Thanks, James, nice to see your stuff again!

Here's the Oppy version of the vanishing-track hazcam movies, sols 1232 and 1254. We're not seeing the complete erasing of tracks, but then there's not as much loose dust on the ground to drift around and cover the tracks at Meridiani as at Gusev:
Click to view attachment
Even though tau has dropped a bit at Meridiani, there's still the risk of settling dust on the arrays reducing power. We need more wind gusts. This comparison of pancams from 1254 and 1255 shows that at least some wind is continuing:
Click to view attachment

I think that's just imperfect matching in the seam in the two images that make up the mosaic.

That's an artifact of my processing. Because the pointing is slightly different on the two observations the blend line between the image pairs is in a slightly different place. So in that little 'uplift' area you are seeing a comparison between the top image of one and the bottom of the other, which, as the the pancam does not rotate exactly about the sensor results in a slightly different viewpoint and hence apparet motion.

Your right about nearly all the movement being from berries in the tracks. I'm sure I did find one in an undisturbed area, but i'll be darned if I can see it now.

James

fredK

The animation of Oppy's vanishing tracks looks very interesting. It appears as if there are more "berries" visible in the tracks of the later image than in the earlier image. It looks to me as if the tracks are being eroded away rather than filled in. Do we have a clean sweep situation here?

It's the terrain returning to its normal state - soil with berries on top.

Berries 'shadow' window from soil
Squish berries into soil
Soil exposed
Soil blown away until berries begin to re-appear
Once they reappear, the soil is shadowed once again.

Doug

I agree, so I will hazard a prediction that the tracks around VC in the next HiRISE images will appear both fainter and darker than in the previous view - a closer match to the background - although the wheel indentations will still be visible to the MER.
In further keeping with the 'clean sweep', I'll predict that the dark streaks in the NE corner will be lighter than before - more closely matching the background - and only gradually darken again as post-storm gusts remove the newly-deposited dust.

We're not having a erosion/deposition debate again. It got overly heated and personal last time. It's not happening again.

Doug

This windy episode has been educational, and all of the before and after pictures/animations people have been posting have been amazing. If the rovers survive this experience and capture some ground truth that can be correlated to the new orbiter imagery, we should learn something.

I'm confused. My MMB update tonight downloaded 79 images from both rovers; more from each of them than we have seen in a while. That would seem to be positive energy news, but the latest Themis dust map from 8/2 to 8/5 doesn't appear to be such good news.

Off topic posts moved - http://www.unmannedspaceflight.com/index.php?showtopic=2549

according to Mark Lemmon: tau = 4.0 on Sol 1255

A new press release from JPL: http://www.jpl.nasa.gov/news/news.cfm?release=2007-087a

Batteries full, temperatures rising and the rover is power positive.

Now that's good news!

I never expected to be so happy with such news (tau=4!).
Let's wait and see if this trend continues and we can see the resume of activities soon.

These little machines (and the people behind them!!!) are really incredible.

For those of you who may have missed it, MSNBC posted an article on the dust storm and MER health on 8 August. Full article is at http://www.msnbc.msn.com/id/20180818/ Excerpts featuring Steve Squyres are posted below:

Of the two rovers, Spirit, in the Columbia Hills of Gusev Crater is doing pretty well, said Steve Squyres, lead Mars Exploration Rover scientist from Cornell University in Ithaca, New York.
"We need to be careful about how much power we consume, so we're not doing any driving for now. But we're making daily science observations, and the vehicle is in good health," Squyres told Space.com. "In fact, in just the past couple of weeks we've made the first observations ever of substantial movement of wind ripples on the Martian surface. So I feel good about Spirit for the moment."
At Opportunity's locale, atmospheric opacity is still very high and solar array power available to the robot remains low, between 130-140 watt hours.
"The power levels have been lower there, and Opportunity is performing only bare-bones survival activities, communicating with us once every three sols," Squyres observed.
Squyres said that there are two concerns with Opportunity: One is that there's need to keep the vehicle "power positive" — to make sure that it generates more power than it consumes. The other is that the rover must keep its electronic innards warm enough.
"The difficult thing about this is that the way you stay power positive is by not consuming energy, and the way you stay warm is by consuming energy. So it's a matter of finding the right balance. We're doing OK so far ... but it's day to day," Squyres said.
If things get really tough for Opportunity, the vehicle will take matters into its own hands, Squyres explained, invoking its onboard "fault protection" capabilities.
"This hasn't happened yet, but if it does, the vehicle will go into a safe mode where it doesn't send us any data at all," Squyres said. "Instead, it will just sleep all day and all night, using what power it has to keep warm and only waking up for a short period each day to listen for commands from Earth. In a situation like that, we might decide to leave it alone for awhile until we got news from orbital images that the skies were beginning to clear."
"I still feel very good about both rovers' chances of survival," he said. "We've got two things going for us. One is just that these are damn tough machines. The other is that even though they're at the mercy of the Martian environment — if they get very low on power — the Martian environment is actually pretty merciful during a major dust storm."
It turns out — given all that dust flittering about in Mars' atmosphere — the temperatures don't get nearly as cold at night. And that means that nighttime survival for a rover becomes much easier than it is when the skies are clear, Squyres advised. "So I think there's a good chance we're going to ride this out," he predicted.
Indeed, late last week, the news from Opportunity was good. Downlink data from the robot showed that the vehicle was in excellent health.
"The batteries are fully topped off, and the minimum nighttime temperatures have
still been within the acceptable range," Squyres reported. "In fact, we may decide soon to use some of that battery energy to conduct some science activities. This has a double benefit: It provides us with improved insight into what the weather is doing, and by running the computer inside the rover it also warms the vehicle."
And if the Mars machinery does conk out — does Squyres have any words in terms of Last Rites?
"Sorry, but you'll get no Last Rites from me unless the time for that has come!"

