14 research outputs found
Forming Planetesimals in Solar and Extrasolar Nebulae
Planets are built from planetesimals: solids larger than a kilometer which
grow by colliding pairwise. Planetesimals themselves are unlikely to form by
two-body collisions; sub-km objects have gravitational fields individually too
weak, and electrostatic attraction is too feeble for growth beyond a few cm. We
review the possibility that planetesimals form when self-gravity brings
together vast ensembles of small particles. Even when self-gravity is weak,
aerodynamic processes can accumulate solids relative to gas, paving the way for
gravitational collapse. Particles pile up as they drift radially inward. Gas
turbulence stirs particles, but can also seed collapse by clumping them. While
the feedback of solids on gas triggers vertical shear instabilities that
obstruct self-gravity, this same feedback triggers streaming instabilities that
strongly concentrate particles. Numerical simulations find that solids 10-100
cm in size gravitationally collapse in turbulent disks. We outline areas for
progress, including the possibility that still smaller objects self-gravitate.Comment: To appear in Annual Reviews. This review is intended to be both
current and pedagogical. Incorporates suggestions from the community; further
comments welcome. v2: Single-space
Automatic recognition of impact craters on the martian surface from DEM and images
Impact craters are the most outstanding and attractive geomorphological features on the surface of the planets, showing variety and complexity of the surface morphology. The accurate recognition of impact craters on Mars is very useful to analyze and understand the relative dating of Martian surface. In this chapter, four crater-detection methods have been presented and discussed with various extent of discrimination ability on Martian images or topography data. Themodified ad boosting approach demonstrates the best performance in classification of craters, while the algorithms based on topography data have low efficiency in automatic detection. Comparing to previous solutions, the modified ad boosting method has greatly improved the detecting performance of the algorithm and reduced detection time. © Springer-Verlag Berlin Heidelberg 2015