Maxwell Boltzmann and the Uranium in Black Shale
The Maxwell-Boltzmann equation describes the velocity distribution of vaporized atoms. The heavier the element the slower its velocity related to other lighter elements. As the heaviest natural element vaporized uranium atoms will always have the lowest probability of escaping the Earth’s gravity well. That is why uranium falls back to Earth while other lighter atoms escape. Uranium falls back and is concentrated in the source rock for oil; black shale. The Uranium is a marker for an interstellar impact flare.
The speed probability density functions of the speeds of a few noble gases at a temperature of 298.15 K (25 °C). The y-axis is in s/m so that the area under any section of the curve (which represents the probability of the speed being in that range) is dimensionless.
I made this image, and am placing it in the public domain. Pdbailey 17:51, 24 Oct 2004 (UTC) From Wikipedia