That was published on Space.com yesterday - and is superceeded somewhat by the Aug 7th JPL article

http://marsrovers.jpl.nasa.gov/newsroom/pr.../20070807a.html

Doug

Now that's a .sig!

Lemmon says Oppy has dropped a tiny bit to 3.9. Perhaps this is the start of the predicted slow decay of tau as the dust gradually settles.

The bulk of the activity seems to have started closer/up-wind from Opportunity, with Spirit's taus responding later with a corresponding increase. Hopefully, the two taus converging represents the storm diffusing on the global scale. I say hopefully because a globally high/medium tau should suppress any further local storms that would add more dust into the atmosphere, giving this dust a chance to settle out.

I, for one, am ready to see the dust finally begin to settle out. If the globally distributed dust will now simply float vertically downward, wouldn't the rovers be best served if they could be parked on slopes that would optimize the photons the panels could collect while minimizing their horizontal surface area?

I certainly hope that not a lot of dust settles down on the rovers. But if a lot of dust did fall on Opportunity, I wonder if it would be worth driving back to the wind-swept/-deposited streaks area for some dustoff.

We got cleaned near to here as well..not just right over at those streaks...probably because of the high winds related to the start of the storm

Doug

I'd tend to agree Doug. I think it's logical to look at the cleaning events seen by both rovers before the storm as indicators that local winds were starting to pick up this time of year. Since less solar energy is reaching the surface now that the skies have gotten uniformly mucky, I'm not sure we can count on the wind to do much in terms of cleaning until this has passed.

Unfortunately, it looks like the deposition has already begun. Here's an L456 color animation of Opportunity's sundial, sol 1219 through 1259. Click through for the full dial image:


The brightness of the images isn't accurate at all, nor do I trust the magnet to be a great indicator of dust on the panels (the magnet is probably pulling down more out of the passing dusty air than is naturally falling onto the rover deck), but even the dial itself looks significantly dustier after sol 1235.

Nice animation Slinted as well as the dust deposition it shows how the shadow of the dial starts out nice and crisp and then blurrs almost to vanishing point and then starts to get sharper again.
Let's hope the shadow gets darker and sharper faster than the dust is covering the the solar panels.

Roy

Judging from the deposits on the sundial
http://qt.exploratorium.edu/mars/opportuni...W0P2124L4M1.JPG
the wind is blowing dust up the bay between Cape Verde and Cape St. Mary and it is falling out on Oppy.

Moving Oppy toward the tip of Cape Verde could help.

We don't know anywhere near enough about the local weather or the fluid dynamics of the crater topography to know where might or might not be good for cleaning.

Doug

All the evidence of winds we've seen in recent weeks has pointed to winds coming roughly from the north, so towards the rim rather than away from it.

On a related topic, after Rocker's post a few posts back, I've had this image in my head of a solar powered rover equiped with basic meteorological equipment, most importantly wind speed/direction, and perhaps a topographic model of its surroundings with the software to do some basic fluid dynamical modelling (yes, that's right; bear with me!). Then my image is of the rover caught in a severe dust storm, with array power dwindling, but the rover deciding where to park to maximize the power it receives, both in terms of incidence angle of light and in terms of winds that can remove dust from the arrays. Presumably it would be best to park with arrays tilted somewhat into the wind, in places where the local topography maximizes wind speed. The rover would have to decide if the energy required to drive would be offset by the power gain at the new location.

The point of this is obviously not a practical one - rover planners have enough on their plates already without having to put together such an elaborate scheme, which could be made moot with rtg's anyway. I just found it a compelling image, with the rover seeming like a living organism trying to survive, perhaps the way a lizard finds a good spot to bask in the sun, but must balance basking with the need to forage for food.

