Life on Mars, a great quest of life in modern times. If life evolved on Earth from nonliving organic molecules, might the same process have taken place elsewhere in our solar system? Scientists have carefully scrutinized the planets and satellites in an attempt to answer this question, and most of the answers so far have been disappointing.
Requirments to Find Life on Mars
One major problem is that liquid water is essential for the survival of life as we know it. The water need not be pleasant by human standards—terrestrial organisms have been found in water that is boiling hot, fiercely acidic, amazingly salty, and even ice-cold — but it must be liquid at least once in a while. In order for water on a planet’s surface to remain liquid, the temperature cannot be too hot or too cold. Furthermore, there must be a relatively thick atmosphere to provide enough pressure to keep liquid water from evaporating. Of all the worlds of the present-day solar system, only Earth seems to have the right conditions for water to remain liquid on its surface.
However, there is now compelling evidence that Europa, one of the large satellites of Jupiter, has an ocean of water beneath its icy surface. As it orbits Jupiter, Europa is caught in a tug-of-war between Jupiter’s gravitational influence and those of the other large satellites. This flexes the interior of Europa, and this flexing generates enough heat to keep subsurface water from freezing. Chunks of ice on the surface can float around on this underground ocean, rearranging themselves into a pattern that reveals the liquid water.
Searching for Life on Mars
The next best possibility for the existence of life is Mars. The present-day Martian atmosphere is so thin that water can exist only as ice or as a vapor. However, images made from Martian orbit show dried-up streambeds, flash flood channels, and sediment deposits. These features offer tantalizing evidence that the Martian atmosphere was once thicker and that water once flowed over the planet’s surface. Could life have evolved on Mars during its “wet” period? If so, could life—even in the form of microorganisms—have survived as the Martian atmosphere thinned and the surface water either froze or evaporated?
In 1976, two spacecraft landed in different parts of Mars in search of answers to these questions. Viking Lander 1 and Viking Lander 2 each carried a scoop at the end of a mechanical arm to retrieve surface samples to look for biological activity. These samples were deposited into a compact on-board biological laboratory that carried out three different tests looking not simply for chemical evidence of water, but for evidence of Martian microorganisms.
The first data returned from these experiments caused great excitement, for in almost every case, rapid and extensive changes were detected inside the sealed containers, suggesting that microbial life had been found on Mars. More in-depth analysis of the data, however, led to the conclusion that these changes were due solely to nonbiological chemical processes. It appears that the Martian surface is rich in unstable chemicals that react with water to release oxygen gas, which could be easily mistaken for biological activity. Because the present-day surface of Mars is bone-dry, these chemicals had nothing to react with until they were placed inside the moist interior of the Viking Lander laboratory
At best, the results from the Viking Lander biological experiments were inconclusive. Perhaps life never existed on Mars at all. Or perhaps it did originate there but failed to survive the thinning of the Martian atmosphere, the unstable chemistry of the planet’s surface, and exposure to ultraviolet radiation from the Sun. (Unlike Earth, Mars has no ozone layer to block ultraviolet rays.) Another possibility is that Martian microorganisms have survived only in certain locations that the Viking Landers did not sample, such as isolated spots on the surface or deep beneath the ground. And yet another option is that there is life on Mars, but the experimental apparatus onboard the Viking Lander spacecraft was not sophisticated enough to detect it.
Water and New Evidence for Life on Mars
The European Space Agency’s Mars Express spacecraft, which went into orbit around Mars in December 2003, used its infrared cameras to examine the ice cap at the Martian south pole. These cameras allowed scientists to see through the ice cap’s surface layer of frozen carbon dioxide and reveal an underlying layer with the characteristic spectrum of water ice. In January 2004, NASA successfully landed two robotic rovers named Spirit and Opportunity at two very different sites on opposite sides of Mars. While the terrain around the Spirit landing site appears to have been dry for billions of years, Opportunity landed in an area that appears to have been underwater for extended periods. Measurements made by Opportunity confirm that some of the very dark surface material at its landing site contains an iron-rich mineral called gray hematite. On Earth, deposits of gray hematite are commonly found at the bottoms of lakes or mineral hot springs. The presence of gray hematite at the Opportunity site reinforces the argument that Mars once had liquid water on its surface, and helps hold open the possibility that living organisms could have evolved on Mars.
