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Moderate Resolution Imaging Spectroradiometer (MODIS) images of Earth snow nice, regular repeating gaps in longitude where the viewing angle is too far for a given pass.

But the large GIF shown in the Washington Post article NASA’s Opportunity rover is fighting for its life in a Martian dust storm shows very irregularly-shaped gaps in the MRO images, and in one of them the black gap is actually partially filled in with data.

Question: Why are the gaps in MRO global images of mars so irregular and sometimes filled in?

below: example MODIS (Aqua) image from an earthobservatory.nasa.gov page, (see the >2 MB full-size image).

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below: frames extracted from GIF in The Washington Post, caption: "This set of images from NASA’s Mars Reconnaissance Orbiter shows the growing dust storm (red) that is kicking up on the planet. (NASA/JPL-Caltech/MSSS)"

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This is essentially down to the difficulties of capturing and returning data from Mars orbit rather than Earth.

These images from MRO are taken by the Mars Color Imager (MARCI), a very wide-angled camera designed to make a (near) global map of the planet at low resolution (1-10 km per pixel from MRO's ~300 km orbit), primarily for tracking Martain weather. Compare this to MARCI's more famous co-passenger HiRise which is designed to produce very high 30-60 cm per pixel resolution images of much smaller areas.

From MARCI's (much less flashy) website:

Black areas in the movie are the result of data drops or high angle roll maneuvers by the spacecraft that limit the camera’s view of the planet. Equally-spaced blurry areas that run from south-to-north (bottom-to-top) result from the high off-nadir viewing geometry, a product of the spacecraft’s low-orbit...

Data drops are part and parcel of interplanetary missions partly due to the challenge of aiming at a distant Earth with the craft's antenna as well as signal drop off, atmospheric attenuation and interference.

The high angle rolls are often necessary for a variety of reasons:

  • The above mentioned aiming of the high-gain antenna.
  • Ensuring MRO's solar arrays are angled optimally to the Sun.
  • Targeting surface features with the narrower-angled imagers on board, particularly HiRise:

Targeting requires rolling the spacecraft up to 30° off-nadir in either direction, waiting (several minutes) for the spacecraft to settle, commanding the instruments to observe, and then rolling back to nadir orientation.

These high-angle rolls may explain the smoothly curved edges of some of the missing data.

I believe the conspicuous horizontal lines through some of the blank regions are due to a combination of how MARCI captures it's images (essentially a slow rolling-shutter scan) and data drops.

More information here, particularly pages 5 and 10.

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  • $\begingroup$ Most of the gaps in most of the images are half-moon-shaped "bite marks" from one side of the scan or the other (the exception is of course the large black area in the top image). Would these "bite marks" more likely be data drops or roll excursions? This may be discussed in one of your links, but it might be good to briefly explain in the answer itself. $\endgroup$ – uhoh Jun 20 '18 at 10:14
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    $\begingroup$ @uhoh good question. I haven't found any information on that, but it may be buried somewhere in one of the links. I'd speculate that the smoother curve shapes would be due to the rolls. I've added a little more info on the horizontal lines too. $\endgroup$ – Jack Jun 20 '18 at 10:18

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