Master of Science
Planetary Science and Exploration
Osinski, Gordon R.
Tornabene, Livio L.
Impact craters and their ejecta deposits offer insights into the structure and composition of planet crusts. Hargraves Crater, Mars, demonstrates an unusual balance of exposure and preservation in its ejecta. Analysing its morphologic, morphometric, thermophysical, and stratigraphic characteristics, we interpret two predominant units: an underlying ballistic lithic breccia and an overlying impact melt-bearing deposit. The lower unit is a lithic breccia composed of sub-angular, unsorted clasts (~10-12 m mean diameter), while the overlying unit is a smooth, dark toned, smaller clast-bearing (~1 m diameter at image resolution) impact melt-bearing unit with polygonal fracturing. There appears to be a sharp contact between these units visible through “windows” in the overlying unit that reveal the underlying breccia. These orbital observations link well with terrestrial field observations. Future study of this “Hargraves-type” ejecta, which reveals the stratigraphy and structure of ejecta deposits, will improve our understanding of ejecta and ejecta emplacement processes.
Summary for Lay Audience
Impact craters are windows into the insides of planets. When an asteroid collides with a planet, the resulting pit is called a crater. The debris around the crater from the impact is termed the ejecta blanket. We study ejecta to learn about the inside of the impacted planet as well as the surface of the planet. One of the topics we do not fully understand about ejecta is how it moves from inside the crater to its final position around the outside of the crater. To study this question, this thesis focuses on the ejecta blanket of a crater on Mars, known as Hargraves Crater. Hargraves Crater offers unusual access to the interior of the ejecta. Using close-up images of the ejecta, we study the shapes, sizes, positions, and rate of temperature change of visible rocks. From the results, we determine that the ejecta is primarily two units: a lower unit called a lithic breccia made of broken rock fragments (~10 m average diameter) with no structure thrown from the crater, and an upper unit, an impact melt-bearing rock, made of a combination of rock fragments (~1 m average diameter) and melted rock that flowed out of the crater. These units show that there are at least two parts to how the ejecta blanket forms, one for each unit. We are able to examine how these two units relate to each other due to holes in the top unit that allow us to observe the lower unit. This unusual visibility of the inside of the ejecta at Hargraves Crater facilitated the observation of both units and how they relate, thus we suggest studying other examples of crater ejecta with interior units visible in order to learn more about the ways that ejecta leaves the crater and forms the ejecta blanket. We also suggest that it may be important to try to understand why only a few craters show the visibility we see at Hargraves Crater. By studying these types of craters, we may be better prepared for future space missions and studies of craters and of Mars.
Sacks, Leah E., "Hargraves Crater Ejecta and Implications for Impact Processes" (2020). Electronic Thesis and Dissertation Repository. 7470.
Available for download on Wednesday, August 31, 2022