Or perhaps the long wait for the storm to subside is starting to affect my mind...

We may be entering the most dangerous part of the storm...when the dust settles. We don't know how much wind will clear the rover during dust settling.

Ya think?

How's about getting one wheel up on a rock, and letting it fall to shake some dust off?

Ummm, have you seen any large rocks on the plains of Meridiani? It didn't seem to help Spirit remove dust at Low Ridge...

That's a good idea; like that. One problem is that fluid dynamics requires a fair amount of computing power. Mind you, the MER team have done amazing things with such puny processors, so who knows. I expect most missions will continue to use solar arrays, with only an occasional large mission like MSL using RTGs (5 kg of Plutonium is not exactly easy to come by ).

You guys actually had me thinking about images I've seen of reptiles and insects which climb sand dunes in a South American coastal desert (Atacama?) to collect and drink condensing dew on their bodies from the onshore winds.

That may be more than we can expect from a Martian rover, but thankfully Opportunity had enough power to return some images today showing changes on the surface nearby.
Click to view attachment

It's not just you. I too find myself wanting to just go outside and sun myself on a hot rock.

Of course, that's not easy in Seattle . . .

--Greg

I suppose in Cosmics animation, the lack of change in the old tracks as opposed to the new tracks, is due to the loose dust from the old tracks having already been blown away before the storm.

Roy

On 6 July when Oppy reached Tau 4 and 255Whr we put out heads in our hands in despair. Now Tau 4 is a reason for rejoicing.

Out of interest, given the thin atmosphere and consequent small dust particle size limitation, just how much dust is there aloft?

When Tau reached 3.3 early July, Steve Squyres mentioned that there was "enough dust to coat the planet to about the thickness of a human hair."

How does that translate into reduced electrical power from the solar panels?

It means that eventually there will be more dust falling onto the panels ( say in a month or two) than there was at the equivalent time last (martian) year but for now the situation is mostly still up in the air. The last confirmed value before the storm was that the panels were producing approx 85% of their theoretical (clean) max. They definitely got cleaner initially and are doing OK right now (probably) but will get dirtier faster this year.

I wonder if we can argue along these lines. Imagine that initially there's enough dust in the air to reduce direct sunlight by some large tau, and zero dust on the arrays. Call the total mass of dust per square metre of ground surface, integrated vertically to the top of the atmosphere, sigma. (There should be some relation between tau and sigma, but that'll depend on the details of the dust.) Let's suppose that some weeks from now, after the storm, we have total mass sigma*A dust on the arrays, where A is the array surface area, and no dust in the atmosphere. (Speaking loosely, "all of the dust falls down" - this is just an approximation to make the problem easier to work out.)

Then does that mean that the amount of direct sunlight that makes it through the dust on the arrays is again given by exp^(-tau)? In other words, would you get the same direct attenuation regardless of whether the dust is up in the air or on the arrays? I would think "yes".

But scattered light is more important at large tau. Will the scattered light that makes it to the arrays be the same before and after? I find this a tough question, and it may depend on the array's ability to absorb at various incidence angles. These arrays are probably designed to reflect least at normal incidence, so this could conceivably make total array power worse after the storm. Or maybe the answer turns out to be that the scattered and direct light are exactly the same before and after. Then we can expect similarly low array power after the storm as we have now.

Of course crucial to this argument is where the dust lands. If it spreads to areas where tau is lower than at Oppy, we should get less dust deposited than is in the air above Oppy now.

And of course as the dust settles more gusts could blow it away. For this to work (which it has in the past!) it has to be easier to remove dust from the arrays than from the ground.

There are some important things to remember, here...

1. The dust mass raised into atmospheric suspension by this storm is not evenly distributed throughout Mars' atmosphere. As has become very obvious, the atmosphere in some places is holding more dust than in others. And while the dust is being circulated semi-globally, circulation patterns will not generate a homogeneous dust suspension around the gobe.

2. The dust in the air column directly above a point on the Martian surface will not fall straight down onto that surface. The air moves in relation to the planet -- often circulating from the evaporating pole to the condensing pole.

3. While a good amount of the dust in the air right now will be deposited all around the globe, a preferential amount will be deposited in places where large-scale eddies in the circulation pattern cause it to drop out of suspension. Also, a preferential amount ought to be deposited at the condensing pole. (The air can hold more dust the warmer it is, so the air over the poles will tend to drop dust more quickly than air over the equator. Add to that the tendency for the airmass to sink as it flows to the condensing pole, and you get decent conditions for preferential pole deposition.)

