Organic cemeteries could dominate ancient Mars
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: July 22, 2008

Two complementary studies based on data from NASA's Mars
Reconnaissance Orbiter support the popular theory that the Red Planet once hosted vast lakes, flowing rivers and a variety of other wet environments that had the potential to support life.

One study, based on data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and the High Resolution Imaging Science Experiment (HiRISE), and published in the current issue of Nature, shows that extensive regions of the ancient highlands of Mars that comprise about half the planet, contain clay-like minerals called phyllosilicates, which can form only in the presence of water. These minerals preserve a record of the interaction between water and rock dating back to the very first billion years of Martian history.

Following the deposition of these water-loving minerals, a drier period followed, dominated by volcanic lavas which buried the clays. But during the period of Heavy Bombardment 3.8-4.6 billion years ago, in which the inner Solar System endured an intense assault from asteroids and comets, and also in later years when the impact flux was more sporadic, these clay minerals were exposed in thousands of impact craters dotted across the Martian surface. Craters act like windows into the past and allow planetary geologists to look at the different layers of minerals and rocks that have built up over time.

This three-dimensional map of a trough in the Nili Fossae region of Mars shows the prolific nature of the phyllosilicate minerals (indicated by magenta and blue hues), largely concentrated on the slopes of steep cliffs and along canyon walls. The abundance of phyllosilicates shows that water played a sizable role in changing the minerals of a variety of terrains in the planet's early history. Image: NASA/JPL/JHUAPL/University of Arizona/Brown University.

It is well known to scientists that organic material can strongly interact with clays, therefore clays are thought to have played a significant role in the emergence of life on the Earth. But owing to to the destructive nature of plate tectonics and erosion on our home planet, the earliest clues to this potential interaction have mostly been destroyed, making the Martian phyllosilicates a unique record of liquid water environments which may have been suitable for life in the early Solar System. Indeed, two favourable sets of conditions – the mild chemistry that protects organic matter from destruction, and the intensity of erosion and activity of liquid water that would have allowed organics to accumulate – work well in the favour of discovering potential organic ‘cemeteries’ in the future.

"The minerals present in Mars' ancient crust show a variety of wet environments," says John Mustard, a CRISM team member from Brown University. "In most locations the rocks are lightly altered by liquid water, but in a few locations they have been so altered that a great deal of water must have flushed though the rocks and soil. This is really exciting because we're finding dozens of sites where future missions can land to understand if Mars was ever habitable and if so, to look for signs of past life."

A colour-enhanced image of a river delta in the now empty lake bed of Jezero Crater. Ancient rivers are thought to have ferried clay like minerals (shown here in green) into the lake, forming the delta. Clays are ideal for trapping and preserving organic matter, making this location a good place to look for signs of ancient life. Image: NASA/JPL/ JHUAPL/MSSS/Brown University.

Another study, published in last month’s issue of Nature Geosciences, supports the idea of wet conditions persisting for thousands to millions of years on Mars. This conclusion comes from the observation that a system of river channels eroded the clay minerals out of the highlands and concentrated them in a delta where the river emptied into the 40 kilometre diameter Jezero crater lake.

"The distribution of clays inside the ancient lakebed shows that
standing water must have persisted for thousands of years," says Bethany Ehlmann, another member of the CRISM team from Brown University. "Clays are wonderful at trapping and preserving organic matter, so if life ever existed in this region, there's a chance of its chemistry being preserved in the delta."

The team also identified three principal classes of water-related minerals dating to the earliest epoch of Martian history, the Noachian Period, as: aluminum-phyllosilicates, hydrated silica or opal, and the more common and widespread iron and magnesium-phyllosilicates. The variations in the minerals across the Martian surface suggest that different processes, or different types of watery environments – such as standing water or flowing water – dominated at different times.

The presence and state of water on the surface of Mars has always been a subject of intense debate because of the direct astrobiological implications, and the recent research that implicates clays into the equation make these deposits very attractive locations for future exploration. Indeed, the results from both studies will be used to compile a list of sites where future missions, such as the Mars Science Laboratory or Exomars, could land and look for organic chemistry that could finally determine whether life has ever existed on Mars.