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It’s amazing what a rover can find laying by the side of the road. The Mars Exploration Rover Opportunity has found a rock that apparently is another meteorite. Less than three weeks ago, Opportunity drove away from a larger meteorite called “Block Island” that the rover examined for six weeks. Now, this new meteorite, dubbed “Shelter Island,” is another fairly big rock, about 47 centimeters (18.8 inches) long, that fell from the skies. Block Island is about 60 centimeters (2 feet) across and was just 700 meters (about 2,300 feet) away from this latest meteorite find. At first look, the two meteorites look to be of a similar makeup; Opportunity found that Block Island was is made of nickel and iron.
This image was taken during Oppy’s 2,022nd Martian day, or sol, (Oct. 1, 2009).
See below for a 3-D version of this image created by Stu Atkinson.
Where is the impact crater?
good question – I guess the answer lies in the dust around the meteorite.
The original crater would have been blown away over the years leaving the rock sitting where Opportunity found it?
Yes, it is a very good question. Here’s another.
How come we continually see impact craters without impactors present, and we see impactors without craters present, but nowhere in the millions of images of such things do we see impactor and crater together?
Predicted answers:
The impactor was destroyed by the impact…
The crater was worn away by erosion…
How convenient…
No, not convenient!
Natural is a better word, I think!
Impactors that leave a crater must have been high-energetic events. They probably hit with high speeds and thus exploded during the impact (I guess, such things are inelastic collisions – meaning the kinetic energy was completely turned into heat).
Impactors that leave no crater have been rather low-energetic events. They hit most likely with rather low speeds, the air slowed them down enough that they weren’t hot and fast anymore. So they survived the impact. But that also means that they just scratched only a little hole into the ground.
This is not convenience, this is physics and a little logical thinking -don’t you think?
Seeing the apparent ease with which Martian meteorites are being found and extrapolating from the total number of these meteorites from the known samples, I wonder how many of these may have originated on Earth. While most of the meteorites on Mars might have their origin elsewhere, a small percentage of them may have been blasted off the early Earth. Setting aside the notion of Earth life hitching a ride to Mars, I think this would be a great boon to planetary geologists studying the early Earth (and possibly the Moon, too.).
@Dr Flimmer: I agree, natural is a better word! After all “This is not convenience, this is physics and a little logical thinking -don’t you think?” Couldn’t have said it better myself 🙂
Earth gets quite a few of these “low-energy” impactors, too, that don’t leave any craters but they do leave a dense rock lying on the ground. And while I find the possibility that these rocks Opportunity are finding come from Earth to be tantalizing, Occam’s Razor leads me to think these guys are more likely coming from the Asteroid Belt (Mars is right next to it, after all, and the composition of the rocks found so far is consistent with what you would find in your typical asteroid).
Nonetheless, I’m very happy to see Opportunity continue to live up to her name!
Joe, my thinking exactly.
When the similar crater question arose on Block Island, I found that the hypothesis (probably from erosion and Opportunity data) is, IIRC, that Meridiani Planum has weathered 1 km down. The idea that the weathered remains from the impact of a 3 Gy old and relatively small impactor such as Block Island would then leave a still visible imprint is a tough sell.
Also, IIRC the dating of Block Island relied on the considerably thicker Mars atmosphere of that era. It couldn’t have survived a high velocity impact in todays tenuous atmosphere.
Besides this being a natural process, as Dr Flimmer reminds us, a contradictory conspirationist idea is always not only unnatural but severely so; it’s nuts.
Btw, I can’t shake the idea that these types of 1D meteorite concentrating conveyor belts, while not as effective as the 2D Antarctica ice variants, show up on Earth as well. Is that part of the reason why deserts are popular hunting grounds for collecting meteorites?
D’oh! “Joe” = Jon. My apologies.
It’s a nice idea, Hanford, but I seem to recall a discussion (here perhaps?) that it’s far less likely for stuff to get blown from Earth to Mars than the other way round.
Mars is really very small compared to Earth, so while it isn’t easy, it’s easier to leave Mars’ gravity. Secondly, I think it’s easier for meteoroids to migrate inwards than outwards in the solar system.
Of course it should be possible to do some back of the envelope calculations for the proportion of the rates of transfer back and forth, and then use the discovered number of Martian meteorites here on Earth to estimate how many are discoverable on Mars.
Sili, Torbjorn Larsson, thanks for the feedback to my post. By no means do I believe significant pieces of Earth lie on Mars (Sili is perfectly correct that its easier to migrate meteorites inward than outward, for example). I was merely alluding to the possible value of such finds once they are made. Of course, some mineralogical changes in the specimens may have taken place if Mars was indeed wetter in the distant past.
On a separate note all these Martian meteorites can tell us a great deal about Mars’ past climate and something about their own composition, age, and parent body.
Good idea, Sili. Pity that the Moon doesn’t have any kind of meteorite conveyor belt I know of. But OTOH small enough Earth impactors could be relatively unharmed and have merely solar wind weathering.
Otherwise I would scour the stable Lagrange points for specimens. Huh, the most interesting ones would be the Earth-Moon L4 and L5 and I see that they are believed to contain more or less stable “Kordylewski clouds” of dust.
“Current knowledge based on scientific observations according to Roach [6] are:
• there is sufficient material at the-Earth-Moon libration points L4 and L5 to
produce a solar counterglow of 20 S10 Vis brightness,
• these libration clouds are about 6 degrees in angular size as seen from the Earth,”
“The clouds extend up to a diameter of approx. 10000 km.”
