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Tom Ames
2004-Mar-21, 05:49 AM
Does anyone have any ideas as to why the dark streaks (presumably dust devil trails) in photos of Gusev seem to mostly originate from surface features such as craters?

Example (http://marsoweb.nas.nasa.gov/landingsites/mer2003/mocs/Images/R07-01606/).

Until the discussion about the dark interior of Bonneville, I had assumed that the streaks were due to the exposure of darker material caused by the dust devils' stripping the light-colored soil from above it.

I'm thinking now that the darker material is of finer grain than the lighter, and finds its way into the interiors of craters. Dust devils that pass over a crater then pick up a large volume of this dust, and distribute it along their paths.

Kaptain K
2004-Mar-21, 06:07 AM
First of all, this is an educated WAG (wild @$$ guess).
Second, I am assuming that I can go from the terrestrial mechanisms to analogous situations on Mars.

1) When air is heated, it becomes less dense and rises.
2) Airflow over a smooth surface is fairly laminar (non-turbulent).
3) When laminar airflow encounters an irregularity (such as a crater) it becomes turbulent.
4) Turbulence + vertical motion + coriolis forces results in a vertical vortex (dust devil), with its point of origin at or near the crater.

* Note: Laminar flow does not need an external catalyst to become turbulent. Once certain criteria are met, turbulence is inevitable. That is why you see tracks without obvious trigger points.

Manchurian Taikonaut
2004-Mar-21, 06:21 AM
it would be great if just before the power on the rovers die out, a massive dust storms and hurricane start and the spirit takes wonderful images of the formations of these stroms

Squink
2004-Mar-21, 06:21 AM
Does anyone have any ideas as to why the dark streaks (presumably dust devil trails) in photos of Gusev seem to mostly originate from surface features such as craters?
In this daytime infrared image (http://www.spaceref.com/news/viewpr.html?pid=7590), the floors of craters look hotter (whiter) than the surrounding areas. That heat could produce the updrafts needed to form dust devils.


Just for the record, the craters and ejecta blankets also stay warmer than surrounding areas at night (http://www.marstoday.com/viewsr.html?pid=7255).

Tom Ames
2004-Mar-21, 06:23 AM
1) When air is heated, it becomes less dense and rises.
2) Airflow over a smooth surface is fairly laminar (non-turbulent).
3) When laminar airflow encounters an irregularity (such as a crater) it becomes turbulent.
4) Turbulence + vertical motion + coriolis forces results in a vertical vortex (dust devil), with its point of origin at or near the crater.

* Note: Laminar flow does not need an external catalyst to become turbulent. Once certain criteria are met, turbulence is inevitable. That is why you see tracks without obvious trigger points.

Thanks! That was very helpful. Your idea is more consistent with the evidence than mine was.

ToSeek
2004-Mar-21, 02:21 PM
it would be great if just before the power on the rovers die out, a massive dust storms and hurricane start and the spirit takes wonderful images of the formations of these stroms

Odds are against - we're actually heading toward winter where the Rovers are, so the odds of getting a dust storm are reducing.

Tripp
2004-Mar-22, 04:55 AM
First of all, this is an educated WAG (wild @$$ guess).
Second, I am assuming that I can go from the terrestrial mechanisms to analogous situations on Mars.

1) When air is heated, it becomes less dense and rises.
2) Airflow over a smooth surface is fairly laminar (non-turbulent).
3) When laminar airflow encounters an irregularity (such as a crater) it becomes turbulent.
4) Turbulence + vertical motion + coriolis forces results in a vertical vortex (dust devil), with its point of origin at or near the crater.

* Note: Laminar flow does not need an external catalyst to become turbulent. Once certain criteria are met, turbulence is inevitable. That is why you see tracks without obvious trigger points.

Good try ("WAG") given commonly recognized mechnics but not really how things work despite all those aspects such as Corriolis, turbulence, and vertical motion being involved.

The thermoclinal forces in dustdevils and larger hurricanes or global Mars cyclonic dust storms are all fueled by thermodynamics. Thermodynamics, is the initial force resulting in the creation of the dustdevils, with the heat absobed by the ground surface causing a temperature inversion, wherein the heat wants to rise. Once this heat begins to rise along with an aspect f the air motion crossing latitudinal lines, then Corriolis becomes involved creating a cyclonic wind pattern.

There is really no specific, singular "trigger" location, as these are determined by the randomness of nature with numerous factors playing a part. What is needed is a sufficiently large area of ground with its aspect inclined at such a degree to the sun to permit build up of a disproportionate heating in comparision to surrounding areas.

While dustdevils might originate in sufficiently large areas of impact craters, and become more likely if this is darker, more thermally absorptive terrain, so too are dustdevils likely to have their thermoclinal flow disrupted by terrain features with sufficient relief.

Below is an image of a dustdevil caught *LIVE* traveling up the wall of a crater. The dustdevil dark path is seen in the lower right quadrant of the image with the dustdevil itself visble midway up the right edge of the image. In all liklihood this dustdevil will not traverse outside the crater and will die on the crater rim as the other evident dustdevils to the north also died. The crater's steep slope inhibits the cyclonic winds of these dustdevils and causes them to dissipate as dioes the lack of thermal energy build up on the incline of the slope remove fueling thermoclinal energies.
http://www.spaceflightnow.com/news/n0208/23mgsdust/dust.jpg

Staiduk
2004-Mar-22, 03:24 PM
Sorry; just gotta say it - that is SO cool!

