by neufer » Mon Nov 22, 2010 10:33 pm
tekic545 wrote:
Ah, I hadn't realized we are looking at sand-ripple scale.
I'm not entirely sure that we are..., however:
http://en.wikipedia.org/wiki/Ripple_marks wrote:
* Normal ripples or impact ripples occur in the lower part of the lower flow regime sands with grain sizes between 0.3-2.5 mm and normal ripples form wavelengths of 7-14 cm Normal ripples have straight or slightly sinuous crests approximately transverse to the direction of the wind.
* Megaripples occur in the upper part of the lower flow regime where sand with bimodal particle size distribution forms unusually long wavelengths of 1-25 m where the wind is not strong enough to move the larger particles but strong enough to move the smaller grains by saltation.
* Fluid drag ripples or aerodynamic ripples are formed with fine, well-sorted grain particles accompanied by high velocity winds which result in long, flat ripples. The flat ripples are formed by long saltation paths taken by grains in suspension and grains on the ground surface.
http://www.marsdaily.com/reports/Sand_Ripples_Taller_On_Mars.html wrote:
Sand Ripples Taller On Mars
<<Mars is kind of like Texas: things are just bigger there. In addition to the biggest canyon and biggest volcano in the solar system, Mars has now been found to have sand ripples twice as tall as they would be on Earth. Initial measurements of some of the Red Planet's dunes and ripples using stereo-images from the Mars Orbiter Camera onboard the Mars Global Surveyor have revealed ripple features reaching almost 20 feet high and dunes towering at 300 feet.
One way to imagine the taller dimension of ripples on Mars is to visualize sand ripples on Earth, then stretch out the vertical dimension to double height, without changing the horizontal dimension. "They do seem higher in relation to ripples on Earth," said Kevin Williams of the Smithsonian National Air and Space Museum. Williams will be presenting this latest insight into the otherworldly scale of Marscapes on Monday, Nov. 3 at the annual meeting of the Geological Society of America in Seattle, WA.
Ripples are common on Mars and usually found in low-lying areas and inside craters, says Williams. On Earth they tend to form in long parallel lines from sand grains being pushed by water or air at right angles to the ripple lines. Dunes, on the other hand, are formed when grains of sand actually get airborne and "saltate" (a word based on the Latin verb "to jump"). That leads to cusp-shaped, star-shaped, and other dune arrangements that allow materials to pile sand much higher.
How exactly Martian dunes and ripples form is still unknown, says Williams, since the images from space give us no clues to the grain sizes or whether they are migrating or moving in any way. Though there are Viking spacecraft images from almost 30 years ago to compare with, the images do not have the resolution to confirm whether ripples have moved much in that time. For now, the dimensions of ripple-forms on Mars are the only indications of whether they are large ripples or small dunes. Williams' results came about from the advantageous combination of image parameters to get the first height measurements of these ripple-like features at the limit of image resolution.
According to Williams, it's likely the doubled heights of Mars ripples relative to their spacing is made possible by the same thing that makes Mars' volcanoes
so tall: lower gravity. With about one-third the gravity of Earth, sand, silt, and dust can theoretically stack up higher before gravity causes a slope failure. However, other differences could play roles in making these large piles of sand as well. "It could also be from different wind speeds, air densities or other factors," said Williams. Mars has a perennially subfreezing, very thin atmosphere in which global dust storms have been known to obscure the surface from view.
The study of Mars dunes and ripples has been underway since Viking spacecraft images of Mars first revealed such features in the late 1970s and early 1980s, says Williams. The primary difficulty of the work continues to be in discerning the close-up details, like the exact heights of features and grain sizes. As with dunes and ripples on Earth, these wind-blown features could reveal a lot about local and regional weather and wind currents if more was known about ripple and dune building under the very un-Earthlike conditions of Mars.>>
tekic545 wrote:
But still, we appear to have very dark sand on top of light (rippled) sand.
Is there a material difference, and if so, what process keeps the light and dark materials separate?
