Ancient Mercury Possessed Massive, Roiling Ocean of Magma

Billions of years ago, Mercury was quite a different place than it is today. It once had a massive, roiling ocean of magma that covered the planet. This new finding comes after researchers analyzed the planet's rocky surface, drawing on the chemical composition of rock features and partially reconstructing its history.

Sun-scorched Mercury is only slightly larger than Earth's moon today. The closest planet to the sun, it zips around the star every 88 days. Scientists at MIT, though, have painted a very different picture of the planet when it first formed about 4.5 billion years ago.

They analyzed data gathered from MESSENGER (MErcury Surface Space ENvironment GEochemistry and Ranging), a NASA probe that has orbited the planet since March 2011. The probe first entered Mercury's orbit during a period of intense solar-flare activity. This allowed the instruments onboard to measure the spectrum of light reflecting from the planet as it took the brunt of the sun's rays.

The researchers analyzed the X-ray fluorescence data from the probe and identified two distinct compositions of rocks on the planet's surface. In order to fully examine these two different rock compositions, the researchers then used the data to recreate two rock types in the lab. They then subjected each synthetic rock to high temperatures and pressures to simulate geological processes.  In order to actually melt the synthetic rocks, they placed them in a furnace and cranked the heat up and down in order to simulate the process that would create crystals and eventually rocks.

From their research, the scientists came up with only one reason that could explain the two compositions. A vast ocean of magma must have covered Mercury. The ocean would have created two different layers of crystals, solidified and then remelted into magma before erupting onto Mercury's surface.

The findings show a new way of looking at the closest planet to the sun. The magma ocean probably existed within the first 1 million to 10 million years of the planet's existence, and may have been created from the violent processes that formed Mercury.  It could allow scientists to better understand planet formation.

The findings are published in the journal Earth and Planetary Science Letters.