Click for original image.

Cool image time! The picture to the right, cropped, reduced, and enhanced to post here, was taken on January 6, 2023 by the high resolution camera on Mars Reconnaissance Orbiter (MRO), and shows a small but typical area of the Medusae Fossae Formation, what is thought to be the largest volcanic ash deposit on Mars.

The picture itself was a “terrain sample,” taken by the MRO science team not as part of any specific research but to fill a gap in the camera’s schedule so as to maintain its temperature. The terrain itself looks like a field of sand that someone had run a fine comb across. In this case, the comb was the winds of Mars, prevailing from the southeast to the northwest. The crescent-like divots in the picture’s lower right are probably caused by some hard underground feature that the winds cannot blow away. Instead, it blows around, like water in rapids flowing around a rock, and takes the ash with it as it does so.

The white dot near the center of the overview map to the right marks this picture’s location, inside Medusae.

Scientists estimate the size of the entire Medusae ash deposit to be comparable to about the size of India and in some places as much as a thousand feet deep. They believe the ash is volcanic pyroclastic ash, material released quickly during a violent volcanic eruption such as the one that occurred at Mount St. Helens in 1980. Nor does all of the deposit have this endless combed look. Because the ash drapes the underlying topography, it sometimes shows hints of those underlying structures. And of course, the edges and gaps in the deposit result in more complex topography. See for example this July 2021 cool image only a few miles to the southwest of today’s picture. This endless plain of ash suddenly ends, as the ground drops down into a canyon.

The mystery is that this formation, while smack dab in the middle of volcano country surrounded by Mars’ biggest volcanoes, it is too far away from those volcanoes for the ash from their eruptions to have been blown here by the wind, based on our present understanding of Mars’ atmosphere. Scientists are therefore unsure how the ash got here. Though much of the terrain here, between these giant volcanoes, is flood lava laid down by nearby vents and not from the big mountainous volcanoes, such flood lava doesn’t produce explosive pyroclastic ash. Instead, this lava flows out from the vents more gently, steadily without explosive events.