In addition to the other excellent responses, Andy, I'll also note that one of the driving forces in global Martian dust storms is the heating factor of the dust in the air. Unlike on Earth, where thick clouds that block insolation *cool* the planet below it, a major dust storm tends to make Mars *warmer*. The air is so thin that solar heating of the dust particles in suspension is one of the main factors in atmospheric heating. Increase the amount of dust entrained in the Martian atmosphere and you raise its overall temperature noticeably. This also increases the volume of the atmosphere, which reduces overall average pressures but raises the top of the atmosphere above non-storm altitudes, sometimes by tens of kilometers.

One of the better (IMHO) theories about how dust storms propogate on Mars has this non-homogeneous heating effect creating streams of "warm" air, which pick up and entrain greater amounts of dust, which then warm up and expand the air mass they're entrained in, which causes it to press more forcefully against adjacent, cooler air masses, causing more unstable air, which causes more streams of "warm" air invading new portions of the planet... until you begin to see very complex and energetic atmospheric circulation patterns that drive the storms along for months. It's the very effect of dust entrainment leading to air heating that contributes a lot of the energy which drives the storms.

-the other Doug

Thanks for all the replies. My slightly more awake afternoon head has digested the incoming posts.

But it clearly leads to more questions!

The self-driving storm cycle is then a classic case of positive feedback - warmer air, more storms, more dust, warmer air - until a planet-wide storm is the result. So it's probably a good thing the orbit is as eccentric as it is. If Mars' orbit didn't allow for aphelion insolation some 69% of the perihelion levels, then presumably the Mars we saw would be constantly enshrouded in dust storms?

Andy

You are right, but you have to consider which layers of the atmosphere are affected by this effect.
As far as I have read, only Mars' upper atmosphere is heated by the dust storms while the surface cools. This is also shown by the Viking temperature data I posted earlier in this thread.
The dust has a similar effect on the surface as have clouds on Earth. They both reduce short wave insolation (the rovers receive less power) and, therefore, lead to a subsequent reduction of irradiation of IR radiation from the ground -> the surface layers' daily average temperature drops. But, they also have the effect to reduce the nightly minima. Clouds and dust scatter part of the IR radiation back to the ground and reduce the amount of energy lost to space at night.

The main difference is that dust absorbs s/w radiation which leads to a heating of the upper layers of the dust. Further below, the effect of reduced s/w radiation dominates.
Clouds, on the other hand, reflect most of the s/w radiation back into space with almost no heating effect.

Michael

Dust heats the lower atmosphere at low optical depths. When optical depths get above something like 2, absorption of sunlight above the dusty atmosphere layer reduces energy reaching the layer and the daytime average "absorbed-watts-per-cubic-meter-of-air" vs tau curve rolls-over and starts back down as tau increases.

New tau numbers for Spirit were just published. Tau dropped another two tenths, and is now at 3.4 (sol 1285). This is the lowest we've seen since sol 1255.

A very slow, but steady, drop. The background colour gradient is based on the RGB values of the sky in that amazing sky-survey mosaic that they released a few weeks ago.

Doug

Note that the spikey fluctuations in tau have damped down. This is consistent with the dust being relatively uniformly distrubuted, also consistent with the storm transitioning into the exponential decay phase.

I wouldn't know exactly what kind of decay to expect, but if the total amount of dust decays exponentially in time I suppose we'd expect (very roughly!) a linear decay in tau. Of course we should expect departures from linearity due to varying winds etc.

Still, I can imagine a roughly linear decay rate in the Viking storm data plotted here.

This storm seems to be having some unanticipated effects on some of our forum members. Perhaps there's something in the martian air...

Perhaps it's wrong - but because I'm at Europlanet and really busy - I'm kind of glad to have a few weeks of 'rover holiday' . I like new pictures and driving and want these things to last forever....but the break has been kind of nice - a bit like an extra Solar Conjunction.

Or am I the only one who thinks that?