[THE KORDYLEWSKY CLOUDS – AN EXAMPLE FOR A CRUISE PHASE OBSERVATION DURING THE LUNAR MISSION BW1, Laufer et al.]
That is still some volume to go through. 😀
(Also, the clouds seem to move around, a dynamic process which doesn’t look good for long-term capture of larger stuff. :-/)
[Takes out envelope and flips it over. Where’s that pen?]
I love the picture! If you look at the horizon at about center, you’ll see a dark shape. any clues what it is?
An anomaly from the photo or maybe phobos deimos?
I remember reading accounts for visual and photographic observing programs undertaken in the late 1950s and the 1960s to look for signs of these ‘Kordylewsky Clouds’ at the L4 and L5 positions. The photographic programs employed conventional 35mm or 4×5 cameras with superfast wide-angle lenses (f/0.95 to f/2), Schmidt cameras, Baker-Schmidt cameras and astrographs. I’ve still not seen a convincing image of these clouds published anywhere. This is not to say that small bodies are not present at L4 and L5, but it may take a dedicated effort to find and analyze them. Only one reference in the paper cited by Larsson was written in the 80s when CCD detectors should have the upper hand in sensitivity for such faint, extended objects. I’ll have to check out the references provided to see if I can find an image of these purported ‘Kordylewsky Clouds’ and read the authors analysis of these images. Seems, though, that with todays supersensitive CCD cameras, professional and amateur astronomers should have no trouble imaging them. It should also be noted that in the paper mentioned by Larsson the actual claim of the discovery and existence of these clouds is still being debated.
BTW, thanks to Torbjorn Larsson for the link to this paper and references therein.
After reading the Wiki page on ‘Kordylewski clouds’ [sic] it appears that photometric and photographic evidence has been obtained seemly favoring their existence. Check out the Wiki page here: http://en.wikipedia.org/wiki/Kordylewski_cloud . But the Wiki piece also points out “The existence of the Kordylewski clouds is still under dispute. The Japanese Hiten space probe, which passed through the libration points to detect trapped dust particles, did not find an obvious increase in dust levels above the density in surrounding space.” Thanks again to Torbjorn Larsson for bringing this arcane but important line of research to the readers of UT.
Logical thinking… so we have enough “air” on Mars to cool and slow down this rock… that’s news to me.
Did anyone mention some conspiracy?
The landscape Opportunity has been crossing resembles Antarctic snow fields where meteors are searched for as it is open, windswept and flat. Perhaps similar to Antarctica, meteors are transported to the surface as ice sublimates, or glaciations retreat. Craters formed in ice deposits more easily eroded? and therefore much less evident or completely missing?
davesmith_au,
Astronomers understand enough of Martian atmosphere to use it successfully in aerobraking for the landers sent to the planet. This knowledge of Martian atmosphere can lead to understanding how meteorites can survive and not make a crater.
The way you worded your first post reminds one of the style used by conspiracy theorists. I can see why the word “conspirationist” was used. You appeared to be trying to discredit current understanding of crater formation to make an opening for your own pet theories.
We found a Martian meteorite here (on Earth), we would be able to recognize a meteorite land there (on Mars)?
@ davesmith_au
There is also another thing to consider, not only air.
What about the relative velocity between Mars and the asteroid? If the asteroid is only slightly faster than Mars on their orbits and, say, catches up from behind, then there is no need for thick air or anything. It would just touch down quite softly and would survive the impact without an explosion or a crater.
Couldn’t that be possible, too?
Or maybe this little feller is a chunk of a bigger one that hit someplace else and busted up. Lotta energy lost that way and it would mean that this smaller chunk of rock wouldn’t have a lot of impact force. Or maybe the Meteorite struck a now nonexistent sheet of ice or pool of liquid, eh who knows, we need boots on the ground people!
There is enough atmosphere to slow down the spacecraft we send to Mars, Most meteroids we observe are brought to subsonic speeds in the stratosphere with pressures similar to the atmosphere of Mars on the surface. A chunk of rock this size could be slowed enough so a crater does not form.
LC
So there are several variables with a meteorite impact. The initial speed before reaching the atmosphere, the atmosphere itself, planet’s gravity, the angle relative to the planet, surface area, density, composition (rocky vs iron/nickel), final speed on reaching the surface and surface composition. These are top the head.
I believe the smaller the object is the greater the role of terminal velocity on the final speed.
Corrections are welcome. Again these are just reasonable guesses.
davesmith_au:
I wish that there was Comic Sans text facility here for the above quote. *Sigh*
Dude, what you are overlooking is the fact that Mars once had a much thicker atmosphere a billion or so years ago — which was probably when that meteorite landed; it did not drop down out of Mars’ sky like, er… last bloody Xmas! — and its atmosphere, being composed chiefly of carbon dioxide from volcanic activity, would probably have been several times denser than even Earth’s atmosphere is today, so the meteorite would have been slowed down sufficiently to just drop onto the ground without causing a big crater.
I suggest that you read this: Meteorite Found On Mars Yields Clues About Planet’s Past
Furthermore, there is evidence that Mars once had shallow seas, as well a thick atmosphere, so if the meteorite had landed into one, it would have absorbed even more of the meteorite’s kinetic energy and it would have just settled at the bottom of the sea, which has since evaporated and the water is now locked up as permafrost at Mars’ poles.
*CRICKETS*