Spectacles to see a moon: NASA select ‘MEGANE’ instrument for MMX

Left: The gamma ray and neutron spectrometer (GRNS) that flew with the NASA Mercury mission, MESSENGER (J. O. Goldsten, 2007 in Science v.131 p.339) and right: GRNS for NASA’s Lunar Prospector mission (Feldman et al, 2004, Geophys. Res. 109). The GRNS on the MMX spacecraft will be similar in design.

NASA has selected their scientific instrument for the Martian Moon eXploration (MMX) Mission.

The US Space Agency called for proposals in March for the design of a gamma ray and neutron spectrometer (GRNS); a top priority instrument for the spacecraft due to its ability to identify the chemical elements that form the Martian moon rock.

The selected design is from the Johns Hopkins University Applied Physics Laboratory (APL) in Maryland on the east coast of the USA. The instrument team is led by David Lawrence, who relayed the group’s enthusiasm for the mission to Mars:

“The scientific project of trying to understand how Phobos and Deimos formed is one of the biggest questions for our understanding of the terrestrial planets,” he said. “Measuring their elemental composition is one of the key ways we can answer this question.”

Neutrons are subatomic particles usually found in the nuclei of atoms. They can become dislodged when the atoms of a planetary surface are struck by cosmic rays, which are high speed particles that originate from the Sun and sources outside the Solar System. Due to their very high energy, cosmic rays can free neutrons from the atomic nuclei; these neutrons either escape or collide with neighbouring atoms, which then shed this sudden acquisition of energy by emitting gamma rays.

Since both neutrons and gamma rays originate from changes in individual atoms, their number and energy reveal the atom type. Therefore, measuring the neutron and gamma rays emitted from a planetary body tells you about the chemical elements in the rock, such as oxygen, potassium and iron.

The GRNS that Lawrence’s team is designing for MMX is named the ‘Mars-moon Exploration with GAmma rays and NEutrons’, giving the acronym ‘MEGANE’ which also means ‘spectacles’ in Japanese. It is an appropriate name for the instrument aiming bring the moon composition into focus. The instrument uses a large crystal of germanium that is cooled to 90 degrees above absolute zero (90K = -183°C) to measure the gamma rays. This high energy radiation induces an electric current in the crystal at a strength proportional to the gamma ray energy. The neutrons are detected with cylinders filled with helium-3 gas; an isotope of common helium that has one less neutron in the atomic nucleus. A distinctive electronic signal is produced when neutrons are absorbed by the gas.

The results from MEGANE will complement the ‘MacrOmega’ near-infrared spectrometer being developed with France’s CNES for the mission. MacrOmega will focus on identifying the rock minerals, while MEGANE will measure the relative quantities of the actual elements.

Lawrence’s team at APL are one of the most experienced instrument builders in the business. Starting with NASA’s Lunar Prospector mission in the late 1990s, Lawrence has participated in experiments to explore the composition of the Moon, Mercury, Mars and the dwarf planet, Ceres. The group’s GRNS instrument on NASA’s MESSENGER mission to Mercury revealed a surprising abundance of volatiles (easily vaporised elements) on our hot innermost planet. This has challenged theories regarding how Mercury formed and is a result our BepiColombo mission with ESA is eager to explore further when it leaves for Mercury next year. The team is also designing the GRNS for Psyche; a new NASA mission to visit the iron-rich asteroid, 16 Psyche.

Writing to congratulate Lawrence’s team on their successful proposal, Masaki Fujimoto, Research Director at ISAS, said he was “happy to have the best GRNS on the globe onboard the MMX spacecraft.”

It is a sentiment everyone here at the MMX team delighted to share.