Ultra-compact, high-resolution Lunar UV spectrometer

This DALI proposal advances the development of a high étendue (E > 1e-9 str m2), ultra-miniature (160 grams) spectrometer, to understand the creation of the tenuous lunar exosphere and the physical processes that maintain it, and the causes of its variability by observing the faint exospheric hydroxyl spectral feature near 3082 Å onboard a small lander or rover.

The proposed instrument, Heterodyne OH Lunar Miniaturized Spectrometer (HOLMS), is based on an interferometric technology called Spatial Heterodyne Spectrometer (SHS) that enables HOLMS to obtain ultra-high sensitivity data from angularly extended and diffused targets such as the exospheric volatiles in the lunar environment. HOLMS enables sensitive, high-resolution spectroscopy on small stationary lunar landers and is applicable to more than one mission and is capable of meeting multiple science objectives. In general, HOLMS is configurable for a variety of gases with a very narrow bandpass anywhere within the near-UV (NUV) to the IR region (Ne, Na, K, O, etc. However, for the purpose of this DALI project, HOLMS is designed to measure the OH emission spectra at 308 nm, the A-X (0,0) band, to study the Lunar water abundance and sources on the lunar environment.

For this DALI project, as the reference mission, we assume HOLMS is mounted on the PUFFER rover, an STMD project which can carry ~ 200 grams of science payload and can operate during the lunar sunlight time for 14 Earth days. Small landers and rovers provide the benefits of low cost, low risk due to multiple units, and several simultaneous measurements; but are challenged because of their low payload mass capability. The total mass of HOLMS is ~160 grams (58 grams for the spectrometer module and ~100 grams for the electronics), which leaves ~20% mass margin for PUFFER.

We leverage on the previous and current similar technology maturation tasks in JPL, our long term science and technology collaborators for developing similar instrumentation and lessons learned from other JPL similar flight projects in the robotics team. A winning strategy for this proposal is the low risk by using subsystems with flight heritage, to ensure the main effort is dedicated to maturing the HOLMS spectrometer and its performance.

This DALI project matures our existing TRL 4 breadboard instrument to the point where it can be proposed in response to future announcements of flight opportunities without additional major technology development as early as ~2024 for multiple future lunar science missions such as CLPS landers, human landers, other landers developed by NASA’s international partners (e.g., JAXA, ISRO), or missions proposed through Discovery and New Frontiers. HOLMS’ footprint (size, weight, and power) is sufficiently small that it can be added to any lunar lander with minimal impact.

HOLMS' data significantly contributes to our understanding of OH migration and formation processes by measuring diurnal observations of OH from various locations. The scientific measurement and data set taken by HOLMS provide a novel set of data, which may result in a revision of our understanding of the physical processes associated with a tenuous exosphere on the lunar environment and other air-less bodies in the Solar System. The current development of Lunar landers from several nations and private companies indicates that the lunar exospheric environment will transition to be mainly artificial within a couple of decades. If the current trend continues, it will not be possible to understand the natural OH environment after this new moon race. Therefore, a HOLMS-like instrument/mission is critically required promptly.