The Chaotic Terrains of Mercury: Unveiling the History of Planetary Volatile Retention and Loss in the Innermost Solar System

1 Overview
Remote-sensing compositional characterizations of Mercury’s surface reveal globally distributed volatile-bearing surface materials.
In addition, conspicuous sublimation pits (hollows) show that volatile compositions locally extended tens of meters into the subsurface (https://www.science.org/doi/10.1126/science.1211681). A fundamental, still unanswered question is whether the volatile-bearing materials might characterize much deeper sections of the planet’s upper crustal stratigraphy. An investigation of Mercury’s most extensive chaotic terrain (https://www.nature.com/articles/s41598-020-59885-5) evokes the presence of volatile-rich layers reaching several hundred meters to a few kilometers thick and attributes the region’s origin to gradual devolatilization. We propose the first global investigation of Mercury’s chaotic terrains. We will document their complete geographic distribution, geomorphologic diversity, volume losses due to their development, chemical composition, and ages. These results will provide new constraints on how the volatile-bearing stratigraphy is distributed across the planet. For the first time, we will produce numerical simulations of chaotic terrain formation to understand the physics and geochemical conditions of landscape deconstruction associated with their origin.

2 Methodology
We will produce detailed geomorphologic maps of Mercury’s chaotic terrains and date each occurrence using crater count statistics. We will thermochemically determine volatile compounds likely to be significant parts of Mercury’s surface and upper crustal stratigraphy. Finally, we will numerically simulate chaotic terrain formation due to upper crustal and surface devolatilization. Our numerical approaches will be tightly connected and constrained by our mapping results. We will also use high-resolution digital elevation models and photometric characterizations as boundary conditions.

3 Significance
Understanding the global distribution and origin of chaotic terrains on Mercury is relevant to various fundamental aspects of the planet’s history: For example, to (1) existing models of upper crustal aggregation, (2) the upper crustal long-term thermal stability relative to changes in geothermal and solar heating, and (3) understanding possible sources of the exosphere gases.

4 Relevance to DDAP
Our project uses and focuses on MESSENGER datasets to investigate Mercury’s chaotic terrains and carefully assess the history of collapse over a global volatile-rich crust. Hence, it is compliant with the purpose of DDAP to enhance the scientific return from the Discovery missions, broaden scientific participation in the data analysis, and provide higher-level, mission-relevant data products. We will archive at the PDS the geomorphologic maps produced during this investigation for future studies.