Some of our friends at the Lakeview Museum of Arts and Sciences in Peoria, Illinois, wonder whether there is life on Mars. This is an idea that has intrigued people for centuries, and one that I, perhaps like you, have wondered about for most of my own life.
Earth is the only place that we know for certain supports life. Many claims have been made by observers who thought they saw evidence of life on Mars, but we now know they were tricked by the very difficult measurements. From Earth, even with our most powerful telescopes, we just cannot see enough detail on Mars to answer this question. We need a close-up look at the planet.
While robotic spacecraft have given us wonderful views, no humans have ever tried to journey to Mars, and no such missions will be attempted for many years. In fact, whoever will turn out to be the first people on Mars may be your age today, and when you are an adult, perhaps you will watch — or even participate!– as people make the first voyage to that planet.
In the meantime, NASA is working hard now to discover whether there is life on Mars. The United States and other countries have been sending spacecraft to orbit or land there since the 1960s, and each mission teaches us more about this fascinating planet. We have learned that even though Mars is more similar to Earth than anywhere else in the solar system, and therefore is a good place to look for life, it is still different from Earth in many ways.
A compass points to the North Pole on Earth because our whole planet acts like a giant magnet, but Mars does not act this way. Besides turning a compass needle, Earth’s magnetic field turns away dangerous particles of space radiation. Without a magnetic field on Mars and with much, much less air than on Earth, more harmful space radiation reaches its surface. Although some measurements tell us there probably is water on Mars, there is far less than on Earth. And it is so cold there that most of the water is probably not liquid but rather is ice. Overall, Mars would be a pretty uncomfortable place to try to live!
In 1976, NASA landed robotic spacecraft named Viking 1 and Viking 2 on Mars. One of these landers worked there for nearly 4 years and the other lasted more than 6 years. Think of spending that much of your life studying another world! Among their scientific experiments were the only ones so far specifically designed to discover whether there was something tiny (like bacteria) living in the soil.
Most scientists agree that the results do not reveal any signs of life. The spacecraft had cameras that returned thousands of images of the surface, showing the changing seasons and details of the rocks and dirt near the stationary landers. While not officially part of the life experiments, the cameras did show us that there weren’t any large creatures wandering around! Future landers will probe underneath the surface to try to find out if there is anything living below ground.
But where is the best place to look for life? Although Mars is smaller than Earth, it is still a very, very big place, so where should scientists aim landers to give them the best chance of finding evidence of life? All life on Earth depends upon water, so spacecraft in orbit and the next few landers will search for more signs of water to help guide later missions to promising locations.
Even if there were no life on Mars, it would be exciting to know whether there used to be life there. So in addition to looking for living bacteria, NASA will be searching for tiny fossils that might indicate life got a start early in Mars’ history but, unlike on our home planet, it did not survive and evolve into larger life forms.
Many of the studies of Mars will involve robots, like the ones that have gone there before, but getting more advanced with each flight. Someday a spacecraft may pick up samples from Mars and bring them back to Earth where they can be studied in our best laboratories. Eventually, humans may make the daring journey, but many important problems have to be solved before trying such an expensive, difficult, and exciting voyage.
Conditions Needed for Life to Thrive
On Earth, all forms of life need water to survive. It is likely, though not certain, that if life ever evolved on Mars, it did so in the presence of a long-standing supply of water. On Mars, we will therefore search for evidence of life in areas where liquid water was once stable, and below the surface where it still might exist today. Perhaps there might also be some current “hot spots” on Mars where hydrothermal pools (like those at Yellowstone) provide places for life. Recent data from Mars Global Surveyor suggest that liquid water may exist just below the surface in rare places on the planet, and the 2001 Mars Odyssey will be mapping subsurface water reservoirs on a global scale. We know that water ice is present at the Martian poles, and these areas will be good places to search for evidence of life as well.
In addition to liquid water, life also needs energy. Therefore, future missions will also be on the lookout for energy sources other than sunlight, since life on the surface of Mars is unlikely given the presence of “superoxide’s” that break down organic (carbon-based) molecules on which life is based. Here on Earth, we find life in many places where sunlight never reaches–at dark ocean depths, inside rocks, and deep below the surface. Chemical and geothermal energy, for example, are also energy sources used by life forms on Earth. Perhaps tiny, subsurface microbes on Mars could use such energy sources too.
Looking for Life Signs
NASA will also look for life on Mars by searching for telltale markers, or bio signatures, of current and past life. The element carbon, for instance, is a fundamental building block of life. Knowing where carbon is present and in what form would tell us a lot about where life might have developed.
We know that most of the current Martian atmosphere consists of carbon dioxide. If carbonate minerals were formed on the Martian surface by chemical reactions between water and the atmosphere, the presence of these minerals would be a clue that water had been present for a long time–perhaps long enough for life to have developed.
On Earth, fossils in sedimentary rock leave a record of past life. Based on studies of the fossil record on Earth, we know that only certain environments and types of deposits provide good places for fossil preservation. On Mars, searches are already underway to locate lakes or streams that may have left behind similar deposits.
So far, however, the kinds of bio signatures we know how to identify are those found on Earth. It’s possible that life on another planet might be very different. The challenge is to be able to differentiate life from nonlife no matter where one finds it, no matter what its varying chemistry, structure, and other characteristics might be. Life detection technologies under development will help us define life in non-Earth-centric terms so that we are able to detect it in all the forms it might take.