Messages from Mercury: what we don’t know yet

Mercury may be first on most lists of planets, but it’s easily the least explored of all the inner planets. The heat and immense gravitational pull of the nearby Sun has made visiting Mercury difficult, but fly-bys from NASA’s Mariner 10 in the late 1970s gave us the first close-up glimpses. Mercury then remained unvisited until 2004 when NASA launched the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) probe. MESSENGER has been orbiting Mercury since 2011, taking photos and running analyses to tackle some of our deepest questions about the smallest and closest planet to the Sun.

The planet Mercury, as photographed by MESSENGER. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury’s past

One mystery for MESSENGER is why Mercury is so dense. The four inner planets (Mercury, Venus, Earth and Mars) are all comprised of a dense metallic core surrounded by a rocky mantle and crust – but Mercury is unusual in having far more metal than rock. Mercury’s metal core is estimated to make up 75% of the planet’s radius, but Earth’s is just 10%.

The core of Mercury makes up the majority of the entire planet, but the Earth's core is comparably tiny.  Figure courtesy of NASA and APL.

Many theories trying to explain this told of a violent past for Mercury – perhaps the rocky surface had been smashed away by massive impacts, or perhaps it was boiled away by the Sun in its temperamental younger days. MESSENGER rendered these theories unlikely by studying gamma rays emitted by naturally occurring radioactive elements on Mercury and finding a high potassium : uranium (K:U) ratio. Potassium is more volatile than uranium and the extreme conditions proposed by both of these theories would have vaporised it first, lowering the K:U ratio.

As a result, it seems Mercury was subjected to no harsher conditions than any of the other inner planets¹ and may instead have formed from different material in the pre-planetary solar system and so never had a thick mantle. Discovering precisely what these differences might be will take more research not only on Mercury but on other planets and moons as well, so the entire story of the origins of the solar system as we know it can be pieced together.

The Pantheon Fossae

The Pantheon Fossae surrounding the Apollodorus crater on Mercury, photographed by the MESSENGER spacecraft. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The Mariner 10 flybys left huge swathes of Mercury unimaged, so there were bound to be a few surprises in store when MESSENGER arrived with better imaging equipment and more time to use them than Mariner 10 had. One surprise came in the form of the Pantheon fossae, formerly nicknamed The Spider. Fossae are long narrow trenches occasionally found on other planets, and the Pantheon has over 200 of them radiating outward from a centre point which is close to a medium-sized crater. The whole structure was found near the centre of the Caloris basin, a vast impact crater filled in with cooled lava.

The name Pantheon refers to the Roman temple with the same name which has a central hole in the domed roof and a radial pattern around it. The central crater is named Apollodorus after the likely architect of the Roman Pantheon. It would be tempting to think that the impact from Apollodorus caused the Caloris basin to fracture like cracks in a windscreen, thus forming the fossae. However there are a number of similar structures seen on Venus and none of them have a central crater like Apollodorus, so it is likely (but not certainly) a coincidence.

What’s less certain is whether the positioning of the fossae in the centre of the Caloris basin is crucial or just a coincidence. It could be that the fossae are an outward expression of an upswelling³ in the mantle of Mercury which was disturbed by the impact which resulted in the Caloris basin. In the absence of higher resolution images (researchers are currently analysing the structures on an almost pixel-by-pixel basis), consensus in the scientific community may be a long way off.

Artist's impression of the MESSENGER spacecraft in orbit around Mercury. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

What’s next for Mercury?

NASA hopes MESSENGER will continue taking readings and photographs until 2015, which happens to be the expected launch date of a new probe. This one is called BepiColombo and is a joint enterprise between the European and Japanese space agencies. Maybe one day we will send a rover like Mars’ Curiosity, however the extreme conditions on Mercury make this unlikely to become a reality soon. In the meantime, the images and data collected by MESSENGER may take many more years to analyse. Mercury has surprised us many times before and will probably do so again!

Emily Coyte helps teach biochemistry at the University of Bristol and has had a fascination with space from a young age. She tweets as @EmilyCoyte and blogs about many aspects of science at Memetic Drift. This article is part of our World Space Week series, but if you'd like to ensure you don't miss our exciting science articles we recommend you sign up to our mailing list.

References
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[1] Peplowski, Patrick N et al. "Radioactive elements on Mercury’s surface from MESSENGER: Implications for the planet’s formation and evolution." Science 333.6051 (2011): 1850-1852. DOI: 10.1126/science.1211576 (PDF - free reg. required)
[2] Klimczak, Christian, Richard A Schultz, and Amanda L Nahm. "Evaluation of the origin hypotheses of Pantheon Fossae, central Caloris basin, Mercury." Icarus 209.1 (2010): 262-270. DOI: 10.1016/j.icarus.2010.04.014
[3] Basilevsky, AT et al. "History of tectonic deformation in the interior plains of the Caloris basin, mercury." Solar System Research 45.6 (2011): 471-497. DOI: 10.1134/S0038094611060025