If Martian life exists, it is likely to be confined to small niches, protected from the worst of the hostile conditions on the surface of the planet. However, Mars may not have always been so inhospitable. Life could have been widespread in the past.
There are few places on Earth as evocative as Mono Lake, California. The glassy surface stretches off into the distance, and eerie rock formations protrude from the water, like gnarled fingers. It sits high in the hills, about 580 kilometers (360 miles) north of Los Angeles, and has been a remarkable witness to the Earth’s geological history. With an estimated age of nearly 800,000 years, the lake has seen an ice age come and go, and has survived the volcanic eruptions that created two islands within it. Walking its banks, you would be forgiven for thinking that you had been transported to another world: Mars perhaps. Certainly that is what the legions of visiting astrobiologists—scientists concerned with finding life on other planets—think as they contemplate this mysterious environment. The landscape is how planetary scientists imagine Mars to have been in the past, before the last of the water was lost from its surface, forcing any Martian life that existed to struggle for survival.
Attracting the most attention are Mono Lake’s eerie fingers of limestone, known as “tufas,” protruding some three meters from the surface of the lake. If astrobiologists could find such structures on Mars, these might reveal whether life ever began on the red planet. The tufas formed underwater when calcium-rich spring water bubbled up through the alkaline lake. The lake water was rich in bicarbonate and this combined with the calcium to form limestone, which aggregated to build the tufas. As the water level in the lake fell, so the tufas broke the surface. Crucially, the tufas contain an abundance of microfossils, entombed as the limestone built up. A tufa landscape on Mars would offer a perfect landing site from which to search for Martian microfossils.
THE TUFAS OF MONO LAKE, CALIFORNIA: SIMILAR STRUCTURES MAY BE FOUND ON MARS.
The initial attempt to find life on Mars was made in the 1970s by a pair of landers called Viking 1 and 2, the first manmade objects to touch down on the red planet. They carried out four experiments to look for life by analyzing the Martian soil, but only one showed any promising results. When a nutrient broth was dripped onto the soil, carbon dioxide gas was liberated, mimicking microbial metabolism. However, subsequent runs failed to repeat the release. Also, the other experiments failed to detect the organic molecules expected in the Martian soil if microbes were present. So astrobiologists concluded that life was not present in the soil sample but this left them with a question: what was it that released the first puff of carbon dioxide? This was to remain unanswered for several decades.
In 2008, NASA’s Phoenix robotic lander detected chemicals called perchlorates in the Martian soil, which could have reacted to give off carbon dioxide. They may also be responsible for breaking up any organic molecules in the “top soil” of Mars. Whatever the exact explanation, the search for life on Mars is proving far more difficult than just landing robots on the planet and scooping up a handful of dust. What has been revealed without doubt is that the conditions on Mars today are extremely hostile by most Earthly standards. Not only are there highly reactive chemicals in the ground, there is no air to breathe, and the surface is scoured by ultraviolet radiation and high-speed particles thrown out by the Sun. There is an apparent lack of water and the temperature swings from around 17 degrees Celsius during the day to lower than minus 100 degrees at night.
In microniches on Mars, however, conditions may be right for organisms that would be considered extremophiles if found on Earth (see Are We Made From Stardust?). Failing that, it is possible that life might have existed on Mars in the past when conditions were different.
Planetary scientists are certain that Mars began its existence as a world similar to Earth, with many bodies of standing water. If life began on Mars soon after the cessation of the late bombardment (see How Did the Earth Form?), as it is believed to have done on Earth, there could have been plenty of time for it to colonize the planet before the conditions changed.
In the quest for life on Mars, the mantra is to “follow the water.” This is because life needs a liquid in which to perform its chemical reactions. Water is an excellent option because it is a simple molecule and is abundant across the Solar System. Until astrobiologists are forced to consider more outlandish alternatives, they have decided to look for water first and then, once they find it, to look for signs of life in that location.
NASA’s Viking orbiters of the 1970s returned pictures that show geological structures on Mars resembling water-cut features on Earth. Some of these, such as the Nanedi Vallis, are indicative that water has run on Mars as rivers. This meandering channel is 2.5 kilometers (1.6 miles) long and displays terraced walls and oxbow curves. It is clearly a place in which water has run repeatedly, perhaps constantly, over long periods of time. Other features, such as the teardrop-shaped “islands” in the mouth of Ares Vallis, show that water has flowed across the surface as floods. The islands appear as if they were fashioned by millions of gallons of water emptying from the valley onto the surrounding plains. This picture was confirmed when NASA landed the Pathfinder rover in the outflow plain near the teardrop islands in 1997. Its images were immediately notable because many of the rocks surveyed were rounded, as they would be by erosion in a watery environment.
