What does a meteorite's weathering grade mean?
A weathering grade, from W0 to W6, describes how much a meteorite has rusted and changed since landing on Earth, from fresh to almost completely altered.
Once on Earth, meteorites begin to change. Their iron-nickel metal and iron sulfide react with water and oxygen to form rust and other minerals. Scientists describe how far this has gone using a weathering grade.
The widely used scale for ordinary chondrites, introduced in 1993, is based on what can be seen in a polished section:
- W0: no visible oxidation of metal or sulfide; usually only very fresh falls. - W1: minor oxide rims around metal and troilite. - W2: moderate oxidation, with about 20 to 60 per cent of metal affected. - W3: heavy oxidation, with 60 to 95 per cent of metal replaced. - W4: almost complete oxidation of metal and troilite. - W5: beginning alteration of silicate minerals, such as olivine. - W6: massive replacement of silicates by clays and oxides.
Antarctic meteorites are often described with a separate letter scale, A, B and C, for minor, moderate and severe rustiness in hand specimen.
Weathering grade matters because it affects many measurements. Weathered meteorites may have lost or gained elements, such as barium and strontium from desert soils, and their oxygen isotopes may be shifted. It also gives a rough clue to how long a meteorite has been on Earth, though climate matters more: a meteorite can weather more in a few decades in a humid climate than in thousands of years in a dry desert.
For collectors, a low weathering grade means a fresh specimen, generally more attractive and stable.
More about the science of meteorites
- How do scientists tell which parent body a meteorite came from?
- How many different parent bodies do meteorites come from?
- Why do iron meteorites come from so many parent bodies?
- Can meteorites tell us what is inside Earth?
- How do scientists cut and study a meteorite?
- What is a regolith breccia and what can it tell us?