Doug

......cue Tumbleweed

First images for a few days:

http://qt.exploratorium.edu/mars/opportuni...W0P1169L0M1.JPG

Shadows look quite prominent now, the sky must have cleared quite a bit.

Hi all
Here is oppys calibration target in L456 Posted on20/08/07 quite a lot of dust around which to me looks a darker colour than normal.
A breeze would be nice
Roy

The gender discussion now has its own thread
http://www.unmannedspaceflight.com/index.p...=4492&st=15

Do these navcam shots show dirty lenses, or clouds overhead? The lighting changes from shot to shot but a lot of the texture is immobile between images, so presumably it's dust on the optics?

e.g.:
http://qt.exploratorium.edu/mars/opportuni...W0P1585L0M1.JPG
http://qt.exploratorium.edu/mars/opportuni...W0P1585L0M1.JPG
http://qt.exploratorium.edu/mars/opportuni...W0P1585L0M1.JPG
http://qt.exploratorium.edu/mars/opportuni...W0P1585L0M1.JPG

The forward hazcams also look very dirty:
http://qt.exploratorium.edu/mars/opportuni...cam/2007-08-20/

fascinating... :.

My guess is they show dirt.

I think they are clouds.

Images taken during the local dust storm that cleaned Spirit's panels back on sol 420 looked similar:
http://marsrovers.jpl.nasa.gov/gallery/all...8P1584L0M1.HTML

I had been watching the Themis site for updates of their 9 micron atmospheric opacity maps, hoping to see signs of the dust storm's end. When I tried to compare their optical depth opacity scale to the tau values we commonly refer to, I realized that the Themis instrument apparently pegs out at approximately a tau of 0.9.

It is probably not fair to strictly compare MER tau measurements to the 9 micron Themis maps, but we probably will not see prettier Themis maps with less red until tau drops below 1.
Click to view attachment

I'd say dirt. Given that these sky images get a particularly heavy ammount of abuse when stretched to become the JPG's we see, I doubt that a calibrated image would show even 1% of the diversity of brightness that these JPG's show.

Doug

Looks like Oppy has been moving its her IDD:

http://marsrovers.jpl.nasa.gov/gallery/all...W0P1159R0M1.JPG

Check out the thick dust coating. Thank goodness the MI has a dust cover...

... and it looks like he/she/it will be moving again on sol 1271 for the first time since sol 1232!
That's how I read the imaging plan for tosol.

What have we done, it used to be so simple!!!

Rocker, where did you find out how to convert the themis opacity to a tau value? Is that from their sentence "The scale bar's values run from nearly clear (0.05) to roughly a one-third reduction in sunlight (0.40)"? A one third reduction in direct sun gives tau = 1.1, although they say "roughly". As you say, who knows how well 9 microns relates to visible, where Oppy get's most of her power.

Also, I don't get your plot - as I understand it tau = optical depth. What are your dots?

Edit: I see, your horizontal axis is the fraction of direct sun that makes it to the ground. Technically that shouldn't be called optical depth.

The individual sol tau's have been removed from Lemmon's site, and replaced with an updated plot with Oppy, Spirit, and Viking.

Gods, what a monster downpour! not to mention the panels!

...thought early on in the storm they might move Oppy close to the edge to take advantage of any turbulence that could be had by the terrain interface, but it looks like we're remaining at the back of the porch pretty much out of the wind (if there were any). but someone has probably already commented on that (yes, ive been out for a while). anyone know what the wattage is looking like now?

Wow...look at the tire tracks! What goes up certainly must come down...but not here!

Something like slightly less than low-to-mid-two-hundred, give or take a few here and there and add some...

You're right, fredk. I've corrected that in my post. I wanted to try to compare tau to the 9 micron opacity, but when I read an article to learn how to calculate tau from the ratio of sunlight intensities, I guess I got my terminology confused.

Now that I look back on it, I think the Themis site's description ("roughly a one-third reduction in sunlight (0.40)") is good. At tau = 0.40, e^-t = .67.