“Mars is there, waiting to be reached.”
BUZZ ALDRIN APOLLO 11 ASTRONAUT
There is even the suggestion that Mars once had a large ocean. Firstly, there is circumstantial evidence in the difference between the two hemispheres of Mars. The southern hemisphere is mostly high ground, with jagged craters and chasms, whereas the northern hemisphere is a low-lying basin, and it is much smoother, implying an earlier ocean. Secondly, there are two possible shorelines. Seen from orbit, each stretches for thousands of kilometers around the northern basin, and they have been estimated to be between two and four billion years old.
Yet today the planet seems to be dry. If we are going to target areas to look for life, then we need to understand what happened to the water and, if any of it is still on Mars, where it could be hiding.
Mars has been badly affected by its lack of a magnetic field. Unlike the Earth, where harmful solar particles are mostly deflected away, these collide with Mars’s atmosphere. This has eroded away the gas by knocking molecules out into space, and as the atmosphere was lost, so the planet’s climate declined. The decrease in atmospheric pressure meant that lakes, seas, even the ocean would have evaporated more easily. With no protective magnetic field or ozone layer, the incoming solar radiation would have broken up airborne water molecules into its constituent hydrogen and oxygen atoms. The liberated hydrogen would have been light enough to escape the planet’s gravity and disappear into space, whereas the heavier oxygen would have sunk to the surface of the planet and combined with the minerals in the rocks. Any water that remained on the surface is thought to have drained downward into the rocks and frozen solid.
Early theories assumed that this had been a relatively gradual process, perhaps lasting billions of years. However, by counting the number of craters in different regions of Mars, planetary scientists are uncovering a different picture: a past history punctuated by planet-wide climatic and volcanic catastrophes. Counting craters seems simplistic but it is useful because during volcanic eruptions areas of the planet were resurfaced with lava, erasing the existing craters and providing a blank sheet onto which meteorites fell during the subsequent eons, creating new craters. By comparing the number of craters in each region of Mars, planetary scientists can estimate the ages of those areas: the surfaces with the fewest craters are deemed to be the youngest.
Instead of undergoing a gradual transition over four billion years from an Earth-like world to the present frigid desert, crater counting suggests that Mars became a desert in less than a billion years. Any widespread life that had developed on the planet would have perished, and the remainder would have been driven into niches, much earlier than previously thought. The planet was then subjected to a violent sequence of volcanic activity, producing a series of giant upheavals. Each one briefly resuscitated the planet, because the sudden outpouring of internal heat would have thawed out the frozen reserves of underground water and driven them upward to the surface, flooding large areas of the planet. According to analysis, there have been five such upheavals during Mars’s history: the earliest took place 3.5 billion years ago; this was followed by another 1.5 billion years ago; then by further events 800 million years ago, 200 million years ago, and 100 million years ago. Each episode may not have lasted for more than a few tens of thousands of years but they have all left ample evidence on the surface of the planet. This is not just seen in the form of lava flows but also in the myriad outflow channels, riverbeds and even shorelines. Perhaps during these periods, Martian microbes flourished before again being forced to return to their deeply frozen conditions.
Today most of the remaining water is thought to be just below the surface in the form of icy plains, or deeper in underground lakes. In 2002, NASA’s Mars Odyssey orbiter showed that ice was indeed buried, perhaps just a few centimeters under the surface, in the northern hemisphere of Mars. When Phoenix touched down in the middle of these plains on May 25, 2008, it swiftly verified that there were large deposits of ice just below the surface. In the spirit of the “follow the water” mantra, this has boosted the possibility of finding ongoing life on Mars.
There has been no similar luck in finding the supposed underground lakes of water. Two radar instruments have been sent to Mars, both designed to send radio waves down to a kilometer or more beneath the surface where they would be expected to bounce back from the supposed boundaries between rock layers and water layers. Thus, they should have created a map of Mars’s network of underground lakes. However, neither radar instrument has seen anything resembling a lake. Perhaps the water is deeper than anticipated, or simply not there; either way, this particular result is not so encouraging to the search for life.
