Mass Spectrometry at Uranus: Scientific Rationale, Instrument Design, and Site Selection for the UOP Mission

The Uranus Orbiter and Probe (UOP) mission has been designated as NASA’s highest-priority new flagship mission for the 2023–2032 decade. Aimed at investigating the origin, structure, and evolution of Uranus, the UOP mission seeks to address key outstanding questions about ice giants—arguably the least understood class of planets in our Solar System. Central to the mission concept is an atmospheric entry probe that will perform in situ measurements of Uranus’s atmospheric composition during descent. A mass spectrometer is the leading candidate for this task, offering the ability to directly quantify molecular and elemental abundances, including critical isotope ratios. These measurements are essential for constraining models of planetary formation, chemical evolution, and atmospheric dynamics, particularly in light of Uranus’s distinctive magnetosphere, subdued atmospheric activity, and unusual energy balance.

Some questions remain open about the mission preparation and execution. Where on the planet should the probe dive in tothe atmosphere? Which are the most important species to detect and measure? A multidisciplinary effort is needed to address such questions, including both theoretical and observational work. The current state-of-the-art atmospheric models for Uranus, derived from thermochemical equilibrium calculations, are severely degenerate due to massive uncertainties in elemental abundances and thermal profiles. They therefore act as snapshots. On one hand, we are working on quantitatively showing how the modelled atmospheres change when examining the whole space of parameters left by those uncertainties. This comparison will be crucial to determine which species will be the most important ones to measure within the probe’s reach, especially the condensibles which are thought to rule the atmospheric stability and consequently, heat transport. On the other hand, we will provide an observational support for this mission, by exploring the atmospheric composition and dynamics using high-resolution spectroscopy. With current and future cutting-edge spectrographs, we can combine spectrally and spatially resolved observations to probe both the composition and winds velocities across the planetary disk. This will help us find the best potential diving locations for the mass spectrometer, which should not present any local feature that would not be representative of the planet’s atmosphere.

On the technical side, in preparation for this mission opportunity, we are developing a dedicated mass spectrometer system as a potential European contribution to the UOP probe payload, building on Europe’s strong heritage in planetary science instrumentation. Our focus is on the design and prototyping of a laboratory system that integrates a time-of-flight mass spectrometer with a high-performance vacuum and gas handling subsystem. The instrument is being engineered for fully autonomous, reliable operation across the extreme and dynamic pressure regime expected during descent—ranging from deep vacuum (10⁻⁷ mbar) to high-pressure conditions (up to 20 bar). To achieve the mission’s scientific goals, the system includes gas pre-processing components designed to remove dominant species such as hydrogen, thereby enabling more accurate detection of minor species and heavy noble gases. A reference gas calibration unit is also being developed to ensure precise and repeatable isotope ratio measurements, addressing one of the most technically demanding aspects of the mission. In addition, we plan to incorporate a tunable diode laser spectrometer to provide complementary spectroscopic validation of selected gas species, enhancing the overall accuracy and robustness of the measurements. In this presentation, we will outline the key scientific drivers for the mass spectrometer experiment, demonstrate how our modeling and observational work directly informs species selection and dive-site planning, present the current status of the instrument prototyping, and discuss system-level considerations including operational sequences, entry conditions, and potential synergies with other instruments aboard the probe.