Tag: geology

Boxwork inside Wind Cave, South Dakota, mere inches across.

Anyone who has ever visited either Wind or Jewel caves in South Dakota has likely seen some wonderful examples of the cave formation boxwork, formed when the material in cracks is more resistant to erosion that the surrounding bedrock, which once eroded away leaves behind the criss-crossing ridges seen in the picture to the right.

Today’s cool image provides us what appears to be an example of boxwork on Mars. However, unlike on Earth it is not in a cave but on the surface. It is also much larger. Instead of the ridges being almost paper thin and stretching for inches or feet, this Martian boxwork is feet wide with ridges extending hundreds of feet in size, as shown by today’s cool image below.
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Cool image time! Last week the MRO science team posted a new captioned image entitled “Bright and Dark Dunes” featuring a particularly large single dune in the floor of a 25-mile-wide unnamed crater located at about 68 degrees south latitude. The photo to the right, rotated, cropped, reduced, and color enhanced to post here, shows that dune. According to the caption, written by Colin Dundas of the U.S. Geological Survey’s Astrogeology Science Center in Arizona,

This image shows a large sand dune with bright patches. Martian dunes near the poles often have bright patches in the spring, when seasonal frost is lingering. However, this image is from late summer, when frost is long gone. What is going on here?

A close-up look with [MRO’s high resolution camera] provides some clues. The bright patches are made up of large ridges that look like wind-blown bedforms. Additionally, the bright patches are yellowish in the infrared-red-blue image. In enhanced color, most sand on Mars is blue but dust is yellow. This suggests that the bright bedforms are either built from, or covered by, dust or material with a different composition.

Thus, the bright patches reveal either aspect of the dune’s underlying structure, either inherent in the bedrock itself, or the texture of its surface that allows it to hold more dust. As Dundas adds, “I think more study would be needed to determine the answer in this particular case.”

There are other aspects of this dune that can be seen by a look at the wider view afforded by MRO’s context camera below.
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Today’s cool image well illustrates the effort of planetary scientists to map out the range of buried ice on the Martian surface. Taken on December 13, 2020 by the high resolution camera on Mars Reconnaissance Orbiter (MRO) and rotated, cropped, and reduced to post here, it shows a 3.5-mile-wide crater located in the southern cratered highlands, but for those cratered highlands at the very high northern latitude of 24 degrees.

The black streaks on the crater’s interior slopes are probably slope streaks, but these are not the subject of this article. Instead, it is the material that covers the crater’s floor. These features resemble the glacial fill material that scientists have found widespread in the latitude bands between 30 to 60 degrees latitude. However, this crater is farther south, where such ice would not be stable and should have sublimated away.

Could there still be ice here? I emailed the scientist who requested the photo, Colin Dundas of the U.S. Geological Survey’s Astrogeology Science Center in Arizona, and asked him what I was looking at. His answer:
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Cool image time! The photo to the right, cropped to post here, was taken on January 2, 2021 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). It shows a windswept sandy region of ridges and dunes with two dark features nestled between ridges.

What are these dark patches? At the available resolution they appear to be deep pits, with the one on the right having a significant overhang. And if these are pits, they would appear significantly different than most of the previously identified Martian pits, which are usually somewhat circular in shape. These features have very complex shapes, as if the pit is conforming itself to the terrain that surrounds it.

The resolution, however, is not good enough to confirm this interpretation. These dark patches could also be exposed volcanic material, darker than the surrounding terrain. The location, as shown in the overview map below, adds weight to this interpretation.
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Time for another cool Martian image. The photo to the right, rotated, cropped, and reduced to post here, was taken on January 3, 2021 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). The section I have focused on is a single crater about a mile and a half wide.

What makes this crater interesting is the material that appears piled up against the crater’s northern half. Furthermore, both the floor of the crater as well as this piled up material looks like it is eroding away, kind of like a block of ice which is having warm water sprayed on it.

So, is this glacial ice?
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Cool image time! The photo to the right, rotated, cropped, and enhanced to post here, was taken by the high resolution camera on Mars Reconnaissance Orbiter (MRO) on October 4, 2020. It shows what appears to be features that are either corroding or eroding away, with the lower areas filled with rippling sand dunes.

The circular features might be ancient craters. The material that partly fills them might be a layer of dust or sand that the wind is slowly blowing away to dig out the depressions along the southern cliff wall.

According to the MRO science team’s interpretation of the colors produced by the high resolution camera [pdf], the dark blue colors here are likely “coarser-grained materials (sand and rocks)”, while the orange-red material on the higher terrain is likely dust.

Could this material be evidence of buried ice eroding away? At first I thought so, and then I took a look at the photo’s location, as shown in the overview image below.
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In a paper just published, scientists are proposing that bright areas seen in the low mid-latitude gullies on Mars are the underground ice table newly exposed as surface dust is removed.

This paper is a reiteration in more detail of a previous presentation [pdf] by these same scientists at the 2019 the 50th Lunar and Planetary Science Conference in Texas and reported here in March 2019.

