MSL - Astronomical Observations, Phobos/Deimos, planetary/celestial observations and more Options

[fredk]

I didn't realize they could trigger from REMS. Very cool. It is hard to say how large it is. All we have to go on is that the entire visible slope of the central mound darkens in frame 2, including the little peak way off to the left. But the visible local horizon doesn't darken noticably. So the dust must be beyond the local horizon, which, given the extent of the darkening, suggests it is very large. Presumably from the timing and some estimate of wind speed we could get an estimate of the size.

Ed - I can't see any way this could be exposure variations. Between the first two frames, eg, the central mound darkens while the foreground stays about the same brightness. Exposure variations would brighten/darken everything uniformly, or maybe in some nonlinear way accounting for lut's and stretching. But whatever the details, all pixels with some particular pixel value must be mapped to the same pixel value after the exposure change (if the scene remains the same, of course). You shouldn't see some parts of the frame with some shade of grey mapped to different shades of grey than other parts with the same original shade of grey.

I'd add one more change to Ed's list. For the first two frames, the sky is brighter than in the other frames. This is all consistent with a gust passing over. The extra dust in the gust would brighten the sky as it passes over, while shading the ground.

[Gerald]

All we have to go on is that the entire visible slope of the central mound darkens in frame 2, including the little peak way off to the left. But the visible local horizon doesn't darken noticably. ...

Similar effects can also be obtained by color filtering.
As example, based on this Sol 364 ML image, switching between the brightness and the blue channel, brightness adjusted.
A local dust cloud may work like a color filter, brightness adjustment by exposure time.

[dvandorn]

Must it have been a dust cloud? The air at Gale has been getting clearer and less dusty lately, I thought.

What are the possibilities of either clouds or one of the larger-size dust devils casting these shadows?

-the other Doug

[Deimos]

REMS cannot currently be used to trigger imaging. The general idea does go way back, but I don't know of any MSL related implementation. Any other images document the event time frame?

[PaulH51]

We caught what appears to be a serious wind gust on sol 368.....

Could this 'event' have been caused by the transit of Phobos? By reducing the light level (when the darkened HazCam was captured) or by the reduced lighting causing an abrupt fluctuation in the air temperature?

Note: the MastCam 100 was taking a sequence of images of such a transit at the same time as the 'event'. Sol 308 Midnight Planets

[dvandorn]

Are you saying that we could have been seeing the shadow of Phobos passing over the terrain?

Cooool!!!

-the other Doug

[Gerald]

First, for a better illustration of the filtering effect of an assumed dd-related local dust cloud, I've combined an rgb image from two Sol 368 grey-scale Rear Hazcam Left B images (red from one image, green and blue from the other, so actually just two color channels) :


Local dust devils don't necessarily increase the overall dust in the atmosphere, as long as there isn't a local or regional dust storm. So there is no contradiction to the air becoming clearer at Gale, in general. It was just a short event of the order of a minute.

A relation to the Phobos transit should be very unlikely, it was sol 363, i.e. 5 sols earlier.

Edit: There was again a transit, very convincing explanation, Paul! So it will have been a shadow of Phobos, instead of a dd.

Interesting, that the channel combination did work, nevertheless.

[elakdawalla]

No, dude, check it out: there was an eclipse, a good one, happening at the same time as those Hazcam images. Super duper cool. Nicely done, @Deimos!

[fredk]

Yeah, this image was simultaneous with the RLB sequence. Very cool indeed! Was this never done with MER?

The darkening moving over to the mound makes sense - the shadow (penumbra) of Phobos is on the order of the size of Phobos, ie 20-odd km. (Actually the penumbra will be somewhat wider.) The mound is several km away.

[Explorer1]

Chalk up another amazing one for the team; feels like Spirit and the dust devils all over again!
Next August is another season of transits; by then Curiosity could be on Mount Sharp itself and right in the path... a chance for MAHLI movies?

[jmknapp]

Are you saying that we could have been seeing the shadow of Phobos passing over the terrain?

Cooool!!!

Per SPICE, here's the ground track of the center of Phobos' shadow (red line) during the transit:



The two red dots are the position at the time of two of the HAZCAM shots. The wedge shows the field of view of the rear HAZCAM.

How wide would the shadow be?

[EdTruthan]

The eclipse thumbnails appear to indicate a maximum conjunction at almost precisely the 03:22:00 P.M. LMST mark, the exact time of the RLB darkening event. Way cool catch by the imaging team.

And speaking of cool - Sol 369 tracks anaglyph - gotta love that repeated JPL Morse Code track. Easy to judge distance too, I believe it's about 1.5 meters per rotation if I'm not mistaken. (50cm wheel diameter x 3.14 = 157cm or 1.57 meters).

[atomoid]

scroll down to get an idea about the size of Phobos' shadow on Mars' surface, i had no idea it was imaged by Viking lander/orbiter as well! stunning!

hopefully more pics will stream in, i suppose there are more hazcam frames and they werent clustered at the very end of the transit as it currently appears. with everything else going on im wondering if it wass technically feasible to stage a navcam sequence as well?

[Deimos]

MER never imaged the ground while imaging a transit. Partly, this is the sheer scale of the event--maybe 60 km diameter, but no sharp edges. Partly it is that there is not a straight line from the science requirements to taking those images. Once in a while, maybe you do something just because you can. There didn't look like a good chance at Endeavor this year, but in principle right Pancam images can be taken in parallel with left RHAZ or FHAZ. You could do the same with NAV, but of course you couldn't point away from the Sun while also imaging the transit. And the sequencing on MER for that seems complex, given the hoops we jump through to get 3 sec/image out of Pancam.

Maybe next year hazcam could be kicked off earlier in the video. There are sequencing constraints, but boundaries aren't usually pushed hard the first time.

You could consider Navcam of a near-miss transit -- but of course the event center is >30 km away. That's a tough sell given the implications of the specific transit timing constraints on sequencing a sol. That's especially true with Opportunity's power limitations, but transits (and near-misses) don't wait for drives or comm passes ... and are less important than either, in general.

[Gerald]

"How wide would the shadow be?"

There is no umbra of Phobos on the Martian surface.
The size of Phobos is 26,8 × 22,4 × 18,4 km, average 22.2 km, distance from the surface 6,000 km.
Diameter of the Sun 1,392,684 km, average distance Sun-Mars 228 million km.
Apparent diameter of Phobos relative to Sun therefore (22.2/6,000) / (1.4/228)=0.61.

Therefore the antumbra will be about 14.2 km (remaining 39% of Sun's apparent diameter).
The whole penumbra adds a tiny bit more than the diameter of Phobos around the antumbra, i.e. 22.2 km, together 14.2 + 2 * 22.2 = 58.6 km.
That's a rough approximation based on average values at noon.
The eclipse was in the afternoon, so we get a roughly elliptical shadow, with the apparent diameter of Phobos a little smaller. Irregular shape of Phobos is to be added.

See also Transit of Phobos from Mars on Wikipedia,
Phobos, Sun, Distance Sun-Mars.