Visiting the French team to design our MacrOmega Instrument

(Article by Dr Sarah Crites, International Top Young Fellow at ISAS)

The Martian Moons eXploration (MMX) mission will bring back samples from one of the two moons of Mars to probe early Solar System history and the processes that formed the Martian moons–but to achieve the mission’s goals, those samples need to be representative of the rest of the surface. How will mission planners decide where to land, and which samples to bring back?

Near infrared spectroscopy from orbit will provide one important input for selecting landing sites for sampling on the surface of the moon. When a planet’s surface is illuminated by sunlight, photons of certain energies are absorbed depending on the surface material, while others are reflected–this is the process that makes plants look green and coffee black to our eyes. By measuring this ‘spectral signature’ of a planet or moon’s surface, scientists can identify what minerals, rocks, or ices make it up, and map their distribution. The Martian Moons eXploration (MMX) mission will include a near infrared spectrometer, developed in partnership with the French space agency Centre National d’Etudes Spatiales (CNES), to map the composition of the Martian moons and help select landing sites that are representative of the whole surface.

The MMX near infrared spectrometer, called MacrOmega, is based on acousto-optic tuneable filter (AOTF) technology, which uses an ultrasonic wave transmitted through a crystal to permit only light of a specific wavelength to pass through. By changing the frequency of the ultrasonic wave in steps, the wavelength measured by MacrOmega is changed as well, building up a hyperspectral image cube containing both spatial and spectral information about the moon. This technology has been used in several currently operating space instruments, including SPICAM on Mars Express and MicrOmega on Hayabusa 2, and offers many advantages over other spectrometer technologies, including the ability to operate with no moving parts, high signal-to-noise ratios, and flexibility in selecting wavelengths of interest to measure.

In June, scientists and engineers from France and Japan gathered at the Institute Astrophysique Spatial (IAS) to discuss how to apply MacrOmega’s unique spectroscopy approach to achieve the science goals of the Martian Moon eXploration mission. At the meeting just outside Paris, the joint team toured IAS facilities, which have been used to build and test instruments on space missions from Mars Express to the Planck telescope; debated aspects of technical design; and participated in a science workshop with Martian moon and planet experts in the French scientific community.

One of the topics discussed at the science workshop was the ways in which the spacecraft and MacrOmega will improve on previous near infrared measurements of Phobos and Deimos. Earth-based telescopes and Mars orbiting spacecraft have measured the spectral reflectance of Mars’ moons, but limitations of spectral range and spatial resolution have thus far prevented detection of diagnostic absorption features which would reveal the composition of the moons’ dark surface. Our mission will get closer to the Martian moons than any mission before–even before landing to gather samples–so the near infrared spectrometer will gather data at extremely high spatial resolution, which will lessen the effects of blurring of spectroscopic features due to averaging over large areas. In addition, MacrOmega will make observations across a wavelength range designed to covering several key absorption features of hydrated minerals and organics that may be present on the moon.

The orbit of the spacecraft, though optimized for observations of the moons, also offers a unique opportunity for Mars atmospheric science. From its position near the inner moon, Phobos, MMX will be able to make full global measurements of the Martian atmosphere up to several times per Martian day, complementing past and ongoing measurements from Mars Reconnaissance Orbiter, Mars Express, and the ExoMars Trace Gas Orbiter. With global high temporal resolution measurements from MacrOmega, which can detect atmospheric species such as H₂O and CO₂, the Martian climate system’s complex cycles of dust, CO₂, and water between atmospheric and non-atmospheric reservoirs can be quantified in an unprecedented way on a daily to seasonal timescale.

As part of the visit to IAS, the MacrOmega team also made laboratory measurements of analog materials to provide a baseline for the planned orbital observations. Martian Moon eXploration Mission science board member Tomoki Nakamura, professor of Earth and Planetary Materials Science at Tohoku University, brought several meteorite samples to IAS to measure their spectral properties with a prototype version of the near infrared spectrometer instrument. Like the Martian moons, the carbonaceous chondrites measured are very low-reflectance, and are made up of minerals and grains that may be present on the surface of the moon. The team also simulated observations under varying illumination conditions to mimic real measurement conditions from the orbit of the moons around Mars. These measurements will provide a valuable reference dataset for the final design of the instrument.

This week-long technical planning meeting and science workshop represents the first step in building a fully unified Japanese-French team for the Martian Moons eXploration (MMX) near infrared spectrometer. Jean-Pierre Bibring, IAS principal investigator of the instrument, summed up the spirit of the meeting: “The intimate integration of our two teams, in their diversity, based on trust and profound respect, hosts a huge potential for joined achievements, for this investigation and beyond.”