The image to the right, from the paper, was taken by the high resolution camera of Mars Reconnaissance Orbiter (MRO) in 2009, and has been cropped to post here. The white streaks are what the scientists propose is that exposed underground ice table. At 32.9 south latitude, this particular gully would be the closest to the equator that such an ice layer has been identified. All the previous ice layer discoveries have been in the ice scarps found at latitudes above 50 degrees. As the paper’s lead author, Aditya Khuller at Arizona State University, explained in to me in an email, “We believe we are seeing exposures of dusty ice that likely originated as dusty snow.” From their paper:

We suggest … that the light-toned materials are exposed H2O ice. … [T]he appearance, and then subsequent disappearance of these light-toned materials, suggests that they are some form of volatile, such as dusty ice, rather than dust alone. … [The appearance] of these light-toned materials is similar to the >100m thick, light-toned ice deposits exposed on steep mid-latitude scarps, indicating that these materials are probably also ice, with some amount of dust on, and within the ice.

The layer would have likely been laid down as snow during a time period (a long time ago) when the rotational tilt of Mars, its obliquity, was much higher than today’s 25 degrees. At that time the mid-latitudes were colder than the poles, and water was sublimating from the polar ice caps to fall as snow in the mid-latitudes.

The overview map below reveals some additional intriguing possibilities.
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The cool image to the right, rotated, cropped, and reduced to post here, was taken on November 3, 2020 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). It shows what the science team labels a “Possible Tongue-Shaped Flow Feature in Protonilus Mensae.” There is no caption, so I will try to provide.

Protonilus Mensae is part of the long string of chaos terrain that runs about 2,000 miles along the transition zone between the southern cratered highlands and the northern lowland plains at about 30 to 40 degrees north latitude, and includes the other mensae regions dubbed Deuteronilus to the west and Nilosyrtis to the east. This region of Mars I like to call glacier country, because almost every high resolution photograph appears to show glacial features. To get an idea what I mean, take a gander at these past posts, their locations indicated by number in the overview map of Protonilus Mensae below:
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Cool image time! The photo to the right of two images taken by the high resolution camera on Mars Reconnaissance Orbiter (MRO) was posted as a captioned image by the orbiter’s science team today.

This crater, dubbed the Happy Face Crater because of the shape of the blobby features within it, is located on the south pole ice cap of Mars, about 200 miles from the south pole itself.

Today’s caption noted how these pictures, taken nine years apart, illustrate the change going on at the Martian south pole.

The “blobby” features in the polar cap are due to the sun sublimating away the carbon dioxide into these round patterns. You can see how nine years of this thermal erosion have made the “mouth” of the face larger. The “nose” consisted of a two circular depressions in 2011, and in 2020, those two depressions have grown larger and merged.

While this caption noted the importance of studying these long term changes in order to understand the evolution of Mars’ climate and geology, it did not give the very specific discovery these changes suggest for Mars globally, a discovery that is actually very significant.

The two ice caps of Mars have some fundamental differences, all presently unexplained. The similarities are obvious. Both have permanent caps of water ice that are presently believed to be in a steady state, not shrinking or growing. Both each winter get covered by a thin mantle of dry ice that sublimates away completely with the coming of spring.

The differences are more puzzling, as shown by the maps of the two poles below.
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Cool image time! The photo to the right, cropped and reduced to post here, was taken on October 27, 2020 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). It was taken not for any particular research project, but as one of the periodic images the camera team needs to take maintain the camera’s proper temperature. When they need to do this, they often will take a picture in an area not previously viewed at high resolution. Sometimes the image is boring. Sometimes they photograph some geology that is really fascinating, and begs for some young scientist to devote some effort to studying it.

In this case the photo was of the generally featureless northern lowland plains. What the image shows us is a scattering of impact craters that appear to have cut into a flat plain likely saturated with ice very close to the surface.

How can I conclude so confidently that these craters impacted into ice close to the surface? The location gives it away.
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Today’s cool image gives us a sample of the strange colorful hills in an even stranger knobby depression on Mars called Ariadnes Colles. The photo to the right, cropped and color enhanced to post here, was taken on September 10, 2020 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). It focuses on just one of those colorful hills. The color strip only covers the western half, which is why that is the only part of the hill in color.

Ariadnes Colles is a patch of chaotic terrain 110 by 100 miles in size, located in the southern cratered highlands due south of Mars’s volcano country, at latitude 34 degrees south. What makes this particular patch of chaos distinct from the many others on Mars is that the hills, knobs, and mesas within it are routinely bright and colorful, compared to the darker surrounding terrain. Moreover, as noted in this Mars Express press release for images of Ariadnes Colles from that orbiter,

In contrast to other chaotic terrains … Adrianes Colles is not a water-source region. It is still debated, therefore, whether Ariadnes Colles was formed by the action of water or wind.

The darker material in the southern areas is most likely sand or volcanic ash; some slopes of the flat-topped features have been covered by this dark material that was blown up on the slopes.

The sand or volcanic ash most likely come from the Medusae Fossae Formation several hundred miles to the north, the largest volcanic ash deposit on Mars. The colors on the hill likely come from a variety of minerals.

The overview map below shows the entire patch, with the location of the hill above indicated by the white dot in the red rectangle that shows the full image location.
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