Rosetta’s measurement of the deuterium-to-hydrogen ratio (D/H) measured in the water vapour around Comet 67P/Churyumov–Gerasimenko. Deuterium is
an isotope of hydrogen with an added neutron. The ratio of deuterium to hydrogen in water is a key diagnostic to determining where in the Solar System an
object originated and in what proportion asteroids and/or comets contributed to Earth’s oceans. The graph displays the different values of D/H in water observed in various bodies in the Solar System. The data points are grouped by colour as planets and moons (blue), chondritic meteorites from the Asteroid Belt
(grey), comets originating from the Oort cloud (purple) and Jupiter family comets (pink). Rosetta’s Jupiter-family comet is highlighted in yellow. The ratio for
Earth’s oceans is 1.56 ×10–4 (shown as the blue horizontal line in the upper part of the graph). Credit: ESA/ATG medialab/Rosetta/NavCam/Altwegg
form of hydrogen with an additional
neutron – that was three times greater than the water here one Earth.
“We knew that Rosetta’s in situ
analysis of this comet was always going to throw up surprises for the bigger picture of Solar System science,
and this outstanding observation certainly adds fuel to the debate about
the origin of Earth’s water,” says Matt
Taylor, ESA’s Rosetta project scientist.
Comets are cosmic time capsules,
harboring protoplanetary material
left over from the early days of planet formation. These icy bodies form
in various regions of the solar system
and contain traces of material from
where they were forged. However,
solar system dynamics does not
make comet origins easy to discern.
Long-period comets can originate
out in far reaches of the solar system
in a region known as the Oort Cloud,
and typically form in the region
around Uranus and Neptune. This
area is far enough away from the
Sun that water ice would be present.
So how do they get to the Oort
Cloud? As the outer planets settled
into their orbits, gravitational interactions between the planets scattered
the comets to the outer solar system.
On the other hand, Rosetta’s
comet, belonging to the Jupiterfamily of comets, is thought to have
formed way out, beyond Neptune,
in a region known as the Kuiper
Belt. From time to time, the orbits
of these comets are disturbed and
as they travel through the solar
system, they are captured by Jupiter’s massive gravitational pull.
Scientists have measured the ratio of deuterium to hydrogen (D/H)
in 11 different comets, thought to
originate in different regions of the
solar system, only the Jupiter-family
of comets, like Comet 103P/Hartley
2, was observed to contain the same
D/H ratio as the Earth’s oceans.
Meteorites within the main asteroid
belt between Mars and Jupiter were
also studied and determined to be
a match to Earth’s oceans. Asteroids
and meteorites tend to have significantly lower water content than
comets, but could still contribute to
the presence of water on Earth.
Rosetta’s instrument Rosetta
Orbiter Spectrometer for Ion and
Neutral Analysis, or ROSINA, determined the D/H ratio of comet
67P is three times higher than that
of Earth’s oceans, higher than the
Jupiter-family comets, and higher
than any Oort cloud comet.
“Our finding also rules out the
idea that Jupiter-family comets contain solely Earth ocean-like water,
and adds weight to models that
place more emphasis on asteroids
as the main delivery mechanism
for Earth’s oceans,” Atlwegg said.
Over the coming months, Rosetta will continue to follow along
with comet 67P, taking measurements and beaming data back
to us on Earth. The operations
team will continue to monitor
how the comet evolves and behaves as it approaches the Sun.
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