Optimized Filter Channels for In-Situ Heat Flux Measurements in Uranus' Atmosphere to 10 Bar Pressure

The recent Decadal Strategy for Planetary Science and Astrobiology 2023-2032 [1] called out the Uranus Orbiter and Probe mission as the highest priority new Flagship mission. This report identified six key science questions in origins, interior and atmosphere of Uranus. Our advanced Ice Giants Net Flux Radiometer (IG-NFR) directly addresses two of these questions i.e., (i) determine the planet's atmospheric heat balance, and (ii) determine the planet's tropospheric 3-D flow. Little is known as to how the energy inputs to Uranus' atmosphere, i.e., via solar insolation from above and the remnant heat-of-formation from below to create the planetary-scale patterns seen on Uranus [2]. In-situ NFR measurements in the pressure range 0.1-to-10 bar, using optimally designed filter channels, will determine the balance between the upward and downward solar and thermal radiation fluxes [3][4][5].

Our IG-NFR, Fig. 1, onboard a probe descending deep into the atmosphere, is designed to measure energy flux in seven spectral channels from 0.2-to-300 µm wavelength, each with a 10° FOV projected into the sky, in five viewing angles (±80°, ±45° and 0°).

Radiative modeling, using NEMESIS [6], of the expected observations, helped to determine the optimal FOV and channel filter band selections. Fig. 2 shows a spectral radiance vs wavelength calculation at 1 bar pressure level looking up and down at 45° to the local horizon and at an azimuth angle of 90° with respect to the Sun and shows the positions of the seven nominal filter channels. Sensitivity studies of the filter properties led to an optimal baseline filter set, Table 1, that covers the bulk of the radiance field from 0.2-to-300 µm, and which also covers the visible to near-IR scattered solar radiation spectrum targeting several key atmospheric properties. Channel 7 is placed to capture all the scattered solar radiation, while channels 6 and 5 are placed to capture light that is progressively more absorbed by gaseous methane.

Ref.: [1] A Decadal Strategy for Planetary Science and Astrobiology 2023-2032. DOI 10.17226/26522. [2] Mousis, O., et al., PSS, 155, 12-40, 2018. [3] Aslam S., et al., Space Sci. Rev. 216:11, 2020. [4] Irwin P. G. J., et al., DOI:10.5194/epsc2020-306. [5] Aslam S., et al., LPI Contributions, 2686, 4001, 2022. [6] Irwin P. G. J., et al., DOI: 10.1016/j.jqsrt.2007.11.006.