A NASA image showing the craggy surface of Mars

Ravines and gullies on the steep slopes of Mars, in an image captured by NASA's Mars Reconnaissance Orbiter (MRO). 

Ancient meteorite gives a glimpse into the crust of Mars

A meteorite sample from the Museum collection is revealing more about how and when the crust of the Red Planet formed.

It shows the different ages and types of different igneous and metamorphic processes on Mars.

The meteorite is one of the only ones known to science that is representative of the average composition of Mars' crust, making it incredibly valuable.

It's a pair (or part) of a meteorite called Northwest Africa (NWA) 7034, and it is one of the oldest Martian meteorites we know of.  It formed approximately 4.4 billion years ago.

NWA 7043 is a breccia, which means it contains a variety of different crustal rocks and minerals that were mixed together and then sintered by heating. It is providing a unique opportunity for experts to study the geological processes of the Red Planet.

Dr Caroline Smith, Head of Earth Sciences Collections and Principal Curator of Meteorites at the Museum, says, '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.

'We have been able to unpick the different ages and different geological processes recorded in this highly complicated rock. 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.'

One of the samples in the study, which has a dark varnished outer surface

One of the samples in the study, which has a dark varnished outer surface. A few crystals show through (the 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.

 

What is a planet's crust?

A planet's crust is its solid outer layer. Humans walk around on the top of Earth's crust, which is made up of many different types of rock.

The crusts of Earth and Mars were both first formed of igneous rocks - meaning they were formed by the cooling of molten magma or lava.

Also like Earth, Mars is a differentiated planet (it has layers). It has a dense metal core made of iron, nickel and sulphur, which is surrounded by a mantle and topped with a crust.

Earth's crust is always changing and moving. It is divided up into tectonic plates that slide over the mantle. Volcanoes, mountains and deep ocean ridges form at the edges of the plates, as they collide or separate.  Earth is still geologically active - magma from below the crust often bubbles up and explodes from the top of volcanoes.

Although giant volcanoes once formed on Mars, scientists believe that the planet is not geologically active any more.

Planetary scientists study Martian meteorites to figure out how the rocks on Mars formed, and how they might be changing.

A view of the Red Planet. Image by NASA.

A view of the Red Planet. Image by NASA.

 

A Martian divide

Mars' crust is split sharply into two. There are many craters in the south, and smoother plains in the north.

The northern lowlands comprise about a 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, which are very mountainous and volcanic.

The average thickness of the Martian crust is 45 km, with 32 km recorded in the northern lowlands region, and 58 km in the southern highlands.

It's thought that the difference was created either by something large impacting the planet (or more than one thing impacting it), or by the flow of material in the planet's mantle, but scientists have not yet figured out which scenario is more likely, or when it occurred.

The study of this meteorite sheds light on when and how the differences in the crust might have emerged.

Scanning electron microscope images of one of the fragments used in this sample.

Scanning electron microscope images of one of the fragments used in this sample © Ben Cohen
 

Dating the split

The team used radioisotopic dating techniques to figure out that the divide between the south and the north formed before the NWA 7034 meteorite did (4.4 billion years ago).

They found that all the rocks that eventually were incorporated into the meteorite were formed in the crust about 4.4 billion years ago. They all came together in one place about 200 million years ago.

But during that time, the rocks on this part of Mars' crust were heated and changed because of a large plume-fed volcanic centre (about 1.7 to 1.3 billion years ago).

The ancient age of the terrain suggests that large volcanic regions near the surface of Mars have remained relatively undisturbed for over 4.4 billion years.

Lawrence Livermore National Laboratory cosmochemist Bill Cassata, lead author of the paper, says, 'If the divide in the Martian crust 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.

Other Livermore researchers involved in the study include Reto Trappitsch, Carolyn Crow, and Joshua Wimpenny. Scottish Universities Environmental Research Centre, the University of Glasgow, the University of St. Andrews, and the Natural History Museum also contributed to the research.

 

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