Digging Deeper into the Planet Four: Ridges Preliminary Results

Today we have a guest blog by JPL research scientist Laura Kerber, one of  our lead researchers on Planet Four: Ridges .  Laura studies  physical volcanology, aeolian geomorphology, wind over complex surfaces, and the ancient Martian climate,

Hello Ridge-Hunters! We have been finding lots of ridges in Nilosyrtis Mensae, and I wanted to give you a bit of an update on our progress. Here is a map showing the images that we have looked through (blue), the places where I thought there might be ridges before the project started (circled in orange), and the spots where you have actually found ridges (purple dots).

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As you can see, most of the ridges were found in the southeast portion of the search area. I took a look at the outliers, and they aren’t the kind of polygonal ridges we are looking for—meaning that all of the polygonal ridges we found have been in a pretty restricted area. Here is a close-up of that area:

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There are a couple of important things that we have already learned from what we’ve found so far. First, we can see that the ridges aren’t correlated with craters. One of the early theories about these ridges was that they were breccia dikes—that is, dikes of broken-up material that was forced through surrounding terrain during a violent impact event. The presence of polygonal ridges both in craters and on the inter-crater plains makes this hypothesis seem less likely.

Here are some great ridges that you found on the intercrater plains:

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And an even closer close-up:

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We also want to know whether or not the ridges are correlated with valley features. At first glance, it looks like valleys and ridges aren’t correlated, because there are plenty of ridges in the inter-valley plains, and valleys like Auqakuh Vallis that don’t have a lot of ridges near them:

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Upon closer inspection, we can see that the ridges are correlated with what we call an “etched” terrain—terrain that has been heavily eroded, leaving bits and pieces of the terrain that came before it. The southern part of Auqakuh Vallis is dominated by etched terrain, and we can even see that part of the valley has been inverted by erosion (what was once the valley floor is now standing higher than everything around it). We can also see that the western branch of Auqakuh Vallis has cut this positive feature, meaning that it was active long after the eastern branch stopped flowing. There were a lot of ridges identified both surrounding the river deposits that make up the top of the inverted Auqakuh Vallis channel and around it. This may suggest that ridges are preferentially forming in old river sediments:

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But why aren’t there ridges further north along Auqakuh Vallis?

Actually… there are! Here is an image further north along the Vallis. We can see that northern Auqakuh Vallis cuts through a ridge-containing unit, but in most of the surrounding area, the ridge-containing unit is capped by a unit with glacial morphology that hides the ridge unit from view:

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Our current hypothesis: The ridge unit formed before or at the same time as the valleys were being cut. Afterwards, glaciers and ice sheets covered the area and deepened and widened the valleys. The glaciers covered the northern Auqakuh Vallis region and most of the terrain north of it, including the western part of the study region.

The next group of CTX images extends our search area to the east. This is the area where this type of polygonal ridges were first mapped, before we had CTX images covering the entire area like we do now. The first mapping project (in 2006) identified ridge lattices inside mostly inside craters, leading to the hypothesis that they were impact-related breccia dikes. The second project (in 2013) mapped ridges along the Nili Fossae trough system, leading those scientists to hypothesize that the original fractures may be related to the trough system. Our study of the ridges to the west has been offering an expanded context for these hypotheses. The other special thing about this region is that we will be covering two of the three remaining potential landing sites for the next NASA Mars rover, called “Mars 2020”. Mars 2020 is carrying a suite of instruments that it will use to search for habitable places on Mars as well as organic material. The new rover will also carry a drill that it will use to take samples of many different rocks and cache them in tubes for a future mission to bring back to Earth. Wouldn’t it be great if they could bring a bit of ridge back for us?

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4 responses to “Digging Deeper into the Planet Four: Ridges Preliminary Results”

  1. John G Keegan says :

    What a great post. Very informative. Thanks Laura.

  2. james secosky says :

    Thank you so much for this update. It makes me feel that I’m really exploring Mars. I read so place that Mars and the moon were expected to have much of the outer crust broken by the billions of years of impacts. I wonder if much of the crust of Mars, at some depth, is full of fractures that later became full of mineral-rick fluids. Today, we can’t see most of these in many places due to thick layers of dust and the ice-rich mantle that covers many regions. When we see the ridges, maybe we are seeing the ones that are not covered. Perhaps the ridges are almost everywhere. I wonder if ridges appear in dust covered areas. Thermal inertia data overlaid on ridge data may be one test of this. From the pictures I’ve obtained under the HiWish program, I seem to notice that the ridges appear at a certain level.

    Thanks again for your update.

  3. Robert Williams says :

    When are you going to look at the ONLY quadrant on Mars that the USGS map legend had NO geological idea about HOW the area formed? In the USGS map legend it had: “Origins:Enigmatic”. Period. From BEFORE Viking and all the way up to 2005, it still has those terms. In 2005 Dr.Tonka added some similar “guesses”; but NO definative explanation of the area. That quadrant should be where the 2020 Rover goes, otherwise it is a waste of taxpayer money. Since science is supposed to seek the unknown to NOT send the 2020 Rover there is Educational & Scientific Cowardice bordering on Criminality. That quadrant is Cydonia.

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