Starship Asterisk*

It is Friday and i am reunning to an irish bar. Gentlemen, slide over an ice cold one, thank you ...

You fools!

It's their invasion fleet about to launch a mercyless assault on the planet Earth!

Run for the hills!

[quote="rstevenson"][quote="Mactavish"]What is the approximate size of one of these horseshoe shaped dunes? (It would be very helpful to provide some sort of scale when presenting this type of image.)[/quote]
It sometimes takes some doing to find out the scale, but it's always there somewhere. In this case, if you click the "above image" link to go to the HiRISE page, and then click the "Grayscale: map projected" link in the JPEG section below the main image, you'll get one with a scale indicator at top. Here's a modified version of a portion of that image.

[attachment=0]dune_scale.jpg[/attachment]
Rob[/quote]

Thanks Rob. I did click on the “above image” link, but no further. It never occurred to me that I could find a scale where you pointed it out. Indeed, it does “take some doing”. I would hope that, with the combined expertise of the APOD folks and the Hi Rise folks, we can find an easier and simpler way to explain that the sand dunes are approximately the size of a couple of football fields. The Hi Rise images are quite fantastic. Thanks again.

As I said, six posts back.
But there some simulations I've linked to.
JOhn

It would be cool to see several shots over time and see the movement....

:---[===] *

First thing I wondered about was the scale of the image. I see someone addressed it in the comments above.

is this stuff like quicksand,,,,,,,,,,,,err,,,,,slowsand ???

i don't see any evidence of disturbance of the ground that the dunes have passed over. Is that evidence that the substrate differs from the loose sand?

The flat-top mesas act like an airplane wing. When air flows over them, a low pressure region is generated on top. Like an airplane wing, tip vortices are generated downstream of the mesa. As the vortices develop and grow, they settle toward the ground and produce the periodic pattern of the barchan dunes. Neat pictures.

[quote="Mactavish"]What is the approximate size of one of these horseshoe shaped dunes? (It would be very helpful to provide some sort of scale when presenting this type of image.)[/quote]

Yes, I agree. It's a strange world there and we have no idea about dimensions, proportions and relations (a typical "desert problem", even on earth). Dear APOD, whenever you present such fascinating pictures of Moons or other planets, please help us to understand and add a metric scale or ruler or something well-known to compare. Thanks! :wink:

Interesting simulation here:
http://www.youtube.com/watch?v=tsNGcjPP0Zg
Takes some time to form any actual barchans, but eventually they are there, complete with 'drips' from the horns.

This is more diagrammatic, but shows how small dunes can "pass through" larger.
http://www.youtube.com/watch?v=Q_7VtpYQFVI&feature=related

Here's a water sluice simulation that shows very clearly the development of the slip face, in differently dark strata.
http://www.youtube.com/watch?v=cJo0fTpJypg&feature=related
There seem to be no time lapse videos of real dunes at all !

Here is a short field trip tutorial on sand dunes.
Your tutor (I don't know him) is speaking exclusively about terrestrial dunes, but I suspect that much of what he says, including the ancient lake bed, will be true on Mars.
http://www.youtube.com/watch?v=sQ9mwsPvElE&feature=related

But some fun.
This shows how running water forms ever changing ripples, as in the sluice:
http://www.youtube.com/watch?v=l76v0kJ8rgo&feature=related

Scale that up - and its surf time!
http://www.youtube.com/watch?v=lImQH0RD0NU&feature=related
John

It's a great APOD. It seems to me there must be enough pics of Mars of different natures that we could have more of them and fewer of the earth pics that bother some folks a lot and me at times.

