Dual Local Oscillator SIS Receiver for Simultaneous Observations of Water Isotopologues in the Solar System

NASA's Planetary Decadal Survey Vision and Voyages has concluded that measurements of isotopic cometary water vapor, and in particular the deuterium-to-hydrogen ratio (D/H) ratio, are an important tool for unraveling the mysteries involving the origin of Earth's water, and the evolution of our solar system. To support this goal, we have developed, through an internal Jet Propulsion Laboratory research program, quantum-limited superconductor insulator superconductor (SIS) receivers covering the important 500-600 GHz submillimeter frequency band. These instruments can detect the (1
₁₀
-1
₀₁
) HDO rotational transition, and the (1
₁₀
-1
₀₁
) ground state (rotational) transitions of Ortho H
₂
¹⁶
O, H
₂
¹⁷
O, and H
₂
¹⁸
O with exquisite sensitivity. However, given the extremely weak HDO emission and the time-variability of the outgassing processes in comets, expeditious low noise D/H ratio measurements of these sources remain extremely challenging. To address this issue, we investigate the possibility of acquiring HDO, H
₂
¹⁷
O, and H
₂
¹⁸
O simultaneously by means of a novel dual local oscillator (2LO) down-conversion process, as an alternative to ultra-broadband IF bandwidth systems.