Controversy has raged about the nature of Mars for hundreds, possibly even thousands of years. There are records documenting that ancient civilizations such as the Egyptians and the Babylonians observed and studied Mars as it appeared in the night sky. The planet was named after the roman god of war and was given its own day, Tuesday, or martis dies (Mars’s day) in Latin. But it was not until the last few centuries that modern telescopes and imaging technology, as well as spacecraft and rovers, enabled us to properly study Mars, albeit from afar, and consider whether there is, or could have been life on our local red planet.
Role models for Martian life
Mars is a dry, practically giant dust bowl of a planet, spotted with humongous volcanoes, mountains and some of the most extreme, yet beautiful, landscapes quite unlike anything you can find here on Earth. Yet it is on Earth we may find clues and hints as to what life may be, or have been like on Mars. Here we find life in all sorts of places, from blistering deserts to the frozen wastelands at our poles, and even at the very depths of our seas. These organisms are referred to as extremophiles due to their ability to survive in, and their penchant for, extreme conditions. Astrobiologists view these organisms as possible models, displaying potential features and characteristics of the kind of life we might find elsewhere in our universe. The life-forms found in these ecological niches on Earth are commonly microbial, although there are eukaryotic and multicellular examples, and some even class penguins and polar bears as extremophiles. Life has been found flourishing in extremes of temperature, radiation, pressure and acidity, such as the algae Ferroplasma acidarmanus that has been found to survive in acidic mine drainage with a pH of near 0 (the lower the pH the more acidic a solution is). This has reaped the organism the sub-classification as an acidophile, and considering our stomach acid has a pH of 2 (with each pH separated by a factor of 10) this is definitely an accurate description. Another common example is Thermocrinis ruber, a pink filamentous bacterium that seems to thrive in the hot springs (83-88°C) of Yellowstone national park. Since the discovery of this bacterial species by Thomas Brock in the 1960s, there have been dozens of groups of “heat-loving” thermophilic species discovered in and around hot springs, and other high temperature environments. To be classed as an extremophile an organism does not have to be able to survive extreme conditions all the time, but just during some stages of its life cycle or when exposed to certain conditions, say at different times of the year. With this, one could apply the term to the majority of plant life who have a seed-like stage in their life cycle, as seeds are commonly resistant to many environmental extremes. More obvious temporary extremophiles include the Tardigrades, also known as water bears, who are able to enter a hibernation phase in which they are resistant to extreme temperatures, pressures and exposure to chemicals such as fluorocarbons.
It is a combination of harsh conditions and almost constant exposure to them that life would have to survive if placed on Mars. The planet itself is exposed, with an atmosphere 100 times thinner than that of Earth, and therefore cold. The surface temperature of the planet can range from -125 to 20 degrees Celsius, at the extremes of the seasons. Even if you can survive the temperature and the thin atmosphere you must then battle with even more of the elements. The soil itself is full of oxidants, possibly hydrogen peroxide and perchlorate to name just a few. Combining this fact with the radiation the surface of the planet is exposed to, the atmospheric conditions, and the low water content of the soil, scientists predict the formation of reactive species that would be very damaging to organic matter both at the surface and beneath it.
The hunt for ancient signs of life
Our search for extra-terrestrial life is an inevitable one. It is hard to imagine when you look up at the night sky that there is not life somewhere out there, possibly looking back. On Mars though, this search is a search of the planets past. In 1976 the Viking landers attempted the first in situ experiments on Mars in an attempt to find signs of biological activity. The spacecraft subjected soil samples to four different tests, but any positive results have been negated by scientists who argue that the data can be explained by chemical processes happening in the soil, partly due to the presence of oxidants or exposure to radiation. Mars looks to us now as if it can no longer support life, (although there is always a chance, extremophiles have previously been found on Earth living inside rocks in harsh conditions) but could it have done so in the past? Scientists have had to break the search down into steps, the first of which is to assess the habitability of the planet billions of years ago when conditions may have been more supportive of life. In doing so they must answer the question of whether the essential building blocks of life were present, including carbon, hydrogen, nitrogen, oxygen, phosphorous and sulphur. Another sign that microbial life was once present would be the finding of redox gradients, and molecules in varying redox states below the surface of Mars, implying that certain microbial metabolisms had once taken place there. Another potential sign of life on Mars would be the detection of certain biomarkers. These are organic molecules that, in medicine, refer to specific signs and molecules present in disease. In this case finding biomarkers would mean the identification of molecules such as amino acids that are the basic building blocks of proteins in all known life. The ExoMars Rover mission, due to launch in 2018, will carry a Raman spectrometer with this aim in mind. Unfortunately this method may present some complications as some inorganic processes, unrelated to life, can produce structures very similar to the biomarkers which scientists hope to find. These biomarkers would be very simple structures that will have subjected to various conditions throughout the last few billion years therefore detection, and accurate identification, will be no easy feat. One positive is that unlike Earth, Mars has undergone no widespread tectonic activity which on Earth has led to the reformation of ancient terrains, making it very difficult to find geological samples, including fossils, older than 3 billion years in condition suitable for accurate analysis.
