Did Life once appear on Mars?

Present:

The special role of Mars as the possible site for extraterrestrial life may well date back to the time centuries ago when, around the year 1600, telescopes were first trained on those "wandering stars", the planets. Galileo Galilei himself was not able to discern features on Mars but within fifty years Christian Huygens began the telescopic mapping of large scale variations in reflectivity (light and dark areas). It was the discoverer of Neptune, William Herschel, who in the late 1700's measured the length of the martian day (fractionally longer than our own) and the tilt of its axis (also similar to Earth's) -- the reason why Mars has seasons. He also observed the seasonal polar caps so that by the end of that century the similarities of Mars and Earth had become recognized. During the last century and into this, patient and skilled astronomers carried out increasingly detailed Mars mapping programs that not only created excellent classical atlases of Mars but also provided more and more information about the seasonal changes of light and dark surface features, of the polar caps, of condensate clouds and dust storms. With such interesting observations to stimulate them some imaginations ran wild (that of the US astronomer Percival Lowell in particular) and ideas were described that conjured up all sorts of fanciful ideas about martian civilzations past and present. Although, even before the space age began, the cold dry nature of the martian surface and the low atmospheric density had been determined from astronomical measurements, it was not surprising therefore that interest would remain high in the possibility of there being life on Mars. By that time, however, expectations had been substantially lowered in the face of discouraging facts and only the possibility of extremely primitive life was contemplated.

Our first spacecraft observations of Mars in the mid 1960's (when Mariner 4 flew by Mars with a simple camera and other remote sensing instruments) were a set back to anyone hoping that Mars would prove to be a distant sister to our own planet. Mariner 4 imaged only a small part of the cratered martian highlands and the resolution was far too poor to see the valley networks that we know about today. Immediately we concluded that Mars was rather a near sister to our own stark Moon. Or so we thought until Mariners 6 and 7 flew by Mars a few years later and sampled some other regions and new kinds of terrain -- including "chaotic" terrains that we now associate with the source regions of some massive floods. These exciting new observations began to make the martian explorers' blood run faster again.

Fast forward to 1971 and the first Mars orbiters, Mariner 8 and 9. Mariner 8 remains Earth-bound (at the bottom of the Atlantic!) but Mariner 9 made it all the way to Mars and truly opened our eyes as to the actual nature of that beautiful and varied planet. A cliff-hangar to the end, Mariner 9 had to spend months in orbit waiting for a planet-wide dust storm to clear before it could see the surface clearly enough to image the volcanoes, canyons, outflow channels, valley networks, vast new kinds of plains, sand dunes and layered polar deposits. Not to mention the many craters, different in character from those of the Moon, and huge basins like Hellas and Argyre that Mars has preserved. As informative too, in its way, was the Mariner 9 discovery of the absence of certain critical geologic features (fold mountains and volcanic spreading centers equivalent to mid-ocean ridges) that would have implied martian plate tectonics. The "heat engine" inside Mars is, not surprisingly, smaller than Earth's and it can get rid of its excess heat without continuously overturning the surface. Mariner 9's results re-energized NASA's already vigorous solar system exploration program and interest in the search for possible extant life on Mars re-emerged as a serious scientific goal.

Before we discuss the first highly ambitious experimental effort to detect life on Mars, it is instructive to recall the mood of the times. Even though in 1969 the US had decisively won the race with the Soviet Union to the Moon, the early 1970's was still a period of great competitiveness in terms of space activities; many more "firsts" had yet to be written into the text books. So, the possiblity that the discovery of life on Mars might be made by a probe launched by Soviet Union had political implications that matched the great scientific and cultural importance. Probably that is why the very first US landings on Mars (the Vikings) were so remarkably ambitious. Today the pace of Mars exploration is proceeding in a much more methodical way.

The first effort to search for life on Mars was conducted by the Viking spacecraft. The two soft-landing Viking spacecraft carried to the martian surface a complex biology experiment package which contained three instruments designed to detect microbial life (specifically, life that was capable of photosynthesis or respiration). Each lander had been meticulously sterilized to make sure we did not simply detect terrestrial organisms that had hitch-hiked to Mars. The landers each carried a super-sensitive Gas Chromatograph/Mass Spectrometer to detect organic material in the martian soil. In the event, all three of the Viking life detection experiments returned interesting signals that indicated some sort of reactivity in the Martian soil. In fact, one of the respiration experiments yielded a response that was perfectly consistent with life. However, the results of the other experiments were not at all readily explained biologically. The "nail in the coffin" proved to be the non detection of any organic material at levels of parts per billion. The accepted explanation is that the martian soil contains a highly oxidizing component (perhaps hydrogen peroxide) created by a photochemical reaction between the soil, atmospheric water vapor and sunlight's ultraviolet radiation

Past:

Some of the most interesting information from Viking related to the question of life on Mars were obtained not by the landers but by the orbiters -- the much improved orbital imaging coverage (relative to Mariner 9) of the ancient valley networks and sinuous channels. These channels attest to the fact that Mars had, and perhaps still has, beneath its surface, copious amounts of water; at one time this water may have been quite stable on the surface and, for sure, flowed freely over long distances. Because liquid water is an essential requirement for all life on Earth, its presence on Mars during the past is of great significance.

Click on this image to have a more detailed view of Samara Valles.

