IPB

Welcome Guest ( Log In | Register )

 
Reply to this topicStart new topic
Mars Meteorite origins
JRehling
post Oct 8 2007, 04:57 AM
Post #1


Senior Member
****

Group: Members
Posts: 1572
Joined: 20-April 05
Member No.: 321



Walking the line between wacky and sure to be unoriginal:

The martian meteorites are quirky in the ages, in that about half of them have ages of roughly 175 million years, whereas most of the martian surface is clearly older than that. There must be a selection effect biasing the samples we find.

Is it possible that altitude (on Mars) is a major selection factor? The top of Olympus Mons is above 90% of Mars's atmosphere. Of course, that represents a very small fraction of the surface area of Mars, but when you add in the heights of the five biggest volcanos on Mars, you get a still small but nonzero area of the surface of Mars at high altitude AND likely to be the last places on Mars to get paved over with lava.

When an impactor strikes these areas, it faces much less air resistance (and spalling) on the way in, and then any ejecta flying skyward also faces much less air resistance on the way out.

Additionally, sheets of lava should make for more elastic collisions than the dusty regolith at lower altitudes.

Could it be that these selection effects favor the highest volcanos so greatly that ejecta from these places outnumber ejecta from the rest of the surface combined?
Go to the top of the page
 
+Quote Post
ngunn
post Oct 8 2007, 01:40 PM
Post #2


Senior Member
****

Group: Members
Posts: 3156
Joined: 4-November 05
From: North Wales
Member No.: 542



Hey! I like it. Wacky or no, it deserves some comment at least. Hope you get some well-informed replies.
Go to the top of the page
 
+Quote Post
elakdawalla
post Oct 8 2007, 01:51 PM
Post #3


Bloggette par Excellence
****

Group: Admin
Posts: 4466
Joined: 4-August 05
From: Pasadena, CA, USA, Earth
Member No.: 454



I think it's arguable that while there are 34 known SNCs, they come from only three main groups; all of the meteorites from one group may represent only one meteorite-launching impact. If there were, in fact, only three impacts, you're bound to have selection effects on the ages.

--Emily


--------------------
My blog - @elakdawalla on Twitter - Please support unmannedspaceflight.com by donating here.
Go to the top of the page
 
+Quote Post
AndyG
post Oct 8 2007, 01:57 PM
Post #4


Member
***

Group: Members
Posts: 573
Joined: 20-April 05
Member No.: 279



It's an interesting idea, and thank you for making me think about it - but it's not one I think is correct.

Firstly, from an altitude and velocity point-of-view, at the top of Olympus Mons the energy needed for rocks to escape Mars is still over 99% of that required by materials thrown out from the bottom of Hellas, around 30km below. Not much to bias things here.

Looking at the atmosphere issue: Martian escape velocity is close to 5km/s. This feels fast, but with Mars' reduced (mean surface) pressure, that's a drag equivalent to a speed of around 500m/s in Earth's atmosphere. I suggest that's warming but not hugely damaging to any rocks that survive being accelerated to escape velocity in a fraction of a second - and at these speeds therefore free of the atmosphere in just a few seconds. That's not very long for drag to have an effect. So it seems there's little statistical bias against materials surviving being cast from lower altitudes, nearer the mean surface.

A quick google later - this old space.com article gives an insight into why younger rocks might be preferentially selected. (The suggestion is that common-sized impactors have to hit younger, less disrupted surfaces in order to create escape velocity debris).

Andy
Go to the top of the page
 
+Quote Post
tty
post Oct 8 2007, 07:43 PM
Post #5


Member
***

Group: Members
Posts: 683
Joined: 20-April 05
From: Sweden
Member No.: 273



Mass ejection from major impacts should not be much affected by the local atmospheric pressure since they literally punch a hole through the whole atmosphere. It might have some effect on smaller impacts.

However I think it likely that the martian meteors we have all come from just a few relatively recent impacts. Most of the stuff in Earth-crossing orbits would be swept up pretty quickly. It could be pure coincidence.

I don't quite buy the argument that an impactor has to hit hard rock to accelerate fragments to 5 kms-1. The Chicxulub impactor landed in limestone/gypsum which is not particularily hard and it still ejected enough material for the re-entering stuff to virtually fry the whole planet, including the antipodes (antipodal fragments must reach more or less orbital speed i. e. 8-11 kms-1 ).

An observation: we have never found any terran meteorites suggesting that most of the stuff ejected by Chicxulub and the group of major impacts at the Eocene/Oligocene border has already been swept up (admittedly a limestone meteorite might not be recognized as such unless it landed in Antarctica).

By the way this might be a good question Emily: just why don't we ever find terran meteorites, one should think that they should be at least as common as martian ones?
Go to the top of the page
 
+Quote Post
nprev
post Oct 8 2007, 08:00 PM
Post #6


Senior Member
****

Group: Moderator
Posts: 7086
Joined: 8-December 05
From: Los Angeles
Member No.: 602



QUOTE (tty @ Oct 8 2007, 12:43 PM) *
However I think it likely that the martian meteors we have all come from just a few relatively recent impacts. Most of the stuff in Earth-crossing orbits would be swept up pretty quickly.

