APOD: A Dust Devil on Mars (2015 Mar 03)

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APOD: A Dust Devil on Mars (2015 Mar 03)

Post by APOD Robot » Tue Mar 03, 2015 5:08 am

Image A Dust Devil on Mars

Explanation: It was late in the northern martian spring when the HiRISE camera onboard the Mars Reconnaissance Orbiter spied this local denizen. Tracking across the flat, dust-covered Amazonis Planitia in 2012, the core of this whirling dust devil is about 140 meters in diameter. Lofting dust into the thin martian atmosphere, its plume reaches about 20 kilometers above the surface. Common to this region of Mars, dust devils occur as the surface is heated by the Sun, generating warm, rising air currents that begin to rotate. Tangential wind speeds of up to 110 kilometers per hour are reported for dust devils in other HiRISE images.

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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by Indigo_Sunrise » Tue Mar 03, 2015 11:45 am

This link, here, states that there are streaks left from tracks of the martain dust devils, but I'm wondering why there are no tracks in today's image? Is it because the image was captured right as the dust devil formed?

Definitely an amazing image, though!
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by geckzilla » Tue Mar 03, 2015 11:48 am

Maybe not all dust devils result in tracks. Or maybe we're looking a little too closely at it and it's covered up in shadow or the dust devil itself.
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by Indigo_Sunrise » Tue Mar 03, 2015 12:26 pm

Ah, good points!
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by rstevenson » Tue Mar 03, 2015 2:43 pm

The tracks that show in may of the Hirise Update images are on sand dunes. We know from other images that many dune areas on Mars have a light coloured thin layer over darker sand, and when a dust devil tracks across such a dune, it whips up the lighter covering, leaving a dark trail. But this image seems to be of a dust devil traversing a rocky area. There'd still be lots of dust to whip up, but the dust could be nearly the same colour as the rock, so little obvious trail would be left.

But having said all that, I think this might be the trail...
MartianDustDevil_mro_960.jpg
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by Stan Schultz » Tue Mar 03, 2015 4:22 pm

I admit that I while I spend a lot of time in a variety of deserts and have seen a lot of dust devils, all the ones I've seen have been more or less the same color as the dust they're deviling. So, why on a reddish brown planet and on arguably brown soil is that dust devil white? I might expect a different shade of dust brown because of the sifting effects of the wind (e.g., the deviled dust may be of different origin or composition than the coarser base material), or by an unfamiliar angle of observation (looking down from overhead rather than horizontally from the side). But, snow white is difficult to rationalize.

Snow white suggests that the dust devil is deviling something other than dust. Is it possible that it's sorting out water ice or CO2 crystals from the substrate? Is this why we see no devilish footprint? Could it be that our little devil isn't strong enough to lift "ordinary" dust, sand, and gravel, but is capable of lifting much less dense (i.e., "lighter") water ice or CO2 crystals into the atmosphere? Could it be that the "dust-covered Amazonis Planitia" isn't so dust covered after all, rather being covered with something completely different?

Or, how about water ice melting to a slush or liquid in the sunlight, but remaining concealed and protected in the substrate. Perhaps being held in a quasi-liquid state on the surfaces of ordinary substrate particles. And the adhering layer of water tends to make the substrate particles stick together and not be lifted into the atmosphere by our little dust devil. But, as the dust devil whirls overhead the sudden drop in pressure and/or chaotic winds lifts a bit of the semi-liquid or liquid material out of the substrate and turns it into a mist or vapor. And the sudden, extreme drop in temperature turns that mist or vapor nearly instantaneously into a swirling cloud of ultra-fine, whitish crystals?