George
2004-Mar-26, 03:37 PM
The thermoclinal forces in dustdevils and larger hurricanes or global Mars cyclonic dust storms are all fueled by thermodynamics. Thermodynamics, is the initial force resulting in the creation of the dustdevils, with the heat absobed by the ground surface causing a temperature inversion, wherein the heat wants to rise. Once this heat begins to rise along with an aspect f the air motion crossing latitudinal lines, then Corriolis becomes involved creating a cyclonic wind pattern.

I would be surprised if the Corriolis effect had much, if anything, to do with a dust devil.

Tripp
2004-Mar-26, 09:55 PM
I would be surprised if the Corriolis effect had much, if anything, to do with a dust devil.


Surprise. Dustdevils here or on Mars, just as torndados and hurricanes and even the Atlantic Ocean's gulf stream as well as the direction water flows when running down your bathroom drain, all owe the direction of rotation to the Corriolis force, which is clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere.

Rue
2004-Mar-26, 10:09 PM
.... as well as the direction water flows when running down your bathroom drain, all owe the direction of rotation to the Corriolis force, which is clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere.

(Cue James Earl Jones)
This is Bad Astronomy.
<link> (http://www.badastronomy.com/bad/misc/coriolis.html)

George
2004-Mar-26, 10:56 PM
Tripp...Your picture is great, btw.

I noticed the solar angle seems almost overhead. If so, that must be one whale of a dust devil. Wow.

Tripp
2004-Mar-26, 10:58 PM
.... as well as the direction water flows when running down your bathroom drain, all owe the direction of rotation to the Corriolis force, which is clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere.

(Cue James Earl Jones)
This is Bad Astronomy.
<link> (http://www.badastronomy.com/bad/misc/coriolis.html)

Your sources are true but not overtly accurate in claiming this is "bad science". To quote the second source at your cited reference "Experiments have been done in both the northern and southern hemispheres to verify that under carefully controlled conditions, bathtubs drain in opposite directions due to the Coriolis acceleration from the Earth's rotation (2)." The Coriolis force is present and in such relative strangth as to be able to effect the rotation; the point is a valid test.

The first source's statement that "The Coriolis force is so small, that it plays no role in determining the direction of rotation of a draining sink anymore than it does the direction of a spinning CD" is both inaccurate and also an obfuscation in intrudcing the rotation of the CD.

Of course there are times the force induced by traversing lines of latitude (Coriolis) is irrelevant to other greater forces --- otherwise there would be more often a slip twixt a cup and a lip.

George
2004-Mar-27, 06:35 AM
.... as well as the direction water flows when running down your bathroom drain, all owe the direction of rotation to the Corriolis force, which is clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere.

(Cue James Earl Jones)
This is Bad Astronomy.
<link> (http://www.badastronomy.com/bad/misc/coriolis.html)

Your sources are true but not overtly accurate in claiming this is "bad science". To quote the second source at your cited reference "Experiments have been done in both the northern and southern hemispheres to verify that under carefully controlled conditions, bathtubs drain in opposite directions due to the Coriolis acceleration from the Earth's rotation (2)." The Coriolis force is present and in such relative strangth as to be able to effect the rotation; the point is a valid test.
Here is the preceding portion of your referenced quote...

To detect its effect on your bathtub, you would have to get out and wait until the motion in the water is far less than one rotation per day. This would require removing thermal currents, vibration, and any other sources of noise. Under such conditions, never occurring in the typical home, you WOULD see an effect.

Eliminating all the other forces is the only time it can be seen, apparently. It would be like saying anyone can drive 100 mph in LA at 5pm without stating that all other cars must be removed first. :-?


The first source's statement that "The Coriolis force is so small, that it plays no role in determining the direction of rotation of a draining sink anymore than it does the direction of a spinning CD" is both inaccurate and also an obfuscation in intrudcing the rotation of the CD.
The purpose of Bad Astronomy is to help others eschew obfuscation. Even if one set up a lab to do the test, you would never get any kind of a "whirlpool" from this force alone.

Tripp
2004-Mar-27, 07:52 AM
The purpose of Bad Astronomy is to help others eschew obfuscation. Even if one set up a lab to do the test, you would never get any kind of a "whirlpool" from this force alone.

Eschew obfuscation? It would be nice if the cited sources knew something about which they speak, do not create irrelvant examples and actually teach the concept positively rather than emphatically state negatives.

Quoting the source, ". This would require removing thermal currents, vibration, and any other sources of noise. Under such conditions, never occurring in the typical home, you WOULD see an effect"

Corriolis applies to any particle moving across latitudinal lines of planet rotation. It is irrelelvant to stationary particles or particles moving paraellel to lines of latitude. It is irrelevant how the particle's motion is caused, be it by vibration or thermal currents, just so long as they are moving. It has nothing to do with CDs spinning nor traffic on the expressway with or without other cars present. What is relevant is the hydrologic gradient (from draining the sink) not being so great that the Corriolis is made entirely irrelevant on the volume of water; i.e. if you drain the basin too quickly there will be no rotation.

