Life on Mars?
Key text
This topic is sponsored by the Kanagawa Prefectural Museum of Natural History.
Recent discoveries on Mars have re-kindled the long-running debate between scientists about the existence of Martian life.
In August 1996, scientists from the National Aeronautics and Space Administration (NASA) announced they had detected possible signs of life on Mars; the announcement caused heated discussions within the scientific community. A series of more recent discoveries has caused scientists to once again debate the possibility of life on Mars.
NASA's Phoenix Mars Lander
(Image: NASA/JPL/UA/Lockheed Martin)
Where was NASA's 'evidence' found?
The early evidence didn't arrive in the form of a little green man or even a little green bug. It was a rock. Not much bigger than a potato and given the dry name of ALH84001, this rock was discovered in Antarctica in 1984. It stands out as a rock among rocks because it is a meteorite and it came from Mars (Box 1: Mars: Earth's cool cousin). We know where it comes from because it has a chemical 'fingerprint' that matches samples taken from Mars.
Life on Mars...PAH!
NASA scientists noticed several intriguing things about ALH84001 which they thought indicated that life may once have existed on Mars.
The first of these were tiny blobs, the size of fullstops. Called carbonate rosettes, they were similar to the rosettes produced by bacteria in ponds on Earth as they metabolise minerals.
In addition, chemical compounds known as polycyclic aromatic hydrocarbons (PAHs) were found in and around the carbonate rosettes. ALH84001 contains an unusual mixture of certain lightweight PAHs. The NASA scientists concluded that these might have been produced as once-living organisms decomposed.
Another piece of evidence centred on the discovery of tiny crystals of magnetite and iron sulphide embedded in places where the carbonate rosettes had dissolved. The scientists noted that some bacteria on Earth also manufacture similar crystals.
The fourth and final piece of evidence was perhaps the most controversial. Using an electron microscope, the NASA scientists found elongated and egg-shaped structures within the carbonate rosettes which they interpreted to be tiny fossils of Martian microbes.
If not life on Mars, then what?
Mars, which once may have had a climate similar to Earth's, has long been the subject of earthly speculation about life on other planets. Nevertheless, most scientists not directly involved in the study of ALH84001 were sceptical when the results were announced amidst worldwide excitement. The scepticism has increased as further tests on ALH84001 have been carried out.
There are several plausible explanations for the phenomena described above that don't require the presence of life. Natural geological processes alone could have caused all the observed 'evidence'. For example, carbonate globules similar to the meteorite's rosettes have formed through chemical reactions in volcanic rocks in Norway.
Another possibility is contamination. Some other meteorites have been shown to be contaminated by chemicals and organisms from Earth. The NASA scientists argued that most of the carbonates contained isotopes associated with Mars and that PAHs were more concentrated inside the rock than on its surface. Others have concluded that the meteorite picked up at least some of the PAHs (which are common everywhere on Earth) as it sat in the ice for 13,000 years. In any event, these compounds occur on dust particles in space where they almost certainly formed non-biologically.
Keep on looking
The debate about life on Mars continued when methane was detected in the Martian atmosphere in 2003. On Earth, over 90 per cent of our methane is produced by living things. Could living things also be responsible for Martian methane? Or was it produced by reactions between rocks and underground water?
Provides a timeline of the various missions to Mars.
(The University of Arizona, USA)
One way to settle the ongoing debate about life on Mars is to go back there. In January 2004 two NASA 'rovers' – robotic geologists – landed on Mars. They found clues suggesting past water activity, essential for life as we know it, on the planet. Then in 2008 NASA's Phoenix Mars Lander confirmed what scientists had long suspected – that water exists on Mars. Phoenix also confirmed that Martian soil contains magnesium and potassium and has a slightly alkaline pH. Some people went so far as to say that we could be growing alkali-loving plants like turnips and asparagus in it. However, the presence of very high levels of salts and perchlorate has raised more questions about Mars' habitability.
Australian scientists have a range of expertise that can help in the search for evidence of life on Mars. Professor Malcolm Walter, Director of the Australian Centre for Astrobiology at the University of NSW, has worked closely with NASA in their astrobiology program and has previously advised them on the best landing sites for probes searching for evidence of life (or former life) on Mars.
'Hot springs are a great place to live if you're a bacterium,' says Professor Walter. 'So what we should be doing on Mars is focusing on former hot springs.' (Box 2: Cyanobacteria: the simple things of life). In fact life on Earth is thought to have originally evolved in similar warm, saline conditions.
The thought of finding bacteria in Martian hot springs may not immediately excite the imaginations of Hollywood movie producers. But the discovery of life elsewhere in the universe – in any form – could have a profound effect on the way we perceive ourselves. Are we alone, or does the universe teem with life? (Box 3: ET, can you speak up?).
Boxes
1. Mars: Earth's cool cousin
2. Cyanobacteria: The simple things of life
3. ET, can you speak up?
Related Nova topic
Astronomy in the deep freeze
Posted November 2008.