A recent Wired Science article referring to research suggesting that certain hardy Earth life forms could not only survive the trip from Earth to Mars, but could actually live for extended periods of time on the Martian surface, reminded me of our dashed hopes for life in our solar system. In the 19th century, astronomers and savants could speculate freely on the possibilities of life on Venus and Mars, suggesting that intelligence was more likely than not. I’m reminded of the predictions of the determinedly rational French biologist Edmond Perrier, a man of the relentlessly rational Third Republic who made a fairly respectable attempt to imagine what living organisms and ecosystems would be like on other worlds. Life on Earth fits a system; therefore, life on Earth’s sister worlds must share this system.
Life must begin under the same conditions on all the planets. Made up of infinitesimal atoms it must have appeared throughout the planets wherever the necessary atoms could be brought together. The sun and the planets being in reality but one body these atoms must behave everywhere in the same manner, and wherever similar conditions are found there will be similar results. Where conditions are dissimilar results will differ in a manner that we can perhaps calculate.
In his reading, the gas giants are too cool and low-density to support life, these worlds’ moons aren’t considered at all, while of the four inner worlds Mercury is too dense and hot. Venus and Mars, thankfully, lie within the ecosphere of Sol, at the inner and outer edges respectively. Venus, a world of perpetual heat and humidity, with habitable poles and desert equatorial lands, is predicted as having the sort of very stable environment that would slow down evolution, with complexity coming about organizations of social insects like bees rather than complex animals like human beings. Mars, with its lower mass, lower global temperature, and a climate prone to greater extremes than Earth’s, would be a world where evolution advanced rapidly, where warm-blooded and furred animals would flourish and Martians would build their advanced civilizations. Yes, they would be pale-skinned and narrow-jawed and broad-chested and slender-legged, there would be “small beauty about them, to our way of thinking, except for the intelligence of their expression,” but these evolved beings would have built a civilization of peace and plenty. As Venus would be Earth’s past, so would Earth be Mars’.
As it happened, both were wrong. Even as Perrier wrote, ground-based observations were revealing Venus’ atmosphere to be decidedly odd, slowly rotating and very cloudy. These clouds were thought to counteract the increased radiation received by a Venus closer to the sun than Earth. After the determination that Venus’ atmosphere was dominated by carbon dioxide, the received opinion was that Venus was covered by a high-pressure ocean made of soda water. Radio observations in the 1950s and the Mariner 2 probe in 1962 confirmed that the planet was a lethally inhospitable desert, with the planet’s possible water oceans evaporating early in its history–perhaps as early as 3.9 billion years ago, perhaps as recently as 2.6 billion years ago. Odd chemical signatures in a potentially relatively habitable upper atmosphere suggest that there may be life there, bacteria and the like.
As for Mars, again, observers in the 19th century believed that they saw, through a clear Earth-like atmosphere, clear markings of life and perhaps even civilization. For whatever reason–perhaps because Mars’ surface was visible–the idea that intelligence resided on Mars was uncontroversial. It was only later, starting with the spectroscopic analysis of the Martian atmosphere by American astronomer William Wallace in 1894 which revealed the air on Mars to be without oxygen or water, followed by the slew of 20th century observations from the ground and later by space probes which confirmed Mars to be uninhabitable (here, too cold and with two thin an atmosphere). There may still be life on Mars–indeed, it’s arguably probable given a relatively Earth-like early history before its atmosphere thinned, the water froze and the temperature dropped–but again it’s likely to be marginal, bacteria trapped in the subsurface.
We may not be alone in the solar system–if “we” is defined as living organisms–but we are almost certainly alone if we’re talking about complex biospheres capable are producing life forms like us. There may well be life in the subsurface water oceans of moons like Jupiter’s Europa and Saturn’s Enceladus, and the mess of complex chemicals scattered on the cryogenic surface of Titan could lead up to something, but I’d be surprised if it led to something much. Earth is unique in the solar system as being a massive world capable of retaining a dense atmosphere and producing a protective magnetic field, located close enough to the sun to receive enough insolation to create habitable temperatures without scorching its surface; Earth is uniquely hospitable to the development of complex life. There may be life everywhere, but only simple life. The dreams of sober scientists like Perrier this time last century were all for naught.
I’ve seen two reactions to this killing-off of an earlier generation’s dreams. The first comes in the idea of terraforming, artificially creating, conditions of habitability on planets, Mars most frequently due to the relative (relative!) simplicity of the task. The Sands of Mars, an Arthur C. Clarke novel from 1951, has Mars–a Mars with a dense atmosphere and even primitive animal life–be warmed by the ignition of one of its moons into a miniature sun. The more realistic Mars trilogy by Kim Stanley Robinson makes use of a variety of techniques to make Mars more Earth-like. Interestingly, Robinson’s focus is as much on the sorts of human societies created on this transformed world as it is on the process of transformation itself. Others worlds–Venus, and the Moon, most often–are imagined as terraformable, though these two (particularly the last) would require even greater efforts. They would produce the beautiful worlds dreamed of by previous generations.
The other approach is to look beyond the solar system, into the wider galaxy, to search for habitble planets. The ongoing and increasingly successful search for exoplanets is exciting hopes in this regard, with the recent possible discovery of the Gliese 581 g, the most Earth-like planet yet discovered, making people excited.
I wonder about the successful of either approach to recovering the rich fertile community of planets of old. Terraforming requires not only a massive industrial base but the will to use it to transform planetary bodies. Will either qualification be met? As for planets beyond the solar system, quite apart from the possible rarity of relatively hospitable environments, these planets are effectively untouchable, to us and to future generations for some time to come. We are, and (we are starting to recognize) will remain, alone, living in a universe too locally inhospitable and too huge beyond our system.