Mars - Concretions or Secretions?

"jonathan" skrev i en meddelelse
...

snip

Hi Jonathan
It's brave of you to take the discussion head on.
Wind and water make dunes and it's not always easy to tell the difference.
If it was, we wouldn't have this discussion.
Waves has two properties that shapes it's dunes: stream and wave - wind only
one.

One initial problem in assessing water as the medium is, that ... the
landscape would have been 'static' since the water disappeared - no changes.
Based on my observation I cannot accept that, so the closest thing is, that
the dunes at hand are a mix of both wind and water. - and since the
windblown sand are obvious, why not assess all of the features to wind?

We may argue about the origin of the spheres, but you owe us a through
explanation for the above statement.


Do you really think this pic shows wind blown dunes?

Yes, if we talk about the transverse, straight dune in the bottom.

Then why
does the dune define a line precisely where water would
stand were it filled?

Not understood.
This is the (heavy) panorama of Boneville.

http://marsrovers.jpl.nasa.gov/galle...x_pubeng_bonne
villecolor-A071R1.jpg

Along the edge, on the inside of the crater is small drifts of sand. When
the wind find lee just inside the crater-edge, it will loose capacity to
carry sand/silt, and it is dropped there. That goes for the sand in the
crater in general. You could put forward that water would do the same trick,
but then you at least would have to change your 'perseption' away from 'a
brim-filled' crater. In water the craterwall would be an obstacle in a large
sea and attract the strong erosive power of waves and current. Eventually
the swash of a surf could expect to separate sand from larger rocks - and as
happens, form a rough coastline up against the wind and a spit in the lee. I
live in a small country with the largest coastline/country-size - and it is
a 'revaletion' to understand general landscape-morphology in such a frame.
Waves as wind has a 'prefered' orientation.

As for the transverse dune in the bottom of the crater: This may be the
strongest indisium that the sand is windborn. The inside is a water-current
protected area - what should give water an abillity to build such a dune?
The transverse dune is a wellknown wind-feature, and wind would not have the
same problem in getting in to shape the sand.

Why is the dune darker on the wall
then on the floor?

The dunes are generally dark on the crest and light i the troughs. Ripples
tend to have a coarse crest and fines in the trough - that conforms to the
light as fine and the dark (presumably basaltic) as sand more difficult to
move by wind.

There is clearly a difference between the
dune on the wall and on the floor, if wind and dust were
the cause of both then there shouldn't be such a well defined
line separating the two.

Are you certain you are not talking about a shadow?

Your wind is very selective.
And your dust clings very well to the side of a steep
crater wall.

http://marsrovers.jpl.nasa.gov/galle...EFF1800P2285R7
M1.JPG


And in this picture there's a rather flat dune,

It's an area covered by sand/dust

are you saying the
wind blew the rocks away? Where did they go? They clearly
sank in the mudflat,
which means that dune formed when
water was still there. And it's stayed there since. Wind blown
dunes are transient features that come and go.

Do you wnat me to take you serious?


http://marsrovers.jpl.nasa.gov/galle...EFF1700P1943R0
M1.JPG



And in this pic the rock appears to be lying on top, why didn't the
wind blown dust form around the rock, or fill in underneath it?
Shouldn't the dust be lying up against the rock?

http://marsrovers.jpl.nasa.gov/galle...ESF1700P2515R1
M1.HTML

I cannot get an over-all impression and know nothing about the ground on
which the rock resides.

And it's clear the dark portions of the rock are continuous with
the dunes. Is this rock half covered with dark dust, or half
covered with lite dust? And explain how dust can define such
a fine line.

http://marsrovers.jpl.nasa.gov/galle...EFF1151P2433R1
M1.JPG

What?

The dark and lite dust seems rather selective

http://marsrovers.jpl.nasa.gov/galle...EFF12BOP2313L6
M1.HTML

http://marsrovers.jpl.nasa.gov/galle...ESF1300P2530R1
M1.JPG

Close to the rock there is a slight turbulence that does not favour settling
of the fines.

There's no way wind did this, the segregation of the rocks by size
can only be done by water.

How segregated are the rocks? Not enough for the rocks to support each other
(which chould be an argument in your favour) - the rocks are embedded in
sand.

Have you ever been to a beach?

It 'looks like' something that could have been shaped by water. Don't you
find it suspect that such a fine and fragile structure would survive in an
environment that is shaped by wind in general? The rocky pavement is a
deflated area and the small dunes may be a drift of the last bit of sand
taken from that area. Could it be the magnetic fragments?


http://marsrovers.jpl.nasa.gov/galle...EFF1159P2213L6
M1.JPG

Jonathan, you shall have to come up with evidence that is more clear than
you have presented here. Or did I change your mind?

Carsten