Five Mercury Mysteries for BepiColombo

Mysterious Mercury

Mercury is the closest planet to the Sun and the smallest in the Solar System, but it has been a bit neglected in the amount of spacecraft sent to it. While both Venus and Mars have had upwards of twenty visits from spacecraft, only two other missions have ever been to Mercury.

Mariner 10 visited three times between 1974 and 1975 and MESSENGER orbited from 2011 to 2015. These missions revealed a strange, mysterious world where ice exists despite high temperatures and the magnetic field baffles scientists. They raised more questions than they answered!

Due to launch in October 2018, with expected arrival at Mercury in 2025, BepiColombo will orbit Mercury for at least a year and hopefully answer many of the questions raised by Mariner 10 and MESSENGER.

BepiColombo is a joint mission between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). On its journey BepiColombo will perform flybys of Earth, Venus, and Mercury before beginning its Mercury orbit in December 2025.

The mission is made up of three main parts: the transport module, the ESA spacecraft and the JAXA spacecraft. These two spacecrafts will fly separately and capture different observations of Mercury at the same time.

The mission is named after Giuseppe (Bepi) Colombo who was the mathematician who first calculated how to get a spacecraft into orbit around Mercury.

Is there water on Mercury?

Mercury can reach temperatures as high 430°C, and so it’s not a place where you would expect to find water! However, due to the small tilt of the planet, there are permanently shadowed areas that remain cold and temperatures can get as low as -180°C – cold enough for ice to form. In 2012 MESSENGER found that the planet’s north pole contains ice that could be up to tens of centimetres thick. BepiColombo will confirm whether it is frozen water or sulphur.

MESSENGER focused on the northern hemisphere but BepiColombo will be able to study the entire planet and confirm whether there is also ice present at the south pole.

Does Mercury have a magnetic field?

When Mariner 10 discovered that Mercury had a magnetic field, it was very unexpected. Mercury should have a solid core due its small size, but a magnetic field implies that some part of the core is molten or liquid.

MESSENGER found that the magnetic field is around three times stronger at its northern hemisphere than its southern hemisphere. This led to the theory that the planet’s core is solidifying from the outside in.

Mercury’s core now is thought to occupy two thirds of its mass and up to 85 percent of the planet’s radius. With such a large core, the magnetic field should also be large, close to the same strength as Earth’s. However, the actual strength of the magnetic field is only about 1 percent of Earth’s. This is probably due to the interactions of the planet with the solar wind. BepiColombo will tell us more about Mercury’s magnetic field and its occasional aurora.

Where is all the iron?

Mercury is the second densest planet in the solar system, after Earth. Because Mercury is so small, its core must be very dense, suggesting it is made from iron.

However, spectral observations made by MESSENGER showed no evidence of this. This means that the surface has very little iron.

One of BepiColombo’s biggest questions is therefore, where is all the iron?

Does Mercury have an atmosphere?

Unlike the permanent atmosphere of Earth, Mercury has a thin ‘exosphere’ which is constantly being blown away and replenished. The exosphere is made from molecules that have been vaporised from the surface, as well as hydrogen and helium from the solar wind. As the exosphere is bombarded by the solar wind, the molecules trail off the planet like a comet’s tail, to be replenished by more from Mercury’s surface, and the whole cycle begins again.

Marnier 10 found hydrogen, helium, and oxygen within the exosphere, and Earth-based observations have discovered sodium, potassium and calcium. MESSENGER also detected magnesium. Continuing the trend, BepiColombo is likely to find other new elements within Mercury’s tenuous exosphere.

Made in Leicester

The Mercury Imaging X-ray Spectrometer (MIXS) instrument on board the ESA spacecraft will look at planet in X-rays, giving us information about the what the surface is made of. This is particularly exciting for us because it has been partially developed and built here in Leicester and the Principle Investigator is Professor Emma Bunce, who specialises in planetary science at the University of Leicester.

Find out more: Learn how lobster eyes have inspired the MIXS instrument.

The image below left shows an visible-light image taken from MESSENGER on the left. The next image is a simulated image in the X-ray range predicting what MIXS will see at the closest approach of the orbit. The third image shows a simulated image of what MIXS will see at the furthest approach, and the fourth images shows what MESSENGER observed in x-ray. This set of images shows how much more we will be able to learn from BepiColombo, compared to what was learnt from MESSENGER.