The statement was that Corriolis does have an effect and can be in evidence even in a sink, and it can. You may want to argue against that position, yet I've clearly cited where even the referenced sources do explicitly contradict you which tends to make these sources themselves more than a little picciune.

George
2004-Mar-27, 02:56 PM
The purpose of Bad Astronomy is to help others eschew obfuscation. Even if one set up a lab to do the test, you would never get any kind of a "whirlpool" from this force alone.

Eschew obfuscation? Ok... I like oxymorons. :)


The statement was that Corriolis does have an effect and can be in evidence even in a sink, and it can. You may want to argue against that position, yet I've clearly cited where even the referenced sources do explicitly contradict you which tends to make these sources themselves more than a little picciune.

No one is saying the Coriolis (I was spelling it wrong, too) effect is not real. However, it's use is misapplied for small bodies of water. (e.g. sinks, bathtubs). The analogies are to emphasize the degree to which it is being misapplied. In every day use, other forces acting on, and within, the water will dominate and determine the actual spin rotation when you....pull the plug in the tub.

PhantomWolf
2004-Mar-28, 06:19 AM
I know that BA doesn't like huge C&P's but hopefully he'll bear with me in attempting to end this arguement. This is the response on a meteorical site about tornados on Earth by a guy called Jim Allsopp. I think he hits it perfectly.


The previous responses were correct in that the coriolis force is the reason air circulates in a clockwise direction around high pressure and
counterclockwise around low pressure in the northern hemisphere. The coriolis force can explain the rotation in large scale high and low pressure areas including hurricanes. However, the rotation of a tornado is much more complicated. Tornadoes in the northern hemisphere can rotate in either direction but counterclockwise rotation is much more common. The rotation is produced by wind shears and pressure forces in and near the parent thunderstorm. Thunderstorms form when warm moist air rises rapidly upward. This upward current of air within a thunderstorm is referred to as an updraft. If sufficient vertical wind shear exists, this updraft will rotate. Vertical wind shear is a change in wind speed and/or direction from the ground up through the atmosphere. If the wind speed increases rapidly with height and/or if the wind direction turns clockwise with height, air being drawn in toward the thunderstorm updraft will develop a spin about the horizontal axis. Think of an imaginary paddlewheel floating in the air. If winds blowing across the top of the paddlewheel are stronger than the winds at the bottom of the paddlewheel, it will spin. As the air rises into the updraft, the spin about the horizontal axis becomes a spin about the vertical axis. Imagine if you take a rope and roll it along the ground (it is spinning about the horizontal). Now pick up the middle of the rope but keep rolling it. Now the two dangling ends are spinning about the vertical. But one end is turning clockwise and the other counterclockwise. Strong winds blowing through the storm produce pressure forces within the storm that enhance or suppress the updraft. Most tornadoes in the United States occur in the warm humid air mass ahead of an approaching low pressure area. Because of the coriolis force, winds usually turn clockwise with height. This wind profile enhances the counterclockwise rotating updraft and suppresses the clockwise portion of the updraft. That is why most tornadoes turn counterclockwise. However if winds are from a nearly uniform direction throughout the depth of the storm, both circulations can be maintained. In this case the storm can split producing
both a clockwise and anti-clockwise rotating tornadoes. This has been
documented with radar.

beskeptical
2004-Mar-28, 09:29 AM
http://www.spaceflightnow.com/news/n0208/23mgsdust/dust.jpg

Is that a giant thumbprint on the photo? :o

The Bad Astronomer
2004-Mar-28, 07:18 PM
I guess I will step in.

Tripp, the Coriolis force has essentially zero effect on a bathroom drain. While it is true that it does exist, and does have some effect, it is negligible compared to the much larger variations in the structure of the sink, as well as eddies in the water. For the Coriolis force to have a noticeable effect, the water must be very still, the sink smooth, the drain perfectly level, etc. If you can do this, and make sure the water drips out very slowly, then sure, it affects the way a sink drains. But you need lab conditions to do that.

In real life, the Coriolis effect can be ignored in sink drains.

As for tornadoes, I talked with a meteorologist about this a few years back, and he concurred with what PhantomWolf quoted. Basically, the Coriolis force plays a role in tornado formation, but an indirect one.

But dust devils are not tornadoes, and form in a different manner. I have seen quite a few myself; I watched one form in an alcove-like setting around a building, where winds would flow around the corner, and were channeled by the other walls in the alcove. This caused a rotation, and a stable system due to the continuous wind feeding energy to the devil (as far as I could surmise). I watched it for about ten minutes, fascinated, as leaves and dust spun in a vortex about three or so meters across. It was a sunny day, by the way, IIRC.

I do not know how dust devils form on Mars. I'll look into it when I get a chance, since they're pretty cool. That image form the crater is amazing. And if the links I have on my Coriolis page are incorrect I'll have to check them too.