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Dawn at Ceres Thread


Frida Space

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Overexposed is an understatement, it looks like someone spilled glow-stick fluid and is shooting at a high Fstop.

I I would love it if they would correct the camera exposure for the bright spots to be correctly exposed but I think they don't want to do that for fear of missing something else. Because the rest of Ceres would surely be pitch black.

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I think they're getting pretty close to accurately resolving them anyway. There's much less of a change this time than during earlier updates.

It'll probably end up being a series of reflective areas roughly in those shapes - now the only question left is what precisely is there that reflects light so well.

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Downloaded PIA image. Gimp, scaled up 800% and 1000%, nothing fancy done otherwise. It gives me the impression maybe a pooled liquid of some kind, or the remnant of the impactor's mount sheared off flat - with something very reflective beneath. I say this because I see no shadow cast (neither by the 'white spot' nor along its border rim), and find the small peak and its shadow to the right of the 'white spot' interesting.

There are other spots of interest in this image as well, and other bright spots as seen in previous imagery. Have they released anything of those in higher resolution yet?

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Am I the only one to think that the bright dots are overexposed? I mean, if they reduced a bit the exposure time of the Framing Camera maybe the rest of the surface would look very dark, but maybe we could get a few hints on the structure and features inside​ the dots.

If we consider that Earth is 1.5 E11 meters from the sun and radiation drops at a rate of 1/d2 and set k = E/m2 (at 1 a theoretical meter)* 1/2.25E22 (IOW k E/m2 = 1 when d = 1 AU. Then the amount for radiation hitting Ceres relative to Earth is 1/(2.77Au)^2 which means the level of solar radiation is 1/8th, that means in order to see the average grey object on Ceres would require an exposure of 8 fold more time than on Earth or a aperture size 2.77 times larger. In terms of F-stop, the F-stop value would be 2.77 fold smaller than on Earth (Larger F-stop allow smaller levels of light, but increase the focal distance). IOW of the Ceres surface material were on the moon we would shorten the exposure time and increase the F-stop.

The second image was taken at a radius perpindicular to the direction of the sun, the sun being in the left image direction, and the right portion of the image is at sunset, because most of the image is at a low incident angle relative to the sun, the exposure for the image is probably on the order of 16 to 32 times longer exposure or 4 to 6.5 times smaller aperture than the setting required for the same camera capturing the same surface on the moon.

In addition the surface of Ceres appears to be very dark, and this would indicate that even more exposure time than the average reflective surface on earth.

There are 6 factors in the exposure then

1. Distance from the sun

2. Incidence angle of sunlight hitting the surface

3. Albedo of the average surface covering on Ceres.

4. The area of the lens capturing the image

5. The capture efficiency of the photocells capturing the photons and spectral sensitivity (for black and white)

6. Aperture size.

[7.] Image correction software use to correct to the final product. (since the intensity is digitized the signals with voltages very close to 0 or very close to maximum-saturation) will begin to lose correctablity. If a voltage comes in at 1 and this is statistically as 50.0% +/- 0.05% then a voltage coming in at 0.01 has a value of 0.5 +/- 0.05% and a voltage of 0.001 has a value of 0 or 0.1% +/- 0.05%. At the high end the problem is more contorted because the reading is the saturation voltage - read voltage. In some instruments the voltage may even drop a few percentages with more intense light. IOW for very low and very high detected intensities the error in correction grows increasingly more severe as one approaches either limit of detection. Thus the cameras are going to record intensities that fit most pixels in the middle of the range.

This is a problem with almost all development based techniques and it overlaps with digital detection devices, the only exception are PMT based gamma counters that can read linearly across a 108 fold scales (the major problem is background at the low end). Your typical detectors on digital cameras are in the 102-104 fold range scale.

Thus is you want to desaturate the intensity of light of the spots you would have to set the average detected hv by the camera to a lower threshold cease value. This would result in a very dark surface with barely resolvable features.

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I find it hard to believe that mission control would ignore these features. I'm confident that at some point in the mission plan there has been inserted a photo recon series to explore these features across multiple spectroscopy vectors.

Hi orbit, low orbit, whatever... I trust they have this in their sights.

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Am I the only one to think that the bright dots are overexposed? I mean, if they reduced a bit the exposure time of the Framing Camera maybe the rest of the surface would look very dark, but maybe we could get a few hints on the structure and features inside​ the dots.

Good point.

Bob Clark

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But Ceres' gravity is surely not nearly strong enough to rip apart meteorites?

The problem is only the largest bodies in the asteroid belt are gravitationally rounded objects, the concretization forces may not be adequate on most bodies, and there is no atmosphere on any bodies (as opposed to earth, venus and mars) so that small projectiles with marginal escape velocities are not slowed down by drag and escape, but not with such velocity that they enter the central solar system and are more likely to impact other bodies or be thrown out of the system by flying past other asteroids and being tossed out by jupiter. Impacts on the smallest bodies probably results in alot of smaller meteorites.

The ice laden meteors will undergo sublimation and you would have soft porous impactors that break up or fail to produce consistent impact crators.

My impression looking at the high res photos is that there are areas where older craters were covered with ejecta from recent impacts or have a long history of impacts with micrometeorites, some disrupting bolders on the margins of impact craters basically lots of small events cluttering the surface. If you are a lander mission you are probably going to focus on the donut region of a fresh impact crater, but this also presents a warning for manned missions, there is prolly a high risk for dangerous bullet size projectiles than on the moon or on the martian satellites.

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We are already getting picures of these other features now. Is it possible to say "okay, to investigate these bright spots specifically, can we take some shots with a faster shutter speed?"

IIUC, at the moment the aren't doing science, they are waiting until Dawn gets closer. The shots that are being taken are primarily for navigational purposes to help the mission controllers judge how fast Dawn is being drawn to Ceres by its gravity. These piccies are just a side-effect of this process.

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Wow !

That is an awesome picture !

I think I can spot a volcano here, isn't it ?

It even seems to have lava flows on the sides.

Could it be water going out that cone ?

I can see the cone too, not really seeing anything that looks like flows coming off the sides though.

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Wow !

That is an awesome picture !

I think I can spot a volcano here, isn't it ?

It even seems to have lava flows on the sides.

Could it be water going out that cone ?

Liquid water can not exist in vacuum. It will partially turn to gas by boiling and one part will solidify. At temperatures encountered on Ceres, water ice will sublimate away in a matter of weeks. Phase equilibrium is totally shifted towards gas.

That does look like a cryovolcano. Remember this is a very small body, with surface older than the Moon's. It's ancient. There is no tectonics, it's probably barely diferentiated inside. It is alone, nothing is squeezing it. There can not be much activity.

Those flows are not lava, but ejecta sliding down. Nothing we haven't seen before.

M154813223RE_thumb1.png

Similar thing in Tycho crater.

http://www.nasa.gov/sites/default/files/images/565812main_tycho_full_full.jpg

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