apodman wrote:Here's a nice picture of Echus Chasma:
Huge image
Indeed, a very interesting image. Unfortunately i can not recognize the area on the
apod of 20080723so i can not say whether this image shows the same area and so whether it is characteristic for the Echus Chasma. What i see in huge image, is a pattern filled with ripples. These are characteristic for a sandy bottom in a river, beach or tidal area. The English wikipedia explanation of ripples is very brief, the Dutch explanation is somewhat longer, the German explanation is excellent. I'll translate some of it, since (my assumption) you will not understand German.
Translated from
http://de.wikipedia.org/wiki/Rippelmarke
Ripples are formed in a wet sandy environment, where the water velocity exceeds the critical velocity slightly. When the critical velocity for the movement of grains of sand is reached, the grains start to move and cluster in small groups. This creates irregularities in the sedimentary surface, being a few grains thick. This influence the flow in the boundary layer (added: between turbulent flow and laminar flow along the bottom). These irregularities lead to small hills. At the top of these hills the flow lines are closer together and the flow velocity increases. The sedimentary grains are transported upward, on to the top of the hill, at the windward side. Their movement is called saltating: rolling and jumping. The grains accumulate at the top of the hill. If too many grains are deposited at the top, the slope of the hill becomes instable and the grains slide down at the lee side of the hill and form a sediment. These thin layers of grains are called foresets and they form natural slopes of about 30–35°.
This process repeats itself and hill after hill is build, separated and laminated by deposits of intermediary layers of fine sediment, raining out of suspension. This leads to the gradual formation of ripples. At the top of the hills or dunes the flow is split. Part of the flow follows the interface between water and sand. The other part flows further and forms turbulent eddies at the lee side and as a consequence the flow hits the surface of the sediment, where, due to the enhanced turbulence, further erosion is generated, forming and deepening the troughs between the hills. Part of the eroded grains may be transported backwards to the foot of the slope at the lee side and are deposited as a thin layer. The other part is again brought into suspension or is transported to the windward side of the hill.
Ripples are divided into three groups or classes, according to their size:
- Ripples, 3-5cm high, wavelength: 4-60 cm
- Large ripples, 6cm - 1.5 m high, wavelength: 0.6-30 m
- Giant or Mega ripples, 1-8 m high, wavelength larger than 30m.
The large and giant ripples are generated when the particle size of the sand is larger than 0.6-0.7 mm. The giant ripples are stationary, the other migrate or 'walk' . This
imageshows a ripple pattern.
(end of translation).
When studying the image
apodman brought to our attention, two things are remarkable. The first is the fact that the ripple patterns are not parallel. There are parts where the angles differ 90º. If the origin of the ripples is aeolian, like the wind blown sand dunes (Giant ripples) in the Sahara dessert, then on a scale of 50 km the predominant wind would have been blowing from completely different directions, even perpendicular.
The second is the scale of the ripples, or rather their wavelength. I rotated the image by 97º, it is then horizontal. In the mid section the wavelength of the ripples is about 10 m (20 ripples in 234 pixels = 234 m). At the right side the wavelength has decreased to about 6 m (33 ripples in 235 pixels = 235 m). If one considers the flow as mildly turbulent, gravity plays hardly any role. The mere fact that the acceleration of gravity on Mars differs from that on earth, is under the assumption of mildly turbulent flow of no importance, and thus the values as on earth, can be used.
