On the Road Again - volume 3 - Page 6

sol 2043 ( Oct 23, 2009 ) R12 4x1 in next drive direction:

Looks like Oppy is finally turned southwards.

If a meteorite impacts at a shallow angle, then won't it impart a considerable momentum to debris in the forward direction? That should fall as a pile some distance from the impact site. There may be a thinner covering extending back to the impact area, but that could get covered by dust more quickly than the main pile, making the pile look isolated.

Horton, Thanks for the 3-D pair of the cobble field. The two ripple-like features seem to be real structures alright though not especially convincing as ripples. It is hard to look like a wind ripple with centimeter scale rocks showing all over. Thinking about which came first, the cobbles or the ripple fines, maybe they arrived already mixed together and originally covered the plain. Then the present ripples are deflation remnants - the parts that haven't been eroded away.

I found the another example of cobble ripples from sol 1792:

There seem to be very coarse grained ripples in the background and the foreground. Their orientation seems to match the small cross-ripples so by my primitive way of reckoning direction they won't match the current pair (sol 2042) which seem to be oriented east-west.

Barsoomer, re your 102, The cobbles are not the same sort of material as the bright bedrock, so for the cobbles to be related to small impacts they would have to come from the impactors that have struck the plains not the bedrock ejected. I agree that it is possible that Mars material exotic to Meridiani (which the cobbles appear to be) could have been moved onto the plains by impact without leaving obvious craters. I accept the possibility that a large piece of secondary debris in a low trajectory might bounce and roll to a stop without leaving an obvious crater. If such were then to disintegrate into many pieces we could also explain the cobble clustering.

The cobbles have been everywhere that Oppy has visited and they have occurred in many places as clusters. I think that impact-surge is a good match for the widespread but often clustered distribution because it covers large areas in a continuous ejecta blanket. It fits better for me than multiple secondary impactors that haven't left craters.

Sol 1792 ( Feb 7, 2009 ) L257 3x1 of cobble field in dunes:

Could the cobbles be more easily explained as lag deposits? We have seen a number of nickel iron meteorites on the surface. Since these should have been buried on impact the most logical explanation seems to be that they have been exhumed over billions of years. But iron meteorites are comparatively rare. If stony meteorites or stony iron meteorites hit and fractured, and the remnants were slowly exhumed, with larger pieces continuing to fracture over time due to thermal changes in the environment then they would form a lag deposit such as we see.
A quick search revealed that Hortonheardawho has posted a number of images showing cobble fields in the process of formation that seem to bear out this explanation. This would also explain why cobbles have been seen on both the bedrock and dune areas.

and another

Thanks Serpens, that's the pile of "road apples" I raferred to earlier.

sol 2045 ( Oct 25, 2009 ) Navcam L0:

A 70+ meter backwards drive southward.

No forward autonav ( long distance ) drives since sol 2002 ( Sep 11, 2009 ).

Would anyone care to explain the relatively large number of chunks of pavement resting on the surface in Hort's 107 image ?

Serpens, Do I read your thought that these "cobble patches" were lowered by erosion as lag from a once higher level.?

If so ,wouldn't they have been concentrated at that higher level and if so by what means?

Higher velocity winds ?? fluvial action?? ice flow ??

Ben, My idea is from fragmented stony / stony iron meteorites (over 90% of meteorite falls) which would fragment during (and after) impact, as well as some possible ejecta fragments. A meteorite fragmenting on impact would have fragments under within and around the final crater. A reasoably localised spread. The fragments would gradually concentrate to a common level with ongoing deflation We have seen a couple of the disintegrating lumps as imaged in reply 105 and if you can see a cobble pile forming in this way then by inference there is a good probability that most cobble fields came from this demonstrable process.

I can't really reconcile the cobble fields as a surge effect, particularly because of the lack of any evidence of surge in the Victoria cliff exposures. But all things are possible I guess.

Ben, re your reply 108, the "chunks of pavement resting on the surface" that you refer to seem to be getting more common hereabouts. We have seen lots of loose blocks that are clearly ejecta from craters but there have also been some all along with no apparent relationship to craters. These are yet another category of objects sitting enigmatically on the plain that, like the cobbles and meteorites, are clearly too big to be directly moved by wind and have no other obvious motivator. If the layered pavement chunks are undercut and toppled by wind erosion why have we seen no hoodoos in an earlier stage of the the process? The loose blocks look mechanically displaced from the bright bedrock by some almost random process.

I've been thinking about the great age of the main plains ripples which has become clear only since the HiRise camera's resolution became available showing the many impact craters in the ripples:

http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1973.pdf

If the ripples take 1 to 3 million years to change enough to erase meter-scale impact craters, how feasible is it that most of the Meridiani ripple material has been pushed 100s of kilometers out onto this plain by wind? It is easy to forget one of the centrally weird facts about Meridiani - that the bright bedrock seems to have contributed very little to the soil except spherules. The basaltic "sand" that makes up much of the ripples is not from around here and would have taken a long time to get here, maybe an implausibly long time, judging from the near immobility of the ripples that hold the sand.

Another centrally weird fact about Meridiani is the way that the ripples seem to be affected by features that should be vastly older and completely irrelevant. Take the annulus of Victoria Crater. Why should the ripples, if they are anything at all like aeolian forms on Earth, suddenly stop at Beagle Crater and be absent on the adjacent annulus? It is as if the main plains ripples could even be older than Victoria Crater - but that's crazy talk. There are some north-south ripples on the annulus, mostly east of the crater where Oppy didn't go. Anyway just thinking.

Speaking of craterless plain's debris, I don't yet see the crater associated with this dark line of rubble just ahead:

Maybe it will look more typical tomorrow.

sol 2045 map position:

with links to next drive direction pan and sol 2042 R1 3x1 pan of the cobble field:

Kye, the 8 meter crater is about 50 meters distant. It looks like the south wall is raised but the north wall is not?

Kye; Thanks for your response. Forces me to acknowledge that I often tend to conjure up very simple explanations for what on Mars may be very complex scenarios.
But for that matter, "surge effects" may be too simple.

Serpens; The disintegrating lump in #105 has no apparent crater associated with it so it must be erosional lag from a previous higher stratigraphic level !
Could one then assume the observed cobble patches are from impacts a very long time ago in Meridiani beds since removed?
If so what happened to all of the subsequently eroded Meridiani beds ?
I am still trying to recocile the situation and keep thinking of a wet period and "fluvial sheet wash" for the cobble patches.

sol 2047 ( Oct 27, 2009 ) R0 3x1:

just east of a small crater after a 45 meter backwards drive southward.

Sol 2048 L0 3x1 after 75 meter backwards drive southward:

with link to location map.

Hort>>>

Very nice 'stitching'.

yt
dx

sol 2049 ( Oct 29, 2009 ) L0 3x1 panorama after 2 hour forward autonavigation drive southwards:

I haven't located the exact map position yet, but I think it is 200+ meters south of the 2048 position. I'll have another look after supper.

From Tesheiner's 2049 route map I was able to confirm a 76 meter southward move from the sol 2048 position using triangulation from the Navcam 3Ds.

Kind'a surprised it was such a short drive in two hours. Maybe the plan was to do a short drive forward to check out the wheel currents?

sol 2050 ( Oct 30, 3009 ) L0 3x1:

Looks like another forward autonavigaton drive - this time SSW about 80 meters?

Again, I can't match the navcam views with a specific point on the map.

sol 2051 L0 3x1 pan looking south after 67 meter drive:

with a link to a location map.

There were drives on the last 5 sols. I wonder what the record for consecutive drives is?