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The evolution of a single raindrop falling below a cloud is governed by fluid dynamics and thermodynamics fundamentally transferable to planetary atmospheres beyond modern Earth's. Here, we show how three properties that characterize falling raindrops—raindrop shape, terminal velocity, and evaporation rate—can be calculated as a function of raindrop size in any planetary atmosphere. We demonstrate that these simple, interrelated characteristics tightly bound the possible size range of raindrops in a given atmosphere, independently of poorly understood growth mechanisms. Starting from the equations governing raindrop falling and evaporation, we demonstrate that raindrop ability to vertically transport latent heat and condensible mass can be well captured by a new dimensionless number. Our results have implications for precipitation efficiency, convective storm dynamics, and rainfall rates, which are properties of interest for understanding planetary radiative balance and (in the case of terrestrial planets) rainfall‐driven surface erosion.

 

Context. Terrestrial exoplanets in the habitable zone are likely a common occurrence. The long-term goal is to characterize the atmospheres of dozens of such objects. The Large Interferometer For Exoplanets (LIFE) initiative aims to develop a space-based mid-infrared (MIR) nulling interferometer to measure the thermal emission spectra of such exoplanets. Aims. We investigate how well LIFE could characterize a cloudy Venus-twin exoplanet. This allows us to: (1) test our atmospheric retrieval routine on a realistic non-Earth-like MIR emission spectrum of a known planet, (2) investigate how clouds impact retrievals, and (3) further refine the LIFE requirements derived in previous Earth-centered studies. Methods. We ran Bayesian atmospheric retrievals for simulated LIFE observations of a Venus-twin exoplanet orbiting a Sun-like star located 10 pc from the observer. The LIFE SIM noise model accounted for all major astrophysical noise sources. We ran retrievals using different models (cloudy and cloud-free) and analyzed the performance as a function of the quality of the LIFE observation. This allowed us to determine how well the atmosphere and clouds are characterizable depending on the quality of the spectrum. Results. At the current minimal resolution ( R = 50) and signal-to-noise ( S / N = 10 at 11.2 μ m) requirements for LIFE, all tested models suggest a CO 2 -rich atmosphere (≥30% in mass fraction). Further, we successfully constrain the atmospheric pressure-temperature ( P–T ) structure above the cloud deck ( P–T uncertainty ≤ ± 15 K). However, we struggle to infer the main cloud properties. Further, the retrieved planetary radius ( R pl ), equilibrium temperature ( T eq ), and Bond albedo ( A B ) depend on the model. Generally, a cloud-free model performs best at the current minimal quality and accurately estimates R pl , T eq , and A B . If we consider higher quality spectra (especially S / N = 20), we can infer the presence of clouds and pose first constraints on their structure. Conclusions. Our study shows that the minimal R and S/N requirements for LIFE suffice to characterize the structure and composition of a Venus-like atmosphere above the cloud deck if an adequate model is chosen. Crucially, the cloud-free model is preferred by the retrieval for low spectral qualities. We thus find no direct evidence for clouds at the minimal R and S / N requirements and cannot infer the thickness of the atmosphere. Clouds are only constrainable in MIR retrievals of spectra with S / N ≥ 20. The model dependence of our retrieval results emphasizes the importance of developing a community-wide best-practice for atmospheric retrieval studies. 

Rocky planets are common around other stars, but their atmospheric properties remain largely unconstrained. Thanks to a wealth of recent planet discoveries and upcoming advances in observing capability, we are poised to characterize the atmospheres of dozens of rocky exoplanets in this decade. The theoretical understanding of rocky exoplanet atmospheres has advanced considerably in the last few years, yielding testable predictions of their evolution, chemistry, dynamics, and even possible biosignatures. We review key progress in this field to date and discuss future objectives. Our major conclusions are as follows: ▪ Many rocky planets may form with initial H 2 –He envelopes that are later lost to space, likely due to a combination of stellar UV/X-ray irradiation and internal heating. ▪ After the early stages of evolution, a wide diversity of atmospheric compositions is expected as a result of variations in host star flux, atmospheric escape rates, interior exchange, and other factors. ▪ Observations have ruled out both the presence of H 2 -dominated atmospheres on several nearby rocky exoplanets and the presence of any thick atmosphere on one target. A more detailed atmospheric characterization of these planets and others will become possible in the near future. ▪ Exoplanet biosphere searches are an exciting future goal. However, reliable detections for a representative sample of planets will require further advances in observing capability and improvements in our understanding of abiotic planetary processes. 

Abstract One of the most interesting questions about the climate and hydrology of early Mars is whether oceans existed and, if so, when. Various geologic features have been interpreted as ancient shorelines, but these features do not follow gravitational equipotentials. Prior work has shown that the elevation of the Arabia level, hypothesized to represent a large, early ocean, better conforms to an equipotential when correcting for global topographic change after its formation. Although the shoreline coordinates underlying these studies are debated, exploring the consequences of these topographic corrections allows additional observable consequences to be identified. Here we show that the topographic corrections cause Jezero crater, the landing site of the Perseverance rover, to be submerged under the proposed Arabia ocean. This precludes the ocean’s existence during known fluvio-lacustrine activity at Jezero and suggests the ocean did not exist during the main era of valley network formation in the Noachian/Early Hesperian. We identify a period of ∼10 8 yr years before fluvial activity at Jezero when the ocean could have existed and discuss potential observable consequences.