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.

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?

Mars Invades Earth - Part One

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

Life and ExoWorlds at SpaceDaily