Physics – Geophysics
Scientific paper
Sep 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.0503a&link_type=abstract
American Astronomical Society, DPS meeting #41, #5.03
Physics
Geophysics
Scientific paper
During terrestrial planet formation most of the colliding matter comes late, in the form of similar-sized planetary bodies colliding at velocities ranging from 1 to a few times their mutual escape velocity (e.g. Wetherill 1985). I consider an edge effect under these conditions, where the next-largest bodies in a hierarchically accreting population (e.g. Earths) grow increasingly exotic as they survive non-accretionary collisions onto the largest bodies (e.g. super-Earths). Collisions between bodies within a factor of several in size, at around v_esc, are extended-source phenomena where the contact timescale equals the gravity timescale, and where for most geometries most of the colliding matter does not intersect. This sets similar-sized collisions, of which the giant impact formation of the Moon may be an example, far apart from point-source cratering impacts. It has been found and confirmed that in similar-sized collisions, hit-and-run is a more common outcome than efficient accretion. If the largest terrestrial planets grow by feeding on the next-largest planets, and if they eventually accrete most of these next-largest planets, then the surviving (unaccreted) next-largest planets are each likely to be survivors of one or more hit-and-run collisions. These events can cause the loss of atmospheres, oceans, crusts and outer mantles, and lead to exotic pressure-release petrology and degassing. This conclusion appears to be relatively scale invariant, provided the random velocities scale to v_esc, supporting the following corollary for solar systems with Super-Earths: If Earth-massed planets roamed among super-Earths, and survive as bounced-off unaccreted remnants from late stage accretion, then Earth-mass worlds will be as stunningly diverse in these solar systems, as the asteroids and smallest planets are (Moon, Mercury, Mars) in our own system, and may be devoid of atmospheres and oceans.
Supported by the NASA Planetary Geology and Geophysics Program and the NASA Origin of the Solar System Program
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