|This is a Viking image of the surface of Mars. The footpad of the Viking lander is visible in the corner of the image.
Image from: NASA
When we ask "Where might we find extraterrestrial life", the first place many scientists turn to, because of its similarity to the Earth, is Mars. Mars may have been like the Earth in its past. Although no signs of life on Mars have been found, scientists will continue to search because they are aware of the potential for life in extreme environments.
What is Life?
This tour wouldn't be complete without looking at...What is life? Does this sound like a strange question to you? Of course we all know what is meant by the word "life", but how would you define it?
Do all living things move? Do they all eat and breathe? Even though we all seem to know what is meant by saying something is "alive", it's not very easy to describe what "life" is. It's almost as hard as describing where life came from.
Even the biologists (people who study life) have a tough time describing what life is! But after many years of studying living things, from the mold on your old tuna sandwich to monkeys in the rainforest, biologists have determined that all living things (at least living things on Earth) do share some things in common:
Therefore, in order for something to be considered to "have life" as we know it, it must possess these characteristics.
Life on Earth...Life Elsewhere?
Life as we know it on earth requires a certain environment to survive. Even so, there are creatures on earth which seem to be able to survive in harsh environments, where the temperatures are very cold, or where there is little water or oxygen. It is environments like these which are similar to those found on other planets.
Sophisticated life forms are relative newcomers on Earth compared to bacteria. Because the environment of other planets is more primitive, life on other planets (if it exists) may be primitive and unsophisticated. If not, life would have developed the ability to withstand otherworldly environments as well as finding material for nutriment. We might have to imagine what such creatures would be like.
In 1996, scientists mistakenly thought that they had discovered life on Mars. But, there has not been any concrete evidence as of yet of life anywhere in the solar system besides Earth.
Life on Mars?
|This is a picture of the meteorite ALH84001.
Image from: NASA
In July, 1996, it was announced that Dr. David McKay, along with a team of scientists at Johnson Space Center (a division of NASA), had discovered possible fossils of bacteria in a meteorite named ALH84001 that came from Mars. It was found in the Allen Hills in Antarctica in 1984 after having landed there 12,000 years ago. While many scientists were excited at first, much of the proof offered fell apart. NASA said that after two years of study "a number of lines of evidence have gone away".
Several different chemicals and molecular structures were exciting because they looked similar to byproducts of life on Earth. However, these chemicals and structures can also be created without life. Some are even present in deep space on comets, and scientists do not think that they came from Martian life anymore.
Small spheres were observed in the meteorite which the scientists in 1996 claimed were the fossilized remains of bacteria. However, they are roughly 1000 times smaller than the smallest bacteria on Earth, so don't resemble any life thought to be possible. Organic (carbon containing) compounds were found with the spheres, but it turned out that the organic compounds became a part of the meteorite after it landed on Earth (possibly when water seeped in a couple times over the 12,000 years the rock laid in Antarctica). Carbon 14, an isotope found on Earth is present in the organic compounds, but not in the spheres.
The environment of Mars in the past was very different than it is today. Conditions then may have been favorable for the existence of life. Even though the Mars meteorite does not prove life once existed on Mars, it does not disprove the possibility.
The Search for Life on Mars
The Viking program of the 1970's was the first to return data that there is currently no evidence of life on Mars.
As part of a more thorough search, the Mars Surveyor Program was put in place. Five spacecraft in all were to be sent to Mars between 1996 and 2005. Those spacecraft were to include the Mars Global Surveyor, and the Mars Climate Orbiter and Polar Lander. Unfortunately, the Mars Climate Orbiter and Polar Lander have been lost. Scientists still hope that after all the information is gathered, they might know more about the evolution of Mars and its potential to harbor life.
Life that Survives in Harsh Environments
|This is an image of smokers, which live at the bottom of the sea on earth.
Image copyright 1997 by the American Geophysical Union. Further electronic distribution is not allowed.
So far we've taken a look at what life is and at some of the negative results in looking for life in a Martian meteroid and on the planet itself. It doesn't seem like there is life now on Mars. But maybe there is life that could live in an extreme environment like on Mars...or maybe Mars was different in the past. This is what we'll look at now.
On Earth, we know that there are many kinds of life which can live in difficult environments. An example of one such creature is shown in this picture. These are smokers, and they live on the bottom of the ocean floor. There it is very cold, it is completely dark, and there is a lot of pressure (think of how your ears feel when you dive to the bottom of the swimming pool).
Maybe lifeforms like these could live on Mars or other planets with extreme environments.
Mars in the Past
|This is an image of ice at the north pole of Mars.
Image from: NASA
In the past, Mars was much different than it is today. Liquid water used to flow on the surface, as shown in this picture. Both the Earth and Mars should have been frozen in their early history because the sun was weak at first, but both planets show that water was flowing, which suggests that they both must have had thick atmospheres in place to keep the surface warm. In this environment life may have once existed.
The atmospheres on both planets came out of volcanoes. There were not many volcanoes on Mars, and those volcanoes were never very active. Compare this to the Earth where volcanism continues today.
The volcanic eruptions produce a lot of water. The water eventually falls to the ground or into the oceans. Mars is small, and so cooled off very rapidly. Mars was sufficiently cold for water to be absorbed into the ground and freeze like tundra in the Canadian northwest. Today scientists estimate that a large amount of water is frozen into the surface of Mars. They estimate this happened by 2.8 billion years ago.
So it is not likely that Mars will become a haven for life in the future...unless it is life unlike that which we know?
What Would Life Need?
What would life need to exist on Mars?
Exploratour - Life in the Solar System
In spite of Mars' similarity to Earth in size and closeness to the sun, the environment of Mars seems unfriendly toward life as we know it on Earth. Mars is small, so there is not much gravity. For this reason, much of the atmosphere of Mars has drifted away. With little atmosphere, and no ozone layer, there is less protection from the ultraviolet radiation of the sun, which is very harmful to life.
With little atmosphere, there is a only a small buffer between the surface and space itself. This means that the temperature above the surface is cold. With little atmosphere, there is also little pressure, which sophisticated life forms such as humans need to keep blood from boiling. (Remember what happened to the characters of the movie "Total Recall" when they encountered the surface of Mars)!
The Viking lander, which carried the first experiments designed to detect life on Mars, did not find evidence of even the simplest organic molecule. The Mars Pathfinder came up with negative results when looking for signs of life on Mars. We know, however, that there are life forms on Earth which can survive in very harsh environments. It's possible that if life ever existed on Mars or if it will ever exist on Mars, it will look very different from life as we know it!Related Resource