EARTH EXPLORER 11 CANDIDATE MISSION SEASTAR REPORT FOR ASSESSMENT

Seastar is a new Earth Explorer mission concept dedicated to observing fast-evolving small-scale ocean surface dynamics in all coastal seas, shelf seas and marginal ice zones. Its science goals are: 1/ to understand the role of fast-evolving small-scale ocean dynamics in mediating exchanges between land, the cryosphere, the atmosphere, the marine biosphere and the deep ocean; 2/ to determine the ocean circulation and dominant transport pathways in the global coastal, shelf and marginal ice zones; and, 3/ to improve understanding of coastal, shelf and marginal ice zones contributions to the global climate system. Through its goals, Seastar contributes to the ESA Living Planet Ocean Challenge O1, O2, O3 and O4, together with Land Surface Challenge L2 and Cryosphere Challenge C1. The ocean forms a central part of the climate system, with the capacity to store, transport and exchange vast quantities of carbon, heat, water, nutrients and other dissolved, suspended or floating matter. At its margins – in coastal, shelf and marginal ice zones – the ocean interacts with land, the cryosphere, the atmosphere and the marine biosphere, making these regions key interfaces of the Earth System. Within ocean margins, high-resolution satellite images reveal an abundance of ocean fronts, swirls, vortices and filaments at horizontal scales below 10 km that decorrelate rapidly within a few days. These fast-evolving small-scale features are the fingerprints of dynamic interactions and exchanges between the ocean, land, the cryosphere, the atmosphere, the marine biosphere and the deep ocean. Research shows that small-scale processes have far-ranging impacts on lateral dispersion, vertical stratification, ocean carbon cycling, and marine productivity, and play a key role in exchanges across these interfaces of the Earth System and in regulating climate. Small-scale ocean processes play a dominant role in coastal seas, where dynamics are more intense and spatially variable due to the influence of coastlines and bathymetry. Interactions with the sea floor, underwater topography and coastlines produce complex current patterns that transform and transport sediments, nutrients, carbon, water and pollutants between coastal seas and the open ocean. Ocean surface winds exhibit greater heterogeneity in coastal seas, with localised effects near land linked to orographic steering, funnelling or temperature, elevation or roughness differences. Strong current-windwave interactions further modify air-sea fluxes, and cause marine hazards like coastal erosion, rip currents and dangerous sea states. Seastar would deliver frequent high-resolution data to advance scientific understanding, improve predictive skills and support sustainable developments (e.g., wind farms), effective coastal management and conservation strategies. Shelf seas connect coastal seas and the open ocean through multiple physical, chemical, and biological processes, prompting some of the most productive and diverse ecosystems on Earth. Submesoscales act as conduits between the turbulent air-sea boundary layer and the deep ocean, affecting ocean heat and carbon uptakes and replenishment with nutrient-rich deep waters that sustain the ocean food chain. At the sea ice edge, atmosphere-ice-wave-sea interactions at scales below 10 km affect mixing, heat balance, momentum transfer and the dynamics of sea ice growth and decay. Small-scale processes also govern the delivery of heat to Antarctic ice shelves, with potentially critical implications for sea level rise globally. The scales, proximity of ice, remoteness and harsh conditions in these regions limit the availability of observations. New high-quality high-resolution current, wind and wave images are needed to better understand these processes, improve their representation in models, increase the reliability of forecasts and reduce uncertainties in projections about the ocean margins response to climate change. The fast-evolving nature, small scales and spatial intricacy of these phenomena present formidable challenges for oceanographic observations, resulting in a striking scarcity of data at relevant temporal and spatial scales, even in coastal seas. The observational challenges of measuring fast-evolving small-scale dynamics at the boundaries of the global ocean means our knowledge of how the ocean interacts with other parts of the climate system is incomplete.