Meteorite shockwaves trigger dust avalanches on Mars

Dec 16, 2011 by By Daniel Stolte
Meteorite shockwaves trigger dust avalanches on Mars
Artist's conception of an asteroid impact on Mars. (Image painted by William K. Hartmann, co-founder of the Planetary Science Institute, Tucson, Ariz.)

(PhysOrg.com) -- Dust avalanches around impact craters on Mars appear to be the result of the shock wave preceding the actual impact, according to a study led by an undergraduate student at the UA.

When a meteorite careens toward the dusty surface of the Red Planet, it kicks up dust and can cause avalanching even before the rock from outer space hits the ground, a research team led by an undergraduate student at the University of Arizona has discovered.

"We expected that some of the streaks of dust that we see on slopes are caused by seismic shaking during impact," said Kaylan Burleigh, who led the research project. "We were surprised to find that it rather looks like shockwaves in the air trigger the even before the impact."

Because of Mars' thin atmosphere, which is 100 times less dense than Earth's, even small rocks that would burn up or break up before they could hit the ground here on Earth crash into the relatively unimpeded.

Each year, about 20 fresh craters between 1 and 50 meters (3 to 165 feet) show up in images taken by the HiRISE camera on board NASA's Reconnaissance Orbiter. The , or HiRISE, is operated by the UA's Lunar and Planetary Laboratory and has been photographing the Martian surface since 2006, revealing features down to less than 1 meter in size.

Close-up of Martian terrain with dark streaks interpreted as avalanches blasted by shockwaves from a meteorite impact. (Image: NASA/JPL-Caltech/The University of Arizona)

For this study, the team zoomed in on a cluster of five large craters, which all formed in one impact event close to Mars' equator, about 825 kilometers (512 miles) south of the boundary scarp of Olympus Mons, the tallest mountain in the solar system. Previous observations by the orbiter, which imaged Mars for nine years until 2006, showed that this cluster was blasted into the dusty surface between May 2004 and February 2006.

The results of the research, which Burleigh first took on as a freshman under former UA Regents Professor H. Jay Melosh, are published in the journal Icarus. Previous studies had looked at dark or light streaks on the Martian landscape interpreted as landslides, but none had tied such a large number of them to impacts.

The authors interpret the thousands of downhill-trending dark streaks on the flanks of ridges covering the area as dust avalanches caused by the impact. The largest crater in the cluster measures 22 meters, or 72 feet across and occupies roughly the area of a basketball court. Most likely, the cluster of craters formed as the meteorite broke up in the atmosphere, and the fragments hit the ground like a shotgun blast.

Narrow, relatively dark streaks varying from a few meters to about 50 meters in length scour the slopes around the impact site.

"The dark streaks represent the material exposed by the avalanches, as induced by the the airblast from the impact," Burleigh said. "I counted more than 100,000 avalanches and, after repeated counts and deleting duplicates, arrived at 64,948."

When Burleigh looked at the distribution of avalanches around the impact site, he realized their number decreased with distance in every direction, consistent with the idea that they were related to the impact event.

But it wasn't until he noticed a pair of peculiar surface features resembling a curved dagger, described as scimitars, extending from the central impact crater, that the way in which the impact caused the avalanches became evident.

"Those scimitars tipped us off that something other than seismic shaking must be causing the dust avalanches," Burleigh said.

HiRISE image of the study area showing the central crater with two dagger-like features extending at an angle (red and blue arrows). Called scimitars, these features most likely resulted from shockwave interference just before impact. (Image: NASA/JPL-Caltech/The University of Arizona)

As a meteor screams through the atmosphere at several times the speed of sound, it creates shockwaves in the air. Simulating the shockwaves generated by impacts on Martian soil with computer models, the team observed the exact pattern of scimitars they saw on their impact site.

"We think the interference among different pressure waves lifts up the dust and sets avalanches in motion. These interference regions, and the avalanches, occur in a reproducible pattern," Burleigh said. "We checked other impact sites and realized that when we see avalanches, we usually see two scimitars, not just one, and they both tend to be at a certain angle to each other. This pattern would be difficult to explain by seismic shaking."

In the absence of plate tectonic processes and water-caused erosion, the authors conclude that small impacts might be more important in shaping the Martian surface than previously thought.

"This is one part of a larger story about current surface activity on Mars, which we are realizing is very different than previously believed," said Alfred McEwen, principal investigator of the HiRISE project and one of the co-authors of the study. "We must understand how Mars works today before we can correctly interpret what may have happened when the climate was different, and before we can draw comparisons to Earth."

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User comments : 14

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gwrede
3 / 5 (4) Dec 16, 2011
Very interesting and real news! Excellent!

However, even excellent news is no excuse for sloppy thinking:
When a meteorite careens toward the dusty surface of the Red Planet, it kicks up dust and can cause avalanching even before the rock from outer space hits the ground
At the speeds meteorites have, and especially considering the thinner atmosphere of Mars which slows them much less than our atmosphere, there simply is no way the avalanches can happen before the actual impact.

As the speed of sound is much slower than that of the meteorite, this could only happen if the meteorite would almost be a grazer, that is, arrive at an extremely low angle and fly almost horizontally before impact. Obviously, that was not the case we see in the pictures here.
Shootist
1.8 / 5 (5) Dec 16, 2011
I think you're right. Avg. meteor speed is 22km/sec (1).
Speed of sound in a near vacuum?

