New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 12

Figure 2.1 Hubble Space Telescope (HST) images showing young stars surrounded
by dust rings thought to be the birthplaces of planets like Earth. Top: The planet
Fomalhaut b orbits the star Fomalhaut (25 light-years away in the southern
constellation Piscis Australis) near a ring of dust similar to the Kuiper Belt that may
contain bodies ranging from dust grains to objects the size of dwarf planets. Bottom:
Light reflected off a debris disk in cross section around the young star AU
Microscopii, HD197481. SOURCES: Top: NASA, the European Space Agency
(ESA), and Z. Levay. Bottom: NASA, ESA, and J. Graham.
Although much effort has been directed toward understanding accretion from
the perspective of solar system dynamics, many related processes that were important
for early Earth have not received the same attention. Accretion models, for example,
often assume that colliding planetesimals simply adhere, ignoring effects like
fragmentation, spin and precession, melting, vaporization, condensation, and
differentiation (Chambers, 2004). There is mounting evidence for these processes,
many of which bear directly on the final chemistry and structure of the accreting body
(Halliday, 2004).