Laurence Bergreen - The Quest for Mars - NASA scientists and Their Search for Life Beyond Earth

Four and a half billion years ago, the Solar System was in its infancy, and the planets were new. In its first billion years of existence, Mars was a warmer and wetter place than it is now. Water flowed freely over its surface and pooled underground, in reservoirs. The flood channels carved into the surface of Mars, some of them many miles in width, left an eloquent record of catastrophic outpourings of water. In all likelihood, the water on Mars was quite salty. (Fresh water on Earth is due to evaporation and rainfall.) Eventually, the floods subsided, and the water drained into Mars’ vast northern plains, where it might have frozen. In the process, it reshaped the Martian terrain until it resembled a desert that had once been flooded, but became bone dry. Nevertheless, its contours preserved geological memories of rivers and oceans and lakes.

The large Martian pools of standing water were subject to peculiar tides caused by the planet’s two small moons, Phobos and Demos. And they were subject to the Martian winds; when they blew, reaching speeds of hundreds of miles an hour, they generated waves with peculiar shapes, higher and steeper, with more pronounced peaks than exist on Earth. The lower Martian gravity, less than half of Earth’s, allowed the slender waves to tower until they resembled the watery shapes in a drawing by Dr. Seuss; they would flop over and spatter, as if in slow motion. The marine scene on Mars was all oddly familiar, and strangely different.

The Martian sky was blue a few billion years ago, and there were a few clouds, just as Mars has now. It was mostly cold, and extremely cold at the poles, except for the equator, where it was warm. Martian volcanoes erupted with regularity, and in the Red Planet’s low gravity they assumed formations that couldn’t exist on Earth; they were larger and higher. In these ancient Martian conditions of two or three billions years ago, life could have formed and evolved, just as life appeared on Earth within a billion years of this planet’s existence. The volcanoes, especially the ones close to reservoirs of water, or polar ice, created hot spots where life would most likely have formed on Mars. No one knows how far it developed, or if it ever got underway. It might have remained dormant most of the time, for tens of millions of years at a stretch. Or it might have progressed beyond simple bacteria; there might have been Martian insects crawling around, adapted to the Red Planet’s lower gravity, lower density atmosphere, and cooler temperatures. These variations suggested life forms that were spindly, similar to insects. The skeletons might have been external, with many legs to take advantage of the lower gravity. As for the cooler temperatures, life on Earth has shown remarkable adaptive creativity. “Some insects winter-proof themselves with glycerol, a common antifreeze used in automobile radiators,” Carl Sagan theorized. “There is no conclusive reason why Martian organisms should not extend this principle, adding so much antifreeze to their tissues that they can live and reproduce in the extremely cold temperatures occurring on Mars.” The ancient Martian atmosphere would have required similar creativity in the creatures’ breathing apparatus. If they had evolved to the point of multi-cellular differentiation, they might have developed enormous gills or lungs, relative to their size. Even if life never reached this advanced stage of evolution on Mars, it is still possible that tiny organisms formed in the water-drenched Martian rock, and then, for some reason, died off, leaving fossilized remains hidden beneath the surface. It was as though Nature initiated an experiment but abandoned it in the early stages.

Ancient Mars was more turbulent than Mars is now. In the young and volatile solar system, it was constantly bombarded by chunks of asteroids. It is possible that at some point in Martian history, an asteroid of such dimension struck Mars and created cataclysmic changes in the planet’s climate and geography that whatever life forms had managed to take hold were snuffed out, leaving only their skeletons, which became fossilized. Or perhaps the death of Martian organisms came about slowly, as the planet lost its atmosphere a little at a time to space, and its water eventually disappeared below the surface, or vanished with the atmosphere, leaving behind a desiccated, celestial sandbox.

If we had been able to observe the first few billion years in the life Earth and Mars from a vantage point in distant outer space, we might have noticed several common trends. We would have seen watery places on both planets. We would have seen volcanoes on both planets, their plumes of smoke, their pollution of the atmosphere. We would have seen clouds on both planets, and we would have detected seasonal waxing and waning of the polar caps. As the eons passed, subtle differences between the two planets would have become apparent. If we had been looking closely, we might have noticed the atmospheric changes. We might have seen the dramatic increase in oxygen in Earth’s atmosphere, and a corresponding spread of vegetation on its surface; if we were very perceptive, we might have noticed the spread of plant life in its oceans, in the form of algae.

At roughly the same time, we would have seen that Mars was losing its nitrogen-rich atmosphere. It was thinning out, disappearing into the frigid vacuum of space. More obviously, we would have seen the great Martian standing bodies of water recede, exposing a complex erosional system of gullies and playas and rearranged boulders, many of them acting as signposts to the water’s former whereabouts and actions. During the last few hundred million years, if we were sufficiently attentive, we might have noticed the spread and intensification of vegetation on Earth, as the biomass increased and diversified, and various life forms competed for natural resources, or evolved ways to cooperate, or both. At about the same time, we might have watched Mars continue to regress to its early state, with some important differences. It contained geological traces of water, and perhaps traces of biology – clues, ultimately, to its origins, and to ours.

Where did the elusive Martian water and its life-giving properties go? Come to think of it, where did the water come from? Where did water on Earth come from, for that matter? At the Lunar and Planetary Institute in Houston, Steve Clifford has spent years studying water on Mars, and he told me there are several schools of thought concerning the origins of water on this planet and on Mars. “One is that after the Earth was formed, comets bombarded the planet, adding volatiles over perhaps the first billion or half billion years.” A comet is basically a celestial snowball, bearing ice from somewhere – God knows where – to here. “The other school of thought is that much of the water we have on the Earth was contained in the early material that formed the planet. As the Earth started to accrete asteroidal material and dust in the early Solar nebula, it gradually reached a size where the quantity of radioactive material was sufficient to heat up the planet and cause it to differentiate. The heavier stuff sank toward the middle, which is how we got an iron core, while the lighter stuff, which may have contained water, was released during the formation of the crust and atmosphere.” A similar process may have occurred on ancient Mars at about the same time it happened on Earth.

Steve surprised me by suggesting that water on Mars may still linger beneath the surface, more than a little. He was talking about “sizable reservoirs of ground ice and ground water.” The evidence he has for large volumes of water on Mars is mostly indirect. He calculates the amount of “pore space” to be found in Martian rocks and soil; water could be stored there. If it is, some of the water could be in liquid form, especially well below the surface. “Like the Earth, Mars is thought to be radiating internal heat due to the decay of radioactive elements, which means the deeper you go below the surface, the warmer the temperature gets,” he says. “And if you go down several kilometers, you could easily get temperatures that are consistently above freezing,” which means liquid water might exist on Mars today. In fact, he thinks there might be two types of water reservoirs on Mars, a region of permafrost near the surface, as well as larger and warmer reservoirs at greater depths. There you might find liquid water in great quantities, and water, of course, leads to life. This subsurface system would act as a powerful preservationist of life, no matter how harsh conditions on the surface. “If life ever evolved on Mars, and adapted to a subterranean existence, then its survival would be assured for the indefinite future.”