Mars may have been arid for more than 600 million years,
making it too hostile for any life to survive on the planet’s surface,
according to researchers who have been carrying out the painstaking task of analyzing
individual particles of Martian soil.
The researchers have 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.
The results of the soil analysis at the Phoenix site suggest
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 earlier history more than three
billion years ago. The team also estimated that the soil on Mars had been
exposed to liquid water for at most 5,000 years since its formation billions of
years ago. They also found that Martian and Moon soil is being formed under the
same extremely dry conditions.
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. This implies that liquid
water has been on the surface of Mars for far too short a time for life to
maintain a foothold on the surface.
Tom Pike, PhD, from the Department of Electrical and
Electronic Engineering at Imperial College London, who is lead author on the
study published in Geophysical Research
Letters, explains: “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.”
During the Phoenix mission, Pike
and his research group formed one of 24 teams based at mission control in the University of Arizona, operating part of the
spacecraft’s onboard laboratories. They analyzed soil samples dug up by a robot
arm, using an optical microscope to produce images of larger sand-sized
particles, and an atomic-force microscope to produce 3D images of the surface
of particles as small as 100 um across. Since the end of the mission, the team
has been cataloguing individual particle sizes to understand more about the
history of the Martian soil.
In the study, the researchers looked for the microscopic
clay particles that are formed when rock is broken down by water. Such
particles are an important marker of contact between liquid water and the soil,
forming a distinct population in the soil. The team found no such marker. They calculated
that even if the few particles they saw in this size range were in fact clay,
they made up less than 0.1% of the total proportion of the soil in the samples.
On Earth, clays can make up to 50% 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.
They estimated that the soil they were analyzing had only
been exposed to liquid water for a maximum of 5,000 years by comparing their
data with the slowest rate that clays could form on Earth.
The team found further evidence to support the idea that
Martian soil has been largely dry throughout its history by comparing soil data
from Mars, Earth, and the moon. The researchers deduced that the soil was being
formed in a similar way on Mars and the moon 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.
