Multiple Probe Measurements at Uranus Motivated by Spatial Variability

A major motivation for multiple atmospheric probe measurements at Uranus is the understanding of dynamic processes that create and maintain spatial variation in thermal structure, composition, and horizontal winds. But origin questions-regarding the planet's formation and evolution, and conditions in the protoplanetary disk-are also major science drivers for multiprobe exploration. Spatial variation in thermal structure reveals how the atmosphere transports heat from the interior, and measuring compositional variability in the atmosphere is key to ultimately gaining an understanding of the bulk abundances of several heavy elements. We review the current knowledge of spatial variability in Uranus' atmosphere, and we outline how multiple probe exploration would advance our understanding of this variability. The other giant planets are discussed, both to connect multiprobe exploration of those atmospheres to open questions at Uranus, and to demonstrate how multiprobe exploration of Uranus itself is motivated by lessons learned about the spatial variation at Jupiter, Saturn, and Neptune. We outline the measurements of highest value from miniature secondary probes (which would complement more detailed investigation by a larger flagship probe), and present the path toward overcoming current challenges and uncertainties in areas including mission design, cost, trajectory, instrument maturity, power, and timeline.

Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres

Ammonia (NH₃) in a terrestrial planet atmosphere is generally a good biosignature gas, primarily because terrestrial planets have no significant known abiotic NH₃ source. The conditions required for NH₃ to accumulate in the atmosphere are, however, stringent. NH₃'s high water solubility and high biousability likely prevent NH₃ from accumulating in the atmosphere to detectable levels unless life is a net source of NH₃ and produces enough NH₃ to saturate the surface sinks. Only then can NH₃ accumulate in the atmosphere with a reasonable surface production flux. For the highly favorable planetary scenario of terrestrial planets with hydrogen (H₂)-dominated atmospheres orbiting M dwarf stars (M5V), we find that a minimum of about 5 ppm column-averaged mixing ratio is needed for NH₃ to be detectable with JWST, considering a 10 ppm JWST systematic noise floor. When the surface is saturated with NH₃ (i.e., there are no NH₃-removal reactions on the surface), the required biological surface flux to reach 5 ppm is on the order of 10¹⁰ molecules/(cm²·s), comparable with the terrestrial biological production of methane (CH₄). However, when the surface is unsaturated with NH₃, due to additional sinks present on the surface, life would have to produce NH₃ at surface flux levels on the order of 10¹⁵ molecules/(cm²·s) (∼4.5 × 10⁶ Tg/year). This value is roughly 20,000 times greater than the biological production of NH₃ on the Earth and about 10,000 times greater than Earth's CH₄ biological production. Volatile amines have similar solubilities and reactivities to NH₃ and hence share NH₃'s weaknesses and strengths as a biosignature. Finally, to establish NH₃ as a biosignature gas, we must rule out mini-Neptunes with deep atmospheres, where temperatures and pressures are high enough for NH₃'s atmospheric production.

Magma Ocean Evolution of the TRAPPIST-1 Planets

Recent observations of the potentially habitable planets TRAPPIST-1 e, f, and g suggest that they possess large water mass fractions of possibly several tens of weight percent of water, even though the host star's activity should drive rapid atmospheric escape. These processes can photolyze water, generating free oxygen and possibly desiccating the planet. After the planets formed, their mantles were likely completely molten with volatiles dissolving and exsolving from the melt. To understand these planets and prepare for future observations, the magma ocean phase of these worlds must be understood. To simulate these planets, we have combined existing models of stellar evolution, atmospheric escape, tidal heating, radiogenic heating, magma-ocean cooling, planetary radiation, and water-oxygen-iron geochemistry. We present MagmOc, a versatile magma-ocean evolution model, validated against the rocky super-Earth GJ 1132b and early Earth. We simulate the coupled magma-ocean atmospheric evolution of TRAPPIST-1 e, f, and g for a range of tidal and radiogenic heating rates, as well as initial water contents between 1 and 100 Earth oceans. We also reanalyze the structures of these planets and find they have water mass fractions of 0-0.23, 0.01-0.21, and 0.11-0.24 for planets e, f, and g, respectively. Our model does not make a strong prediction about the water and oxygen content of the atmosphere of TRAPPIST-1 e at the time of mantle solidification. In contrast, the model predicts that TRAPPIST-1 f and g would have a thick steam atmosphere with a small amount of oxygen at that stage. For all planets that we investigated, we find that only 3-5% of the initial water will be locked in the mantle after the magma ocean solidified.

