Issue of
December 2, 1998

Geophysicist Sleep: Martian underground may have harbored early life

BY FRANCES COLE

In the early years of the solar system, when giant chunks of rock and ice pummeled the planets, the best place for life to survive -- if life existed at all -- was in underground niches on Mars.

If Martian microbes existed, and survived the large impacts by hiding in the Martian subsurface, they could have later traveled to Earth, via meteorites, and seeded terrestrial life.

Those are two contentions put forth in a new paper by Norman H. Sleep, professor of geophysics at Stanford, and Kevin Zahnle at NASA-Ames Research Center in Mountain View. The paper, "Refugia from asteroid impacts on early Mars and the early Earth," appeared in the Nov. 25 issue of the Journal of Geophysical Research (Planetary Sciences Section).

"Early Mars may have been safer than the early Earth and probably was habitable," Sleep says. That's because of the Red Planet's smaller size and lack of large oceans. Earth's oceans, critical for supporting life under normal conditions, may have been the planet's greatest liability in the event of a really large impact -- a so-called "ocean-boiling impact," according to Sleep and Zahnle.

Between about 3.8 billion and 4.5 billion years ago, no place in the solar system was safe from the huge arsenal of asteroids and comets left over from the formation of the planets. Sleep and Zahnle calculate that Earth was probably hit repeatedly by objects up to 500 kilometers across -- the distance from Los Angeles to San Francisco. Objects that big probably missed Mars altogether, because it was a smaller target. But if they did hit, the damage would have been less severe, because Mars lacked large oceans to convert to a thick, long-lasting and sterilizing steam atmosphere.

If primitive life was wiped out, possibly repeatedly, on Earth, but managed to survive on Mars, it would not have been hard for Martian life to re-seed Earth. Recent computer simulations have shown that, during this early period, large amounts of material must have been exchanged between the two planets. Some of the material blasted from Earth in meteoritic impacts would have landed on Mars, and vice versa. Even today, a Martian meteorite hits Earth about once every three days, Sleep says. Several billion years ago, the impact rate was a thousand times greater. That means millions of Martian meteorites made the trip from Mars to Earth in just a few years, a short enough period for spores or possibly even intact microorganisms buried deep in the meteorites to survive, he calculates.

Sleep notes that even modest-sized impacts have been known to decimate life on Earth. The asteroid that caused the mass extinction at the end of the Cretaceous Period was only 10 to 20 kilometers across -- small by early solar system standards. Yet, when it hit the Yucatan peninsula 65 million years ago, it ignited a fireball that spread out across most of North America and launched tremendous amounts of pulverized rock almost into orbit. As that rock dust fell back to Earth, enough heat was released to set forest fires worldwide and to evaporate about a meter of water from the world's oceans, Sleep says. The dinosaurs were cooked -- literally.

That is mild compared to the damage from an object hundreds of kilometers across, like those that were hurtling through space about 4 billion years ago. Hit the Earth with a 500-kilometer wide asteroid, and it vaporizes the rocks at the site of impact and creates a rock-vapor atmosphere that would radiate at temperatures of 2,000 degrees Celsius, like a star, Sleep says.

Such tremendous heat would boil off all the world's oceans and create a steam atmosphere that would persist for about 3,000 years. That is long enough to sterilize the Earth's outer crust down to nearly one kilometer, according to Sleep and Zahnle. The planet is, in effect, steam-cleaned.

On Mars, an object 500 kilometers across would vaporize the rock at the site of impact, as on Earth, but no long-lasting steam atmosphere would result, so the thermal radiation would dissipate much more quickly. Only the upper few hundred meters of the planet would be sterilized, compared to the upper one kilometer on Earth, Sleep and Zahnle say.

What, if any, forms of life could have survived such devastation? Sleep and Zahnle think the logical answer is the thermophilic -- that's Greek for heat-loving -- organisms. Modern thermophilic organisms thrive at temperatures up to about 100 degrees Celsius on Earth, Sleep says.

Such critters might have survived at a depth of about one kilometer underground during the largest impacts on Earth, but the habitable zone would have been quite narrow -- any shallower and they would have been cooked by the energy of impact, and any deeper and they would have been cooked by Earth's hot interior, the authors say.

By comparison, thermophilic organisms could have survived in a much broader depth range on Mars -- from several hundred to several thousand meters below ground, partly because of Mars' relatively cool interior, but also because of Mars' lower gravity, which allows cracks to extend deeper into the planet's interior and so provides living space for microbes at greater depths than on Earth.

Sleep first began thinking about thermophilic organisms in the mid-1980s, when colonies of these creatures were discovered in hot water vents at the bottom of Earth's deepest oceans. Sleep imagined that such undersea vents might provide a good refuge during asteroid impacts, because they would be the last place to get boiled away by the intense heat of a large impact. He reasoned that the thermophilic organisms in such niches might survive large impacts.

Sleep thinks it is no coincidence that these creatures occupy the main trunk on the tree of life, where the branches of more recent life are joined. "Clearly bacteria and archaea root into thermophile organisms -- and thermophile organisms are exactly the ones you'd expect to survive if the ocean gets darn hot or boils," Sleep says.

The idea that Martian microbes may have traveled to Earth as stowaways on meteorites sounds like science fiction, but it may be a good idea. George Thompson, professor emeritus of geophysics, says "the travel time and conditions in transit are probably tolerable for beasts like those found in bore holes," that is, thermophilic organisms. And the recent discovery of evidence of possible life forms in Martian meteorite ALH84001 makes interplanetary immigration of organisms from Mars seem less outlandish.

The study was funded by the NASA Exobiology Program. SR