Planetary scientists at Massachusetts Institute of Technology have proposed that Mercury may have harbored a large ocean of magma shortly after its formation about 4.5 billion years ago.
Composite image of Mercury taken by NASA’s MESSENGER spacecraft (NASA / APL)
In 2011, a group of scientists analyzed X-ray fluorescence data from NASA’s MESSENGER spacecraft and identified two distinct compositions of rocks on the surface of Mercury.
The MIT team then used the compositional data to recreate the two rock types in the lab, and subjected each synthetic rock to high temperatures and pressures to simulate various geological processes.
The scientists translated the chemical element ratios into the corresponding building blocks that make up rocks, such as magnesium oxide, silicon dioxide and aluminum oxide.
“We just mix these together in the right proportions and we’ve got a synthetic copy of what’s on the surface of Mercury,” said Prof Timothy Grove, who co-authored a paper reporting the results in the journal Earth and Planetary Science Letters.
They then melted the samples of synthetic rock in a furnace, cranking the heat up and down to simulate geological processes that would cause crystals — and eventually rocks — to form in the melt.
“You can tell what would happen as the melt cools and crystals form and change the chemical composition of the remaining melted rock. The leftover melt changes composition,” Dr Grove said.
After cooling the samples, the researchers picked out tiny crystals and melt pockets for analysis. They initially looked for scenarios in which both original rock compositions might be related. For example, both rock types may have come from one region: One rock may have crystallized more than the other, creating distinct but related compositions.
But Dr Grove found the two compositions were too different to have originated from the same region, and instead may have come from two separate regions within the planet. “The easiest explanation for what created these distinct regions is a large magma ocean, which over time likely formed different compositions of crystals as it solidified. This molten ocean eventually remelted, spewing lava onto the surface of the planet in massive volcanic eruptions.”
“The thing that’s really amazing on Mercury is, this didn’t happen yesterday. The crust is probably more than 4 billion years old, so this magma ocean is a really ancient feature,” Dr Grove said.
The team estimates that this magma ocean likely existed very early in Mercury’s existence – possibly within the first 1 million to 10 million years – and may have been created from the violent processes that formed the planet. As the solar nebula condensed, bits and pieces collided into larger chunks to form tiny, and then larger, planets. That process of colliding and accreting may produce enough energy to completely melt the planet – a scenario that would make an early magma ocean very feasible.
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Bibliographic information: Bernard Charlier et al. Phase equilibria of ultramafic compositions on Mercury and the origin of the compositional dichotomy. Earth and Planetary Science Letters, vol. 363, pp. 50–60; doi: 10.1016/j.epsl.2012.12.021
