This is the first in a series on the history of our understanding of Mars.
Is Mars Habitable?
At the start of the twentieth century, Percival Lowell was making observations of Mars and writing books claiming that the red planet harbored an intelligent civilization. The last in this series is where I got the title of this post: “Mars as the Abode of Life.” This was the most thorough of Lowell’s publications on the subject, and included arguments as to how Mars became habitable and why Lowell thought it was inhabited. He presented a compelling vision, that of a civilization working against the dying out of its world for a lack of water had build a massive infrastructure to move melting water from the poles to agricultural fields at the equatorial regions. These conclusions were largely driven by observations of long, straight lines on the surface by Giovanni Schiaparelli, “confirmed” by Lowell’s own observations. They were also supported by arguments made by Lowell on the habitability of Mars, including estimates of the Martian surface temperature. However, within a short amount of time, improved imaging of Mars showed no evidence for Schiaparelli’s canals, a search for water in the atmosphere turned up empty, and it was suggested that the lines “observed” by Schiparelli and confirmed by Lowell were mere optical illusions… nothing more than tricks of the light and the eye and the mind. These refutations were captured by a response to Lowell entitled “Is Mars Habitable?” that exposed and explained errors in Lowell’s logic, observations, and calculations. This piece took Mars from Lowell’s vision of a living planet complete with a global civilization to a desolate world that probably could not support life. (Cool tidbit: this piece was written by Alfred Russell Wallace, the same guy that is better known for his research and thinking on evolution…. Talk about a multidisciplinary scholar!) Thus ended speculations amongst most scientists that Mars harbored an intelligent, canal-building civilization.
“The Viking Death of Mars”
Although most scientists thought Lowell’s arguments had been thoroughly dismissed by the middle of the century, scientists could not rule out the possibility that single-celled organisms were living at the surface. Mars, after all, had an atmosphere, and there was some data suggesting that it had a history of liquid water on its surface. If that represented a recent history, and if that water were in at least periodic contact with the surface, then perhaps “simpler” single-celled life could exist at or near the surface.
We sent a mission to Mars to test this possibility. That mission consisted of two orbiters, each containing a lander. The orbiters first scouted for landing sites, and then released their payloads to descend to the surface. Upon landing, the mission conducted our first astrobiological experiments on another planet. They searched for both life and habitability. The biological experiments produced mostly ambiguous results; while biology would have been consistent with the data, there were explanations for the results other than biology.
The “non-biological” explanations and interpretations of the Viking biological experiments have since been favored because a search for organic material came up empty. This was shocking because there was (and is) plenty of delivery or organics to the surface of Mars, even in the absence of biology. Thus, this negative result implied that some mechanism was actively destroying those compounds. If any life existed, its organics should have been detected – even dead cells would have been picked up by this experiment. Furthermore, this complete absence of organics suggested that the Martian surface wasn’t just uninhabited – it was entirely uninhabitable. After all, how could life exist in an environment in which it’s very structure is actively and rapidly destroyed? To most of the scientific community, the absence of organics on Mars provided a “nail in the coffin” for the prospects of life at the surface.
Some members of the community have always held that “Mars is alive” and that certain experiments showed it to be so. Gil Levin, the PI for the “Labeled Release” experiment, repeatedly points out his experiment’s positive result. If his interpretation is correct, claims that his experiment was the first to discover alien life would also be correct. However, most in the community have remained unconvinced of his arguments.
For the last few decades, considerations of life and habitability on Mars have been limited to other domains in time and space. They focus either on the past, when Mars may have had liquid water at the surface… or in the subsurface, where the damaging radiation that is thought to destroy organics is not a factor. However, the last decade has brought us reasons – both new and old – to believe the Martian surface may currently be habitable. That will be the focus of my next post in this series.