Anyway I was intrigued by the claim these guys could pass through each other. I looked around and found this short paper, [url=http://pvermees.andropov.org/solitons/][i]Solitary wave behavior in sand dunes observed from space[/i][/url] by Pieter Vermeesch. It includes a couple of film strips with interpretive sketches which follow. The first is of a "head-on" collision (really a rear ender) and the second of an off-center collision. Remember the smaller dune moves faster.
[table][tr][img]http://pvermees.andropov.org/solitons/fig1.png[/img][img]http://pvermees.andropov.org/solitons/fig2.png[/img][/tr][/table]

[quote="Mactavish"]What is the approximate size of one of these horseshoe shaped dunes? (It would be very helpful to provide some sort of scale when presenting this type of image.)[/quote]
It sometimes takes some doing to find out the scale, but it's always there somewhere. In this case, if you click the "above image" link to go to the HiRISE page, and then click the "Grayscale: map projected" link in the JPEG section below the main image, you'll get one with a scale indicator at top. Here's a modified version of a portion of that image.

[attachment=0]dune_scale.jpg[/attachment]
Rob

Today's APOD reminds me of heads of wheat! :D 8-) :wink:[img2]http://upload.wikimedia.org/wikipedia/commons/thumb/6/6c/WheatFlower1.jpg/220px-WheatFlower1.jpg[/img2]

[url=http://i.imgur.com/pu9Rz.jpg]Here’s a copy[/url] for the curious.

The link didn't work for me, either.

Ann

The downwind link worked for me; perhaps it was a momentary blip.

The "downwind" link does not work.
http://image1.masterfile.com/em_w/00/86/42/600-00864258w.jpg
gives
"Forbidden

You don't have permission to access /em_w/00/86/42/600-00864258w.jpg on this server.

Apache/2.2.9 (Unix) mod_ssl/2.2.9 OpenSSL/0.9.8h PHP/5.2.14 Server at image1.masterfile.com Port 80"

What is the approximate size of one of these horseshoe shaped dunes? (It would be very helpful to provide some sort of scale when presenting this type of image.)

[url=http://apod.nasa.gov/apod/ap120422.html][img]http://apod.nasa.gov/apod/calendar/S_120422.jpg[/img] [size=150]Flowing Barchan Sand Dunes on Mars[/size][/url]

[b] Explanation: [/b] When does Mars act like a liquid? Although liquids freeze and evaporate quickly into the thin atmosphere of Mars, persistent winds may make large [url=http://blogs.discovermagazine.com/badastronomy/2009/04/13/dune-mars-2/]sand dunes appear[/url] to flow and even drip like a liquid. Visible on the [url=http://hirise.lpl.arizona.edu/PSP_007676_1385]above image[/url] right are two flat top mesas in southern Mars when the [url=http://www.planetary.org/explore/topics/mars/calendar.html]season was changing[/url] from Spring to Summer. A light dome topped hill is also visible on the far left of [url=http://www.psi.edu/pgwg/images/apr09image.html]the image[/url]. As winds blow from right to left, flowing sand on and around the hills leaves [url=http://apod.nasa.gov/apod/ap080303.html]picturesque streaks[/url]. The dark arc-shaped [url=http://apod.nasa.gov/apod/ap010226.html]droplets[/url] of fine sand are called [url=http://en.wikipedia.org/wiki/Barchans]barchans[/url], and are the interplanetary cousins of similar Earth-based sand forms. [url=http://photography.nationalgeographic.com/photography/enlarge/barchan-dunes-photography.html]Barchans[/url] can move intact a [url=http://image1.masterfile.com/em_w/00/86/42/600-00864258w.jpg]downwind[/url] and can even appear to pass through each other. When seasons change, [url=http://planetary.org/blog/article/00001908/]winds on Mars[/url] can kick up dust and are [url=http://news.softpedia.com/news/Martian-Dust-Storms-Threatens-the-Twin-Rovers-109587.shtml]monitored[/url] to see if they [url=http://apod.nasa.gov/apod/ap011017.html]escalate[/url] into another of Mars' famous planet-scale [url=http://www.youtube.com/watch?v=BD7IedPZvt8]sand storms[/url].

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