ALMA2040: Submillimeter-wave Limb Sounding of Venus and Mars

Venus and Mars are unique laboratories for understanding the evolution and diversity of terrestrial planets. Despite their broadly similar rocky compositions, their atmospheres, climates, and surface environments have evolved into dramatically different states. Understanding the physical and chemical processes responsible for these divergent pathways is essential for interpreting terrestrial exoplanets and their habitability. By the 2040s, spacecraft missions will provide unprecedented close-up observations of Venus and Mars, while ground-based facilities can provide the long-term monitoring needed to place these snapshots in the context of atmospheric variability and planetary-scale circulation. ALMA2040 can uniquely bridge these two approaches by combining high-resolution submillimeter heterodyne spectroscopy with routinely accessible, extremely high angular resolution. In particular, high-resolution limb sounding would enable vertically resolved measurements of atmospheric winds, temperature, and trace species from the ground. An angular resolution of approximately 0.01" would allow spatial scales comparable to the atmospheric scale heights of Venus (around 5 km) and Mars (around 11 km) to be resolved. This capability would enable three-dimensional studies of Venusian upper-atmospheric circulation and Martian global dust storms, as well as systematic investigations of CO2 photochemistry, water loss through HDO/H2O fractionation, and Venusian sulfur chemistry and cloud formation. By repeatedly observing different local times, seasons, and solar-activity conditions, ALMA2040 would extend spacecraft measurements from snapshots to long-term four-dimensional characterization. It would therefore provide a unique observational framework for directly connecting spacecraft discoveries with the temporal and spatial evolution of planetary atmospheres.