A Message from Mercury
True colours of Mercury. Credit: NASA

A Message from Mercury

19/03/2021Written by Malika Andress

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In 2015, NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft, the first spacecraft to orbit the planet Mercury, ended its 11-year mission by slamming into the surface of the solar system’s innermost planet.

Missions to Mercury

Missions to Mercury
An artist’s impression: MESSENGER flying over a colourful Mercury. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Between the MESSENGER and the Mariner 10 missions (which, with three flybys, managed to observe less than half the planet), scientists have data that will keep them occupied for many years to come, at least until the European Space Agency’s BepiColombo mission reaches the planet in 2024, and probably for a long time thereafter.

As the least explored of our terrestrial planets, this information not only helps us understand the formation of Mercury, but all of the terrestrial planets.

The MESSENGER spacecraft orbited Mercury for more than four years. Among its accomplishments, the mission determined Mercury’s surface composition, revealed its geological history, discovered details about its internal magnetic field, and verified its polar deposits are predominantly water-ice.

Water on Mercury

Water on Mercury
Mercury, acquired by the Mercury Dual Imaging System (MDIS) aboard NASA's MESSENGER mission Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

MESSENGER entered orbit around Mercury on March 18, 2011, becoming the first spacecraft to do so. The craft was protected by an innovative ceramic-cloth sunshade against temperatures of up to 450°C, providing working conditions around average room temperature.

After orbital insertion an eighteen-day commissioning phase took place, proving that the craft have survived the journey to the planet, along with the onboard scientific instrumentation.

Scientists were surprised to discover a huge amount of water ice considering how close Mercury is to the Sun. With temperatures reaching over 400°C over a trillion tonnes of frozen water is hidden, permanently shadowed in impact craters at the planet’s poles.

Before the MESSENGER mission scientists believed that Mercury may contain volatile chemicals such as potassium, sodium, sulphur and chlorine. However, these theories were in question, as they could not be part of a planet so close to the sun.

MESSENGER showed these chemicals are present on Mercury and can be found around the edges of impact craters. Excitingly, the ratios of some of the chemicals are identical to those found in Martian meteorites.

An Iron Core

An Iron Core

It is believed that the water, along with these other materials on the planet surface, arrived after Mercury was formed, thanks to comet and asteroid bombardments. It seems a lot has happened in the 4.5 billion years of the planets existence.

Once MESSENGER had completed mapping the surface of Mercury, the data showed that the planet used to be bigger. Wrinkles on the planet’s surface, known as scarps, show that Mercury is 7km smaller than it was 4.5 billion years ago.

Although there had been theories that Mercury had been a lot larger historically, another discovery, thanks to MESSENGER, helped resolve the question around the large iron core, that occupies almost 50% of its volume (over twice as much as Earth’s core).

Mercury formed the same way as the other terrestrial planets Venus, Earth and Mars, however, Mercury formed at an iron line in a region that is so hot, where iron condensed out of the protoplanetary nebula.

Mariner 10 had observed that despite having a large, thought to be solid, iron core, Mercury had a magnetic field. However, MESSENGER confirmed this fact and that the magnetic field is very similar to our own here on Earth.

Our own magnetic field is produced by the movement of the liquid iron outer core around the solid iron inner core, so if Mercury has a magnetic field similar to our own, it must have a large liquid iron reservoir to generate it. But how part of the core could remain liquid is still a mystery.

It could be due to radioactive decay in the planets crust, effectively creating a hot ceramic mantel, that insulates the iron core and stopping it solidifying.

Mission End

Mission End
BepiColombo rotates solar panels. Credit: NASA

Although MESSENGER’s primary mission to Mercury ended in March 2012, with the Sun entering a a period of maximum solar activity, the mission was extended to watch the planet for changes.

MESSENGER ended its mission by crashing into the surface on 30 April 2015, however, that was not the end of the mission here on Earth, as multiple organisations, academic establishments and scientists continue to use the data to continue learning more about Mercury and the other planets in our Solar System.

Peter Bedini, a programme manager in APL’s Space Exploration Sector and a member of the Principal Professional Staff, was MESSENGER Project Manager from 2007 through the end of the first extended mission in 2013, said; “More than most missions, MESSENGER represented the Discovery Program’s charter to develop low cost, innovative approaches to planetary exploration. It was an ambitious concept that ultimately exceeded all expectations and filled a serious gap in the understanding of the terrestrial planets.”

Magnetosphere of Mercury

Magnetosphere of Mercury
Dr Suzie Imber, BBC Two's 'Astronauts' finalist

In 2014 Dr Suzie Imber, Associate Professor in Space Physics at the University of Leicester, was awarded a Leverhulme Trust Fellowship, “Rough Winds do Shake the Magnetosphere of Mercury” to utilise MESSENGER data to understand more about Mercury’s magnetosphere.

She is also a co-investigator on the Mercury Imaging X-ray Spectrometer (MIXS), an instrument designed and built at the University of Leicester, currently on board the European Space Agency’s next Mercury mission, BepiColombo, which launched on 19 October 2018.

This instrument is designed to determine the composition of the surface of Mercury in unprecedented detail (aimed at resolving key questions about Mercury’s formation and evolution), and will also measure Mercury’s x-ray aurora, a phenomenon recently discovered by Suzie’s research team studying the magnetosphere of Mercury.