Mars and the Earth are similar in many respects. For example both planets have a metallic core surrounded by a mantle of silicate minerals. Based on seismic studies, it is known that the Earth’s core consists of a solid inner part and a liquid outer part. On the other hand the experts are as puzzled as ever about the structure of the core of Mars.

A liquid past

A group of researchers at the Institute for Mineralogy and Petrology of ETH Zurich led by Max Schmidt, Professor of Crystalline Geology, has now used high pressure experiments (1) to make a more detailed study of the conditions in the interior of our planetary neighbour. In this study, doctoral student Andrew Stewart’s group exposed samples consisting of iron, nickel and sulphur – the elements of which Mars’ core is essentially composed – to high pressures and temperatures similar to those found in the centre of the planet. As the researchers report in the latest issue of the scientific journal “Science” (2), these experiments show that the core of Mars must be completely liquid at the present time. Thus in contrast to the Earth, Mars does not have a solid metallic inner core of iron-nickel, because no solid phases are stable under the conditions prevailing in the centre of Mars.

The results also show that the core of Mars must also have been completely liquid four billion years ago. Based on satellite observations it is known that Mars must have had a strong magnetic field at that time, because strongly magnetised rocks from this period were discovered on the planet’s southern hemisphere. How a magnetic field can form in a fully liquid core, and why it subsequently collapsed is still as unclear as ever. Probably it occurred as a result of powerful convection in Mars’ core, which was extremely fluid at that time. Later the situation then changed dramatically, presumably because the flow of heat from the interior decreased.

Inwards from the outside…

However, the experiments in the high pressure laboratory also allow the researchers to see into the future. Mars will cool down further over geological periods of time. This will cause the core to solidify. Exactly how this happens depends on the total amount of sulphur in the core of Mars.

If the sulphur content is between 10 and 14 percent, crystals of iron-nickel will initially form at the outer edge of the core. Because these crystals are denser than the surrounding molten material, they will sink down to the planet’s centre. Over time this will cause a solid iron-nickel core to form in the interior. (3) Since the composition of the crystals is different to that of the original melt, the latter’s composition will change as the cooling increases. The more iron-nickel crystals have formed, the higher is the sulphur content in the remaining melt. This process continues until the melt reaches what is known as the eutectic composition. The second phase starts at this point: sulphide minerals are now also formed, instead of only iron-nickel crystals. Thus a mixture of various different crystals having an overall sulphur content of about 14 percent forms around the central iron-nickel core, which is already solid.

A cross-section through the interior of Mars. The Figure shows how the core, which is still liquid today, could solidify. Yellow: solid iron-nickel alloy; orange: solid iron sulphide (Fe3S); red: liquid iron-nickel sulphide (molten); blue: solid silicate mantle; black: crust. large

The researchers were able to predict the future conditions in the interior of Mars based on samples of this kind. The photographs show sulphide grains embedded in an iron-rich matrix. The sample was subjected to a temperature of 1323 Kelvin and a pressure of 40 GPa. large

…or outwards from the inside?

The solidification takes place in a quite different way if the sulphur content of the present-day Mars core is more than 14 percent, i.e. it lies above the eutectic composition. In this case crystallisation begins in the interior: a solid core of iron-nickel sulphide with a sulphur content of approximately 16 percent forms in the centre of the planet in a first phase. This in turn leads to a change in the composition of the residual melt, but this time in exactly the opposite direction. The sulphur content decreases continuously until the eutectic composition is reached. From this point onwards a crystal mixture having an average sulphur content of about 14 percent again forms in a second phase.

The researchers do not exclude the possibility that the formation of a solid inner core during the first phase could trigger convection flows in the residual liquid core. This could result in the formation of a kind of dynamo as is happening with the Earth at present. Thus in this case of Mars this would again produce a strong magnetic field as already occurred at one time four billion years ago.