Just remember, from the best HiRISE images we have of them, neither of the Viking landers was significantly dust-covered prior to this storm.

And they've been through 31 years of Martian weather.

I think the rovers will be relatively clean again. Whether that happens in time for the batteries to remain fully operational, I can't say...

-the other Doug

They should get some nice panoramic pictures. It must look like freshly fallen snow there, albeit a thin layer of it.

Transient or not, this sort of dust accumulation is a force to be reckoned with in future designs. Solar panel effects aside, in the distant future people definitely don't want to be breathing this stuff, ever; gonna have to figure out a way to minimize (or ideally repel) this crap.

Intentionally inflamatory comment regarding manned v unmanned exploration deleted - Doug

At last!
Our brave friend is alive and kicking, with enough power to drive right (or near) to Duck Bay after almost 40 sols of stationary activity.

http://qt.exploratorium.edu/mars/opportuni...cam/2007-08-22/

It's time to resurrect the "Opportunity Route Map" thread.

The left navcam is still well covered with dust by the look of it.

...And it is dust. Here's the sky cropped from five recent navcam images.



The first frame is from the left navcam, as are the last three images, taken in different directions up to nearly two minutes later. The second image is from the right navcam, taken at the same time as the first.

I see a pattern!

Andy

On the road again...!!!

V quick and v v messy stitch before heading out to work...

Click to view attachment

Amazing machines, amazing people, amazing adventure.

Welcome back Oppy. Go get 'em gal!

At last she moves

Never mind the messy, it's great to see a changing view at last ... even if some Entry Pool swimmers might be lamenting...

Really good stitch, Stu, precisely because it's so messy.

Gives you the feeling you're peering out at the surface through a dusty little window... which in a sense we are.

Thanks for the quick map update Tesheiner, and for the pan Stu!

I am surprized at how much dustier the left navcam is than the right. I mean, the left and right navcams are pointing in the same direction only centimetres apart. Does that mean the dust is blowing in clumps? Fortunately the right navcams are always sharper than the left!

We're used to seeing dust on the hazcams, which are low to the ground. For the hazcams, the dust is obviously dust since each grain apears as a large circular disk. For navcam, it's much harder to make out the individual grains, since the focal length is much larger, so depth of field is much lower. That's why those navcams looked to some like they might be showing clouds.

It will be interesting to see how the pancams are doing dust wise, although since their focal lengths are even longer the effect may be even less obvious.

It'd be a shame to have the next rover design not have a small ultrasonic shaker for the front glass of the cams, but I understand harsh environmental conditions might preclude that addition.

Re: Dust in clumps -- I'm thinking more of differential electrostatic charge, perhaps based on position relative to wind direction. Just a wild guess, though.

Maybe, it was due to the angle position of Oppy with respect to the wind direction.

One quick thought before racing out the door (late to work)... perhaps the right camera benefited a bit from the position of the arm.

Yeah, I could see that if the wind was blowing roughly "from the side" of the navcams, ie perpendicular to their optical axes, then if you developed eddies around the PMA that could effect the two navcams differently.

Looking at the dunes under Cabo Verde I wonder if this approach to a reasonably steep segment of the Bay isn't designed to try and find some channeled wind to clean off a bit of the dust before the last of the heavy winds die off.

Opportunity: Daily Update - 8/23/07

Brightening Skies Bolster Opportunity
Opportunity Status for sol 1256 - 1265

Lets all hope that the vibrations of movement will shake most of the dust off..

Unlikely - they're not exactly SUV's doing 60mph over rough ground.

Doug

Perhaps engineers could build this functionality into future solar powered rovers:

YouTube

Rover is runing very very slow on a very smooth floor. You can see all spherules are almost sunken under the trail. Hence the rover won't have much vibration unless it get over on a rock and somewhat fast. These conditions won't happen. Now the engineers are testing with a new technique "blobs away" campaign to remove dust from the lens.