How can I teach about water on Earth using meteorites?
Some meteorites contain water-bearing minerals and water with hydrogen isotopes similar to Earth's oceans, so pupils can weigh the evidence that asteroids delivered water.
Where Earth's water came from is a big, open question that pupils can explore using evidence from meteorites.
The puzzle. Earth formed close to the Sun, where it was too hot for ice to condense easily. So how did it end up with oceans? Possible sources include water locked in Earth's original building blocks, delivery by comets, delivery by water-rich asteroids, or chemical reactions with hydrogen from the young Sun's surroundings.
Evidence from meteorites. Carbonaceous chondrites contain clay minerals that hold water, sometimes up to about 10 per cent of their weight. The Winchcombe meteorite, which fell in England in 2021, contains water whose hydrogen isotopes, the ratio of deuterium to ordinary hydrogen, are close to those of Earth's oceans. Samples from asteroid Ryugu show similar results.
Evidence from comets. The Rosetta mission found that the water of comet 67P has about three times as much deuterium relative to hydrogen as Earth's oceans, which argues against comets like it being the main source. Some other comets have values closer to Earth's.
Evidence from enstatite chondrites. Research in 2020 found that these meteorites, which resemble the material that built the inner planets, contain enough hydrogen to have supplied a significant share of Earth's water, suggesting Earth may have been born with much of it.
Activity. Give groups evidence cards for each hypothesis and ask them to rank the hypotheses, explaining their reasoning. Emphasise that scientists are still debating the answer, and that combinations of sources are possible.
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