New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 11

Earth made possible by exploring other planetary objects. Collaboration between NASA and
NSF in supporting such projects can only be positive.
Figure B2.1 Ice geysers erupt on Enceladus, the bright and shiny inner moon of Saturn. This
image presents a backlit view of the moon's southern limb, where icy plumes were
discovered by the NASA Cassini spacecraft mission in November 2005. Cryovolcanism is
evidence that the 500-km-diameter Enceladus has active internal tectonics. SOURCE: NASA
Jet Propulsion Laboratory/Space Science Institute.
Accretion of Earth
The birthplace of Earth was a protoplanetary accretion disk, a cloud of gas and
dust surrounding the early Sun. Modern astronomy provides a glimpse of what this
environment may have been like, in the form of debris disks that surround young
stars, some of which have been imaged by the Hubble Space Telescope (see Figure
2.1). Accretion disks are subject to instabilities driven by powerful gravitational and
electromagnetic forces that collect dust particles into planetesimals, typically 1-
kilometer-sized objects that were the fundamental building blocks of Earth and the
other terrestrial planets. Once a sufficient density of planetesimals developed in the
nebular cloud, increasingly violent collisions began to dominate the accretion process,
forming an ever-smaller number of growing planetary embryos that swept up most of
the remaining nebular debris.