By the way this might be a good question Emily: just why don't we ever find terran meteorites, one should think that they should be at least as common as martian ones?


You may have answered your own question here, Tom. There probably hasn't been an impact energetic enough to boost a truly significant amount of terrestrial surface materials to escape velocity since Chicxulub, and 65 My is plenty of time to sweep up the remnants...

Of course, we may find a surprise or two on the Moon someday. smile.gif


--------------------
A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
Go to the top of the page
 
+Quote Post
tty
post Oct 8 2007, 08:17 PM
Post #7


Member
***

Group: Members
Posts: 683
Joined: 20-April 05
From: Sweden
Member No.: 273



QUOTE (nprev @ Oct 8 2007, 10:00 PM) *
You may have answered your own question here, Tom. There probably hasn't been an impact energetic enough to boost a truly significant amount of terrestrial surface materials to escape velocity since Chicxulub, and 65 My is plenty of time to sweep up the remnants...

Of course, we may find a surprise or two on the Moon someday. smile.gif


Actually the Eocene/Oligocene impacts (Chesapeake, Popigai etc) ought to have been plenty powerful enough, and so should Ries and Steinheim (Miocene) have been.

You may be right about surprises on the Moon. It has alway seemed to me that the only decent chance we have of ever finding out anything about conditions on Earth before c. 3800 MA BP is to search on the Moon. A fairly substantial proportion of Earths Hadean crust and perhaps mantle ought to have ended up there during the Late Heavy Bombardment. In a way I'm a bit surprised no Terran material turned up in the Apollo samples, though after 3800 million years of impact "gardening" most of it would be buried and/or included in impact breccias. We might even find microfossils of the first Terran life-forms up there....
Go to the top of the page
 
+Quote Post
elakdawalla
post Oct 8 2007, 08:29 PM
Post #8


Bloggette par Excellence
****

Group: Admin
Posts: 4466
Joined: 4-August 05
From: Pasadena, CA, USA, Earth
Member No.: 454



I'm trying to figure out who to ask about the dynamics of Earth ejecta vs lunar ejecta and so forth. Renu Malhotra is a name that's popped up after some Google searching; does anyone else interested in this question see any other researchers who might know?

--Emily


--------------------
My blog - @elakdawalla on Twitter - Please support unmannedspaceflight.com by donating here.
Go to the top of the page
 
+Quote Post
tty
post Oct 8 2007, 09:08 PM
Post #9


Member
***

Group: Members
Posts: 683
Joined: 20-April 05
From: Sweden
Member No.: 273



Perhaps Jay Melosh at the University of Arizona?

http://www.lpl.arizona.edu/faculty/melosh.html
Go to the top of the page
 
+Quote Post
elakdawalla
post Oct 8 2007, 09:15 PM
Post #10


Bloggette par Excellence
****

Group: Admin
Posts: 4466
Joined: 4-August 05
From: Pasadena, CA, USA, Earth
Member No.: 454



Yeah, that's a good one. Thanks for the suggestion.

--Emily


--------------------
My blog - @elakdawalla on Twitter - Please support unmannedspaceflight.com by donating here.
Go to the top of the page
 
+Quote Post
JRehling
post Oct 9 2007, 02:58 PM
Post #11


Senior Member
****

Group: Members
Posts: 1572
Joined: 20-April 05
Member No.: 321



QUOTE (AndyG @ Oct 8 2007, 06:57 AM) *
It's an interesting idea, and thank you for making me think about it - but it's not one I think is correct.

Firstly, from an altitude and velocity point-of-view, at the top of Olympus Mons the energy needed for rocks to escape Mars is still over 99% of that required by materials thrown out from the bottom of Hellas, around 30km below. Not much to bias things here.

Looking at the atmosphere issue: Martian escape velocity is close to 5km/s. This feels fast, but with Mars' reduced (mean surface) pressure, that's a drag equivalent to a speed of around 500m/s in Earth's atmosphere. I suggest that's warming but not hugely damaging to any rocks that survive being accelerated to escape velocity in a fraction of a second - and at these speeds therefore free of the atmosphere in just a few seconds. That's not very long for drag to have an effect. So it seems there's little statistical bias against materials surviving being cast from lower altitudes, nearer the mean surface.

A quick google later - this old space.com article gives an insight into why younger rocks might be preferentially selected. (The suggestion is that common-sized impactors have to hit younger, less disrupted surfaces in order to create escape velocity debris).

Andy


Yes, I definitely didn't posit that there was any significance to the altitude getting us higher in the gravity well. Even Olympus Mons is pretty small on that scale. </sacrilege>

Per the drag issue, I'd raise another couple of points: In any impact where some ejecta is capable of escaping the atmosphere, there must be other ejecta that are going not quite fast enough to escape, and even a little drag could make the difference.

I don't have any expertise in the area, but if it turns out that a typical one-in-five-million-years impact to strike Mars can only eject a tiny fraction of its mass, then even a slender margin could have a major effect on how much debris escapes. By analogy, how many humans can run the 100 meters in less than 10 seconds vs. how many can run it in less than 9 seconds (about five people and absolutely zero, respectively).

