a retrorocket would weight only about twice as much as the weight of the actual orbiting sample return container.

And, as compensation, the container would not need a heat shield, a parachute or any shock-absorbing material -- it could be almost as lightweight as the tiny thin-walled container which the currently planned Mars sample return mission is supposed to inject into low orbit around Mars for rendezvous and pickup by a separate Mars orbiting spacecraft.

Its only additional requirement would be a solar-powered radio tracking beacon (if that).

I think that this idea might, for a quite small additional cost, provide a considerable additional element of safety to the Mars sample return mission. And it would certainly be greatly reassuring to the public -- which, I uneasily suspect, will soon raise unholy hell when it finally sinks into the public consciousness that NASA intends to drop a can of Mars soil into the Utah desert in less than nine years, and to continue doing so at four-year intervals.

Just imagine what the tabloids (or Congress) will do with that fact! If NASA wishes to avoid a legal and public-relations nightmare, it may well be worthwhile for it to go to a bit of added engineering expense right now to make this solution possible -- and it seems to me that it is the solution which is most proportionate to the possible impact of any actual danger from extraterrestrial sample returns from possibly habitable alien worlds.

In-situ Saves The Day
One additional note should be added, though. Dr. Levin agrees with DiGregorio than an additional worthwhile precaution would be for NASA, before returning Mars samples to earth, to carry out at least some additional Mars landing probes with additional in-situ experiments to determine whether (as he thinks) there are indeed substantial number of still-living microbes on the Martian surface -- which would give us a great deal of additional insight into just how cautious we should be in returning samples.

Viking biology team director Harold Klein thinks the same thing for different reasons; although he thinks unlikely that such microbes exist (at least in Mars' upper soil layers), or that they are likely to be harmful if they do exist, he thinks that the negative results he expects from such tests would greatly simplify NASA's future procedures for sample return and handling (and thus reduce their cost).

I think there is a good case to be made that NASA, for publicity reasons, is indeed rushing too quickly into Mars sample return -- especially since there are other arguments that can be made for this.

(Planetologist John F. Kerridge, for one, thinks that the chance of returning Mars samples with meaningful fossil evidence of ancient life during the first few sample-return missions is very small -- and that, since such missions are very expensive and there are likely to be only a few of them for some time to come, we should first carry out a considerably more detailed survey of the planet to locate those areas, and mineral deposits, that are the most likely to contain biological evidence.)

It is a fact that NASA actually does plan to carry some in-situ experiments on the 2003 and 2005 sample return landers to check the local surface for chemical evidence of life in addition to returning samples from that area to Earth -- and it might very well make more sense for such in-situ experiments to be carried instead on several preliminary landers (as well as surveying Mars' surface more thoroughly with landers, and perhaps with small balloons or airplanes) before we take the major and very expensive leap of returning any samples from it.

MARSDAILY.COM
Life on Mars: Will It Survive First Contact
Cameron Park - October 7, 1999 - When we send probes to other worlds in search of life, great attention needs to be made to ensure we don't contaminate any life forms on these worlds. Lest we kill ET on first contact

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