Click for original images.

Cool image time! The picture to the right, cropped and reduced to post here, was taken on July 5, 2026 by the high resolution image on Mars Reconnaissance Orbiter (MRO). Labeled simply as a terrain sample, the picture was likely taken not as part of any specific research project but to fill a gap in the camera’s schedule. In order to maintain its proper temperature the camera team needs to take pictures at a regular cadence, and if there is a gap they must fill it with something.

When they need to do this, they try to take pictures of interesting terrain, but don’t always succeed. In this case, the image focuses on a cliff wall with numerous slope streaks, both bright and dark. These features are unique to Mars, and though they resemble avalanches, they produce no change in the topography, have no debris pile at their base, and more resemble stains that appear suddenly and then fade with time.

The white dot on the overview map to the right marks the location, in the southern cratered highlands. This cliff face is about 1,000 feet high, and is part of a straight escarpment that runs for about 200 miles, slicing into the sides of several large craters. It is the straight line running from the northwest to the southeast in the inset.

That this cliff at this spot exhibits both light and dark streaks increases the puzzle of these formations. They occur all year, start out either very dark or very bright, and fade with time. They do not appear to be linked to any specific seasonal event.

The most popular but unconfirmed theory now is that some event at the top — for example a freeze-thaw cycle or a dust devil — causes a pebble or rock to move, and this in turn causes a dust avalanche down the slope. The example one scientist gave me was to watch water dripping down the outside of your car windshield. If a drop touches another drop they combine and flow together, and can gather other drops as well. This theory supposes the same process is happening with dust.

No theory at present is accepted as a conclusive explanation, and this theory doesn’t explain the existence of both dark and light streaks.