Re: How do we know some rock layers are 'extra-terrestial?'

Date: Sat Dec 12 14:12:54 1998
Posted By: Brian Kane, Post-doc/Fellow, Astronomy, AstroPlace, Inc.
Area of science: Earth Sciences
ID: 913132909.Es

Message:

While it's true that the same elements we all know from the Periodic Table exist both on Earth and in space, there are more subtle differences which can distinguish terrestrial rocks from exterrestrial ones, which include, NOT in implied order of importance:

(1) Alloys

Often, as in the case of metallic meteorites, the metals come in alloys which do not form naturally on Earth. For instance, the proportion of nickel to iron, and the arrangement of nickel and iron atoms in small-scale structures within the rocks, can show extraterrestrial origin.

(2) Inclusions

The extreme conditions of space - low or high temperature, and very low density - allow a peculiar chemistry to take place and exotic molecules to form. These molecules, both organic and inorganic, don't naturally form on Earth and can be captured as inclusions within meteoric material.

(3) Relative elemental composition

Elements as they occur on Earth are well-studied, and their relative proportions as studied from surface and near-surface rocks are very well known. Hence, any rock which is found to have one or more particular elements which are extremely super- abundant, is likely to be extraterrestrial in origin. A good example is the layer of sediment on the Cretaceous-Tertiary Boundary which contains very high proportions of iridium. No chemist or physicist knows how such high proportions of that element could be produced locally on Earth, so the theory is that the layer was produced by a brief but intense period of bombardment from comets and/or asteroids.

Current Queue | Current Queue for Earth Sciences | Earth Sciences archives

Try the links in the MadSci Library for more information on Earth Sciences.