Odd Martian crater type made by impacts into ancient ice

Odd Martian crater type made by impacts into ancient ice
Double-layer ejecta craters could form when ejected material slides down steep crater walls and across ice, forming a top layer. Striations, common in landslides on Earth, radiate out from the crater rim. Credit: NASA

Geologists from Brown University have developed a promising new explanation for a mysterious type of crater on the surface on Mars.

Double-layered ejecta craters or DLEs, like other craters, are surrounded by debris excavated by an impactor. What makes DLEs different is that the debris forms two distinct layers—a large outer layer with a smaller inner layer sitting on top. These distinctive craters were first documented in data returned from the Viking missions to Mars in the 1970s, and scientists have been trying ever since to figure out how the double-layer pattern forms.

A new study by Brown graduate student David Kutai Weiss and James W. Head, professor of geological science, suggests that DLEs are the result of impacts onto a surface that was covered by a layer of tens of meters thick.

"Recent discoveries by planetary geoscientists at Brown and elsewhere have shown that the climate of Mars has varied in the past," Head said. "During these times, ice from the is redistributed into the mid-latitudes of Mars as a layer about 50 meters thick, in the same place that we see that the DLEs have formed. This made us think that this ice layer could be part of the explanation for the formation of the unusual DLE second layer," Head said.

In the scenario Weiss and Head lay out, the impact blasts through the ice layer, spitting rock and other ejecta out onto the surrounding ice. But because that ejected material sits on slippery ice, it doesn't all stay put. Weiss and Head believe the layering occurs when material near the top of an upraised crater rim slides down the slippery ice and overtops material on the lower slopes. That landslide, enabled by steep slopes and a slick ice layer, creates the DLEs' telltale two-layered appearance.

"I think for the first time since DLEs were discovered in the 1970s we have a model for their formation that appears to be consistent with a very wide range of known data," Weiss said. An understanding of how these and other crater types formed could help researchers to reconstruct the environmental conditions at the time of the impacts.

The research will be published in the journal Geophysical Research Letters. An early version of the paper went online on July 25.

A good fit to the data

The landslide scenario explains several of the distinct features of DLEs. Most directly, it explains radial striations—grooves radiating out from the crater rim—that are common on the inner ejecta layer of DLEs. Striations are common in landslides on Earth, Weiss said, "especially landslides on glaciers."

That got Weiss and Head thinking that ice could be a key ingredient for making a DLE. Ice would reduce the coefficient of friction on the slopes of crater rims, increasing the likelihood of a slide. "When I did a quick calculation, I realized that the landslide wouldn't be expected to happen [on crater rims] unless the ejecta was landsliding on an ice layer," Weiss said.

The scenario also requires a steep slope on the outside of a crater rim. A raised rim is partly a function of crater size, with larger craters generally having less uplift. Weiss calculated that craters larger than about 25 kilometers probably wouldn't have steep enough rims to cause an icy landslide. With those results in hand, he surveyed about 600 known DLEs and found that nearly all of them are between one and 25 kilometers in diameter.

The ice model also accounts for other distinctive features of DLEs. For example, unlike other crater types, DLEs tend not to have secondary craters surrounding them. Secondary craters are the result of big chunks of ejecta blasted out of the main crater, leaving gouges in the surrounding surface when they land. But if that surrounding surface were covered by ice, evidence of shallow secondary craters would disappear when the ice disappeared.

The model also appears to explain the locations of DLEs at middle or high latitudes—areas where scientists believe there may once have been glacial on Mars millions of years ago.

Ultimately, understanding how DLEs and other crater types are formed could lead to a better understanding of Mars' past.

"There are over 600 DLEs on the Martian surface, so reconciling how they formed with our knowledge of the climate of Mars is pretty important," Weiss said. "It could tell us a lot about the history of the martian climate on a global scale."

Explore further

Water in a Martian desert

More information: onlinelibrary.wiley.com/doi/10 … 2/grl.50778/abstract
Journal information: Geophysical Research Letters

Provided by Brown University
Citation: Odd Martian crater type made by impacts into ancient ice (2013, August 5) retrieved 17 October 2019 from https://phys.org/news/2013-08-odd-martian-crater-impacts-ancient.html
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User comments

Aug 05, 2013
Well, there's another "explanation" offered up to explain the many anomalous features that don't match the impact hypothesis. Strangely, this along with every other crater anomaly as well as rilles, canyons, fulgamites, equatorial ridges, mesas and numerous other geologic features found on "geologically inactive" and dry worlds can be explained by electric discharge.

