The Solar System's “Little Guy” is Shrinking

The Sun’s closest neighbour is still geologically active, according to new data collected by the MESSENGER probe, as it fell towards Mercury’s surface in 2015.

On an ordinary Thursday in April 2015, the small MESSENGER craft is roaring towards the planet of Mercury at a speed of some 14,000 km/h. The craft only just manages to send the last of a total of 289,265 photos to Earth, before the 1 tonne space probe crashes onto the surface, leaving a wide crater.

The crash is the culmination of the probe’s four year exploration of Mercury. On Earth, in the mission control room at the Johns Hopkins University in Maryland, USA, astronomers cheer. The probe’s dramatic death is the planned termination of a very successful mission, during which the probe – following two extension periods – runs out of fuel. Up until this moment, the small NASA craft has orbited Mercury 4,100 times, using its arsenal of digital cameras, spectrometers, laser altimeters, and other gear to collect no less than 10 terabytes of data about the planet.

The mission has been highly successful, as the new data has revealed that Mercury is slowly shrinking and remains geologically active just like Earth.

MERCURY IS SLOWLY SHRINKING

The sensational discovery was published in a scientific article in the Nature Geoscience

journal in 2016, in which NASA astronomers say that in its lifetime, Mercury’s diameter has shrunk 14 km, and the process is by no means about to stop.

The “slimming” is due to the fact that Mercury’s hot outer core is cooled, making the planet contract slightly. Astronomers discovered this, when they took a close look at photos taken by the space probe during the last 1.5 years of the mission. At that time, the craft was orbiting at a lower altitude, enabling it to take more detailed photos. The images reveal relatively small, less than 100-m-wide trenches, markedly smaller than the older faults on the surface of the planet. As the small rocky planet is regularly subjected to intense meteorite and comet bombardments, the trenches can only exist, if they are relatively young.

So, the small faults prove that the rocky planet is still geologically active. Scientists used to think that Earth was the only geologically active planet in the Solar System.

SURFACE IS YOUNG AND OLD

The young trenches are closely related to one of the questions that have bothered scientists for years: the odd mixture of young and old areas on Mercury’s surface.

The different ages of the surface is inconsistent with astronomers’ usual theory of planetary formation. According to the theory, planets are so hot in their youth that they almost melt. Over time, as they become colder, the cooled material crystallizes into different, layered minerals. The Moon is a good example of a world, on which the cooling has caused layers. On the other hand, Earth is not layered, which may be due to the active surface, where plate tectonics is constantly causing changes.

With a diameter of only one third of Earth’s, Mercury is a relatively small planet, whose interior should have cooled into a solid, layered shape long ago.

However, the MESSENGER probe’s detailed magnetism and gravity measurements have shown that the core is huge and still liquid, and that has made astronomers think that Mercury could be made up of another material than Earth. In the lab, astronomers from the American Johnson Space Centre subjected a special type of meteorite – of which the planets of the Solar System are probably made up – to the pressure and temperature in different parts of Mercury’s interior. The scientists conclude that Mercury could easily be made up of the same material as Earth and still be liquid.

The majority of the iron in Mercury’s interior is probably bound in metal or sulphur compounds instead of oxygen like on Earth. Particularly sulphur will affect the melting process in a way that is consistent with the fact that the older parts of the sur-face were formed between the core and the mantle, whereas the younger, volcanic plains were formed at the surface.

NEW PROBE

The MESSENGER probe has solved many of the major mysteries about the Solar System’s innermost planet, but the exploration of Mercury certainly will not stop here.

Already next year, the European Space Agency, ESA, and its Japanese equivalent organisation, JAXA, will launch the BepiColombo probe towards Mercury, which is going to study the planet even closer upon its arrival in 2025.