New mineral points to a wetter Mars
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: October 30, 2008

NASA's Mars Reconnaissance Orbiter (MRO) has observed a new category of minerals spread across large regions of Mars that point towards prolonged periods of water covering the red planet as recently as two billion years ago.

The new mineral, known as opaline silica, was detected on Mars by both the Spirit rover and by the MRO’s CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) instrument earlier in the year, but in the November issue of the journal Geology, the CRISM team provide authoritative evidence for the presence of widespread opal and not some other water-containing (hydrated) mineral.

Opal like minerals have been detected in and around the giant Valles Marineris canyon system, appearing as a light-toned cream colour in this false colour image. The presence of opal in this location suggests water was present on the surface of Mars as recently as two billion years ago. Image: NASA/JPL-Caltech/Univ. of Arizona.

“Finding large outcrops of opal (hydrated silica) on Mars is a great discovery because it shows that the opal detected by the Spirit rover is not a unique occurrence and that this material is apparently quite common in some regions of Mars,” says lead author Dr Ralph Milliken of the Jet Propulsion Laboratory. “Also, the presence of opal and other silica-rich minerals has been predicted to exist on Mars by geochemical models, and now we've finally seen evidence of it on the surface from orbit.”

The hydrated mineral deposits are telltale signs of where and when water was present on ancient Mars, and the MRO observations reveal its presence in long outcrops in the relatively young plains outside of the large Valles Marineris canyon system, as well as some locations near dry river channels. “In some places the opal is found with iron sulfates, which tells us that the deposits may have formed from evaporation of very acidic water,” Milliken tells Astronomy Now. “However, in some locations we only see the opal, so in these places the water might not have been as acidic and may have been more favorable for life. Finding opal in these relatively young deposits tells us that there was liquid water on Mars as recently as 2-2.5 billion years ago, which means that the window for life or habitability on Mars has opened up a bit more than we previously thought.”

Until now, only two major groups of hydrated minerals, phyllosilicates and hydrated sulfates, had been observed by spacecraft orbiting Mars. Clay-like phyllosilicates formed more than 3.5 billion years ago where igneous rock came into prolonged contact with water, and are suited to trapping and preserving organic matter. Hydrated sulfates formed from the evaporation of salty and sometimes acidic water. The newly discovered opaline silicates are the youngest of the three types of hydrated minerals and formed where liquid water altered materials created by volcanic activity or meteorite impact on the Martian surface. The variations in the compositions of the minerals suggest that different types of watery environments created them, showing just how diverse conditions on Mars were in the last few billion years.

The key result of the opal discovery is the implication for the duration of water on Mars, and by association, the ‘extra’ time inferred for life to have taken hold on our neighbouring planet. "What's important is that the longer liquid water existed on Mars, the longer the window during which Mars may have supported life," says Milliken. "The opaline silica deposits would be good places to explore to assess the potential for habitability on Mars, especially in these younger terrains."

NASA’s Mars Science Laboratory will launch in 2009 and although the landing site is yet to be decided, it will be targeting sites most likely to yield signs of past life.