[X] My Assistant

[sattrackpro]

a post script...

I've lived for many years in the American Southwest - in the desert areas of Texas, New Mexico and Arizona - where many times I have seen whirlwinds operative well after sunset, one specifically that I remember well, that nearly took me off of a telephone pole near Lordsburg, NM at just about midnight.

At other times, I've seen multiple twisters in the moonlight northwest of El Paso, TX and southwest of Globe, AZ.

None of this means DD's happen at night on Mars, but I've seen nothing that proves that they don't either.

[dvandorn]

I think the likelihood of whirlwinds forming on Mars at night is a lot less than them forming at night on Earth. There is a lot more energy in the terrestrial system than in the Martian system, held both in the ground and in the air. Mars gets significantly less insolation than Earth and the ground re-radiates stored heat much faster than on Earth, due to the far thinner atmosphere.

Dust devils have not been observed at Gusev during the daytime in winter months. It doesn't get warm enough for them to form until late morning, at that. One can conclude from that fact that there is not enough energy in the system, even during daily daily heating, during the winter months to form them. I'm certain that physics can describe the reasons in terms of the energy needed to energize updrafts sufficiently for creation of dust devils.

There is a lot more energy in the system during daylight hours in the winter than there ever is at night during the summer. If dust devils cannot form during the day in the winter, physics requires that they cannot form at night, even in the summer. It's not necessary to bring in the McGuffin of inability to observe them at night, especially since it's impossible to prove a negative. For example, if you want to try and go hang gliding at night, even though physics tells you that there won't be any thermals to ride, heck, you can't observe whether they're there at night or not, so, hey -- go for it. You first.

-the other Doug

[djellison]

Are you sure you're talking about small thermal driven DD's STP? It's the heat of the early afternoon that triggers them.

Maybe you're right - but until I hear differently, I'm going on the science teams words.

Doug

[CosmicRocker]

Among other things, the May 14th Spirit Update said,

Plans called for Spirit to coordinate searching for dust devils on the ground with overhead scans of terrain by the Mars Reconnaissance Orbiter on sol 1191 (May 10, 2007). Also on sol 1191, the rover was to combine searching for dust devils with searching for clouds. This activity was meant as a stress test, as Spirit has already successfully completed such searches independently.

I hope that went well.

[dilo]

This DD isn't negligible! (Sol1195 - Already highlighted by Alan).
Look at the the pictures sequence (slightly enhanced), seems going straight toward Spirit! :


And about cleaning event (OT?), there is a net improvement in the appareance of last sundial pictures...

probably, is only an apparent change due to uncalibrated colors, but I would like is real

[djellison]

Certainly not enough difference in those sundial images to say if there was any sort of event. With the major cleaning events near Larrys Lookout it was obvious - streaks of blown dust behind objects and half the magnet blown clear.

Doug

[CosmicRocker]

Yeah. I can't say I can see much of a difference between the sundial images, Marco. I hope to see it soon, but Spirit may need to go to a new location to find a breeze. There are new DDs in the recent images and perhaps she'll get lucky. At least we see that Spirit saw DDs on sol 1191, when MRO was supposed to be looking down at Gusev.

[dvandorn]

Problem is, I don't think that the straight-line winds that actually cause cleaning events really get down into the bowl in which Spirit is currently sitting -- Home Plate lies generally in the topographic low of the Inner Basin, and straight-line winds are somewhat dissipated before they reach our current location.

If trying to engineer a cleaning event is ever a goal of the Spirit team, they'd probably try to analyze the prevailing winds and place Spirit at an angle into them, on top of the highest spot they can now reach with the bum wheel.

-the other Doug

[Shaka]

Doth mine eyes deceive me, or is that a gust in the latest navcams heading our way?!
Gust Angel?
Come to Papa! Right now we can risk a shaking up, if it means a good cleaning.
Can somebody put together the 'animation'?

[hortonheardawho]

The brightness changes are 4x the base frame.

[Astro0]

I've been playing around with some new software at home and thought others may be interested in seeing a little test movie I did of a 'dust devil'.

I used a background taken by Spirit, generated a panorama, added sky and sun and then put in several layers to produce the whirlwind effect. There are also a few little foreground dust movements as well.

Just for effect I've added some titles and music track.
As I say just a test, next thing to add would be a ground shadow and a tracking move across the panorama - but hey, it's my first try at this.

Movie File Here: 5.89mb

Enjoy
Astro0

[ngunn]

I enjoyed that movie, Astro0, thank you.

One comment I would make relates to the effects of perspective on time and motion. Because in the distance horizontal distances are very foreshortened your dust devil appears (to me) to have a very fast groundspeed at first, getting slower and slower over the ground on approach. It might be useful to construct a perspective grid as follows:
Start with a horizontal line along the bottom of the image and another along the horizon, then add another half way between the two, then a whole series more, always halving the remaining distance from the previous line to the horizon. To a first approximation these should represent equal increments of distance which would presumably be traversed in (approximately) equal times by an approaching dust devil.

I don't know how much the NASA images constrained your movie - perhaps the DD actually did slow down - so if my comments are irrelevant please ignore . . .

[Astro0]

Thanks ngunn.
I think I see what you're getting at.
Just my first attempt at this type of animation.

My take on it is that as the DD builds in height its sheer volume causes it to slow its movement across the ground. Looking at DDs here on Earth, smaller more defuse ones tend to move faster where as larger ones drag along. I think I was going for a more artistic representation while I learn the software.

As I said before, I want to work on creating a more realistic motion, shadows, dust movement etc for future versions.

Thanks for the input.
Astro0

[ngunn]

Actually, my recipe for the perspective grid needs correcting - what I described in fact generates lines that progressively double in distance on the ground, whereas you need uniform ground intervals - at least as a starting point. Here's the correct version. Taking the vertical distance in the picture plane from horizon to bottom of frame as one unit, mark the following points, measuring downward from the horizon: one half, one third, one quarter, one fifth, one sixth, etc. These will represent equal distance increments on a horizontal ground surface. Hope that is useful, and sorry for getting it wrong yesterday!

[helvick]

I think it's very cool however I think Nigel has a very good point - the DD appears to slow down far too much when it hits the foreground. For realism you might be able to take timed points from the ground track of one of the real DD's and map the effect over that.