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Scientists glimpse into 4.4 billion years of Mars' history from ancient meteorite

Through looking at an ancient Martian meteorite that landed in the Sahara Desert, the Museum’s Dr Caroline Smith, and collaborators led by Lawrence Livermore National Laboratory, CA, have determined how and when the red planet’s crustal topographic and geophysical divide formed. 

The research appears in the May 23 edition of the journal Science Advances.

Northwest Africa (NWA) 7034 and other paired samples, is the oldest Martian meteorite discovered to date, at approximately 4.4 billion years old. The meteorite is a breccia (it contains a variety of different crustal rocks that were mixed together and then sintered by heating), and is the only sample from Mars with a composition that is representative of the average Martian crust.

The meteorite provided the researchers a unique opportunity to study the ancient crust on Mars.

'This multi-disciplinary study, combining both traditional and innovative geochemical techniques has provided us with some exciting new insights into the timings of major processes that shaped young Mars,' said Dr Caroline Smith, Head of Earth Sciences Collections and Principal Curator of Meteorites at the Natural History Museum.

The team carried out mineralogical analyses and applied a number of sophisticated radioisotopic dating techniques to determine that the divide (or dichotomy) between the heavily cratered southern highlands of the planet and the smoother plains of the northern lowlands formed from prior to the formation of NWA 7034 at 4.4 billion years ago. This ancient age is consistent with a giant impact origin for the crustal dichotomy. 

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The exterior of one of the samples used in the study, showing the dark varnished outer surface. A few crystals show through (lighter colors) . Desert varnish has been removed from the lower left portion of this meteorite to reveal the dark gray color of the stone's interior. © Ben Cohen, University of Glasgow 

The dichotomy is a sharp contrast between the southern hemisphere and northern. The two hemispheres' geography differs in elevation by 1 to 3 kilometres (km). The average thickness of the Martian crust is 45km, with 32km in the northern lowlands region, and 58km in the southern highlands.

The northern lowlands comprise about one-third of the surface of Mars and are relatively flat.  The other two-thirds of the Martian surface are the highlands of the southern hemisphere. The difference in elevation between the hemispheres is dramatic (the highlands are very mountainous and volcanic). Three major hypotheses have been proposed for the origin of the crustal dichotomy: endogenic (by mantle processes), single impact, or multiple impacts.

The team set out to determine when and how the crustal dichotomy formed.

Based on new radioisotopic measurements and in conjunction with other published data, the team determined that all the rocks that were eventually incorporated into the NWA 7034 breccia were emplaced about 4.4 billion years ago in the “source terrain” (the crustal source region that the different breccia components are derived from).

The results show that this terrain was subject to prolonged metamorphism associated with a large plume-fed volcanic centre from ~1.7 to 1.3 billion years ago. The areal extents of large, plume-fed volcanic centres on Mars are thousands of square kilometres, and the source terrain was likely comparable in size. 

Finally, they showed the rock was brought together ~200 million years ago or more recently. When viewed together, the data from NWA 7034 demonstrated that large volcanic terrains survived within a few km of the Martian surface since >4400 Ma. This indicates that the dichotomy formed prior to 4.4 Ga, as near surface rocks would have been buried or destroyed by the dichotomy-forming event.

Prof Darren Mark, of the Scottish Universities Environmental Research Centre and St Andrews said, ‘This study nicely demonstrates the power of adopting a multi-chronometer approach to resolving the timescale of various planetary processes on Mars using meteorites.

Recovering the data using a variety of mass spectrometer techniques is only half the battle. Critical is the interpretation - how we use the numbers to reconstruct the geological history of this rock fragment. Geological ages come with inherent uncertainties and any interpretation has to take account of such uncertainty.’

Lawrence Livermore National Laboratory cosmochemist Bill Cassata, lead author of the paper, said ‘If the Martian crustal dichotomy formed as a result of a giant impact, and available data and modelling suggest this is likely, the history of NWA 7034 requires that it formed very early in the planet’s history, before 4.4 billion years ago.’

This team’s results have important implications for our understanding of when and how one of the oldest, and most distinctive, global geologic features on Mars was formed.

‘This study demonstrates that multiple radioisotopic dating systems that are reset by different metamorphic processes can be used to tease out the thermal history of a sample over billions of years,’ Cassata said.

‘We have been able to unpick the different ages and different geological processes recorded in this highly complicated rock,’ said Dr Caroline Smith. ‘Although the sample is in itself small, and the individual fragments we measured are tiny, we have been able to peek into the past and gain a glimpse of approx. 4.4 billion years of Mars’ history.’

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Images: Please download and credit: © Ben Cohen, University of Glasgow (Link not for publication)

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