Rounded and Subrounded Boulders on Asteroid 433 Eros
Abstract
Impact cratering is well known to be one source of many surface features on small bodies. While impacts cause craters, they also produce ejecta which may remain on the surface in the form of various sized rootless positive relief features, or "boulders", which are then potentially shaped by weathering. Different from Earth, weathering on airless bodies is thought to be caused by, meteorite and micrometeorite impacts, interactions with cosmic rays and solar-wind particles, and thermal cycling. Boulders on airless bodies are generally thought to be eroded by thermal cycling and micrometeorite impacts. On comets and asteroids with semi-major orbit axes close to 1 AU, all of these mechanisms may have significant effects on the rounding of boulders. On more distant asteroids, it is not clear that thermal cycling is a large enough effect to be a dominant cause of boulder rounding. On Asteroid Eros there is a lack of boulders at larger scales and a low amount of rounded boulders overall. Other Near Earth Asteroids such as Itokawa have rounded and subrounded boulders. While most of the rounding on Itokawa is explained by its formation mechanism, there are still processes that can cause boulders to lose their sharpness over the lifetime of the body. These mechanisms should in theory affect Eros and lead to rounded boulders on its surface. An unanswered question is why Eros's boulders have not been significantly rounded. The terrestrial sedimentology community's definition of roundness is a ratio of the average radius of curvature of the edges or corners to the radius of curvature of the maximum inscribed sphere. Marshall and Rizk 2015 defined rounding as a ratio between the weathered edge and the diameter of the boulder. By using the different rounding criteria such as these, we can compare the effectiveness of different methodologies at determining the true roundness of boulders on these small bodies, and characterize how round the boulders on Eros are. Finding the amount of rounding can also determine if there are sub-rounded boulders, which can mean that erosion of these boulders through some mechanism either occurred in the past or are slowly occurring over time.
- Publication:
-
AAS/Division for Planetary Sciences Meeting Abstracts #50
- Pub Date:
- October 2018
- Bibcode:
- 2018DPS....5031206R