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Posted by: rlorenz Nov 16 2012, 01:59 AM

Evidence of the diurnal pressure cycle in the radiation detector data, and the suggestion of dust devil encounters in the meteorology package (REMS) data.

http://www.jpl.nasa.gov/news/news.php?release=2012-361&cid=release_2012-361

Suggests 'over 20' events with one or more in-situ signatures of dust devil encounters (e.g. pressure drop, wind direction change etc.)
In 12 weeks that means only ~0.25 events per day, which is pretty low compared with Phoenix and Pathfinder
(more like 1-3 / day, depending on event threshold e.g. see http://www.lpl.arizona.edu/~rlorenz/dustdevilpressuredrops.pdf) but we'll need to wait and see this published properly to understand what thresholds were used and what correction factors for observation duty cycle need to be applied.

Posted by: Eyesonmars Nov 16 2012, 10:49 AM

Can we infer anything about the diameter/size of these dust devils ? The press conference slide showed only the wind direction and pressure changes over time but nothing about horizontal wind speed. The DD diameters at Pathfinder, Phoenix(and Gusev ?)varied by over an order of magnitude but that covered an entire DD season( 3 seasons at Gusev, 80 days for PF). Have we reached the peak DD season at Gale yet ?

Posted by: Eyesonmars Nov 16 2012, 11:11 AM

In the press conference it was stated that the RAD levels at the surface were about 50% of cruise values. Is it the dust in the atmosphere that is responsible ? If so the RAD levels should be more sensitive to atmospheric opacity than to surface pressure. Is tau being measure ? If so, from the rover or from orbit? It will be interesting to see what happens to the RAD levels during major dust storms.

Posted by: AndyG Nov 16 2012, 11:17 AM

At the surface 50% of the nominal cruise radiation is being blocked by Mars. It doesn't sound like the atmosphere and/or dust is doing much at all.

Andy

Posted by: Eyesonmars Nov 16 2012, 11:27 AM

So are you saying that it is simply because the planet blocks half the "celestial sphere" ?

Posted by: Drkskywxlt Nov 16 2012, 12:26 PM

Also interesting that the semidiurnal tide seems to be significantly weaker than the diurnal (just from eyeballing), in contrast to some of the other landers. I haven't looked closely enough yet to see if this is a seasonal effect, but this should be the time that both tides have higher amplitude as Mars is approaching equinox.

Posted by: AndyG Nov 16 2012, 01:41 PM

Not simply, Eyesonmars - the RAD "telescope" points upwards. I can't find out how directional it is. But the higher energy cosmic rays will be Mars-shielded.

Andy

Posted by: Eyesonmars Nov 16 2012, 03:17 PM

Andy, the detector has a 65 degree cone of view normal to the rover body ( upward hopefully( i'm kidding))

Here is a quote from the Nov 15 press conference that is the core of my confusion.

"We see a definite pattern related to the daily thermal tides of the atmosphere," said RAD Principal Investigator Don Hassler of the Southwest Research Institute's Boulder, Colo., branch. "The atmosphere provides a level of shielding, and so charged-particle radiation is less when the atmosphere is thicker. Overall, Mars' atmosphere reduces the radiation dose compared to what we saw during the flight to Mars."

Hence my original question. What is it about the atmosphere that provides the shielding ? The gas or the dust? It should be easy to answer once we get RAD data over a wider range of tau and pressure values

Posted by: Eyesonmars Nov 16 2012, 03:46 PM

http://marsmobile.jpl.nasa.gov/msl/multimedia/images/?ImageID=4874

In the above image, from the press conference, it looks like the lowest areas are dominated by dark basaltic dunes. I would think that this would work against typical slope winds. From personal experience living on a mountain slope the diurnal mountain valley breeze is completely suppressed after the mountain slopes get a fresh snow cover when the valley does not. In other words the albedo induced thermal wind and daytime slope winds cancel each other out. Loosely speaking

Posted by: mcaplinger Nov 16 2012, 03:57 PM

I think you'll find that the mass of the dust is negligible from a shielding perspective. From http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.26.4090&rep=rep1&type=pdf if you precipitated all the dust in the middle of a severe dust storm it would only form a layer 18 microns thick.

