Luke Dones of the Southwest Research Institute discussed a subject of intense interest for the past decade. That's the fact that giant (and even modestly giant) impacts on Mars through its history are now known to have blasted rocks off its surface into solar orbit, a small fraction of which have eventually wandered all the way to Earth -- and the possibility that these might actually provide a route by which living Martian microbes could be carried to Earth, or vice versa.

It's become increasingly clear from computer simulations since 1995 that a significant number of the rocks blasted off Mars into solar orbit by a giant impact can eventually find their way to Earth -- perhaps 2.5 percent of them over just the next 10 million years after such an impact, wandering to Earth at a roughly steady rate during that period.

And even a "small" impactor like Earth's Dinosaur Killer can blast 40 billion or so meter-size rocks off Mars -- which means that billions of such chunks of Mars must have rained down on Earth over its history. (As with meteorites of all sorts, we have so far found only a microscopic fraction of the Mars meteorites that must still be sitting on Earth's surface, before 10,000 years or so of Earth's weathering conditions break them down unrecognizably.)

Thus such an impact could transport about a million meter-size chunks of Mars to Earth within only 10,000 years -- 100 of them getting to Earth within only a year! About another 400,000 more of the rocks launched from Mars by such an impact would return to Mars itself within 10,000 years.

Despite the remarks I've made about almost all the ejecta from a giant impact being melted or even vaporized -- or at least heated red-hot -- a small fraction of them will not be, having been launched from the narrow "spall zone" around an impact site where shock waves from the impact actually interact with and cancel each other out to minimize the heating of the rocks ejected from that area. About one-quarter of the chunks of Mars we've identified on Earth so far were not even heated to the boiling point of water by their launch, or by the frictional heat as they blasted upward through early Mars' dense atmosphere.

And this raises the very serious possibility that -- if Mars ever did have microbial life -- a significant number of Martian microbes may have been carried into space still alive within such chunks of rock, and a small fraction of those may well have survived as spores within the rocks long enough to still be alive when they rained down on Earth. Indeed, Kevin Zahnle remarked in 2000 that "this view... has gone from speculation to truism in less than five years."

(The layer of melted glaze formed on the rocks' outer surfaces during their initial outward rip through Noachian Mars' dense atmosphere -- if it sealed their interior pores off from the vacuum of space -- might even have extended the period during which bacterial spores could survive inside them in space from just a few hundred years to several hundred thousand.)

This means that -- if Noachian Mars did have microbes -- there was another citadel besides the Martian subsurface where they could have avoided being completely exterminated by the giant impacts of the period. Those very impacts would have launched a large number of boulders into space carrying still-living Martian germs -- and a substantial number of them would have returned to Mars long after its surface had cooled back to normal levels, carrying still-living samples of Martian microbes to reseed the planet. And another significant set of them would have seeded Earth itself with living Martian microbes.

The same thing may have happened for Earth during this period, providing another shelter for any life that evolved on Earth during the heavy bombardment age to survive our own planet's giant impacts even if they were big enough to completely boil its oceans away -- similar chunks of Earth rock carrying still-living microbes would have been launched into solar orbit, some of them returning to Earth thousands of years later after the planet's surface had cooled down again, while others would have carried still-living Earth microbes to Mars.

Dones' DPS talk also discussed the efficiency (as shown by the latest computerized orbital simulations) with which giant impacts on Earth can launch Earth rocks to Mars, which turns out to be a good deal less efficient than the other way around -- ten times less efficient than the Mars-to-Earth route, in fact, thanks largely to the fact that Mars is a much smaller planet and thus a harder target for a solar-orbiting meteorite to hit. But this would still have allowed a very large number of Earth rocks to crash onto Mars during the early giant-impact era, many of them quite possibly still carrying living Earth germs.

All this, however, is assuming that microbial life even got a chance to evolve on either planet at all during the bombardment era. Although the most genetically primitive microbes on Earth today are thermophilic -- which indicates that all life on Earth today did evolve from microbes that survived the final few smaller roasting impacts during the trailing-off end of the heavy bombardment period -- there is a good chance that life simply never got a long enough period between giant impacts during the heavy bombardment age to evolve on Earth out of nonliving chemicals in the first place, and so did not start to evolve on Earth until near the end of the bombardment era 3.8 billion years ago.

But ancient Earth, between those impacts and after they ended, would have been a pretty friendly environment for microbes (if not for complex life). Its oceans might have been largely covered with ice floes, but there is no doubt at all that they were mostly nonfrozen -- and cold water is still a perfectly friendly environment for microbes to live in.

(Once life did get started on Earth, some species of those cold-water microbes would have evolved to live instead in the hot water near the volcanic vents on the floors of our oceans -- and those bacteria would have survived the final few scattered giant impacts when all other life had once again been exterminated by them.)

On the other hand, at about the same time the heavy bombardment era ended on ancient Mars, the Noachian era did too -- the planet's atmosphere quickly disappeared almost completely, and its temperature (without the greenhouse effect from the original thick air) dropped to the far colder levels that still exist today, with virtually every speck of surface water (except around the few remaining volcanic hot springs) frozen solid.

And during the tens of millions of years between its own surface-sterilizing impacts during the bombardment era, even Noachian Mars would have had far less nonfrozen liquid water on its surface than Earth did during that same period. The long-running streams that (if Carr is right) carved many of its valley networks during Mars' obliquity cycles, the crater lakes that many of them drained into, and the possible shallow water ocean that filled the northern lowlands would all have been completely covered by fairly thick sheets of ice. Liquid water during Mars' Noachian days must all have been either imprisoned below a surface layer of ice, buried fairly deep underground between pores of soil or rock, or existing in relatively short-lived geothermal hot springs.

So, if life did appear on ancient Mars, it would almost certainly have had to evolve in an underground (or under-ice) environment during the heavy bombardment period -- whereas on ancient Earth, after the giant impacts slacked off, life would have had a gigantic open-topped reservoir of surface liquid water to evolve in. And we simply do not know remotely enough about the complex and still-mysterious chemical processes by which life on Earth first evolved out of nonliving organic chemicals to say whether life could have evolved in the first place in the much more limited environments that could have supported microbes on Noachian Mars.

It may be that underground water, or the hot water near volcanic springs, is the environment in which life first evolved -- in which case it could very well have appeared on Earth and/or Mars before the end of the heavy bombardment.

Or it may be that, to evolve in the first place, life requires very large amounts of water in an cool ocean, and exposure to the Sun's ultraviolet light and the rain of complex organic compounds sprinkled down on the ancient planets' upper surfaces by comets and low-temperature carbonaceous asteroids.

If the latter is true, then life probably never even had a chance to evolve in the first place on Earth until the heavy bombardment era ended -- and the odds that it ever evolved at all on ancient Mars are very poor indeed. (The complex intermediate-stage chemicals involved in the gradual initial development of one-celled life, even if they were blasted into space inside rocks, would have had tremendously less ability to survive there -- without breaking down chemically due to radiation, vacuum or temperature extremes -- than bacterial spores that had developed through evolution to endure such tough environmental conditions.)