Post contributed by Dr. Ryan C. Ewing, Department of Geology and Geophysics, Texas A&M University
Ripples cover the surfaces of sand dunes on Earth and Mars. On Earth, ripples formed in typical aeolian sand (e.g., 0.1 and 0.3 mm) range in wavelength between 10 and 15 cm and display a highly straight, two-dimensional crestline geometry. Ripples are thought to develop through a process dominated by the ballistic impacts of saltating sand grains in which wavelength selection occurs through the interplay of grain size, wind speed, the saltation trajectories of the sand grains, and ripple topography.
Figure 1: Wind-blown impact ripples from Mesquite Flat Sand Dunes, Death Valley, USA. Pen is ~15 cm. Inferred transport direction is to the right on the image. Image credit: Ryan C. Ewing
Posted by suja82 on July 30, 2016
The discovery of dunes on Mars:
Mariner 9, launched on May 30, 1971 conducted an intensive orbital reconnaissance of the red planet between November 14, 1971 and October 22, 1972. One of the astonishing discoveries of Mariner 9 was vast dune fields all over the Red Planet. Viking discovered many more dune fields in the late 1970s. However, the resolution of the images taken by Mariner 9 and Viking 1 and 2 was very poor and one dominant dune type of barchan and transverse dunes was chiefly discerned by this low resolution (Tsoar et al, 1979). The Mars Global Surveyor (MGS) was the next successful mission to Mars, launched 20 years after Viking, in November 1996, and operated for 10 years. The Mars Orbiter Camera (MOC) on board MGS acquired high resolution images of the sand dunes on Mars and revealed some other dune types that were not known before. The latest mission to Mars, the Mars Reconnaissance Orbiter not only reveals the variety of dunes but its high resolution camera (HiRISE) allows us to see the smaller ripples on the dunes.
Image 1: Barchan and linear dunes west of Hellas Planitia near 41.8°S, 315.5°W, formed on the floor of a crater and extending from a mesa. Note the breakdown of the rectilinear dune into barchans with distance from the flow obstruction. HiRISE Image PSP_007676_1385.
Posted by megafloods2013 on September 11, 2013
Posted by Dr. Jani Radebaugh, Department of Geological Sciences, Brigham Young University, Utah, USA
(Re-posted from IAG Image of the Month, September, 2007)
The Cassini spacecraft is in orbit around Saturn, and occasionally flies close to one of its many icy moons. Because of specially designed instruments on Cassini, the surface of Saturn’s largest moon, Titan, enshrouded in a thick, hydrocarbon haze-rich atmosphere, has been observed for the first time by this spacecraft.
Cassini RADAR SAR image is north up, with resolution ~300 m. RADAR illumination direction and inclination angle is indicated by the open arrow. Image courtesy of the NASA Cassini Project.
Posted by megafloods2013 on July 19, 2013