Although the Viking Mars landers found no sign of Martian life, some planetary scientists have wondered whether or not life exists elsewhere on the planet. That hope now seems very dim.
A joint Chinese-US research project has looked into this question thoroughly. It concludes that ''taken together, the evidence suggests that the sterility of the Viking landing sites is representative of the entire planet.''
Furthermore, the researchers demonstrate what they suggest is likely to be the reason no organic material was found at the two Viking sites. A combination of solar ultraviolet (UV) irradiation and the catalytic action of Martian sand would rapidly break down organic material into inorganic compounds, they say.
This research involved cooperation between Kevin D. Pang and Joseph M. Ajello at the NASA Jet Propulsion Laboratory, Sandy F. S. Chun of the University of California, San Francisco, Zhao Nansheng at the Bejing Planetarium, and Liang Minji at the Bejing Glass Institute. Between them, these researchers studied data from Mars, including Viking lander findings, and simulated in the laboratory the process they say breaks down organic material on the Red Planet.
As they explain in presenting their conclusions in Nature, there is much organic material in the universe other than what is found on Earth. Many organic molecules have been identified in interstellar dust. Organic compounds have been found in some meteorites. Thus one would expect to find organic material in Martian soil, even if it were not indigenous to the planet, because of meteorite infall.
''The fact that Viking found no organics at the landing sites means that a very efficient process must be operating on Mars to destroy the organic molecules,'' the scientists note. This is the process of catalytic destruction. They show that titanium dioxide, which exists in Martian soil, facilitates the breakdown of organics in the presence of oxygen and UV radiation. Indeed, ordinary sand can act as such a catalyst.
Given this ''very efficient process'' for destroying organic material, the researchers say they see little reason to expect organic life as we know it - that is, organic life based on carbon - to exist anywhere on Mars. They back up this pessimistic conclusion by showing that Earth-based and spacecraft observations of infrared (heat) radiation emitted by the planet do not indicate organic material. Neither do UV reflections from the Martian surface suggest life's presence. The wavelengths at which a planetary surface emits infrared radiation or reflects UV rays are characteristic of the material that make up that surface.
These findings do not absolutely rule out the possibility of life on Mars. But they do very strongly discount it. And in doing this, they reinforce the most important finding space science has yet made - the unique role of our own planet in the solar system as a habitat for abundant life.