Okay, so you don't like my hypotheses. I challenge you to invent better ones! :lol2:

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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by MarkBour » Tue Mar 03, 2015 4:54 pm

One of the links ("local denizen") shows a lot more images. One sequence appears to be an artist's animation, but the others appear to be video made from a sequence of images of this dust devil. If that is correct, the track is faint, but the devil is proceeding down and to the right over time. The region rstevenson has circled appears to be part of the shadow from the plume.
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by BMAONE23 » Tue Mar 03, 2015 5:47 pm

MarkBour wrote:One of the links ("local denizen") shows a lot more images. One sequence appears to be an artist's animation, but the others appear to be video made from a sequence of images of this dust devil. If that is correct, the track is faint, but the devil is proceeding down and to the right over time. The region rstevenson has circled appears to be part of the shadow from the plume.
"Animation 1" on the linked site clearly shows a Lighter Track left in the Wake of the Dust Devil

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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by JohnD » Tue Mar 03, 2015 5:53 pm

What would a wind speed of 110kilometers/hour feel like on Mars, where the atmospheric pressure is 0.6% of Earth's?

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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by BDanielMayfield » Tue Mar 03, 2015 6:50 pm

JohnD wrote:What would a wind speed of 110kilometers/hour feel like on Mars, where the atmospheric pressure is 0.6% of Earth's?

John
I'll hazzard a guess that it would feel like a 110 km/hr x 0.006 / 0.376 g (to factor in Mars' reduced surface gravity) which is 1.75, so I think it would feel like like a gentle 1.75 km/hr breeze does here to us.

No astronauts are liable to be blown over by Marsian winds. We're heavy worlders as far as Mars is concerned. :)

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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by neufer » Tue Mar 03, 2015 7:42 pm

BDanielMayfield wrote:
JohnD wrote:
What would a wind speed of 110kilometers/hour feel like on Mars, where the atmospheric pressure is 0.6% of Earth's?
I'll hazard a guess that it would feel like a 110 km/hr x 0.006 / 0.376 g (to factor in Mars' reduced surface gravity)
which is 1.75, so I think it would feel like like a gentle 1.75 km/hr breeze does here to us.

No astronauts are liable to be blown over by Martian winds.
We're heavy worlders as far as Mars is concerned. :)
Wind strength factors as the square of the wind speed
(i.e., "wind speed" equivalency as the square root of the density [= pressure/temperature]).

"wind speed" equivalency = 110 km/hr x [sqrt(0.006) / 0.376] ~ 14 mph

A "moderate breeze" on the Beaufort scale
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by JohnD » Tue Mar 03, 2015 8:35 pm

Nice, Art; nice try Daniel, but why bring in gravity? A wind in micro-gravity would still be a wind.

But Art, wind strength is proportional to the square of density?
Mars - 20gms/m^3
Earth - 1200gms/m^3

So "Earth strength" = 110 x 1200^2 = 1.584 x 10^8 (units??)
And "Mars strength"= 110 x 20^2 =44000 = 4.4 x 10^4

So, relatively 2.78 x 10^4 times weaker, on Mars.
Beaufort scale 1, a "Light Air", is 1 metre/sec = 0.001kms/sec = 0.06kms/min = 3.6kms/hour
so "Earth strength " = 3.6 x 1200^2 = 5.2 X 10^6
Which is 120 times stronger than that Mars wind.

Unless, of course, someone knows of another way of working it out!

Sorry, my Q was an idle one, but your diligence challenged me!

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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by neufer » Tue Mar 03, 2015 9:03 pm

JohnD wrote:
Nice, Art; nice try Daniel, but why bring in gravity?

A wind in micro-gravity would still be a wind.
Gravity is the only thing prevent one from being knocked over (or blown away) by the Wind Force.
JohnD wrote:
But Art, wind strength is proportional to the square of density?
Correct:

(Equivalent Wind Force/gravity)E = (Wind Force/gravity)M

{1 atm x (14 km/hr)2}/(1 g) = {0.006 atm x (110 km/hr)2}/(0.376 g)

14 km/hr = 110 km/hr x sqrt(0.006/0.376)
[note: slight correction from previous formula]

Daniel got everything right except for the fact that
the mass of air impinging on one per second is proportional to the Velocity
so that the momentum (= mV) is proportional to the Velocity squared.
Last edited by neufer on Tue Mar 03, 2015 10:05 pm, edited 1 time in total.
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by Ron-Astro Pharmacist » Tue Mar 03, 2015 9:55 pm

