MIRS Imaging Spectrometer for the Martian Moon Explorer (MMX) Mission

The Martian Moon Explorer (MMX) is a sample return JAXA mission that is devoted to the exploration of the Mars system. MMX will be launched in 2024, inserted into Mars orbit in 2025, and will investigate the martian system during 3 years, focusing mainly on Phobos, the principal target of the mission. The main goals of MMX are to return samples of Phobos, and, throughout both the in situ detailed investigation of Mars satellites and the further laboratory studies of Phobos samples on Earth, to clarify the origin of the Mars satellites and the process of planets formation in the Solar System. Observations of Mars will also be performed to investigate its evolution history and its atmosphere. To reach these goals, MMX has a complex onboard instrumentation including imaging systems, infrared, neutron and gamma-ray spectrometers, a lidar, a mass spectrum analyzer, a dust monitoring instrument, and a rover for in situ investigation, beside the sampling and retrieval devices.

We present in this work the design and performances of the MMX InfraRed Spectrometer (MIRS), which is an imaging spectrometer operating between 0.9 - 3.6 microns.  MIRS is provided by CNES and built at LESIA-Paris Observatory in collaboration with four other French laboratories (LAB, LATMOS, LAM, IRAP-OMP), and in close collaboration with JAXA and MELCO. 

MIRS is a spectrometer that uses the push-broom acquisition principle. A single detector acquisition (2D matrix) provides the image of a strip in one direction (spatial), and the spectrum of each point of the strip in the second direction (spectral). The second spatial dimension results from the motion of MIRS Line of Sight in the along-track direction either thanks to the spacecraft speed or by actuation of a scanner mounted on the instrument, which allows ± 20° of optical amplitude respect to the boresight.

The optical design includes a telescope with two free form mirrors focusing the target on the entrance slit of the spectrometer, a collimator, a low-density groove grating working at first order, and a couple of dioptric objectives. The first one projects the spectral image on a filter that sorts the grating orders. The second one projects this spectral image on the detector, but also images the pupil on a cold stop in order to limit the background flux due to the thermal emission of the spectrometer. The detector is a hybrid CMOS made of 500 columns by 256 lines with square pixels of 30 µm pitch, sensitive from 0.45 to 3.8 µm. Both the detector and the cold stop are encapsulated in a cryostat and cooled down to 110 K. A shutter is placed in the slit plane in order to close the spectrometer cavity after the telescope and acquire background images that can be subtracted to science data. The instrument includes also a front cover to limit dust pollution when landing on Phobos, as well as an internal calibration lamp. The MIRS field of view (FOV) is ≥ ±1.65°.

MIRS will observe Phobos and Deimos in the 0.9-3.6 μm range with a spectral resolution better than 20 nm and with a spatial resolution of 0.35 mrad/px. For Phobos, MIRS will acquire spectra at SNR > 100 up to 3.2 micron in about 2 seconds of integration time for observations carried out at phase angle of 30°. The spatial resolution is of 13-30 m/px during the Quasi Satellite Orbit-Medium global survey (altitude varying from 37 to 84 km), down to meter to sub-meter for the selected sampling sites candidates or the sampling spot. The spectral radiometric absolute accuracy is expected to be of 10%, and the relative accuracy of 1%.

Thanks to the large wavelength coverage and high SNR, MIRS will detect faint absorption features associated to minerals and materials that will help in understanding the satellite origin and composition, like anhydrous and/or hydrated silicates, water ice and organic matter, if any. The high SNR and the unprecedented spatial resolution achieved by MIRS will permit to fully characterize the composition and mineralogy of Phobos and Deimos, and to further investigate the local compositional heterogeneity associated with the different geomorphological features across Phobos surface. MIRS will also study Mars atmosphere, in particular the spatial and temporal changes such as clouds, dust and water vapor.