Submillimeter-wave Line-spectrometer for Isotopologue Measurements (SUBLIME)

Hypothesis: Determination of how Earth got its water is both fascinating and scientifically important. Fig. 1 shows the large variability in the deuterium-to-hydrogen (D/H) ratio of water within different solar system bodies, indicating no clear source of the Earth’s water. The aim of the proposed task is to develop a Submillimeter-wave Line-Spectrometer for Isotopologue Measurements (SUBLIME), a first ever receiver system that can simultaneously measure water isotopes including HDO, and will allow higher precision in the D/H measurements of 10’s of comets. This will facilitate testing the hypothesis that the water on Earth was deposited by comets. Measurement Technique: Submillimeter heterodyne spectroscopy has proven to be an incredibly powerful technique in determining local chemical, physical and dynamical processes remotely. Of particular interest is the fractionation of the light isotopes (e.g. H, C, N, O, & S). The enrichment or depletion of rare isotopes relative to the main isotope provides critical insight into the evolution of a planetary body.

Detector Development: An accurate and definitive D/H measurement requires simultaneous determination of the water abundance and that of the HDO isotope. Outstanding detector sensitivity is required as the abundance of rare isotopologues can be as little as 1/10,000 of the main isotopologues. Building on our recently demonstrated (Kooi et al., 2022) niobium (Nb) based Superconductor-Insulator-Superconductor (SIS) mixers with near quantum-limited sensitivity to ~600 GHz, we propose to develop a novel 2-color SIS receiver that will allow for water isotopes (including HDO) to be measured simultaneously. This has never been achieved before. Table 1 lists the deliverables from this task.

Space Mission: For measuring D/H on single comets it’s possible to get close to them and thus room temperature detectors are sufficient. However, to measure tens of comets one needs exquisite sensitivities as the comets will be much further away. The proposed receiver system provides the sensitivity required to measure D/H in 10’s of comets in a nominal 5-year mission with a 2–3 m class telescope. Cryogenically cooled detectors were not practical for planetary missions in the past. However, recent development in low-power and reliable 4K cryo-coolers, partly funded by NASA, has enabled practical solutions that are commercially available and ideal for submillimeter-wave instruments, where the heat load is typically only a few mW (Narasaki et al., 2016 ).

TRL: Entry TRL is 2; Exit TRL is 4.

Developing Instrument technology to improve measurements for future planetary science missions

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