THE PROSPECTIVE CANDIDATE(S)

A scientist at NASA wrote:

"Would we recognize martian sediments or felsic rocks as meteorites if we found them in Antarctica? Probably not, because they would look too much like Earth rocks. The case is even worse for recognizing possible Earth or Venus meteorites". (5)

The statement above is a very powerful indication of how difficult it may be to find one of those meteorite types and recognize it as such. So how would we even begin to recognize a possible Earth or Venus meteorite?

First of all we would need to find a specimen or specimens that looked like they could be meteorites. In the case of Venus they would be a basaltic type meteorite because Venus has been, and to some extent may still be an active volcanic planet. This would not necessarily be true of an Earth meteorite because many different forms and types of rock exist on our planet. So a meteorite originating from Earth may be any number of rock (lithic) types.

So once a likely looking candidate is found, we need to see if it has any attributes that match that of a meteorite. Does it look like it has been melted on the surface? Does it have primary and/or secondary fusion crust characteristics?

A primary fusion crust is a layer of solidified melt glass coating the exterior. Such glassy surfaces are very thin, commonly less than a millimeter, except for solidified pools of melt on the trailing edges of oriented meteorites.(13)

A secondary fusion crust develops when a meteoroid breaks up in flight. If this happens at a high altitude while the meteorite retains much of its cosmic velocity, the freshly broken face will also form a fusion crust, called a secondary crust. It can usually be recognized because it is thinner than the primary crust, having formed later in flight. Also, the broken face suffers less ablation and is usually not as smooth as the primary crust.(11)

Excellent display of smooth primary crust and coarser secondary fusion crust in photos. Same specimen below

Another characteristic in a meteorite we could look for, as stated above, is to find an oriented specimen and see if we could identify the trailing edge of the meteorite and witness for ourselves if the trailing edge has more accumulation of solidified melt glass than is on the sides of the specimen or on the face of the leading edge.

Very nice accumulation of material on trailing edge.

The crust on stony meteorites varies in thickness from an average of less than a millimeter to several millimeters, and often varies in thickness from front to rear on oriented meteorites. The forward end has the thinnest crust and the rear has the thickest, since the melted glass accumulates towards the rear.(11) Much like what we can see in the photo above and below.

Upon closer inspection we can see if these meteorite candidates have any flow structures. Flow structures are indicators of flight direction and are most common on oriented specimens. Flow structures are best seen with a hand magnifier or mineral microscope. Only a relatively fresh meteorite will retain these features.(11)

The above 2 photographs were taken on areas of specimens that were protected from the elements of nature on secondary fusion crust sites. They look like a coarse interiors beginning to melt as the ablation phase was ending. You can also tell the direction of travel based on the signature of the flow structures, going from top to bottom and left to right, much like ocean waves when viewed from an airplane.

The external features that typify a meteorite, or what makes it different from the other rocks around us and make us take note and pick it up for a closer look are found in the above rocks. Our candidates do have the same features as meteorites. But do they actually have a fusion crust that we can see?

It can be seen that the candidates do have the same external features as meteorites. What needs to happen now is to verify that our specimens have an actual fusion crust. We need to make sure they are not just signs of terrestrial weathering processes such as wind or water abrasion or a weathering rind of some sort.

Preferably, in order to actually see if a fusion crust does exist, a thin section of the rock is made. This is a slice of rock mounted on a glass slide and so thin you can actually see through it. However, certain precautions have to be made in order not to destroy the fusion crust during this process. Below are 2 photos , taken through a microscope, of the candidates fusion crust. Although the meteoritic candidate has been subjected to harsh environmental weathering effects, enough fusion crust remains to make it identifiable as such.

TS1.

Both of these photos were taken at 100X. The left edge of the top photo will match the right edge of the lower photo. See crystal structure for matching the photos. 

The probability of the candidates being meteorites are increasing with what we have seen up to this point. There is no reason we should end our pursuit of testing now. We have visually looked at the specimens and they do have the external features of a meteorite. We have also made a thin section of the specimens, and again, they have the characteristics of a meteorite, fusion crust characteristics.

At this point we should authenticate that this really could be a fusion crust, we can see it looks like a fusion crust, but we should have an XRS (x-ray spectroscopy) analysis done on the crust just to be sure. Sometimes the composition of the meteorite can be surmised by examining the crust.(11)

X-ray spectroscopy conducted at a commercial lab on the suspected fusion crust revealed that this "alteration" layer is not a reflection of a terrestrial weathering rind or desert varnish, but is in fact consistent with a fusion crust. An XRS examination was performed on the alteration layer titled as "surface", see XRS 01-surface. Another examination by XRS was conducted on the interior of the thin section to check bulk composition of the basalt, see graph titled -interior-bulk XRS 02. This "alteration" layer was identified as being an iron-aluminum-silicate layer, consistent with the bulk composition of the specimen and matching that of a fusion crust.

XRS 01-surface

XRS 02 - interior

At this stage we should feel pretty confident we have a good candidate for choice to have a Cosmic Radiation Exposure Analysis test performed. Unfortunately, there is no commercial lab available to (us) the public for this type of test. Later on in our presentation we will see how this test can establish the validity of our specimens as being meteorites.

* Authors note* A recent examination confirms this alteration crust is in fact a glass layer.

Below is the piece which the thin section was made from. If you look closely you can see white areas around the perimeter of the specimen where leaching of minerals from the surrounding environment into the specimen has occurred.

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