Science Payload for Ice Giant Entry Probes

To discern the origin and evolution of the solar system including the formation of the terrestrial planets, an understanding of giant planet formation and evolution is needed. Among the most important measurements are the atmospheric composition, structure, and processes of the ice giant. Noble gas abundances in particular are diagnostic of the conditions under which the giant planets formed, and the abundances of cloud-forming (condensable) species are indicators of both the characteristics of the protosolar nebula at the time and location of planetary formation as well as the mechanisms by which additional heavy elements might have been delivered to the planets. Although many key properties of ice giant systems can be accessed by remote observations from flyby and orbiting spacecraft, measurements of the abundances of the noble gas and key isotopes as well as deeper thermal structure, dynamics, clouds, and other atmospheric processes require direct in situ exploration by an atmospheric entry probe.

Entry probe measurements can be classified as either Tier 1 or Tier 2. Tier 1 represents the minimum, threshold science required to justify the probe mission. Tier 2 is high value science that would complement and enhance the Tier 1 measurements, but alone are not enough to justify the entry probe mission.

Tier 1 measurements include atmospheric abundances of noble gases (including helium), key noble gas isotope ratios ²²Ne/²⁰Ne, ³⁶Ar/³⁸Ar, ¹²⁹Xe/total Xe, ¹³¹Xe/total Xe, and ¹³²Xe/total Xe, additional key isotopic ratios D/H, ³He/⁴He, and ¹⁵N/¹⁴N, and the atmospheric thermal structure along the probe descent trajectory. To achieve the Tier 1 measurements, the probe payload must include a mass spectrometer, a helium abundance detector, and an atmospheric structure instrument including pressure and temperature sensors and an atmospheric acoustic properties sensor for speed of sound measurements from which the ratio of ortho- to para- molecular hydrogen can be determined. Depending on mission architecture and probe-carrier telecom design, Tier 1 science can be achieved with a relatively shallow probe descending to several bars.

Tier 2 science includes additional key isotopic ratios such as ¹³C/¹²C and ¹⁸O/¹⁷O/¹⁶O, abundance of condensables, and additional atmospheric structure and processes including the dynamics of the atmosphere (winds and waves), the net balance of upwelling thermal infrared and downwelling solar visible radiative fluxes, and the location, structure, composition and properties of the clouds. The presence of the disequilibrium species such as PH₃, CO, AsH₃, GeH₄, and SiH₄ is primarily due to atmospheric convective upwelling, and abundance measurements would help constrain both the composition of the very deep atmosphere and deep atmosphere chemistries. Additional instrumentation necessary to fully achieve the Tier 2 objectives includes a net flux radiometer, a Nephelometer, and an ultrastable oscillator (USO) as part of the telecommunications system to enable probe Doppler tracking for measurements of atmospheric dynamics.

To address all the Tier 1 and Tier 2 science objectives, a deep probe to 10 bars and beyond would provide measurements of atmospheric thermal structure, dynamics, and processes at levels beyond the direct influence of sunlight that are out of reach of remote sensing.