Solar winds will help ESA probe smell what Mercury's cookin'

Sun's exhalations whip up molecules on planets with weak magnetic fields

Researchers at the Technische Universität Wien in Austria have found that solar wind can do far more than project lights in the Earth's night sky.

The work, published in the journal Icarus, found that while we on Earth are treated to displays such as the Northern Lights, bodies that lack the Earth's protective magnetic field, such as the Moon or Mercury, have a considerably tougher time of it.

Professor Friedrich Aumayr, from the Institute of Applied Physics at TU Wien, explained:

"The solar wind consists of charged particles – mainly hydrogen and helium ions, but heavier atoms up to iron also play a role." On Mercury or the Moon, these particles can strike the surface at between 400 and 800km per second, gradually eroding rock and flinging up numerous other atoms. These settle above the planet, creating a very thin atmospheric layer.

This layer (known as an exosphere) is hugely interesting to planetary scientists because it allows them to ascertain the composition of the surface without having to land a probe.

However, using the exosphere in this way needs a precise understanding of the effects of solar wind. Until now, the assumption had been that it was the kinetic energy of particles that did the damage.

By investigating ion bombardment on wollastonite, a typical Moon rock, the team at TU Wien showed that electrical charge also plays a role, explaining why particles are doing more damage than thought.

Charged particles carry a large amount of energy, which is released in a flash on impact. "If this is not taken into account, the effects of the solar wind on various rocks are misjudged," said Paul Szabo, a PhD student on the team. Failing to consider this could result in an inaccurate conclusion about surface rocks from the composition of the exosphere.

The research will prove useful for the team behind the European Space Agency's (ESA) BepiColumbo mission to Mercury, due to launch in October 2018. The spacecraft, part of a joint mission for ESA and the Japan Aerospace Exploration Agency (JAXA), arrived in French Guiana last month for processing prior to being flung into space aboard an Ariane 5 rocket.

BepiColumbo will consist of two spacecraft, the Mercury Planetary Orbiter (MPO), built by ESA, and the Mercury Magnetospheric Orbiter (MMO) constructed by JAXA.

Among the payloads carried by the spacecraft are instruments specifically designed to probe the exosphere, with objectives including understanding its production mechanisms.

Scientists face a bit of a wait before getting to play with their toys for real. Despite Mercury's proximity to Earth (compared to giants such as Jupiter or Saturn), orbital mechanics and a desire to avoid sending the probe screaming into the Sun mean an arrival in orbit will not happen until December 2025, assuming October's launch goes to plan. ®

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