Mars is being invaded! Already our orbiters are orbiting, our landers are landing and our rovers are roving- and more of them are on the way. Over the next few years, the red planet will be subjected to an array of tests by various machines, some in space and some on the ground. We have already looked at some of these in our past articles- the ESA’s Mars Express probe, for instance. The biggest goal, not surprisingly, is to determine if Mars has life, or has ever had it in the past. This leads to another goal, ascertaining if Mars is habitable for human beings, with the thought of future colonies. This goes beyond the search for water, as important as that is. It also involves assessing environmental hazards that could pose problems for colonists. The purpose of the robot invaders from Earth is to address these questions, because the robots will ultimately be followed by people.
Some of the invaders are already there; others are in the planning stages. One project which is currently being planned, and which will hopefully be launched next year, is the ExoMars mission. This is a project of the European Space Agency, with contributions by NASA. Its purpose is to subject Martian soil and rocks to the most precise and detailed tests yet, in an attempt to discover if they contain the chemical signatures of past or present life. ExoMars will involve both an orbiter and a lander, with the most sophisticated and sensitive detection devices ever sent to Mars. If there is anything alive there, or if there ever was, this is the device that could find it.
The search for life on Mars has had its ups and downs. When the Viking landers arrived in 1976, the samples of Martian soil that they collected showed none of the chemical traces that would be left by life. At that point, all of the planetary scientists on Earth let out a collective groan: Mars was a dead planet! For some years these results stood, leading to the prevailing opinion that Mars did not have life now, and probably never had it.
However, the Viking results, and their interpretation, have come under criticism in the intervening years. For one thing, Viking contained three separate tests for signs of life, and while two of them gave negative results, the third one, called the Labeled-Release Experiment, gave ambiguous results that might be indicative of life. Scientists have never gotten very excited about this, because there is a high probability that the results were produced by natural chemical processes unrelated to life- but the fact remains that the three instruments did not produce uniformly negative results. Until further investigations are conducted, it is impossible to say with absolute certainty that Mars is a dead world, and always has been.
When contemplating the Viking results, these objections have also been raised:
Maybe the instruments on the Viking landers just weren’t sensitive enough to pick up faint traces of biological chemicals in the Martian soil. Since then, new devices have been developed which can detect chemicals in amounts of only a few atoms. If instruments like that were used on Mars, the results might be different.
Perhaps the Viking samples just weren’t taken in the right place. There are certain extreme environments on Earth- the Atacama Desert in Chile, for instance- where soil samples might yield the same results, but that doesn’t mean that Earth is a dead planet.
Another interesting point is that since Viking, we have learned that life is more versatile than we used to think. Extremophile organisms have turned up in places on Earth that are at least as hostile as the environment of Mars. They have been found in places of extreme heat and cold, in permanent darkness, and in chemical environments that would kill most other life. They have even been found encased in rock, where they apparently live by metabolizing the very rock itself. If life can exist in those places, why not on Mars?
A third point, which could invalidate the Viking results, is that even if the chemical signatures of life are not detectable on the surface of Mars, they might be found just underground. Maybe they were present on the surface at one time, but have since been broken down by the harsh UV radiation of the sun, or by oxidation. In that case, they might not be detectable unless you take a sample from underground, or from the inside of rocks.
ExoMars will attempt to address these issues. The lander will be equipped with a drill that can penetrate up to two meters of soil or rock. Samples from that deep should be safe from the both the sun’s UV rays and the effects of oxidation.
The drill is actually three devices in one: a drill, a sample collector and an infrared spectrometer. Once it drills below the Martian surface, it can actually perform spectroscopic analysis within the bore hole. If it spots something that looks interesting, there is a chamber within the drill shaft with an internal shutter which can be used to get a sample.
However, once the sample is taken, the next challenge is to figure out what we’ve got. One problem in identifying primitive organisms, even on Earth, is that they are so simple that their remains can be mistaken for non-living mineral precipitates. Because of this, a visual examination, even on a microscopic level, probably will not be enough to be conclusive. In fact, given the extreme importance of this evaluation, it is likely that no single piece of evidence will be enough to convince all scientists. Therefore, the ExoMars lander will employ several different lines of investigation in determining if a sample contains evidence of life,
including geological and environmental investigations to evaluate possible habitats, visual examination of samples (morphology) and spectrochemical composition analyses.
One of the indicators of life that ExoMars will look for is homochirality. This is based on the fact that two of the main chemicals involved in making life, amino acids and sugars, can exist in left- and right-handed forms which are mirror images of each other. Amino acids and sugars can be created by non-living processes, but when that happens, the resulting substances are split evenly between the two forms. However, living organisms on Earth all use one form or the other- left-handed for amino acids and right-handed for sugars. This is necessary so that the molecules can fit into the biochemical mechanisms of living organisms. If an amino acid or a sugar is of the wrong form, it won’t fit, and the organism can’t use it.
So, if ExoMars finds amino acids or sugars on Mars, and the chirality is evenly split between right-handed and left-handed forms, we will know that the chemicals were made by some non-living chemical process. But if samples are found which have sugar or amino acids that are all left- or right-handed, it will be a conclusive sign that the chemicals were created by biological processes.
To conduct these chemical studies, ExoMars will use an instrument called the Urey Mars Organic and Oxidant Detector (named for Dr. Harold Urey, biochemist at the University of Chicago). This device uses several flat surfaces coated with chemical films, which are attached to each other to form a box. When a sample is placed in the box, the degree of reaction on each surface will be monitored, and this will tell us if there are any biochemicals present.
Besides the basic search for biochemicals, ExoMars will also be trying to spot potential problems for future human explorers. For instance, are there corrosive substances on Mars that could degrade equipment, or cause health hazards for humans? And what about the Martian dust? When astronauts went to the moon, the moondust got into everything, even in places that were supposedly sealed airtight. If that’s going to happen on Mars, we need to know about it ahead of time, and take measures to prevent it.
ExoMars is a big step forward in our journey to the red planet. It will tell us what Mars used to be like, and it may even find the signs of life, past or present. It will also tell us what we can expect there, and what we will have to do to survive. ExoMars and its mechanical brethren are paving the way for us, and the information it obtains may be crucial in the survival and prosperity of future colonies.
In addition to that, ExoMars is looking back into the past of Mars. It is now well established that Mars looked a lot more like Earth at one time: there were large bodies of liquid water, and that means that the air pressure was much higher than it is today. We can’t help but wonder: just how similar was it? Was there life, and did it look like us?
The answers are waiting on the red planet, and ExoMars is going to help us find them.
Sources:
NASA Science Missions: ExoMars Urey Instrument at the NASA website: science.nasa.gov/missions/exomars/
Mars Oxidant Instrument at the NASA website: nasa.gov/centers/ames/multimedia/images/2007/moi.html
“Sensor Being Developed to Check for Life on Mars” at the NASA website: nasa.gov/centers/ames/research/2007/mars_sensor.html
ESA Aurora Program: ExoMars at the webiste of the European Space Agency: esa.int/esaMI/Aurora/SEM1NVZKQAD_0.html
“ExoMars: Searching for Life on the Red Planet” (information packet downloaded from ESA website)