Gamma ray spectroscopy prompts rethink on Mercury formation

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A Gamma-Ray Spectrometer (GRS) on board the first spacecraft to ever orbit Mercury has caused scientists to re-evaluate their theories on how the planet was formed. The instrument, developed by researchers from Lawrence Livermore National Laboratory (LLNL) and Johns Hopkins University Applied Physics Laboratory (JHUAPL), discovered higher than expected levels of potassium, sodium and chlorine across Mercury’s surface, contradicting previous formation theories involving high-temperature processes which would have removed these elements.

The spectrometer is part of a suite of seven instruments onboard NASA’s Mercury Messenger (short for Mercury Surface, Space Environment, Geochemistry and Ranging) spacecraft. The Messenger began its 4,000 orbits around Mercury in March 2011, and is due to complete its mission in the next month.  

At the heart of the GRS is a high-purity germanium sensor system developed by LLNL scientists.

One key finding revealed by data from the GRS has been the discovery of levels of potassium, sodium and chlorine across Mercury’s surface.

Before this, scientists envisioned a number of formation mechanisms for Mercury that involved high-temperature processes to account for the planet’s unusually large iron core. But, because such processes would have removed elements such as potassium, sodium and chlorine from the planet, the GRS observations have forced scientists to reconsider Mercury’s origins.

GRS data has also been used to produce a map of potassium distributions across Mercury’s surface. This was the first elemental map made of Mercury’s surface and is so far the only map to report elemental compositions in absolute quantities.

‘The GRS has exceeded expectations,’ said Patrick Peplowski, instrument scientist for the GRS with JHUAPL and an analyst for the Messenger data. ‘While we all expected to measure elements like potassium, iron and aluminium, no one expected minor elements like sodium and chlorine or a map of potassium.'

During its one year of operation, the GRS measured gamma rays from Mercury that were emitted from the planet’s surface. Since Mercury is not protected by an atmosphere, it is bombarded by cosmic rays that interact with the planetary surface, resulting in gamma-ray emissions from normally stable elements.

The main GRS technical challenge for the LLNL team to overcome, according to LLNL physicist Morgan Burks, was the need to cool the germanium crystal to -200°C and keep it there for a year while orbiting the closest planet to the sun, which can have surface temperatures as high as 400°C.

To achieve its goal, the team developed a thermal and mechanical cooling design that allows the germanium crystal to operate at -200°C while rejecting 98 per cent of the infrared heat and energy from the broiling surroundings.

Prior to reaching Mercury’s orbit, the GRS was turned on four times during its six-and-a-half year voyage.

‘After six-and-a-half years of space travel, once the Messenger spacecraft finally reached Mercury’s orbit, the GRS team was just about on the edge of their seats,’ Burks said. 'Even though the instrument had been tested extensively on the ground and in flight, it was quite exciting when the instrument was turned on and we waited for the first results.’

Just as the Mercury Messenger mission is ending, LLNL and JHUAPL have received a three-year, $3 million grant from NASA to develop and space qualify a new high-purity germanium detector known as ‘GeMini Plus'.

The new GeMini Plus, which is set to be built by LLNL and JHUAPL over the next three years, will be smaller, lighter weight and use less power than the Messenger GRS. It is expected to weigh 2.5kg and use 5.5W of power.

The instrument will be commercialised by Tennessee-based Ortec, a subsidiary of Ametek.

‘Gamma-ray spectroscopy can provide foundational science results for future missions such as landers, rovers and orbital reconnaissance missions at a wide variety of planetary bodies,’ said JHUAPL planetary scientist David Lawrence.

‘However, many of these missions will be resource limited and current gamma-ray instruments have relatively large mass and power requirements that will limit their use to missions with large available resources. GeMini Plus is a new type of gamma-ray spectrometer that brings the power of high-resolution germanium to missions with low resources.’

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