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NITROGEN in Earth’s atmosphere was carried here by comets, say scientists
who’ve been studying Jupiter’s atmosphere. This supports the idea that other
ingredients of life arrived in the same way, in a kind of cosmic starter
pack.

Researchers can trace the source of nitrogen by comparing the ratio of
different isotopes. In our atmosphere, there are about 3 nitrogen-15 atoms for
every 1000 nitrogen-14 atoms. But finding out the ratio elsewhere in the
Universe is tricky, because light from distant nitrogen molecules is not greatly
affected by their atomic mass.

So a team of researchers led by Tobias Owen of the University of Hawaii
looked at data on nitrogen in Jupiter’s atmosphere from NASA’s Galileo probe.
They found the ratio of nitrogen-15 to nitrogen-14 to be about 2 out of every
1000—lower than on Earth.

The isotopic composition of Jupiter’s atmosphere is thought to reflect that
of the cloud of gas and dust from which the Sun and its planets formed. So where
did Earth’s extra nitrogen-15 come from?

Owen says the obvious candidates to deliver it are comets that originate in
the gas clouds of interstellar space and swing into the inner Solar System.
These clouds contain nitrogen bound up in compounds such as cyanide and ammonia.
And because nitrogen-15 forms compounds slightly more readily than nitrogen-14,
the compounds have a higher isotope ratio.

“The nitrogen-15 collects in nitrogen compounds,” agrees astrochemist Eric
Herbst of Ohio State University in Columbus. Once a comet hits the nascent
Earth, Owen says, the heat of impact would break up the compounds and release
the gas.

Owen says the same comets could also have delivered the water and organic
compounds necessary for life to get started on Earth: “This result supplies an
important context.”