Impact ejecta emplacement processes and the potential for sampling deep lunar lithologies and cryptomare
One of the most characteristic, but poorly understood, aspects of the impact cratering process is the generation of ejecta deposits. The lack of understanding is due, in part, to the scarcity of ejecta at the majority of the known terrestrial impact structures. Observations of impact ejecta deposits on other planetary bodies provide a complementary data set with which to study the emplacement of impact ejecta. Understanding the mechanism of impact ejecta generation and emplacement is important as these deposits provide a natural method to sample the subsurface of planetary bodies. It is, therefore, critical to understand the depth of origin of ejecta materials from any particular impact site.
Proximal ejecta deposits are rare on Earth due to post-impact erosional processes, but are common on other planetary bodies. It is generally accepted that proximal ejecta deposits on airless bodies, such as the Moon, are emplaced via ballistic sedimentation. In this model, the ballistic emplacement of primary crater-derived ejecta results in secondary cratering and the incorporation of local material (so-called “secondary ejecta”), and considerable modification of the local substrate.
However, an important observation that deserves attention is that one, or more, layers of ejecta may overlie the continuous ejecta blanket around complex impact structures. This is particularly common on Mars, where so-called double and multiple layered ejecta structures are observed. This is also true for several terrestrial craters, including the Chicxulub, Haughton, and Ries impact structures. On the Moon, impact melt deposits overlying the continuous ejecta blanket may also be thought of as ejecta deposits. If the Bunte Breccia and similar deposits represent the initial ballistic ejecta, then what is the origin of the overlying deposits? Recent work at the Haughton and Ries impact structures suggests that this second layer of ejecta, including the melt ponds on the Moon, are emplaced as melt-rich flow deposits during the final (modification) stages of crater formation. Importantly, the material in this ejecta layer is derived from deeper levels and is more highly shocked than the underlying ballistic ejecta, which has implications for estimating the depth of materials sampled in such deposits on the Moon. This suggests that the current depth estimate provided by Croft (1980) of depth of excavation (de) = 0.1 x current rim diameter (Da) may over exaggerate the depth of origin of materials sampled in lunar ejecta deposits.
Research by CLRN members will address this objective by carrying out a systematic survey of lunar ejecta deposits, particularly those around complex impact structures, to look for evidence of melt ponds and other ejecta materials. These observations will be ground-truthed by field and analytical observations of terrestrial impact structures and impactites. Structures currently being studied by various team members include the Haughton (Canada), Mistastin (Canada), Ries (Germany), and Saint Martin (Canada).
An additional important aspect of the “Ejecta” theme is investigating evidence for hidden mare deposits, known as cryptomaria. Specifically, we intend to explore automated methods for cryptomare exploration. Identification and delimitation techniques that are conducive to automation would allow for a systematic global survey of lunar cryptomaria. Our goal is to identify and develop techniques that are accurate and robust enough to be applied globally to the entire Moon, using simple batch or scripting processes which require no additional modifications or human supervision, such as the various Fe formulas that can be used to adequately estimate the iron content of the lunar surface. We intend to work with spectral mixing methods to find similar techniques for estimating cryptomare content reliably. Additionally, non-spectral data may be incorporated using hierarchical mixing methods, to help increase the accuracy and robustness of the techniques. This will further our understanding of the importance of volcanism in the geological history of the Moon and to explore the distribution and nature of volcanic products around the Moon.