Life in the Solar System

In 1976, two spacecraft from Earth, Viking I and II, landed on the surface of Mars. They carried several different experiments to test for the presence of living organisms. (Including cameras to photograph any passing wildlife!) No evidence of life was found at either landing site. (Photo courtesy of NASA) For more information about the Viking Mission to Mars, see: National Space Science Data Center

Of all the planets, Mars is the most similar to Earth, at least on the surface. A good thing to remember is that its size is about halfway between Earth and Moon. Also, many of its features remind us of both Earth and Moon. When we look at its face, we seem to recognize familiar phenomena--craters and ejecta like those on the Moon, volcanoes and valleys like those on Earth. There are four seasons much like on Earth, as the tilt of the Mars axis differs little from that of Earth. A Martian day is only slightly longer than a day on Earth (24.6 hrs). The average daytime temperature on Mars is a balmy 86°F. Nice for sitting outside and contemplating the scenery from the comfort of a spacesuit! Nights reach a chilling –211°F. Better pack a sweater!

Perhaps the most intriguing class of morphological features comprises those suggesting the activity of water on Mars. All sorts of valleys that resemble dry river courses have kept discussions going, with arguments going back and forth regarding the relative importance of erosion and tectonics. But recently the gullies of Mars, beautifully captured in images sent by the Mars Global Surveyor, clearly indicate the action of some kind of erosion, presumably by water. (The possibily of CO₂ gas-driven regolith flows, similar to volcanic "lehars" on Earth, cannot be discounted.) The temperature on Mars, averaging day and night, is –157°F because of the distance to the Sun (1.5 times greater than the Sun-Earth distance). Thus, any water on Mars is frozen solid, and is thought to be rather far below the surface. (Water near the surface would sublimate into the atmosphere and dissociate, with the hydrogen escaping.) But the gullies remind us that there must be ways to bring liquid to the surface, at least occasionally. The question then arises whether, perhaps, in the early history of Mars water was available more readily. If so, Mars might have been a good laboratory, with sufficient similarity to the early conditions on Earth, to check the idea that Life must arise, eventually, if conditions are right.

East Gorgonum Crater on Mars showing channels created by flowing water. The lack of small meteor impact craters indicates that these features are extremely young. It is possible that water is still seeping up to the surface from the layered rocks below. (Courtesy: NASA.)

Are there microfossils or chemofossils hidden away in some old Mars rocks? Some think that the answer is yes. Heated discussions about a fossil-like structure in a meteorite from Mars are still going on; what is evidence to some is just a morphological oddity to others, with no significance as a fossil.

If Mars was more like Earth in the distant past, its conditions have moved in direction of the Moon in the time since. The atmosphere is thin, and at night and near the poles the temperatures drop to extremely low values. The air is very dry. The ice caps are made of dry ice, frozen carbon dioxide. The fact that carbon dioxide is the most abundant gas suggests there is no photosynthesis going on.

The planet looks inhospitable. However, we know that on Earth there are living things in environments readily identified (by us) as very inhospitable: hot springs, highly acidic waters, films of metal oxides, high pressure conditions deep below the surface, poisoned by hydrogen sulfide. If Life ever existed on Mars it might "hang on", with resting spores bridging times of stress and waiting for the periods when water makes gullies.

Besides Mars, there are two other planetary bodies that might bear Life in the solar system. These are Europa and Titan, two big moons belonging to Jupiter and Saturn, respectively. They too may have water.