Welcome to the new Martian chronicles.
Jim Garvin’s collection of Jim Bunning baseball cards got me to thinking about what is perhaps the most famous baseball card of all – the 1909 portrait of Honus Wagner in a Pittsburgh Pirates uniform. When I gaze at the face of this young man, who seems to be staring into space, I find myself asking, “What was it like to be alive in 1909?” I have little idea, although it was just the other day, in geological terms. All I have to go on are artifacts, such as this famous baseball card. I can’t watch Wagner play baseball, and I can’t hear his voice; all I can see is a fuzzy image of the athlete in a uniform, a trick of light and shadow, an impression of life as it once was. To fill in the gaps, I would have to look beyond the card, but if I’m relying on the card, and only the card, I have precious little data.
The card, and its limitations, call to mind the tantalizing images of fossilized bacteria in a 3.9 billion year-old meteorite from Mars – images that may be the first scientific evidence of life beyond Earth. Fossilization occurs when minerals replace organic elements in once living things. The morphology remains, although the chemistry is different. Still, scientists can learn a lot from fossils. They can detect the approximate age, which is crucial, and, by studying fossils in their natural setting – in situ – they can extrapolate a great deal about the geological, chemical, and biological circumstances surrounding them. “Fossils are the autographs of time,” wrote the American astronomer Maria Mitchell. For these reasons, fossilized bacteria from Mars – if that’s what they are – have great appeal; they are our best indication of life beyond Earth. Like the antique baseball card, they offer only a very narrow glimpse into the past. 1909, the year of the Honus Wagner card, wasn’t very long ago, but it’s long enough past to seem quite mysterious. How much more difficult it is, then, to construct a scenario for the existence of life on Mars several billion years ago from the evidence contained in a meteorite.
If the fossilized bacteria are genuine artifacts of Martian life, they raise more questions than they answer. If life started on Mars, how did it begin? Is it still there? If not, when did it end, and why? If it’s on Mars, where else in the universe might it be, and what form does it take? Did it originate on Mars, on Earth, or somewhere else? All these Genesis Questions point up how much scientists have yet to learn about how life began. In the course of asking questions of various scientists who study these problems for a living, almost every reply I received began this way: “No one knows, but …” That’s followed closely by, “There are several possible scenarios,” and “Well, current speculation has it that …” The answers are all variations on the theme that no one knows, yet. But scientists have hypotheses. They have scenarios. The meteorite from Mars has inspired a widely accepted scenario – I’ll call it the Best Guess Scenario – concerning the origins of life on the Red Planet, and how it came to be transported to 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