High Temperature Superconductor Bolometers for Planetary Science (HTSBPS)

This work is a design study of an instrument optimized for JPL's novel high temperature superconductor bolometers. The work involves designing an imaging spectrometer that spans the infrared and offers capabilities in imaging and spectroscopy that are unprecedented for planetary science. The sensitivity and the use of passive cooling in the proposed far IR hyperspectral imager make this instrument a strong candidate for several missions to the outer Solar system proposed in the Planetary Science Decadal Survey: the Comet Surface Sample Return, the Trojan Tour and Rendezvous, the Saturn Probe, and the Uranus Orbiter and Probe. Furthermore, looking beyond this decade, the survey reads: "it is important to make significant near-term technology investments in the […] Titan Saturn System Mission, and Neptune System Orbiter and Probe." The high sensitivity of our instrument would allow resolving narrow and weak spectral features, acquiring spectra in short time windows (such as during brief flybys), and imaging fast dynamic processes (such as moving clouds or storms or during rapid flybys). When applied to observe storms in the atmospheres of the giant planets, our team expects our far IR hyperspectral imager to be able to acquire broadband images of storms in minutes, resolving chemical species at different depths to provide information about composition and depth with unprecedented detail. The proposed instrument may also be applied to measure thermophysical properties of icy moons (such as Europa and Enceladus), asteroids, and comets. Applications include measuring temperature and thermal inertia to study geology and surface evolution while also performing spectroscopy to study composition of the surface of these objects and their thin atmospheres. Additionally, the ring particles of the giant planets are of interest. Thermal inertia measurements probe the ring dynamics. Spectroscopy using emission, solar reflectance, and stellar occultation measurements of ring particles probe composition.

The Planetary Science Decadal Survey strongly emphasizes the exploration of the outer solar system, which addresses some of the basic questions of planetary science: “How do the giant planets serve as laboratories to understand Earth, the solar system, and extrasolar planetary systems? Important objects for study: Jupiter, Neptune, Saturn, and Uranus.”; “Beyond Earth, are there contemporary habitats elsewhere in the solar system with necessary conditions, […] to sustain life, and do organisms live there now? Important objects for study: Enceladus, Europa, Mars, and Titan.” One of the seven proposed New Frontiers missions (the Saturn Probe) aims at studying the atmosphere of Saturn and one of the three proposed large-scale Flagship mission (Uranus Orbiter and Probe) aims at studying the atmosphere of Uranus. Furthermore, looking beyond this decade, the survey reads: "it is important to make significant near-term technology investments in the […] Titan Saturn System Mission, and Neptune System Orbiter and Probe." The hyperspectral imager we envision would allow studying the atmospheres of Saturn, Uranus, Neptune, their moons and rings with higher spectral and spatial resolution than any other existing technology, and would also be applicable to studies of other outer solar system objects such as Trojan asteroids or comets. Broadband spectroscopy, as offered by the bolometers described below, is vital to studying the thick atmospheres of giant planets and moons because a wide range of wavelengths is needed for probing different depths into the atmosphere. The atmospheres of these bodies are opaque at shorter wavelengths due to the pressure-broadened absorption lines of H2 and He and, in some cases (e.g. Neptune and Titan), methane. High spectral resolution optics, will allow for the probing of a wide range of depths, along with relevant spectroscopy of numerous molecular lines. For this reason, the composition of the atmosphere of the gas and ice giants and their moons has been studied by remote sensing using far infrared (FIR) thermal radiance spectroscopy. The hyperspectral imager we envision would allow studying the atmospheres of Saturn, Uranus, Neptune, their moons and rings with higher spectral, spatial, and temporal resolution than any other existing technology, and would also be applicable to studies of other outer solar system objects such as Trojan asteroids or comets and their thin atmospheres. The unprecedented high sensitivity of the superconducting bolometer technology instrument will allow capture dynamic phenomena or rapidly acquire image and spectra during brief flybys. The main features of the instrument we envision are: (1) broadband spectral range, potentially spanning from the visible (0.5 μm) to the far infrared (500 μm), (2) one hundred times higher sensitivity in the far infrared than existing radiometers, (3) ten times higher sensitivity than narrow band mid IR detectors, (4) potential for large (kilo pixel) arrays of detectors, and (5) operating temperature of 55 K or higher, accessible using a passive cooling, as deployed on the Cassini VIMS instrument. The high sensitivity of our instrument would allow: (1) resolving narrow and weak spectral features, (2) acquiring spectra in short time windows (such as during brief flybys), and (3) imaging fast dynamic processes, such as moving clouds or storms. The large spectral bandwidth of the instrument would allow acquiring hyperspectral images spanning three orders of magnitude in wavelength with the same focal plane, which avoids the problem of stitching together data acquired from different focal planes (as in VIMS and CIRS instruments on the CASSINI mission. The proposed instrument is based on arrays of yttrium barium copper oxide (YBCO) high temperature superconducting kinetic inductance bolometers (KIBs), which our team had recently pioneered. Each of these bolometers responds to power from the interferometer by shifting the microwave resonance frequency of a resonator in which it is embedded. Hundreds of resonators can be read out on a single feedline because each is assigned a frequency that is distinct from the others. Readout is performed by measuring the transmission of a microwave signal through the feedline. Based on the current performance of high temperature KIBs, we expect these detectors to: (1) operate at 55 K, a temperature that has already been attained by passive cooling on planetary flight missions; (2) be sensitive from the visible to the far infrared; (3) achieve 100 times higher sensitivity than existing detectors operating at comparable temperatures in the far IR; and (4) yield kilo pixel arrays, thanks to a resource efficient multiplexing scheme [8]. The development of the design involves laying out the optics to focus broadband infrared on an array of bolometers, while simultaneously achieving precision interferometry. This also requires optical coupling to the bolometers such as by use of a feedhorn array. A radiator and thermal design to cool the bolometers and optics to the required temperature are also being developed. The bolometer array design is being developed to match the optics. The design is being optimized to meet the power and mass requirements of a planetary mission. Together, these provide comprehensive design for the optical thermal systems of the instrument.