Aug 05, 2013
"Electric discharge" - Can'tDriveTooStupid

Yup, electricity is magic like Leprechauns, fairies, and Onion Rings.

Aug 06, 2013
Yup, lightning is "magic", usually the ignorant and religious resort to "magic" and "god". Here's some evidence of "electric discharge", a nice little dendritic channel excavated into concrete, complete with the charred blobs of displaced material.

Here is a crater excavated from a rock, note the spherules.

Here is one that nearly matches the article crater ejecta identically;

Electric discharge is scalable to many orders of magnitude, Valles Marineris, the Grand Canyon, and Olympus Mons attest to that;

The craters are electric

And you?
"If your hate could be turned into electricity, it would light up the whole world." Nikola Tesla

Aug 06, 2013
Yup, electricity is magic like Leprechauns, fairies, and Onion Rings

I like onion rings.

When I saw this story and the included picture, then read about the glaciers, I immediately thought of a much simpler explanation.

What if the glaciers themselves did this? As glaciers advance and retreat they leave a scree line at the points of farthest advance, as well as striations cut into the ground. Sublimation rather than melting would explain the lack of run-off streams. This would also account for the lack of secondary craters, since the glaciers would erase them

A close inspection on the ground would decide this in no time at all.

Giant lightning bolts from space. lol. I think these are more likely to be signs of LGM activity. (Little Green Men) This is obviously the Martian version of crop circles. Of course, in 50 years we will find out that the whole planet Mars was a joke played on us by 2 drunk alien red-necks in a UFO pickup truck. They probably don't remember it.

Aug 06, 2013
Uhhhhh, heh. Well, it's somewhat comforting to know that cantdrive85 hasn't actually been serious this whole time, but it's also somewhat disturbing that anybody would spend so much time intentionally trolling phys.org comments with bunk just to get a rise.

Nobody is ignorant enough to actually believe that the grand canyon was created by a giant lightning bolt.

Aug 06, 2013
This is the best explanation I have found which can explain "giant lightning bolts".

To be sure, I'm not serious about everything, however I am serious that PC deserves more respect for it's predictive prowess. I rather hold the view that I can't believe many mainstream theories are held in such high regard when so much magic is necessary for them to work.

Aug 07, 2013
but it's also somewhat disturbing that anybody would spend so much time intentionally trolling phys.org comments with bunk just to get a rise

it's called dissocial disorder:


It's a specific sub-type of antisocial behavior. Simply being around people more often in real life and having normal social interaction on a regular basis can go a long way towards helping out with this type of problem. In other words, he needs to get out more. Unfortunately, his behavior probably makes people avoid him, so making friends probably takes a lot of effort.

My daughter and her mom's side of the family have something like this that's hereditary, though what they have is probably more serious than what cantdrive seems to have. These kinds of disorders are kinda like different shades of gray, where they all share indicators at varying degrees, so it's hard to diagnose with certainty. Dissocial disorder is about the mildest form

Aug 07, 2013
Nobody is ignorant enough to actually believe that the grand canyon was created by a giant lightning bolt.

Nobody? Love those completely unsupported, instantly incorrect assertions there dude.

And please don't get trapped in the logic loop of "if you believe that, you must be ignorant", because that simply exposes your blindness.

Fact is: A great many non-ignorant people believe this, having been led to such an outlandish conclusion - sometimes dragged there kicking and screaming - by the facts. By hard evidence.

Deny the evidence all you want, but then please don't turn and point at someone else and label them "ignorant". Rebut the evidence. Present counter-evidence. But saying "Nobody is ignorant enough to actually believe..." about anything in this wild woolly world is not only hyperbole, but an instant fail because you come across as actually meaning it.

Aug 08, 2013
Maybe you missed every other comment thread on this site where cantdrive85 blabbers on about how his pet theory explains everything, then everybody actually bothers to do just that. He doesn't care.

I'm simply pointing out that at this point it's obvious that he doesn't care because he isn't even serious. And yes, you squarely qualify as ignorant(or whatever is wrong with cantdrive85, I'm not buying ignorance anymore) if you are willing to not only state that the grand canyon was formed by a giant lightning bolt, but in doing so, also asserting that even the most basic knowledge about the grand canyon's formation, abundantly obvious simply by looking at it but solidly proven by science as well is all wrong, even if they came to that conclusion by "studying" the "evidence" on some crackpot's geocities-looking page.

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