Did Anyone See It Land? Why That Distinction Matters
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Meteorites are divided into those seen to arrive and those found later, and the difference determines how much they are worth scientifically. Witnessed arrivals are rare and the evidence they carry is not recoverable otherwise.
The distinction and why it exists
A fall is a meteorite recovered after its arrival was observed, and a find is one recovered without anybody having seen it come down. The distinction matters because everything about a meteorite begins degrading the moment it lands. Terrestrial weathering rusts the metal, dissolves soluble minerals, and introduces contamination from soil, water and the atmosphere, which affects exactly the delicate chemistry that makes these objects interesting. A fall collected promptly is therefore close to its original condition, while a find of unknown residence time carries alteration that must be identified and allowed for. Falls are also dated exactly, which finds are not, and their arrival trajectory can sometimes be reconstructed.
What a witnessed arrival supplies
Observation adds information the rock alone does not carry:
- •The date of arrival, which allows short-lived radioactive isotopes to be measured before they decay
- •Uncontaminated material, if collected quickly and handled properly
- •The trajectory, if enough observations or camera records exist
- •The orbit before arrival, calculated from the trajectory, which locates the source region
- •Association between a specific rock type and a specific orbit, which is otherwise unavailable
- •Fewer than fifty meteorites have well-determined orbits, out of tens of thousands catalogued
Where finds come from
Most catalogued meteorites are finds and they come overwhelmingly from two kinds of place. Hot deserts preserve them because there is little water to weather them and little vegetation to hide them, and a dark rock on a pale desert pavement is conspicuous, so systematic searching in the Sahara, in Oman and in Australia has recovered enormous numbers. Antarctica is better still, because meteorites falling on the ice sheet are buried, carried along by flow and concentrated where the ice is forced upward against mountains and ablates away, leaving them exposed on blue ice fields in concentrations far above the fall rate. Organised Antarctic searching since the 1970s has recovered tens of thousands, which transformed the available sample and included the first recognised pieces of the moon and of Mars.
Telling one from an ordinary rock
Most objects brought to museums as meteorites are not, and the checks that eliminate the majority are simple. Genuine specimens are usually denser than ordinary rock and almost always attract a magnet, since nearly all contain some metallic iron. A recent fall carries a thin black glassy crust formed by melting during descent, which is distinct from the interior and frequently shows flow lines. The interior of most types shows small metal flecks on a cut or ground surface. What they do not show is vesicles, meaning the rounded gas bubbles that riddle slag and volcanic rock, which is the single most useful negative test since industrial slag is the commonest false identification. Quartz never occurs, so a rock containing visible quartz crystals is terrestrial.
What to do if one lands
The handling advice matters and is frequently ignored in the excitement. Photograph the object in place before moving it, since position and orientation carry information. Handle it with clean gloves or a bag rather than bare hands, since skin transfers organic contamination that ruins certain analyses. Do not wash it, apply anything to it or use a magnet on it, since a strong magnet destroys the record of magnetisation the rock carries. Keep it dry and store it in a sealed bag away from heat. Record the location precisely. Report it to a museum or university, which will identify it and which in most countries has no power to take it, though ownership rules differ substantially between jurisdictions and several countries treat meteorites as belonging to the state.
The takeaway
A fall is recovered after somebody saw it arrive and a find is not, and the difference matters because weathering begins immediately and destroys the chemistry that makes these rocks interesting. Observation supplies a date, an uncontaminated sample and sometimes an orbit, and fewer than fifty have well-determined orbits. Most catalogued specimens come from deserts and from Antarctic blue ice.