(1) French B. M. (1998) Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite
Impact Structures. LPI Contribution No. 954, Lunar and Planetary Institute, Houston. 120 pp.
Vendicar_Decarian
3.7 / 5 (3) Dec 16, 2011
Speed of sound in the lower atmosphere of mars 240 m/s approx.

Computed impact velocity.

For 31 gram objects at the top of the atmosphere, the surface impact velocity is 430 m/s

For a 3.1 Kilogram object, impact velocity is 5,400 m/s

For a 31 Kilogram object, impact velocity is 6800 m/s

For a 310 Kilogram object, impact velocity is 7300 m/s

http://adsabs.har...81..399D

In all instances the computed impact speed is hypersonic at the surface of mars and hence the impact shock wave will be coincident or lag the actual impact.

Crazy_council
3 / 5 (2) Dec 16, 2011
would it still have a bow shockwave in front of it ? as mars has some atmosphere, it would still compress in front of it wouldnt it ?
nkalanaga
5 / 5 (3) Dec 16, 2011
I think the article is guilty of sloppy English. All three of you are right that the impactor would be travelling faster than the local speed of sound. The bow shock would arrive at the impact point before the impactor, but only by a fraction of a second.

It sounds like the article is discussing avalanches in existing craters due to a new impact. If that's the case, the air shock from the arriving object would affect them before the ground shock, even if the object impacts before the air shock reaches a given location. And, if it's not a vertical impact, at least some of the existing craters will receive the air shock ("sonic boom") before the object impacts.
Pirouette
1 / 5 (4) Dec 16, 2011
Those vertical "streaks" in the second picture are NOT avalanches, trust me. They are something else, which I don't have time to talk about here. McKewen could have provided pictures with higher resolution and less fuzziness from the HiRise camera. My group and I have pictures from HiRise that are crystal clear with no fuzziness, which is why I doubt that second picture is from the HiRise at all. More likely it is from the Mars Orbiter Camera (MOC) that was aboard the Mars Global Surveyor orbiter, that disappeared from Mars orbit on November 2006 and then HiRise took its place. The MOC was cutting edge of its time, although the pictures all had a bit of a flattish, slightly fuzzy quality anyway. It was built by MSSS. If you Google the MOC images, you will see what I mean that they have less resolution than HiRise. It took the HiRise to give the images a sharper, clearer quality that MOC didn't have.
MarkyMark
2.3 / 5 (3) Dec 17, 2011
For those that dont know the above poster Piroette is part of a groupe that belives that giant Transparent people and Frank Herberts Dune worms live on mars.
Vendicar_Decarian
3.4 / 5 (5) Dec 17, 2011
"Those vertical "streaks" in the second picture are NOT avalanches, trust me" - Spirochete

Ya, they are trees and the Evils Gubderment and the Corrupt NASA administration is trying to hide the fact from the American People under pressure from the Grey Aliens and their Socialist Moon Men coalition.

There is no bounds to Spirochete's Libertarian Delusion.
Pirouette
1 / 5 (3) Dec 17, 2011
@VendiNutzoid. . . . .that's fine. No one is stopping you from having your own opinions. Say whatever crosses your "mind".
:O
Vendicar_Decarian
3 / 5 (2) Dec 19, 2011
"Say whatever crosses your "mind"." - Spirochete

I am far too polite to do that.

Give us a link to some of your mars men pictures Spirochete.

Woo Hoo....
bluehigh
1.8 / 5 (5) Dec 19, 2011
... and some Mars females too please. I kinda feel like an exotic thrill. Kinky thinking that I could see whats happening through certain semi-transparent body areas.

Of course Vendi, if you prefer the men that's okay.

Pirouette
1 / 5 (2) Dec 19, 2011
LOL. . . .who the hell is spirochete? I don't know any spirochete, do you, bluehigh?
Pirouette
1 / 5 (1) Dec 19, 2011
"Say whatever crosses your "mind"." - Spirochete

I am far too polite to do that.

Give us a link to some of your mars men pictures Spirochete.

Woo Hoo....

For some strange reason, VendicartoonCharacter thinks he is too polite. I wonder how and where he gets a weird idea like that. Oh yes. . . .LOL. . .I remember it all now. His idea of being polite is to call everyone a "TARD". I'm still trying to figure out what TARD means. Could it mean that someone is a "LEOTARD" or a PETARD, or that he's just a "TARD 0UT" little boy, or maybe he's been "TARD AND FEATHERED" at some time in his past when he says he left America decades ago to become a naturalized immigrant in Texas. Why, that's about the equivalent of Obama and his 57 states, or that Australia is in Asia, or saying how nice it was to be back in Texas, when he was actually in Kansas.
Woooohoooo :)
bluehigh
1 / 5 (1) Dec 20, 2011
I never had VD so I'm not familiar with Spirochete which is a kind of bacteria that causes syphilis.

Perhaps a giant condom to wrap Vendi Dickhead and protect us all from his slide into delusional madness.

PS: c'mon Vendi, gimme a TARD award, its my turn. You don't play fair!

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