Meteorites (The Life Carrier)
Meteorites are one of the important factors to bring life on Mars. A dozen or so meteorites that appear to have formed on Mars have managed to make their way to a variety of locations on the Earth. These meteorites are called SNC meteorites after the names given to the first three examples found (Shergotty, Nakhla, and Chassigny). What identifies SNC meteorites as having come from Mars is the chemical composition of trace amounts of gas trapped within them. This composition is very different from that of the Earth’s atmosphere but is a nearly perfect match to the composition of the Martian atmosphere found by the Viking Landers.
How could a rock have traveled from Mars to Earth? When a large piece of space debris collides with a planet’s surface and forms an impact crater, most of the material thrown upward by the impact falls back onto the planet’s surface. But some extraordinarily powerful impacts produce large craters—on Mars, roughly 100 km in diameter or larger. These tremendous impacts eject some rocks with such speed that they escape the planet’s gravitational attraction and fly off into space.
There are numerous large craters on Mars, so a good number of Martian rocks have probably been blasted into space over the planet’s history. These ejected rocks then go into elliptical orbits around the Sun. A few such rocks will have orbits that put them on a collision course with the Earth, and these are the ones that scientists find as SNC meteorites.
Meteorites and Life on Mars (The Presence of Liquid)
Liquid water can bring life on Mars. Using a radioactive age-dating technique, scientists find that most SNC meteorites are between 200 million and 1.3 billion years old, much younger than the 4.56-billion-year age of the solar system. But one SNC meteorite, denoted by the serial number ALH 84001 and found in Antarctica in 1984, was discovered in 1993 to be 4.5 billion years old. Thus, ALH 84001 is a truly ancient piece of Mars. Analysis of ALH 84001 suggests that it was fractured by an impact between 3.8 and 4.0 billion years ago, was ejected from Mars by another impact 16 million years ago, and landed in Antarctica a mere 13,000 years ago.
ALH 84001 is the only known specimen of a rock that was on Mars during the era when liquid water most likely existed on the planet’s surface. Scientists have therefore investigated its chemical composition carefully, in the hope that this rock may contain clues to the amount of water that once flowed on the Martian surface. One such clue is the presence of rounded grains of minerals called carbonates, which can form only in the presence of water.
In 1996, David McKay and Everett Gibson of the NASA Johnson Space Center, along with several collaborators, reported the results of a two-year study of the carbonate grains in ALH 84001. And that was one of the important pieces of evidence on the way to life on Mars. They made three remarkable findings. First, in and around the carbonate grains were large numbers of elongated, tubelike structures resembling fossilized microorganisms. Second, the carbonate grains contain very pure crystals of iron sulfide and magnetite. These two compounds are rarely found together (especially in the presence of carbonates) but can be produced by certain types of bacteria. Indeed, about one-fourth of the magnetite crystals found in ALH 84001 are of a type that on Earth are formed only by bacteria. Third, carbon-based molecules are present—just the sort, in fact, that result from the decay of microorganisms.
McKay and Gibson concluded that the structures seen in the picture are fossilized remains of microorganisms. If so, these organisms lived and died on Mars billions of years ago, during the era when liquid water was abundant.
Are McKay and Gibson’s conclusions correct? Their claims of ancient life on Mars are extraordinary, and such claims necessarily require extraordinary proof. With only one rock like ALH 84001 known to science, however, such proof is hard to come by, and many scientists are skeptical. They argue that the structures found in ALH 84001 could have been formed in other ways that do not require the existence of Martian microorganisms. Future spacecraft may help resolve the controversy by examining rocks on the Martian surface. For now, the existence of microscopic life on Mars in the distant past remains an open question.