Prof. Patrick Irwin

Abstract:

A range of concepts for long duration aerial missions, using high altitude balloons operating in the clouds of Venus, have been studied by NASA and JPL for the Planetary Science and Astrobiology Decadal Survey and for NASA's competitive New Frontiers and Discovery programs. These concepts offer a rich set of scientific opportunities in atmospheric chemistry, astrobiology, atmospheric dynamics, seismology and sub-cloud surface imaging. The Venus aerobot would be sustained in flight by a variable-altitude balloon and carry a payload of instruments at altitudes between 52 and 62 km. The aerobot would fly in the cloud layer containing sulfuric acid aerosols and be subject to large temperature extremes as it traverses a range of altitudes and latitudes at different times of day. To achieve the desired lifetime on the order of one Venus day we have defined a solar power system that would supply power over the full altitude range while the aerobot is circumnavigating the planet. We have initiated development of the requisite technology, including rechargeable batteries, solar arrays, and a peak power tracker for this challenging mission. Specifically, we have fabricated triple-junction inverted metamorphic (IMM) solar cells optimized for power generation in the unique spectrum of light expected at 51.5 km altitude and measured 34.0 mW/cm2 power output at room temperature in initial testing. We developed a coating to protect aerobot solar panels from corrosion in sulfuric acid and demonstrated survival without performance degradation after 96 hours in 96% aqueous sulfuric acid at room temperature. Initial performance data were obtained on a peak power tracker showing 96% power conversion efficiency. In addition, we have developed specialized lithium-ion cells intended to operate between -30 and 100°C and demonstrated 80% capacity retention after 90 cycles at 100% depth of discharge at 100 deg C. These cells were incorporated into a 4s1p battery module and successfully tested under expected flight-like random vibration and thermal vacuum conditions. These results represent key steps in the process of developing the power system technology needed to bring the Venus aerobot mission to fruition.

Abstract:

<jats:title>Abstract</jats:title> <jats:p>We present radiative transfer analyses of IRTF-TEXES and SOFIA-EXES mid-infrared spectra of Jupiter's mid- to high latitudes recorded between 2019 April 16 and 2023 July 20. The spectra were inverted across a photochemical model grid of varying eddy diffusion coefficient profiles, and the quality of fit of the synthetic spectra to the observed was used to constrain the CH<jats:sub>4</jats:sub> homopause level. For a subset of latitudes/dates, we find that the CH<jats:sub>4</jats:sub> homopause level is elevated in the region enclosed inside of, or magnetospherically poleward of, the northern ultraviolet main auroral emissions (MAEs) in comparison to the region outside or equatorward of the MAE. For example, using SOFIA-EXES results on 2021 June 10, we derived a CH<jats:sub>4</jats:sub> homopause level of log(<jats:italic>p</jats:italic> <jats:sub>H</jats:sub>(nbar)) = 1.54<jats:inline-formula> <jats:tex-math> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.69</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.51</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> </jats:inline-formula> or <jats:italic>z</jats:italic> <jats:sub>H</jats:sub> = 453<jats:inline-formula> <jats:tex-math> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>76</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>128</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> </jats:inline-formula> km above 1 bar poleward of the northern MAE at 68<jats:sup>∘</jats:sup>N compared to a lower limit of log(<jats:italic>p</jats:italic> <jats:sub>H</jats:sub>) &gt; 2.43 and upper limit of <jats:italic>z</jats:italic> <jats:sub>H</jats:sub> &lt; 322 km derived equatorward of the northern MAE. We therefore conclude that the region poleward of the northern MAE is, at times, subject to enhanced vertical transport resulting from auroral energy deposition. The exact mechanisms responsible for the enhanced vertical transport in Jupiter's auroral regions are uncertain: time-dependent circulation modeling of Jupiter's polar atmosphere is required to better understand this phenomenon. Poleward of the southern MAE, derived homopause levels agreed within uncertainty with those at equatorward locations. However, we consider this result a spatial sampling artifact rather than concluding that the southern auroral region is not subject to enhanced vertical transport.</jats:p>

Abstract:

<jats:p>Voyager-2 radio occultation measurements have revealed changes in the atmospheric refractivity within a 2-4 km layer near the 1.2-bar level in Uranus and the 1.6-bar level in Neptune. These changes were attributed to the presence of a methane cloud, consistent with the observation that methane concentration decreases with altitude above these levels, closely following the saturation vapor pressure. However, no clear spectral signatures of such a cloud have been detected thus far in the spectra acquired from both planets. We examine methane cloud properties in the atmospheres of the ice giants, including vertical ice distribution, droplet radius, precipitation rates, timescales, and total opacity, employing microphysical simulations under different scenarios. We used a one-dimensional (1D) cloud microphysical model to simulate the formation of methane clouds in the ice giants. The simulations include the processes of nucleation, condensation, coagulation, evaporation, and precipitation, with vertical mixing simulated using an eddy-diffusion profile (K_eddy). Our simulations show cloud bases close to 1.24 bars in Uranus and 1.64 bars in Neptune, with droplets up to 100 μm causing high settling velocities and precipitation rates (∼370 mm per Earth year). The high settling velocities limit the total cloud opacity, yielding values at 0.8 μm of ∼0.19 for Uranus and ∼0.35 for Neptune, using K_ eddy = 0.5 m^2 s^-1 and a deep methane mole fraction (μ_CH_4) of 0.04. In addition, lower K_ eddy or μ_CH_4 values result in smaller opacities. Methane supersaturation is promptly removed by condensation, controlling the decline in μ_CH_4 with altitude in the troposphere. However, the high settling velocities prevent the formation of a permanent thick cloud. Stratospheric hazes made of ethane or acetylene ice are expected to evaporate completely before reaching the methane condensation level. Since hazes are required for methane heterogeneous nucleation, this suggests either a change in the solid phase properties of the haze particles, inhibiting evaporation, or the presence of photochemical hazes.</jats:p>