Composition Measurements of Uranus’ Atmosphere

Knowing the composition of the giant planets is important in understanding their forma­tion and evolution history. The abundances of heavy elements, of noble gases, and isotope ratios reveals the physical and chemical conditions and processes that eventually led to their formation. The current knowledge of the composition of the giant planets is limited, with Jupiter being best studied thanks to the Galileo probe. Much less is known for Saturn, and almost nothing is known for Uranus and Neptune. Uranus and Neptune contain substantial hydrogen and helium at­mos­pheres, with bulk mass frac­tions of 5–20%. The remainder is thought to be "ices" and rocks, such as H₂O, CH₄, H₂S, and NH₃. Ura­nus and Neptune are the least-investigated planets in the solar system, but may be representative of similarly sized pla­nets com­mon in the population of exo-planets, thus provide some ground-truth.

Measurement of abundances in the atmosphere can be derived through a variety of re­mote sensing techniques, which is restricted to the upper layers of the atmosphere, but the number of useful observations from Earth is very limited. The most significant step forward in our know­ledge of giant planet internal composition was achieved with the Galileo probe into Jupiter’s atmos­phere. The prime instrument to probe the atmospheric composition on an descent probe is a mass spectrometer experiment (MSE), which comprises the actual mass spectro­meter for gas analysis, possible extensions by a gas-chromatographic pre-selection of the gaseous species, a cryogenic trap to enhance the measurement of noble gases and their isotopes, and an aerosol col­lector and pyrolysis system giving access to the composition of cloud and haze particles. To improve on the isotope measurements of selected species, a Tunable Laser Spectrometer can be added to measure the isotopic ratios with accuracy of selected molecules.

The atmos­pheric probe will enter on a specific location into Uranus’ atmosphere. Aside from technical constraints, what would be the scientific considerations for the locations? Entering at lower latitudes, perhaps near the equator where the zonal flow is retrograde or at higher latitudes with fast pro­grade zonal flows, or at a pole with very limi­ted horizontal flow, which might be easily ac­ces­sible because of Uranus’ rotation axis being close to the eclip­tic plane; at places with clouds running at constant lati­tudes or at cloud-free areas; at a dark spot (an anti­cyclonic storm) possibly providing upwelling from material; or other unique features observed on the surface.

An Uranus orbiter will provide complementary information of the atmosphere via remote sensing, e.g. mapping the “surface” of Uranus, tracking storms, clouds, and eddies in reflected sunlight, maps of key species, abundances of hydro­carbons in the photolysis layer, and some more. This will put the entry location of the probe in a global per­spective, is its entrance at a unique surface feature, is there presen­ce of clouds and hazes, and the temporal evo­lution during the orbital observations, like con­vec­tion, upward and downward energy flow, atmospheric wave activity, which shape atmospheric features such as cloud bands and vortices. In addition, micro­wave soun­ding might probe deep inside the atmosphere.