Mars has been arid for more than 600 million years, making the planet's surface too dry and hostile to support any life, according to new research based on soil analysis.
The analysis was published Friday in the journal Geophysical Research Letters and will be presented at a European Space Agency (ESA) meeting on Feb. 7.
The team spent three years analyzing data on Martian soil that was collected during the 2008 NASA Phoenix mission to Mars. Phoenix touched down in the northern arctic region of the planet to search for signs that it was habitable and to analyze ice and soil on the surface.
Soil analysis at the Phoenix site shows the surface of Mars has been arid for hundreds of millions of years, despite the presence of ice and the fact that previous research has shown that Mars may have had a warmer and wetter period in its history more than three billion years ago.
The soil on Mars had been exposed to liquid water for at most 5,000 years since its formation, and it was on the surface for far too short a time for life to maintain a foothold on the surface, said Tom Pike of Imperial College London, who led the study.
"We found that even though there is an abundance of ice, Mars has been experiencing a super-drought that may well have lasted hundreds of millions of years. We think the Mars we know today contrasts sharply with its earlier history, which had warmer and wetter periods and which may have been more suited to life. Future NASA and ESA missions that are planned for Mars will have to dig deeper to search for evidence of life, which may still be taking refuge underground."
By comparing soil data from Mars, Earth and the moon, the team found further evidence that Martian soil has been largely dry throughout its history.
The researchers also believe that Martian and moon soil has been formed under the same extremely dry conditions, because they were able to match the distribution of soil particle sizes. On Mars, the team inferred that physical weathering by the wind as well as meteorites breaks down the soil into smaller particles. On the moon, meteorite impacts break down rocks into soil, as there is no liquid water or atmosphere to wear down the particles.
For the study, the researchers looked for the microscopic clay particles that are formed when rock is broken down by water. Even if the few particles they saw on the Mars rocks were in fact clay, they made up less than 0.1 per cent of the total proportion of the soil in the samples.
On Earth, clays can make up to 50 per cent or more of the soil content, so such a small proportion in the Martian samples suggests that the soil has had a very arid history.
Satellite images and previous studies have proven that the soil on Mars is uniform across the planet, which suggests that the results from the team's analysis could be applied to all of Mars.