That Mars once had liquid water flowing on its surface in quantities sufficient to carve huge outflow channels and complex valley networks suggests that the martian atmosphere may have been thicker and warmer billions of years ago than it is today. Let's think about one possibility, namely that warmer conditions on Mars may have been due to the presence of a thick carbon dioxide (CO2) atmospheric "blanket", over several hundred times thicker than the current atmosphere. Our own concerns on Earth about the greenhouse properties of carbon dioxide ("global warming") indicate that CO2 is indeed very effective in warming a planetary atmosphere. We know that both Earth and Venus have outgassed large quantities of CO2 (still in the oven-like atmosphere in the case of Venus and, on Earth, mainly trapped now as carbonate rocks). Mars was formed from similar material to that from which Earth and Venus accreted so there is every reason to expect that Mars would have outgassed lots of CO2 also. So, supposing that early Mars also had a thick CO2 atmosphere AND had standing bodies of liquid water, then, what would have happened? By analogy with the Earth, the CO2 would have dissolved into the water to form a weak solution of carbonic acid. In turn this would have led to chemical interaction with the sediments in the water to create carbonate rocks which would have been permanently deposited on the floors of lake and ocean basins. As just described, most carbon dioxide that started out in the Earth's atmosphere has been transformed over time into carbonate rocks. But there is this key difference between Earth and Mars: on Earth subduction (the convective plunging down of surface rocks into the interior of a planet) associated with plate tectonics recycles these sediments (through reheating at depth and subsequent volcanic emission of released gases). Mars, by contrast, is a "one-plate planet" with no mechanism to recycle the carbon dioxide it lost from its atmosphere. So the atmosphere would have become thinner and the climate cold. Presumably the water that flowed once on the surface then became trapped below the surface as permafrost and at the ice caps. Perhaps too, at depths of several kilometers there may still be aquifers charged with liquid water.

An instructive approach to investigating the question of life on early Mars is to compare conditions with those of the early Earth. On Earth there is definite evidence for life at 3.5 billion years ago -- within a billion years of planetary formation. This evidence is in the form of fossilized remains of algae mats (stromatolites) as well as microfossils. There is even some provocative evidence in the isotopic signature of sedimentary carbon that life was present on Earth 3.8 billion years ago (biologic carbon has more of the heavy carbon isotope C14 relative to the common isotope C12). Unfortunately, the ancient sediments in question have been so altered that the case cannot be made with certainty. Considering that 4 billion years ago the Earth was still being blasted by titanic asteroidal impacts capable of boiling oceans, it is clear that life evolved almost as soon as the dust settled, so to speak.

Turning now to Mars we note with interest that the period during which life originated on Earth (about 3.8-3.5 billion years ago) is a time when liquid water was active on Mars (in fact, water was still flowing episodically a billion years later). The apparent similarity of Earth and Mars during this early period leads naturally to the possibility of a parallel evolution of the biogenic elements on Mars. With this in mind, scientists have been developing a strategy to systematically determine the early climate of Mars, to understand in detail what geologic processes were taking place that might have created benign locations (for example, thermal springs) for the biogenic elements to evolve toward life, and even to find microfossils that could seal the case.

Where to search for Life on Mars ?

A lake bed is one ideal location (thermal spring deposits would be another but we have yet to discover such deposits) for searching for fossils because the sediments that accumulated on the lake floor provide the necessary protection to physically preserve organic materials. Without preservation, no fossils! Furthermore, our terrestrial experience suggests that the shores of a martian lake may have provided a suitable habitat for microbial colonies (which on Earth form algal mats) that could have been preserved in recognizable form.

Early martian lakes have another attractive characteristic: they could have provided habitable environments (that is, have been a source of liquid water) long after the rest of the surface of Mars had cooled to intolerable levels. We see this today on Earth in the ice-covered lakes of the Antarctic dry valleys where in spite of average temperatures well below the freezing point there is liquid water all year round under a thick ice cover. These dry valleys are extremely cold and dry and, even before the discovery of the ice-covered lakes, were considered to be the best terrestrial analogs to conditions on Mars.

The ice covered lakes of Antarctica are worth a little side trip here. Let's begin with the mean temperature in the dry valleys, which is about -20 degrees C. Precipitation of snow is highly variable, averaging about 2 cm of equivalent water. Four to six meters of thick perennial ice cover the lakes and act to trap heat within the lake. The liquid water (and its large latent heat of solidification) is refreshed by the trickle of melt water from a nearby glacier associated with the few days each summer that temperatures climb above freezing. These few days, and their "degree-day" product with temperature above freezing, are the key to the persistence of the ice-covered lakes. Simple climate models suggest that on Mars the degree-days above freezing would have been comparable to the situation today in the Antarctic dry valleys for about 500 million years after the mean annual temperature fell below freezing. Of course, we also have to consider the (slow) rate at which the ice cover would have sublimed into the thin cold martian atmosphere. For lakes that began with depths of several hundred meters, similar time scales of hundred's of millions of years are involved. Thus we conclude that ice covered-lakes could have been the ultimate refuge for life on Mars. One of the most exceptional examples of an ancient martian lake is Gusev crater.

Permafrost and ground-water ice:

A second location that may provide evidence for early life on Mars is the permafrost. Near the south pole is it likely that there is the material that have been frozen continuously since the early, possibly biological, period of Mars history. It is observed in the polar regions of Earth that microorganisms are trapped in the active layer and frozen into the permafrost and can be viable after long periods of time, up to 3 million years at -10 degrees. On Mars the timescale for preservation is longer, 3.5 billion years, but the temperature is also much colder (about -70 degrees). Thus, it is possible that well-preserved organically-intact microorganisms, if not viable life, would be found 30 to 50 meters below the martian permafrost. Preserved at this depth, the organisms would be shielded from cosmic and solar radiation (but would still suffer from radioactivity from the martian crust) and would be immune to transient warming events lasting thousands of years.

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