A study of hydrogen isotopes has yielded evidence that the
Earth's water originates from asteroids in the inner solar
system, rather than icy comets. Hydrogen comes in two stable
variants -- regular hydrogen and heavy deuterium. We know that
water originating further from the sun (in comets and asteroids
in the outer solar system) has a higher deuterium content, due
to the extreme cold in which it forms, than water which
originates in the inner solar system. That difference allowed a
team from the Carnegie Institution of Washington, led by Conel
Alexander, to compare the ratios of the two isotopes in
fragments of asteroids known as chondritic meteorites with those
measured in comets. "We can measure the hydrogen isotope ratio
of ice in comets remotely by infrared and submillimetre
spectroscopy. We can measure the ratios in meteorites by
analysing the hydrogen that is there -- which is typically in
the form of hydrated silicates, such as clays, that are the
remnants of ancient water," explained Across 86 meteo