Abstract
AbstractDawn's framing camera observed boulders on the surface of Vesta when the spacecraftwas in its lowest orbit (Low Altitude Mapping Orbit, LAMO). We identified, measured, and mapped boulders in LAMO images, which have a scale of 20 m per pixel. We estimate that our sample is virtually complete down to a boulder size of 4 pixels (80 m). The largest boulder is a 400 m‐sized block on the Marcia crater floor. Relatively few boulders reside in a large area of relatively low albedo, surmised to be the carbon‐rich ejecta of the Veneneia basin, either because boulders form less easily here or live shorter. By comparing the density of boulders around craters with a known age, we find that the maximum boulder lifetime is about 300 Ma. The boulder size‐frequency distribution (SFD) is generally assumed to follow a power law. We fit power laws to the Vesta SFD by means of the maximum likelihood method, but they do not fit well. Our analysis of power law exponents for boulders on other small Solar System bodies suggests that the derived exponent is primarily a function of boulder size range. The Weibull distribution mimics this behavior and fits the Vesta boulder SFD well. The Weibull distribution is often encountered in rock grinding experiments and may result from the fractal nature of cracks propagating in the rock interior. We propose that, in general, the SFD of particles (including boulders) on the surface of small bodies follows a Weibull distribution rather than a power law.
Highlights
IntroductionBoulders may be created by spallation during large impacts and are typically found in and around fresh craters
Boulders on small Solar System bodies provide a window into the interior
Using the Clauset et al (2009) test with dmin = 80 m, we found that the power law fails to fit the size-frequency distribution (SFD) of all four craters with more than 100 boulders larger than 4 pixels (Antonia, Licinia, Marcia, and Pinaria). 3.4.2
Summary
Boulders may be created by spallation during large impacts and are typically found in and around fresh craters. They do not survive forever but are gradually eroded into dust by exposure to the space environment (Basilevsky et al, 2015; Delbo et al, 2014). The surface of near-Earth asteroids is densely populated by boulders of all sizes, all thought to originate from the destruction of a parent body (DellaGiustina et al, 2019; Mazrouei et al, 2014; Michikami et al, 2019). The exponents of asteroid boulder SFDs are generally found to be close to this value. What clues does a particular value of the exponent provide to the composition and physical properties of the surface?
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