SPLASH - The Origin of Water in the Inner Solar System

• Applicant: Dr. Max Mahlke
• Subject Area: Astrophysics and Astronomy
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 575226829

The SPLASH (Sources of Planetesimals for Significant Hydration) project seeks to answer a fundamental question: How did Earth and the inner Solar System get their water? While the early Solar System was too hot for water to form near Earth, our planet is covered in oceans. The leading theory is that water was delivered by icy, volatile-rich asteroids originating from the colder, outer regions of the Solar System. However, identifying these water-carriers and tracing their journey has been impossible due to a lack of observational data. SPLASH will solve this by leveraging unprecedented data from two space missions: NASA's recently launched SPHEREx and ESA's Gaia. SPHEREx will provide the first-ever all-sky spectral survey in the near-infrared, a wavelength range crucial for detecting water and hydrated minerals on asteroid surfaces. This project will harness this transformative dataset to: 1. Create a new, definitive asteroid taxonomy. By combining SPHEREx, Gaia, and albedo data, SPLASH will produce the most detailed and comprehensive compositional map of the asteroid belt ever created, with a specific focus on identifying hydration-bearing asteroids. 2. Forge links between volatile-rich bodies in the inner and outer Solar System. Using water-related spectral features as a common thread, the project will identify which asteroid classes in the Main Belt have genetic relatives in the outer Solar System, confirming their origin and status as implanted bodies. 3. Model the delivery of water. The new compositional and orbital map will provide strong, concrete constraints for numerical simulations. By testing competing models of planetary formation and migration, SPLASH will determine the specific mechanisms and timeline for how and when these water-rich planetesimals were implanted into the inner Solar System. This project is perfectly timed to capitalise on a new era of asteroid spectroscopy thanks to the SPHEREx mission. With direct involvement in the SPHEREx science team, the project will unite observations with dynamical theory, aiming to deliver a conclusive, testable narrative for the origin of water on Earth.

• DFG Programme: Research Grants