Additionally, the drag would only be minimal for the stuff that's popping straight up, which should also be a tiny fraction of the mass (zero, even?). There should be much more mass traveling more tangential to the surface, so it would have to cover a ground track of many tens or even over 100 km before rising out of the atmosphere. Essentially, the same sort of horizontal path through the atmosphere that makes the Sun look so much redder and dimmer before sunset. I believe on Earth, the light path to the Sun at sunset involves about 15-30 times as much air mass as takes place when the Sun is at zenith. Now contrast the situation in the martian lowlands with what you'd have atop Arsia Mons. The horizontal path off the top of Arsia would almost be in the vacuum of space already, whereas the stuff flying at shallow angles off the lowlands would have to spend around half a minute in the atmosphere of Mars.

Finally, per the space.com article, the favorability of lava sheets is what I was referring to with my comments on inelasticity of the surface. Another factor favoring these same locations.

I'd like to see a model of the spallation that gives the full 3-dimensional account of how much stuff sprays out at all angles, and apply the atmospheric drag as a factor for lowlands and the loftiest heights. It seems to me that there are some possible effects making the multiplier a very large one, but the devil is in the details. If I change my 100 meters example to 47 seconds vs. 46 seconds, then 1 second isn't such a big deal anymore. The question would be if the maximum velocity of ejecta would be very close to the important threshold here (escape velocity). The closer it is, the more a tiny factor like air pressure would make on the amount of ejecta escaping.
Go to the top of the page
 
+Quote Post
AndyG
post Oct 10 2007, 11:35 AM
Post #12


Member
***

Group: Members
Posts: 573
Joined: 20-April 05
Member No.: 279



To put some figures to this intriguing idea, I've used a basic model of the Martian atmosphere and run density/quantity measurements for straightline paths at a variety of starting altitudes and exit angles. The results are tabulated in a jpg here. (I think straight lines are acceptable in this initial instance, where hypersonic material which isn't going to fall below escape velocity, is being slowed for just a few seconds).

In the table, alt is in kilometres above or below the reference radius, the angle of ejection goes from 0 degrees (parallel to the surface!) to 90 degrees (straight up). Results are in % of the vertical amount of atmosphere above the 0km altitude.

The higher the percentage figure, the greater the effects of drag. As you point out, higher altitudes allow for "wider exit cones" depending on whichever drag figure marks a cut off for any particular launch velocity - but with regards to material "popping straight up" it's interesting to see that material released at lower angles than 90 degrees doesn't get badly affected by drag until the angle really is quite low: drag effects are only 10% worse at 65 degrees, 40% worse at 45 degrees: finally doubling at 30 degrees. (In the real - i.e. non straightline, and slower particle - world, exit cones would be, I accept, narrower than these.)

The Martian atmosphere is not great at retarding rocks. I've seen figures suggesting that the freefall terminal velocity for Earth meteorites is around the 180m/s mark. Under lower Martian gravity, but with far thinner air, it would seem likely that (spent) meteorites which hit Mars come in around the 1km/s figure - a fifth of escape velocity.

You mention spallation models, and I agree that this is the key to a greater understanding - though I wonder if a 3d model is necessary, since planetary rotation effects (+/- 250m/s at the equator) could be handled along with drag and exit angle in a 2d case just as easily. What is rather intriguing is the effect of drag, given that it rises with the square of the velocity. There's a chance that - for a bell-curve of initial material speeds and mass distributions (and is that right?), a wider cross section of material launched at different speeds and angles could reach minimal escape velocities...though, as you suggest, are we just seeing the tip of the Olympus-berg?

Andy
Go to the top of the page
 
+Quote Post
marsbug
post Oct 12 2007, 12:46 PM
Post #13


Member
***

Group: Members
Posts: 284
Joined: 5-January 07
From: Manchester England
Member No.: 1563



QUOTE (tty @ Oct 8 2007, 10:08 PM) *
Perhaps Jay Melosh at the University of Arizona?

http://www.lpl.arizona.edu/faculty/melosh.html



If it hasn't been run across already, this article, reproduced from Melosh's contribution to the july 94 planetary report, has some interesting ideas that might apply to a mars impact, particularly in the updates down the bottom where he talks about how spallation might not operate alone to launch particles , but in combination with the impact plume.


--------------------
- "I am a child, and I like to ask childish questions!" Albert Einstien

My Blog
Go to the top of the page
 
+Quote Post

Reply to this topicStart new topic

 



RSS Lo-Fi Version Time is now: 23rd October 2014 - 01:37 AM
RULES AND GUIDELINES
Please read the Forum Rules and Guidelines before posting.

IMAGE COPYRIGHT
Images posted on UnmannedSpaceflight.com may be copyrighted. Do not reproduce without permission. Read here for further information on space images and copyright.

OPINIONS AND MODERATION
Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators.
SUPPORT THE FORUM
Unmannedspaceflight.com is a project of the Planetary Society and is funded by donations from visitors and members. Help keep this forum up and running by contributing here.