It's always tempting to think of other planets in general terms, and we need to remember that they're usually a heck of a lot more complex than we expect.

-the other Doug

Too true, oD, and there remains only the extension of your large scale considerations to the smaller scales:
4. For a given horizontal wind velocity, any portion of the Martian surface - from the scale of a canyon or crater bottom to the lee side of a pebble or ripple - will act as a dust source (i.e. eroding) or a dust trap. Assuming that wind velocities, after a storm, decline gradually over time, the dust will not just "drop", but will be caught - one grain at a time - in traps, leaving the sources more or less clean.
Let us hope that the MER solar arrays remain as sources, not traps, at the gust velocities.

Other Doug - you mention that the air can hold more dust the warmer it is, but my early-morning head says that's counter-intuitive. Cold air is denser - dust will fall through it more slowly. So isn't the carrying capacity for cold air, other things being equal, going to be higher than that for warm air?

Andy

With very small particles, as they must be on Mars, one of the forces keeping particles suspended is Brownian motion. These forces increase with temperature. Maybe that would account for it

http://en.wikipedia.org/wiki/Brownian_motion

The main reason that warmer air has a higher carrying capacity is that it is inherently more energetic. That is basically the slightly more fundamental reason behind Abalone's point about Brownian motion. Warmer air also leads to more energetic atmospheric dynamics ie higher winds which also increases the kinetic energy of the molecules. The dust being carried by this air is being kept aloft by both the higher kinetic energy of the warmer molecules (ie the brownian motion) and also being lifted\lofted by the much higher winds.

Indeed. That's why it's important to break the problem into more managable pieces.

In this case, we have (at least) two big sub-questions:

1. What will be the distribution (global and local) of dust settling, which determines how much total mass of dust will collect on Oppy's solar arrays by the time the storm ends.

2. How much effect will that accumulated dust on the arrays have in terms of reducing power?

In my post above I was only concerned with question 2. It's certainly a crucial part of this. If after the dust settles some fraction x of the total dust currently suspended above Oppy ends up on the arrays, what effect will that fraction x have on power? (Of course the dust doesn't fall straight down! But a global storm of this sort should homogenize quite effectively, so the dust suspended "here" should be pretty much the same as the dust "there". What matters is how much dust settles!)

My thinking is that the fraction that does settle on the arrays will have roughly the same effect on power as the equivalent amount suspended in air. If I'm right, our only hope is question 1, that is inhomogeneous deposition or additional gust-driven removal from the arrays as I said above.

I don't think the dust attenuation is the same. I'm just going to let my diagram do the talking...

Excellent sketch Edward. I had a similar idea, but was not able to express it as clearly.

Would a water analogy work here? If you had a fixed amount of sediment in a column, say 100 meters high and the same amount of sediment collected on a flat surface, water would be able to penetrate the tall column of sediments much more readily than the thin, more densly packed layer.

Exactly the sort of thing I was wondering about. This should depend on how many "layers tall" the dust will be on the arrays. If it's just one or a fraction of one, I'd think the power loss would be the same as for the equivalent amount of suspended dust.

But I suspect it's more than one, if we're talking about a layer corresponding to a few tau. But I'd think the dust is probably not very spherical, maybe quite rough, so it's not clear to me how scattered light would propagate through it. On top of that, when the dust is stacked on the arrays, wavelength scale effects are very important, ie diffraction. That can completely throw off your intuition - light doesn't just travel along rays and bounce.

A related question is how many multiple scatterings does each photon typically experience. Off the top of my head I'd guess it's of order tau.

I understand that I have over simplified with the round dust particles. If, for example, the dust were extremely fluffy, there would be almost no difference between being suspended and being settled. I suspect that the answer is somewhere in between. At least some of the potenial light paths will be cut off due to particles being in contact with one another.

The two-D diagram also seems to cut paths off that wouldn't be in the realworld (three-D). I still think it illustrates what I wanted to say - which is that once the density gets high enough, the light paths start to disappear.

Fredk's point about wave length issues are well taken. That will be highly dependant on particle sizes, the wave lengths the solar panels use, and how thick the dust layer ultimately is. All of which I am not sure about.

Frankly, I don't think we're going to see that much greater an accumulation rate on the solar panels after the storm than we did in prior dusty seasons. And I definitely don't think it will accumulate in a different fashion that it already has.

We have some decent (if indirect) evidence that winds blow around the inside of Martian craters; Oppy did't get nearly as dirty as Spirit during the time she spent in Endurance, right? That was probably due to wind enhancement within the crater bowl.

I have a feeling that the best place for Oppy to go, once tau lightens up a bit, will be *down*... into the windy upper reaches of Duck Bay. Yes, dust will fall, but with higher winds, it will be less likely to stick.

-the other Doug