Water on Mars
Ever since the first detailed maps of Mars were produced it has been hypothesized that Mars was once a planet that was able to support an environment in which water was present in liquid form. In fact much of the terrain can be seen to have formed as a result of water activity. Confirming this was the task given to, and completed by the rovers Spirit and Opportunity, who launched back in 2003 and arrived on the planet on the 25th January 2004. While Spirit lost contact in 2010, Opportunity is still functioning and sending back valuable data, even though its initial mission was only 90 days long. Joining Opportunity on the planet (although separated by some distance) is Curiosity, who touched down at Gale crater in 2012. This rover, nearly the size of a car, is practically a mobile inter-planetary chemistry laboratory; with ten distinct machines all geared towards assessing whether the ancient aqueous environments confirmed by Spirit and Opportunity could have supported life. These ancient environments would have formed relatively early on in the planets life, perhaps within the first billion years after planetary formation. This prediction has arisen from comparing the conditions to those on Earth when microbial life first appeared, and the possible conditions present on ancient Mars. Habitable after all A recent number of publications in Science, coinciding with the 10th anniversary of the landing of Opportunity and Spirit, have described a system of ancient environments, at Gale crater, that the researchers say, could have been inhabitable by chemoautotrophic microorganisms during the Hesperian age, about 3.7 billion years ago. Chemoautotrophs are organisms that use inorganic compounds to produce energy, such as iron or hydrogen sulfide and obtain carbon from carbon dioxide. Using readouts from Curiosity, scientists now believe that there was water flow around the crater rim that pooled at the bottom, with a neutral pH low levels of salt. With organisms being found on Earth that can survive the harshest acidic conditions as well as the most hypersaline (high salt) waters this is a promising find. Interestingly the data also indicates that there was a colder and/or drier environment at the time, which is quite impressive considering the environment today is cold and dry already in comparison to our planet. Thermal decomposition of rock powder collected by the rover revealed the presence of materials bearing carbon and nitrogen that may have been generated by organic materials.
Overall the work of the rovers over the last ten years and the many fly-bys by spacecraft before that, as well as our initial, and modern observations from Earth, all seem to indicate that ancient Mars was indeed habitable, if only for perhaps a period as short as tens of thousands of years. Of course there are no confirmations as to whether the planet actually was inhabited at this time and it may be many years and missions yet until this question is answered. To find fossils from this ancient time would be a great achievement and one that will require a lot of work. One of the major problems is that due to the radiation levels the planet’s surface is exposed to, the top few meters of the surface are penetrated by ionizing radiation that could have damaged any preserved fossils. Now the focus of the missions on Mars turns towards understanding how organisms would have decayed, been preserved as fossils, and then how they would have been affected by factors such as this radiation, in order to begin to understand where we might be able to find any fossils that may have escaped the ravages of time unharmed. Mars may be the second smallest planet in our solar system with a mass of 0.107 of Earths but it is not a small search area. The terrain is rough, varied and realistically rovers can only move so fast (Curiosity has an average speed of 30 feet per hour). There is also the chance that scientists could conclude fossils would not have survived close to the surface and so we may have to search deeper and so design spacecraft capable of this. It is the European Space Agency’s ExoMars mission, planned for 2018 that will next shoulder that task of searching for these ancient signs of life now that habitability has been confirmed by its predecessors. So far the search for life on Mars has been met with no ultimate success, although proving the planet was habitable in some areas is a major step forward. Yet it is quite simple to put a positive spin on this. In 1976 the Viking landers did not search the entire planet, and the 2018 mission will launch with much more advanced equipment with controllers and scientists behind its operation and design armed with much more knowledge and experience. It is a long process but proving there is life on Mars is ultimately easier than proving the planet is, and always has been lifeless. One sample, one exquisite find and the years of searching will be vindicated. Even if we search the entire surface of the planet and find nothing with todays, or even tomorrows technology, that will not be enough to dishearten the men and women involved in the search. It can only lead to new thoughts, new theories, and new missions looking in perhaps even more difficult to reach places.
Author: David Busse
Originally published at http://dbsci.wordpress.com