Mars

Some of the most exciting recent advances in our understanding of the Solar System are coming from studying Mars.  Scientists have used new data from telescopes and space probes to map the martian surface and deduce the chemistry of rocks and the atmospheric pressure at different times in Mars’ history. In addition, 55 meteorites are now accepted to have come from Mars.  One of the particular fascinations about Mars is that much of the new information points towards the occasional presence of liquid water during the planet's history.  At the Space Research Centre in the Department of Physics and Astronomy we study the evolution of the martian atmosphere and surface in order to understand how the Mars climate has changed since the planet’s formation.  We do this from involvement in current and planned lander missions such as Mars Science Laboratory (the 900 kg rover which will landed in Gale Crater in August 2012), ExoMars, CASSIS (stereo, colour camera for the 2016 orbiter) and Mars Sample Return, studying the effects of Mars water on meteorites from Mars and using images and infrared spectroscopic data returned by spacecraft (especially Mars Reconaissance Orbiter).  The Space Research Centre is heavily involved in building instruments for upcoming space missions to Mars and other planets in the Solar System.  Undergraduate projects in planetary science options give the chance to participate in the department’s Mars research activities. 

 This famous image of Mars is a mosaic of thousands of Viking Orbiter photos.  It shows the western hemisphere of Mars with 3 of the great shield volcanoes of the Tharsis province in the west and the 4500 km long Valles Marineris Canyons near the equator. 

One of the great breakthroughs in Mars science has come through the identification of layered sedimentary rocks.  We now know from the results of the Opportunity Mars Exploration Rover (MER) that sedimentary and evaporite layers do exist on Mars in Noachian rock units. Thus some of the layers we see in the Valles Marineris canyons and in other parts of ancient Mars are sedimentary rocks.  At the Space Research Centre we are studying such ancient sediments through digital terrain models (with collaborators from the Mullard Space Science Laboratory and the British Geological Survey) to determine the past climate on Mars.  We also use infrared spectral data from the CRISM instrument of the Mars Reconaissance Orbiter to determine the mineral composition of sediments and other Mars surfaces. 

Layered sediments in Becquerel Crater (351^oE, 21.5^oN).  This is part of THEMIS image 20020528a.  One of the questions we are addressing of such sediments is were they deposited by wind or water?

Another way of studying the past climate of Mars is through martian meteorites.  These provide over 100 kg of Mars for study in laboratories such as the Space Research Centre.  The SNC meteorites (named after three representative members of the group: Shergotty, Nakhla, Chassigny) are a group of 55 achondrite meteorites. They have a diverse range of compositions but are known to be grouped and derived from the same parent body (which is almost certainly Mars). This grouping has been recognised because of their oxidised mineral assemblages. Most meteorites contain Fe-Ni metal but in the SNCs, Fe and Ni have combined with oxygen and other elements so no metal is present.

How do we know SNC meteorites come from Mars?

The SNC class of meteorites is distinct from primitive achondrites for a number of reasons. They have a range of crystallisation ages based on radiometric dating from 4.5 Ga (billion years) to 160 Ma (million years). In contrast chondrites and primitive achondrites all date from the earliest stages of the Solar System (4.5 - 4.6 Ga). The long range of ages over which the SNCs crystallised suggests that they formed on a large, geologically active planet. That is because smaller asteroidal bodies (e.g. hundreds of km in diameter) cooled within about the first ten million years of their formation. The mineral assemblages and chemical compositions of the SNC meteorites are also more fractionated than the primitive achondrites. The achondrites are mainly composed of Mg-rich silicate minerals with Fe-Ni metal and feldspar. In contrast the SNC meteorites have a more varied and fractionated mineralogy.  A feature of both martian meteorites and rock analyses made by landers is that they are iron-rich compared to terrestrial rocks. 

Fragment of the Nakhla martian meteorite which fell in Egypt in 1911.  This sample is being researched at the University of Leicester because it contains mineral relicts of brines that were present on the surface of Mars.  This in turn gives clues to the past climate of Mars.  Natural History Museum Sample BM1913, 26.  Field of view 8 cm. 

The age range and mineral compositions of the SNC meteorites both suggest that they formed on a planetary body larger than known asteroids. The possible planetary bodies can be considered. Venus has a very thick atmosphere, which means that little material has been ejected from its surface into space.  Some of the Jupiter moons show volcanic activity but very little material could escape from the orbit of Jupiter. That leaves Mars as the most obvious parent body. However, in order to demonstrate their martian origin a final bit of geochemical evidence has been needed. The composition of gases trapped within the shock-melted glass of some of the SNCs (shergottites) is identical within experimental error to that of the martian atmosphere determined by the Viking landers. Since that relationship was demonstrated in the 1980s the martian origin of the SNCs has become generally accepted.

Image taken with a scanning electron microscope of a fracture within a nakhlite martian meteorite (called Lafayette) through which a brine flowed for a short period of time depositing smectite, serpentine, amorphous gel and carbonate.  The carbonate represents trapped atmospheric CO₂ that dissolved in the brine near the surface of Mars. 

By using a transmission electron microscope (TEM) we can examine 80 nm thick wafers extracted from veins.  Measuring the atomic lattice spacings can be used to identify different mineral types – in this case the sheets silicate serpentine, which is a hydrous mineral formed by alteration of mafic rocks.