[quote="tekic545"]
Ah, I hadn't realized we are looking at sand-ripple scale.[/quote]
I'm not entirely sure that we are..., however:
[quote=" http://en.wikipedia.org/wiki/Ripple_marks"]
* Normal ripples or impact ripples occur in the lower part of the lower flow regime sands with grain sizes between 0.3-2.5 mm and normal ripples form wavelengths of 7-14 cm Normal ripples have straight or slightly sinuous crests approximately transverse to the direction of the wind.
[color=#0000FF]* Megaripples occur in the upper part of the lower flow regime where sand with bimodal particle size distribution forms unusually long wavelengths of 1-25 m where the wind is not strong enough to move the larger particles but strong enough to move the smaller grains by saltation.
* Fluid drag ripples or aerodynamic ripples are formed with fine, well-sorted grain particles accompanied by high velocity winds which result in long, flat ripples. The flat ripples are formed by long saltation paths taken by grains in suspension and grains on the ground surface.[/color][/quote]
[quote=" http://www.marsdaily.com/reports/Sand_Ripples_Taller_On_Mars.html"]
[c][size=150]Sand Ripples Taller On Mars[/size][/c]
<<Mars is kind of like Texas: things are just bigger there. In addition to the biggest canyon and biggest volcano in the solar system, Mars has now been found to have sand ripples twice as tall as they would be on Earth. Initial measurements of some of the Red Planet's dunes and ripples using stereo-images from the Mars Orbiter Camera onboard the Mars Global Surveyor have revealed ripple features reaching almost 20 feet high and dunes towering at 300 feet.
One way to imagine the taller dimension of ripples on Mars is to visualize sand ripples on Earth, then stretch out the vertical dimension to double height, without changing the horizontal dimension. "They do seem higher in relation to ripples on Earth," said Kevin Williams of the Smithsonian National Air and Space Museum. Williams will be presenting this latest insight into the otherworldly scale of Marscapes on Monday, Nov. 3 at the annual meeting of the Geological Society of America in Seattle, WA.
Ripples are common on Mars and usually found in low-lying areas and inside craters, says Williams. On Earth they tend to form in long parallel lines from sand grains being pushed by water or air at right angles to the ripple lines. Dunes, on the other hand, are formed when grains of sand actually get airborne and "saltate" (a word based on the Latin verb "to jump"). That leads to cusp-shaped, star-shaped, and other dune arrangements that allow materials to pile sand much higher.
How exactly Martian dunes and ripples form is still unknown, says Williams, since the images from space give us no clues to the grain sizes or whether they are migrating or moving in any way. Though there are Viking spacecraft images from almost 30 years ago to compare with, the images do not have the resolution to confirm whether ripples have moved much in that time. For now, the dimensions of ripple-forms on Mars are the only indications of whether they are large ripples or small dunes. Williams' results came about from the advantageous combination of image parameters to get the first height measurements of these ripple-like features at the limit of image resolution.
According to Williams, it's likely the doubled heights of Mars ripples relative to their spacing is made possible by the same thing that makes Mars' volcanoes
so tall: lower gravity. With about one-third the gravity of Earth, sand, silt, and dust can theoretically stack up higher before gravity causes a slope failure. However, other differences could play roles in making these large piles of sand as well. "It could also be from different wind speeds, air densities or other factors," said Williams. Mars has a perennially subfreezing, very thin atmosphere in which global dust storms have been known to obscure the surface from view.
The study of Mars dunes and ripples has been underway since Viking spacecraft images of Mars first revealed such features in the late 1970s and early 1980s, says Williams. The primary difficulty of the work continues to be in discerning the close-up details, like the exact heights of features and grain sizes. As with dunes and ripples on Earth, these wind-blown features could reveal a lot about local and regional weather and wind currents if more was known about ripple and dune building under the very un-Earthlike conditions of Mars.>>[/quote]
[quote="tekic545"]
But still, we appear to have very dark sand on top of light (rippled) sand.
Is there a material difference, and if so, what process keeps the light and dark materials separate?[/quote]
[img3="[size=135]The size, density & shape of the particles determine
whether they move by [color=#FF0000]suspension[/color], [color=#FF00FF]saltation[/color] or [color=#0000FF]creep[/color].[/size]"]http://cmg.soton.ac.uk/assets/people/gtj104/gallery/saltation-small.JPG[/img3]