First it was Unusual Plumes and now it's Dust Devils. I was wondering where the UFO took them.
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by geckzilla » Wed Mar 04, 2015 12:32 am

Stan Schultz wrote:I admit that I while I spend a lot of time in a variety of deserts and have seen a lot of dust devils, all the ones I've seen have been more or less the same color as the dust they're deviling. So, why on a reddish brown planet and on arguably brown soil is that dust devil white?
Another factor to consider is that when you are looking at the dust devils in the desert, you are looking at them against a bright background such as the sky or the sun shining on the ground from directly above. In the APOD picture, we've got the sun at a fairly oblique angle against a rough surface which is the ground. That makes the ground look darker. Meanwhile, the dust devil is sticking up from the ground and is taking sunlight directly, making it appear brighter. The rest of it is just light scattering by the small particles in the devil.
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by MarkBour » Wed Mar 04, 2015 1:40 am

geckzilla wrote:
Stan Schultz wrote:I admit that I while I spend a lot of time in a variety of deserts and have seen a lot of dust devils, all the ones I've seen have been more or less the same color as the dust they're deviling. So, why on a reddish brown planet and on arguably brown soil is that dust devil white?
Another factor to consider is that when you are looking at the dust devils in the desert, you are looking at them against a bright background such as the sky or the sun shining on the ground from directly above. In the APOD picture, we've got the sun at a fairly oblique angle against a rough surface which is the ground. That makes the ground look darker. Meanwhile, the dust devil is sticking up from the ground and is taking sunlight directly, making it appear brighter. The rest of it is just light scattering by the small particles in the devil.
I find that explanation quite plausible -- the lighting angles are very different, and that could account for it.

How tall are dust devils on Earth? I would guess the tallest dust devil I've seen has been maybe 50-100 feet tall, so it is still pretty near the ground compared to these beasties ... reaching 10 or 20 km above the ground! So that plume is tall enough to reach more than twice the altitude difference from sea level to the top of Everest. I'm not saying that should have any particular effect, just that there is a potentially relevant difference between this picture and what I've seen in the Mojave desert.
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by Stan_Schultz » Wed Mar 04, 2015 2:30 am

geckzilla wrote:... Another factor to consider is that when you are looking at the dust devils in the desert, you are looking at them against a bright background such as the sky ...
Often, yes. But there are plenty of times when Earth based dust devils are observed against hills or mountains in the distance. And, I distinctly remember seeing a video of a dust devil on the African veldt taken from a hot air balloon (a Nat Geo documentary?). In all those cases the dust devil had a distinct tinge correlated with the substrate, not snow white.
geckzilla wrote:... In the APOD picture, we've got the sun at a fairly oblique angle against a rough surface which is the ground. That makes the ground look darker. Meanwhile, the dust devil is sticking up from the ground and is taking sunlight directly, making it appear brighter. The rest of it is just light scattering by the small particles in the devil.
I will grant you your point with only one further comment: Even if we account for the sun's angle and our angle of observation, one would expect brown dust particles to reflect a tannish or brownish light, certainly an off white of some sort. That is clearly not the case in the photo.

A further observation: In the photo on APOD (presumably directly downloaded from Mars) the "dust" cloud appears snow white, and there is no obvious trail on the substrate's surface. However, in both animations not only is a trail present, but there is a distinct, dark core in the funnel cloud. And while the CGI isn't bad, the rotating devils are just too mechanical, very much like the tornadoes in Jan de Bont's Twister. I vote that both animations are merely some NASA artist's conception of reality with the trails and the dark cores inappropriately Photoshopped in, and should not be considered as an indication or evidence for anything in real life.

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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by Nitpicker » Wed Mar 04, 2015 2:45 am

I just did a Google image search for dust devils and saw a variety of different swirling dust colours on Earth, ranging from "snow white" to "deep orange".