It is ironic that as planetary scientists collect more and more data from Mars, they find themselves presented with an increasingly confusing picture as to whether the red planet is habitable or not. They press on in the quest because there is so much at stake. Not only would finding the evidence for past or present life on another planet be a tremendous achievement in itself, it could also fill in some important gaps in our knowledge of life on Earth and specifically how life first formed here.
On Earth, the fossil record of life’s origin has been lost because of our planet’s restless surface. Driven by the decay of radioactive elements in the Earth’s interior, our continents float on a semimolten layer, moving by a few centimeters each year in a process called plate tectonics. If the continents rub past one another they produce earthquakes; if they push against each other, they build mountain ranges. If one plate rides roughshod over another, it forces it down into the interior of the Earth, where the rocks are melted and recycled in the form of lava that bursts through volcanoes at the surface.
This global recycling has destroyed nearly all of the truly ancient rocks on Earth, taking with them any traces of the first life forms. This may not be true for Mars; being a smaller planet, there was not enough radioactive heat to begin a full-scale plate tectonics process. Lacking this constant “recycling machine,” parts of Mars—like the Moon—must be primordial, dating back to the original formation of the planet. A meteorite called ALH 84001, discovered near the Allan Hills in Antarctica in 1984, bolstered this belief. Dating ALH 84001 showed that it was around 4.5 billion years old, placing it at the very origin of the Solar System. Small bubbles of gas were trapped inside the rock and researchers examined their content. Astonishingly, the gas displayed the same composition as the Viking landers had registered for the Martian atmosphere. The rock appeared to have come from Mars.
The story now envisaged for the long life history of the meteorite is that the rock solidified as part of Mars’s original surface but was blasted from its site of formation during an impact in the late bombardment, around 4 billion years ago. But this did not loft it into space; instead it fell back to Mars and remained on the surface for nearly the whole of the subsequent history of the Solar System. Then, 13 million years ago, another impact sent it careering into space, where it crossed paths with Earth some 13,000 years ago. It fell as a meteorite, landing in Antarctica, where it was entombed in a glacier and finally returned to the surface as the glacier struck the Allan Hills. And, if that story is not amazing enough, ALH 84001 may even be showing us the very way life began.
In midsummer 1996, NASA showed the world scanning-electron-microscope images of tube-like structures found in a sample of meteorite ALH 84001. They were clearly distinct from the surrounding rock and looked eerily reminiscent of bacteria fossils found on Earth, and chemical evidence suggested that these tubes might indeed once have been alive. But there was no consensus. These “fossils” were only 20–100 billionths of a meter in diameter, making them similar in size to the nanobes from Earth that were simultaneously being puzzled over (see Are We Made From Stardust?). As a result, the same argument against their once having been alive was presented: namely that they were too small to contain the DNA copying mechanisms considered necessary. Of course, the same counterargument was proposed as well: that these organisms might carry out their copying in a more primitive way, eventually superseded by the reproduction methods of today’s microbes. Other scientists claimed that such structures could be made through simple crystallization processes, without the intervention of life.
Another important development in the debate about life on Mars became widely acknowledged by scientists early in 2009. It concerns methane, a gas readily found in Earth’s atmosphere, produced either by volcanic activity or by the metabolism of life forms. For several years planetary scientists had been detecting methane on Mars. It is localized to three regions of the planet, rather than spread thinly throughout the whole atmosphere, and this strongly suggests that it is being produced at those locations, otherwise it would have been distributed around the planet by winds. If it is being produced now, then either there is current volcanic activity inside Mars, or there are colonies of living Martian microbes metabolizing under the surface. Either option is quite mind-blowing. If it is life, then the discovery is obviously momentous. But the volcanic option would be big news, too, because Mars was thought to be geologically dead; the volcanic episodes that characterized its past were considered impossible today.
The excitement is compounded because, during follow-up observations, the methane was seen to have disappeared. It had not been blown around the planet; it had totally disappeared. Something had destroyed it. The ultraviolet light from the Sun could not have broken it down that quickly, so scientists think the cause must be highly reactive chemicals in the soil. Calculations show that the methane was destroyed 600 times faster than scientists were expecting, so to have built up the original quantity in the atmosphere requires a production mechanism that worked 600 times faster than previously assumed. If microbes generated it, there must be 600 times more of them than originally thought. But until we can get to Mars and determine hands-on whether it is life or volcanic activity producing the methane, we will remain uncertain of whether there has been, or still is, life on Mars.