Live Dust Devil? Options

[Guest_Sunspot_*]

Heres the JPL animation of the dust devil on sol 486

http://www.jpl.nasa.gov/missions/mer/images.cfm?id=1682

JPL .mov animation

[dilo]

Cool!!!

[Jeff7]

Heres the JPL animation of the dust devil on sol 486

http://www.jpl.nasa.gov/missions/mer/images.cfm?id=1682

JPL .mov animation

I wonder how big that thing got higher up - it cast a pretty large shadow around itself.

[Tman]

I wonder how big that thing got higher up - it cast a pretty large shadow around itself.

The DD's local time was 11.48 a.m. at the beginning of the animation. I guess that is nearly sun culmination. Does someone know the angle that sun currently appears by culmination at Gusev?

My guess for the DD's height is circa 170 meters.

[MichaelT]

The DD's local time was 11.48 a.m. at the beginning of the animation. I guess that is nearly sun culmination. Does someone know the angle that sun currently appears by culmination at Gusev?

My guess for the DD's height is circa 170 meters.

Currently, the sun's culmination height is about 88° at Sprit's location (I used Guide 8). So it's directly overhead at midday. Twelve minutes earlier the sun's elevation is about 87°. So what we see is rather the horizontal extension of the dust devil projected onto the plain than its hight.

Michael

[Tman]

Thanks! This sun position makes sense by comparing the whole course of the shadow. If so the DD was sometimes very wide in its higher part or it had sometime a rolling shape. Maybe about 100 meters wide, but I'm unsure which part of the DD the JPL caption means "about 35 meters in diameter".

[dilo]

Last DD (Sol-501) seems very far and huge! Here NavCam details of the same position taken about 3 min apart (no DD enhancement, only some retouch in order to improve luminosity/contrast and reduce jpeg artifacts):

In the last frame, DD base is only 6 pixel (0.28deg) below horizon... I do not remember actual height of Spirit above the plain, but assuming 50m, the distance should be about 10 Km, so DD width is at least 350m
Do someone can confirm these numbers or/and make enhanced version?

[wyogold]

Last DD (Sol-501) seems very far and huge! Here NavCam details of the same position taken about 3 min apart (no DD enhancement, only some retouch in order to improve luminosity/contrast and reduce jpeg artifacts):
In the last frame, DD base is only 6 pixel (0.28deg) below horizon... I do not remember actual height of Spirit above the plain, but assuming 50m, the distance should be about 10 Km, so DD width is at least 350m 
Do someone can confirm these numbers or/and make enhanced version?

Wow cool stuff. I hope they catch one of those that etch the huge tracks you can see from space.

[MichaelT]

Do someone can confirm these numbers or/and make enhanced version?

Assuming an elevation of 50 m I get the same results. The enhanced version looks like this:

The dust in the left section does not belong to the big one which develops 100 s later.

Looks like a devilish day at Gusev Crater

The three large dd in frame 1 have diameters of 180, 160 and 260 m, approximately (see below).


Also very interesting is the change of brightness between frames 4 and 5. Using an enhancement level of 20 it becomes obvious that there are very week dd moving through, which are almost invisible otherwise.


Michael

[AndyG]

In the last frame, DD base is only 6 pixel (0.28deg) below horizon... I do not remember actual height of Spirit above the plain, but assuming 50m, the distance should be about 10 Km, so DD width is at least 350m 
Do someone can confirm these numbers or/and make enhanced version?

For the 16 degree field of view pancam, with 1024 pixels, I get 6 pixels equalling 0.09375 degrees.

A little trigonometry:

Horizon distance = SQRT((h*h)+(2*R*h))

...where h is the height and R the radius of Mars (3397000 metres). If an object appears "n" pixels below a distant (and assumed flat) horizon, then the distance "x" to that object is given by:

x = R*cos(A)+SQRT((R*R*cos(A)*cos(A))+(h*h)+(2*R*h))

...where A is 90+(16*n/1024) degrees.

So in this instance, if h is 50, then x is about 13.5 km. Gusev appears to be fairly flat, so the assumption feels about right.

Andy G

[MichaelT]

For the 16 degree field of view pancam, with 1024 pixels, I get 6 pixels equalling 0.09375 degrees.

Thanks for these equations

The dd images used here are Navcam images (2nd character of the file name is "N") which have a field of view of ~45 degrees. Therefore the angle per pixel is 45°/1024 px = 0.043945 °/px.

Using that number for the calculation yields a distance of 8.5 km for the dust devil. Which is still close to 10 km.

Michael

[AndyG]

Thanks for these equations

The dd images used here are Navcam images (2nd character of the file name is "N") which have a field of view of ~45 degrees. Therefore the angle per pixel is 45°/1024 px = 0.043945 °/px.

Ah! Mea culpa! ;-)

Andy G

[Mongo]

So these 'dust-devils' can reach 300m in diameter, and according to this website, Martian dust-devils can reach nearly 8km in height.. That seems awfully large for a dust-devil. From what I can find on the Web, on Earth the largest dust-devils reach about 100m in diameter, and maybe 1km in height.


It does seem to be the rule that on Mars, everything is an order of magnitude bigger than on Earth.

Bill

[um3k]

It does seem to be the rule that on Mars, everything is an order of magnitude bigger than on Earth.

"Mars is the planetary equivalent of Texas."

[dilo]

Michael, your images are really impressive , congratulations!...
I'm amazed by clear stripes visible in the last animation, suggesting weak foreground DD... at this point, is very probable that Spirit pictured others like this in previous Sols but nobody noticed it up to now!
Anyway, should be very interesting to take a PanCam image of the largest, bigger DD like last one! I think that it can be done through a modification of the "DD catcher software" already used fo NavCam...
AndyG, can you kindly demonstrate the second formula you report? thanks...