What’s special about planet Mercury?

When it comes to exploring the Solar System, planets like Mars, Jupiter, and Saturn tend to steal the limelight. Mars could have once been habitable, and Jupiter and Saturn are compelling in their sheer size and number of icy moons.

But have you ever considered Mercury?

Most people have probably only seen a few pictures of Mercury – a seemingly dead world with a grey, rocky crust. In fact, Mercury is so unfamiliar that people sometimes mistake it for the Moon! But what if this small, first planet from the Sun has more to offer when we look deeper?

The first mission to investigate Mercury was Mariner 10 that performed three flybys of the planet in 1974. It mapped out about half of Mercury’s surface and discovered a large iron core within.

MESSENGER then entered Mercury’s orbit in 2011 and performed a variety of experiments, including observing the magnetic field and analysing the chemical composition of the crust. A remarkable discovery was that the magnetic field is offset from the centre, towards Mercury’s north pole. But there are still many mysteries…

What’s special about Mercury?

By sending missions to other planets, we have learnt much about the history of the Solar System, including how the Earth was formed. The more we know about the history of the Solar System, the more we learn about other star systems and how life comes to be.

But Mercury’s formation is shrouded in mystery. Having a huge iron core compared to the size of the crust is a real outlier in our current understanding of planetary formation.

We need more detailed knowledge of Mercury’s composition in order to build up a better picture of how Mercury has formed and evolved over the past few billion years.

Find out more: Five Mercury Mysteries for BepiColombo

Leicester's role

The BepiColombo spacecraft holds a whole host of scientific instruments for studying the surface of Mercury and the solar wind that surrounds it.

One of the key instruments was actually designed and built just down the road at the University of Leicester. This is the Mercury Imaging X-ray Spectrometer or MIXS for short.

MIXS’ mission is to determine what the surface of Mercury is made of. It will use craters on the planet to not only analyse the surface material, but also the layers many kilometres beneath the surface.

This is possible thanks to the craters that pockmark the surface. When a meteorite hits the planet, a crater is formed and material from underneath the surface is thrown out, landing back on the planet in a burst pattern of ‘ejecta’.

Mercury has been bombarded by meteorites in the past, conveniently excavating material from deep underground which MIXS will be able to analyse.

While our eyes see grey rocks, MIXS will see a very different picture. MIXS is equipped with X-ray eyes…

X-ray vision

Intense X-ray light from the nearby Sun constantly blasts the surface of Mercury.

When an X-ray bumps into the atom of a rock, the electron inside that atom gains some energy. The electron doesn’t hold onto this energy for long, and it re-emits it as an X-ray photon, but with less energy. This process is called fluorescence, similar to how glow-in-the-dark materials shine after absorbing light.

MIXS uses a unique design to collect X-rays (inspired by lobster eyes!) in order to gain a much better image resolution than ever before, far beyond the capabilities of the X-ray spectrometer on MESSENGER.

MIXS will be able to detect the elemental ‘fingerprint’ of the re-emitted photon from the rocks in order to determine what Mercury is made of. Because MIXS will be looking at ejecta from craters, it can probe the internal layers of Mercury without having to land and gather samples.

Once BepiColombo arrives at Mercury in 2025, the images and data it gathers should help us unravel the mystery of how Mercury formed and evolved over the past 4.5 billion years.