Atmospheric characterization of terrestrial exoplanets in the mid-infrared: biosignatures, habitability, and diversity

Exoplanet science is one of the most thriving fields of modern astrophysics. A major goal is the atmospheric characterization of dozens of small, terrestrial exoplanets in order to search for signatures in their atmospheres that indicate biological activity, assess their ability to provide conditions for life as we know it, and investigate their expected atmospheric diversity. None of the currently adopted projects or missions, from ground or in space, can address these goals. In this White Paper, submitted to ESA in response to the Voyage 2050 Call, we argue that a large space-based mission designed to detect and investigate thermal emission spectra of terrestrial exoplanets in the mid-infrared wavelength range provides unique scientific potential to address these goals and surpasses the capabilities of other approaches. While NASA might be focusing on large missions that aim to detect terrestrial planets in reflected light, ESA has the opportunity to take leadership and spearhead the development of a large mid-infrared exoplanet mission within the scope of the "Voyage 2050" long-term plan establishing Europe at the forefront of exoplanet science for decades to come. Given the ambitious science goals of such a mission, additional international partners might be interested in participating and contributing to a roadmap that, in the long run, leads to a successful implementation. A new, dedicated development program funded by ESA to help reduce development and implementation cost and further push some of the required key technologies would be a first important step in this direction. Ultimately, a large mid-infrared exoplanet imaging mission will be needed to help answer one of humankind's most fundamental questions: "How unique is our Earth?"

Modeling Repeated M Dwarf Flaring at an Earth-like Planet in the Habitable Zone: Atmospheric Effects for an Unmagnetized Planet

Understanding the impact of active M dwarf stars on the atmospheric equilibrium and surface conditions of a habitable zone Earth-like planet is key to assessing M dwarf planet habitability. Previous modeling of the impact of electromagnetic (EM) radiation and protons from a single large flare on an Earth-like atmosphere indicated that significant and long-term reductions in ozone were possible, but the atmosphere recovered. However, these stars more realistically exhibit frequent flaring with a distribution of different total energies and cadences. Here, we use a coupled 1D photochemical and radiative-convective model to investigate the effects of repeated flaring on the photochemistry and surface UV of an Earth-like planet unprotected by an intrinsic magnetic field. As input, we use time-resolved flare spectra obtained for the dM3 star AD Leonis, combined with flare occurrence frequencies and total energies (typically 10^30.5 to 10³⁴ erg) from the 4-year Kepler light curve for the dM4 flare star GJ1243, with varied proton event impact frequency. Our model results show that repeated EM-only flares have little effect on the ozone column depth but that multiple proton events can rapidly destroy the ozone column. Combining the realistic flare and proton event frequencies with nominal CME/SEP geometries, we find the ozone column for an Earth-like planet can be depleted by 94% in 10 years, with a downward trend that makes recovery unlikely and suggests further destruction. For more extreme stellar inputs, O₃ depletion allows a constant ∼0.1-1 W m^-2 of UVC at the planet's surface, which is likely detrimental to organic complexity. Our results suggest that active M dwarf hosts may comprehensively destroy ozone shields and subject the surface of magnetically unprotected Earth-like planets to long-term radiation that can damage complex organic structures. However, this does not preclude habitability, as a safe haven for life could still exist below an ocean surface.

The search for signs of life on exoplanets at the interface of chemistry and planetary science

The discovery of thousands of exoplanets in the last two decades that are so different from planets in our own solar system challenges many areas of traditional planetary science. However, ideas for how to detect signs of life in this mélange of planetary possibilities have lagged, and only in the last few years has modeling how signs of life might appear on genuinely alien worlds begun in earnest. Recent results have shown that the exciting frontier for biosignature gas ideas is not in the study of biology itself, which is inevitably rooted in Earth's geochemical and evolutionary specifics, but in the interface of chemistry and planetary physics.