Posted by: marsophile Nov 16 2012, 04:10 PM

Still nothing on humidity?

Posted by: Eyesonmars Nov 16 2012, 04:26 PM

It is amazing that only 18 microns of dust can completely obscure the surface. I guess the extremely small particle size is the key. So then we should not see the RAD values affected by major dust storms. Still, it will be interesting to watch.
I suppose they rigourously modeled and tested how dust settling on the detector affects results. So i guess that it doesnt matter too much if that 18 um is in the atmosphere or on the detector

Posted by: djellison Nov 16 2012, 09:05 PM

Dust storms will do other things - not just put more dust in the atmosphere. That atmosphere will get warmer, and taller. It's a complex interplay. There will be an effect, I'm sure.

Posted by: serpens Nov 16 2012, 09:49 PM

Since the comparison is cruise to surface intensity, does anyone know if there is any residual magnetic field loop in the vicinity of Gale that would provide a localised magnetosphere effect? If so then would atmospheric pressure changes affect the level of local ionization and hence deflection?

Posted by: Eyesonmars Nov 16 2012, 09:59 PM

A tongue in cheek answer is that the relative humidity is always 100% at sunrise and 0% at midday.
But I'm sure you mean absolute humidity. If/When we do get numbers they will be only a few precipitable microns. Typical earth values are a few precipitable centimeters by comparison.

Posted by: marsophile Nov 16 2012, 10:30 PM

Not sure why it would necessarily be 100% at sunrise unless there was overnight frost, which so far has not been reported.

But the intent of my question related to whether any results were reported at the telecon for the humidity sensor. (None have been posted on the weather site.) An earlier post on this blog drew assurances that the sensor was not damaged, even though it is on the damaged boom. Did no one ask about it at the telecon? Is it still being commissioned?

Posted by: Fran Ontanaya Nov 17 2012, 01:23 AM

MGS's magnetic field map is http://www.nasa.gov/centers/goddard/images/content/135896main_pnas_102_42_connerney_fig1.tif.

Posted by: mcaplinger Nov 17 2012, 01:43 AM

I think even the strongest remnant fields are so weak (~30 nT as opposed to Earth's field of ~30,000 to 60,000 nT) as to have little effect.

Posted by: Reed Nov 17 2012, 02:45 AM

On a somewhat related noted, it was mentioned here http://astrogeology.usgs.gov/news/item/sol-74-update-on-curiosity-from-usgs-scientist-ken-herkenhoff-squeezing-more-in that they planned early morning chemcam observations to look for frost.

Posted by: MahFL Nov 17 2012, 03:25 AM

Re radiation. they said Mars and the thin atmosphere does block some radiation. But different heavier particles are created with the atmosphere interaction which are as or more dangerous than the cruise radiation. Over all it seems that a trip to Mars and back would be the lifetime limit for an astronaut. But if you've gone to Mars and back I'd think you'd be wanting to spend a lot of time on Earth, lol.

Posted by: Zelenyikot Nov 17 2012, 04:46 AM

It is interesting that it will be better as protection against radiation: residual magnetization or thicker layer of the atmosphere in lowlands, for example in Hellas.

http://www.esa.int/esaCP/SEMLQ71DU8E_index_0.html

Posted by: serpens Nov 17 2012, 05:52 AM

Very weak indeed, but wouldn't this, combined with the solar wind induced magnetosphere boundary provide for a large part of the observed RAD attenuation?

Posted by: mcaplinger Nov 17 2012, 06:27 AM

I think the column density of the atmosphere is a much more significant effect. At least that's the only one we looked at when we did our radiation analyses for the electronics.