Space Rocks. How do they know?

Originally posted by Taledus
@ Essan:

The links bring up pages, but not the abstract. In order to get the pdf you have to sign up

. Any way you could just post the abstract?

Source

Oblique Impact: A Process for Obtaining Meteorite Samples from Other Planets
JOHN D. O'KEEFE 1 and THOMAS J. AHRENS 1

1 Seismological Laboratory 252-21, California Institute of Technology, Pasadena, CA 91125.

Cratering flow calculations for a series of oblique to normal (10° to 90°) impacts of silicate projectiles onto a silicate halfspace were carried out to determine whether or not the gas produced upon shock-vaporizing both projectile and target material would form a downstream jet that could entrain and propel SNC meteorites from the Martian surface. The difficult constraints that the impact origin hypothesis for SNC meteorites has to satisfy are that these meteorites are lightly to moderately shocked and yet have been accelerated to speeds in excess of the Martian escape velocity (more than 5 kilometers per second). Two-dimensional finite difference calculations were performed that show that at highly probable impact velocities (7.5 kilometers per second), vapor plume jets are produced at oblique impact angles of 25° to 60° and have speeds as great as 20 kilometers per second. These plumes flow nearly parallel to the planetary surface. It is shown that upon impact of projectiles having radii of 0.1 to 1 kilometer, the resulting vapor jets have densities of 0.1 to 1 gram per cubic centimeter. These jets can entrain Martian surface rocks and accelerate them to velocities greater than 5 kilometers per second. This mechanism may launch SNC meteorites to earth.

[url=http://www.sciencemag.org/cgi/content/abstract/198/4323/1249-a] Source [url]

Meteorite Impact Ejecta: Dependence of Mass and Energy Lost on Planetary Escape Velocity
JOHN D. O'KEEFE 1 and THOMAS J. AHRENS 1

1 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena 91125

The calculated energy efficiency of mass ejection for iron and anorthosite objects striking an anorthosite planet at speeds of 5 to 45 kilometers per second decreases with increasing impact velocity at low escape velocities. At escape velocities of >105 and >2 x 104 centimeters per second, respectively, the slower impactors produce relatively less ejecta for a given impact energy. The impact velocities at which ejecta losses equal meteorite mass gains are found to be approximately 20, 35, and 45 kilometers per second for anorthosite objects and approximately 25, 35, and 40 kilometers per second for iron objects striking anorthosite surfaces for the gravity fields of the moon, Mercury, and Mars.

They're just examples I found. Sorry I don't have access to the full papers.

Also found this which provides a basic explanation of martian meteorites

www.theguardians.com...

Originally posted by Taledus
Got any clue how large an object has to be in space for it to still be a rock after burn-entry into the atmosphere?

It depends on many factors:- Velocity, density, composition, and angle of entry. Obviously the slower and denser meteoroids that are made of tougher material and enter at a relatively shallow angle will have a much better chance of making it to the ground. Too shallow and it'll either skip off the atmosphere and go back out to space or it'll burn up trying to make it trough the atmosphere. Too steep, and it may disintegrate completely due to the extreme forces associated with a very steep entry angle.

I would say, that a rock as large as a bowling ball and with the correct attributes as described above, could quite easily survive.

Originally posted by Taledus
Edit: OMG!!! I just realized that I have completely taken over this thread...I apoligize to the OP for that.

Its all good. This has become way more interesting than my initial question anyways...

The concept of a large object hitting Mars and sending particles this way is one I have heard about for a while, so I have no doubt of the possibility of the rocks being here. I was just in doubt of how they could be so sure.

Addressing the 'new' topic at hand, another way to look at this, is if the 'impact' was say millions of years ago, the atmosphere on mars may have been very different. IE thinner or even non existent. This may make a difference.

If the rocks sent here are from deep within the ground of mars, the composition would be different to that of the surface would it not? That I thought would have made a difference.

Now back to the original topic, and not wanting to dispute for the sake of disputing, peacejet said (and I apologise for the large quote)

The reason why I am saying this is to understand the uniqueness of the planets, so venus is too hot and mars is too cold and the earth is just right for life and this temperature determines, the way in which elements are formed, for example, mercury has a surface and core full of iron, which didnt rust, because of the lack of water vapour due to the vicinity to the sun, similarly the kuiper belt objects are too far away from the sun and are merely ice ball orbiting the sun, so, the composition of a rock from mars cannot be the same of that of the kuiper belt object.

My thoughts (more an abstract theory) on this are that it could be possible that a K.Belt object for the sake of arguement may take quite some time to make its way to earths orbit therefore 'changing' its make up as it gets closer to the sun and heating up, or from attracting foreign objects and various collisions on its journey. I know this is not put very scientifically, but I hope you get the point.

Originally posted by VIKINGANT

Addressing the 'new' topic at hand, another way to look at this, is if the 'impact' was say millions of years ago, the atmosphere on mars may have been very different. IE thinner or even non existent. This may make a difference.

Great point! Now this makes it a logical occurence that it can happen, just a loooooong time ago. I am just finding it difficult to believe that it can happen with the current atmospheric properties of Mars now, without something very large (semi-truck) travelling into Mars at the speed of light in order to propel the mass of a kilometer-long object off the surface and into space. But I am in no way an expert, and don't wish to have it thought that I have a whole bunch of experience dealing with this. I just find it very perplexing that it can happen, and the variables that take place when it does.

reply to post by VIKINGANT


Yes, I get your point, but for the kuiper belt objects only the outer layer of ice will melt away, just the like one seen with comets, and to have a change in chemical nature, the rock has to be melted entirely and allowed to cool, which is possible only in the initial stages of the universe and not in the present time.