A PASSION FOR SPACE
Prof Suzie Imber on Mercury’s mysteries.
For many years our attention has been grabbed by the rings of Saturn, the ice caps of Mars or Jupiter’s Great Red Spot. Missions to these planets have provided a wealth of information about our Solar System. However, there is an anomalous planet that continues to perplex scientists, so much so that we have just launched our second dedicated mission there this century: Mercury.
Understanding the formation and evolution of Mercury may provide the key to unlocking some of the outstanding mysteries of our Solar System. It’s so close to the Sun in the sky that the majority of our current knowledge has come from in situ spacecraft observations. Even getting a satellite to Mercury presents a huge challenge, requiring a seven-year journey incorporating several flybys of Earth, Venus and Mercury to slow it down enough to be captured by Mercury’s weak gravitational field. And that’s just the first step; the environment around Mercury presents huge engineering and operational challenges. Chief among these is the heat: orbiting spacecraft must endure temperature swings from +450°C to –180°C in periods as short as 30 minutes, while most of the instruments on board are designed to operate at room temperature.
Mercury is the densest planet in the Solar System (accounting for gravitational compression), as its metal-rich core takes up a far greater fractional volume of the planet than any other. This could be the result of a huge collision that removed the outer layers of rock, but Mercury’s surface appears to be rich in the volatile elements that would be the first to disappear as a result of such a collision. How then – and where – did Mercury form?
Enduring a solar assault
Over the four years it orbited Mercury from 2011-2015, NASA’s MESSENGER spacecraft made the remarkable discovery that Mercury’s magnetic field is offset to the north of the planet. It also revealed tantalising hints that Mercury’s space weather may be fundamentally different to Earth’s, because Mercury’s giant iron core is fending off the powerful solar wind in the inner heliosphere. Can the solar wind from the Sun ever scour the surface of the planet, and what impact would this have on the planetary atmosphere? Are the mysterious X-ray emissions on the night side of the planet similar to Earth’s aurora, and if so, what can these features tell us about Mercury’s space weather? Also, what exactly is the dark material that MESSENGER discovered on Mercury’s surface, and does it play a role in the formation of mysterious holes – known as ‘hollows’ – on the planetary surface? On 20 October this year I witnessed the launch of the joint European-Japanese BepiColombo mission, carrying a suite of instruments (including one built at Leicester) to Mercury. It’s a pioneering mission composed of two spacecraft that will travel together until they arrive at Mercury in December 2025, then split apart and orbit the planet separately.
This configuration will enable some ground-breaking studies of the surface and environment, seeking to explain Mercury’s formation, evolution and dynamics, with implications for our Solar System and beyond. It’s a long journey, but I can’t wait to see what discoveries our new mission will bring to light!
BepiColombo launched in October 2018 and is due to reach Mercury in 2025