A message for you: Mercury's turbulent past

The dichotomy of ancient messages

X-ray spectroscopy and astronomical analyses reveal that Mercury - the planet closest to the Sun - may have had a vast, roiling ocean of liquid rock, or magma, during its very early history some 4.5 billion years ago.

There's nothing like a neat astronomical acronym to kick-start a science project and scientists have been making the most of a NASA probe that has orbited the planet Mercury since March 2011, which goes by the name of MESSENGER, alluding, of course, to the character of Greek mythology. The acronym stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. Regardless of the construction of abbreviations, the probe's X-ray fluorescence data from Mercury’s rocky surface has allowed the scientists to reconstruct, partially at least, the balmy planet’s history over the last few billion years. By revealing the chemical composition of rocky features on the planet’s surface, scientists at Massachusetts Institute of Technology (MIT) have reasoned that Mercury may had liquid coating not too long after it first formed.

Rocky recreation

The MIT team used the compositional data to recreate the two rock types discerned on Mercury's surface which represented something of a puzzle - how might two distinct rock types be present. The team subjected each "synthetic" rock to high temperatures to simulate various geological processes. The results suggest that only a magma ocean that formed two distinct layers followed by re-melting and eruption on to the planet's surface could have given rise to the rock types the data reveal.

Postdoctoral researcher Bernard Charlier has worked with Tim Grove and Maria Zuber on the study and they publish details in the journal Earth and Planetary Science Letters.

The planet was amenable to the initial analysis because MESSENGER entered orbit during a period of intense solar-flare activity during which the rocks on Mercury's surface reflect an intense X-ray fluorescent spectrum. In September 2011, the MESSENGER science team gleaned three spectral peaks associated with particular elements, which led to the identification of two main types of rock on the surface of the planet.

The team wanted to unearth precisely how two rock types might be present and so translated the element ratios into the corresponding building blocks that make up rocks containing magnesium oxide, silicon dioxide and aluminium oxide. They then generated a range of finely powdered oxides to duplicate the Mercurial surface dust here on Earth. Repeated heating and cooling of these oxides generated different mixtures of crystals depending on the specific conditions simulating magma melting, cooling, re-melting etc.

Mercurial leftovers

"You can tell what would happen as the melt cools and crystals form and change the chemical composition of the remaining melted rock," Grove explains. "The leftover melt changes composition." The team found that two compositions obtained in this way analogous to surface rocks were two different to have originated in the same region and so must have derived from elsewhere on or in the planet. The easiest explanation is to invoke a vast magma ocean that would generate different rock compositions that would be spewed on to new surface rocks through volcanic activity as the crust solidified billions of years ago. The researchers suggest that this magma ocean existed during the first 1 million to 10 million years.