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Damn good question. Short answer: The images are brightened visible light. The wikipedia article has more information: https://en.wikipedia.org/wiki/Bright_sp

Damn good question. Short answer: The images are brightened visible light. The wikipedia article has more information: https://en.wikipedia.org/wiki/Bright_spots_on_Ceres https://en.wikipedia.org/wiki/Bright_spots_on_Ceres Notable points: In infrared the spots are darker or equal than surroundings, thus cooler or equal. In lower resolution pictures the bright spots are considerably larger than their now apparent size, thus they're throwing out massive amounts of light, likely reflection of sunlight. They seem to reflect equally well from any given viewing angle, thus whatever their material is seems to exhibit some kind of cat's eye effect. ( https://en.wikipedia.org/wiki/Cat's_eye_(road) https://en.wikipedia.org/wiki/Cat's_eye_(road) )

mturmon 11y ago
About where the images come from, don't forget that there are two cameras on Dawn: -- Framing Camera (FC) which is high-resolution and has a 7-position filter wheel (which allows for specific wavelengths to be chosen) as well as allowing white-light images. FC is used for navigation and general mapping of the asteroid geometry. -- A Visible and Infrared Spectrometer (VIR) which is lower resolution, but better calibrated radiometrically. It produces spectrally-resolved images to determine mineral composition. More at http://dawn.jpl.nasa.gov/technology/science_payload.asp http://dawn.jpl.nasa.gov/technology/science_payload.asp The images you're seeing in this press release seem to be from FC. I say this because they have high resolution, and some other press releases with similar images say outright that they used FC. The FC spec is 94 micro radians/pixel which, at a distance of 4400 km, works out to be about 410 meters per pixel, and that seems compatible with the feature sizes of ~10km noted in the press release. But, some of the images in the wikipedia page you linked, which is very good, are in infrared, and are of lower resolution, and probably therefore come from VIR. The temperature plot also seems like VIR (you can get a temperature if you have a spectrum). Finally, about processing, I notice that the images in the press release seem to have a lot of pixels. This is a clue that they might have composited a lot of individual images to get this one image. If so, there had to be a geometric correction (at the very least) because of course the camera is in motion while the images are taken. This requires very good geometry information. Additionally, there might have been a projection into a standard coordinate system (so that "north" is up). You might be interested to know that there is controversy about asteroid coordinate systems (for Vesta, https://en.wikipedia.org/wiki/4_Vesta#Coordinate_systems https://en.wikipedia.org/wiki/4_Vesta#Coordinate_systems). About the white areas -- yes, probably they saturated the detector.
CamperBob2 11y ago
In infrared the spots are darker or equal than surroundings, thus cooler or equal. We can't really say this, though, without knowing the material's emissivity. And since we don't know what it is...
- mturmon 11y ago
  This is true in the abstract, and a good point. But in this case, they have a pretty densely sampled and well-calibrated spectrum from one of the instruments (http://dawn.jpl.nasa.gov/multimedia/vir_gallery.asp http://dawn.jpl.nasa.gov/multimedia/vir_gallery.asp -- there appear to be ~100 spectral samples), so they would be able to make a black-body assumption, derive a temperature, and then validate the extent to which that is correct. In other words, it's not just a plain IR image, it's a whole spectrum.
oofabz 11y ago
The cat's eye effect only works when the light source and the viewer are in roughly the same place. In a car, light from the headlights is reflected back at the driver. It wouldn't work for the Dawn probe because there is no light source on the spacecraft.
- Pfhreak 11y ago
  Couldn't the probe be between the sun and Ceres, causing a similar effect?

mturmon 11y ago

About where the images come from, don't forget that there are two cameras on Dawn: -- Framing Camera (FC) which is high-resolution and has a 7-position filter wheel (which allows for specific wavelengths to be chosen) as well as allowing white-light images. FC is used for navigation and general mapping of the asteroid geometry. -- A Visible and Infrared Spectrometer (VIR) which is lower resolution, but better calibrated radiometrically. It produces spectrally-resolved images to determine mineral composition. More at http://dawn.jpl.nasa.gov/technology/science_payload.asp http://dawn.jpl.nasa.gov/technology/science_payload.asp The images you're seeing in this press release seem to be from FC. I say this because they have high resolution, and some other press releases with similar images say outright that they used FC. The FC spec is 94 micro radians/pixel which, at a distance of 4400 km, works out to be about 410 meters per pixel, and that seems compatible with the feature sizes of ~10km noted in the press release. But, some of the images in the wikipedia page you linked, which is very good, are in infrared, and are of lower resolution, and probably therefore come from VIR. The temperature plot also seems like VIR (you can get a temperature if you have a spectrum). Finally, about processing, I notice that the images in the press release seem to have a lot of pixels. This is a clue that they might have composited a lot of individual images to get this one image. If so, there had to be a geometric correction (at the very least) because of course the camera is in motion while the images are taken. This requires very good geometry information. Additionally, there might have been a projection into a standard coordinate system (so that "north" is up). You might be interested to know that there is controversy about asteroid coordinate systems (for Vesta, https://en.wikipedia.org/wiki/4_Vesta#Coordinate_systems https://en.wikipedia.org/wiki/4_Vesta#Coordinate_systems). About the white areas -- yes, probably they saturated the detector.

CamperBob2 11y ago

In infrared the spots are darker or equal than surroundings, thus cooler or equal. We can't really say this, though, without knowing the material's emissivity. And since we don't know what it is...

mturmon 11y ago
This is true in the abstract, and a good point. But in this case, they have a pretty densely sampled and well-calibrated spectrum from one of the instruments (http://dawn.jpl.nasa.gov/multimedia/vir_gallery.asp http://dawn.jpl.nasa.gov/multimedia/vir_gallery.asp -- there appear to be ~100 spectral samples), so they would be able to make a black-body assumption, derive a temperature, and then validate the extent to which that is correct. In other words, it's not just a plain IR image, it's a whole spectrum.

oofabz 11y ago

The cat's eye effect only works when the light source and the viewer are in roughly the same place. In a car, light from the headlights is reflected back at the driver. It wouldn't work for the Dawn probe because there is no light source on the spacecraft.

Pfhreak 11y ago
Couldn't the probe be between the sun and Ceres, causing a similar effect?