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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by geckzilla » Wed Mar 04, 2015 2:57 am

Stan_Schultz wrote:I will grant you your point with only one further comment: Even if we account for the sun's angle and our angle of observation, one would expect brown dust particles to reflect a tannish or brownish light, certainly an off white of some sort. That is clearly not the case in the photo.
Ok, you are interpreting the colors of the devil as being accurately reflected by what you see on your monitor. That is not necessarily the case. While we can say it is certainly lighter than the ground below, we would need to take a measurement to determine if it is actually as bright as snow. To me I see plenty of variations in brightness within the vortex and I would not say it is as bright as snow just from looking. I'm not sure how to measure its brightness but you definitely wouldn't want to do it with this picture. You'd need raw data.

And a telescopic view looking straight down is still quite a different thing from a wide field view one would obtain from a hot air balloon.
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by Chris Peterson » Wed Mar 04, 2015 5:15 am

neufer wrote: "wind speed" equivalency = 110 km/hr x [sqrt(0.006) / 0.376] ~ 14 mph
Yes, in terms of the bulk force we would feel. But it's still a 110 km/hr wind. If you're caught in this dust devil, you're still pelted by abrasive dust particles at 30 m/s. So it might feel like a moderate breeze, but it could chew up your spacesuit or equipment pretty quickly.
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by BDanielMayfield » Wed Mar 04, 2015 11:17 am

neufer wrote:
JohnD wrote:
Nice, Art; nice try Daniel, but why bring in gravity?

A wind in micro-gravity would still be a wind.
Gravity is the only thing prevent one from being knocked over (or blown away) by the Wind Force.
JohnD wrote:
But Art, wind strength is proportional to the square of density?
Correct:

(Equivalent Wind Force/gravity)E = (Wind Force/gravity)M

{1 atm x (14 km/hr)2}/(1 g) = {0.006 atm x (110 km/hr)2}/(0.376 g)

14 km/hr = 110 km/hr x sqrt(0.006/0.376)
[note: slight correction from previous formula]

Daniel got everything right except for the fact that
the mass of air impinging on one per second is proportional to the Velocity
so that the momentum (= mV) is proportional to the Velocity squared.
I'm glad correcting my hazardous guess re John's good question tempted Art out of his abstention from commenting. Welcome back nuefer.

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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by JohnD » Wed Mar 04, 2015 11:46 am

Glad to be corrected! My result FELT too small!

Now, Chris, damage by flying sand grains proportional to velocity and mass? I'm on my phone so finding mean size of terrestrial and Martian grains non-trivial. But if significant. diff. then less damaging on the planet with smaller grains?
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by neufer » Wed Mar 04, 2015 7:35 pm

Click to play embedded YouTube video.
neufer wrote: "wind speed" equivalency =
110 km/hr x [sqrt(0.006) / 0.376] ~ 14 mph
Correction: "wind speed" equivalency =

110 km/hr x [sqrt(0.006/0.376)] ~ 14 km/hr
Chris Peterson wrote:
Yes, in terms of the bulk force we would feel. But it's still a 110 km/hr wind. If you're caught in this dust devil, you're still pelted by abrasive dust particles at 30 m/s. So it might feel like a moderate breeze, but it could chew up your spacesuit or equipment pretty quickly.
Surely, spacesuits & equipment are made of sturdier stuff
and are mostly cleansed in the process.
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by orin stepanek » Wed Mar 04, 2015 7:58 pm

Dust Devil, Dust Devil, picking up the dust; On the Martian surface, looking just like rust! :lol2:
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Re: APOD: A Dust Devil on Mars (2015 Mar 03)

Post by BMAONE23 » Wed Mar 04, 2015 10:02 pm

neufer wrote:
Click to play embedded YouTube video.
neufer wrote: "wind speed" equivalency =
110 km/hr x [sqrt(0.006) / 0.376] ~ 14 mph
Correction: "wind speed" equivalency =

110 km/hr x [sqrt(0.006/0.376)] ~ 14 km/hr
Chris Peterson wrote:
Yes, in terms of the bulk force we would feel. But it's still a 110 km/hr wind. If you're caught in this dust devil, you're still pelted by abrasive dust particles at 30 m/s. So it might feel like a moderate breeze, but it could chew up your spacesuit or equipment pretty quickly.
Surely, spacesuits & equipment are made of sturdier stuff
and are mostly cleansed in the process.
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