by Jim Nintzel
More great snapshots of Mars taken by the UA Lunar and Planetary Lab's HiRISE camera aboard the Mars Reconnaissance Orbiter. Zilair Crater? The dunes of Aonia Terra? Who names these places, Edgar Rice Burroughs?
The first shot features gullies at the edge of Hale Crater. Shane Byrne explains:
Several years ago gullies carved into hill-slopes and the walls of impact craters like the ones pictured here were discovered. Scientists are excited to study these features because, on Earth, they usually form through the action of liquid water - long thought to be absent on the Martian surface. Whether gullies form under today's cold dry conditions is a major question that planetary scientists are trying to answer.
The gullies pictured here are great examples of what a typical Martian gully looks like. You can see wide V-shaped channels running downhill
(from top to bottom) where the material that carved the gully flowed. At the bottom of the channel this material empties out onto a fan-shaped mound. Several gullies are visible here and the fans from each gully overlap one other in complicated ways.
At the tops of the channels, large amphitheater-shaped alcoves are carved in the rock. The material removed from these alcoves likely flowed downhill to the aprons through the gullies.
Gullies at this site are especially interesting because scientists recently discovered examples at similar locations to be still active. Images separated by several years showed changes in the appearance of some of these gullies. Today, planetary scientists are using the HiRISE camera to examine gullies for ongoing change and investigate what that might mean for the occurrence of liquid water on the surface of Mars.
The middle pick shows a close-up view of the terrain northeast of Zilair Crater.
Andrea Philippoff explains:
In lower-resolution images, from the Context Camera (also on board the Mars Reconnaissance Orbiter), this area looks quite rough. The reasons for this rough texture are difficult to determine; however, clues from the surrounding area help solve this mystery. Many large impact craters surround this image, but there are no volcanoes nearby making it unlikely that the rough surface is from fresh, blocky lava flows.
The high-resolution image shown here reveals a few fresh, irregularly-shaped craters on top of this rough terrain. An irregular crater, approximately 600 meters (2000 feet) in diameter, is visible just right of center, about a third of the way up from the bottom of the image. Its shape suggests that it formed from a body that impacted the surface at a relatively low angle and slow speed. This makes it likely that it is a secondary crater (a crater that forms from debris blasted out of a nearby crater).
The rough terrain here is therefore likely to be a mix of impact material and secondary craters from nearby impacts, such as the one that formed the nearby large crater Zilair.
The bottom shot shows colliding sand dunes in Aonia Terra.
Circe Verba and Paul Geissler tell us:
This dune field in Aonia Terra shows sand dunes with a variety of morphologies.
These complex shapes often indicate that the dominating winds change direction, either over time or from one location to the next. Many of the dune fields in this region of Mars are banked up against topographic obstacles such as crater rims. The location of the dunes within impact craters gives an indication of the average regional winds, while details such as slip-face orientations and superposed ripples are controlled by the present day local winds.
The outlying dunes in this image appear as simple arc-shaped barchans formed by southerly winds. As the dunes migrated northwards, they encountered other dunes crowded against a confining scarp. Many of the barchans climbed up onto the upwind (stoss) slope of the next row of dunes. This migration is shown by the avalanches that have occurred on the downwind (slip) face of the dunes. The collision of the dunes altered the morphology of the barchans to star-shaped dunes with multiple arms that took on a linear appearance as the horns were modified.
There is also the presence of faint dark tracks from dust devils picking up a very thin veneer of dust on the dunes. The faintness of the tracks could be an indication that the dunes have actively saltating sand grains that clean much of the settled dust from the dunes.