The origin of Earth's water is hotly debated. New discoveries suggest that our planet's seas could once have been giant icebergs floating through space.
ESA's Herschel infrared space observatory has found water in a comet with almost exactly the same composition as Earth's oceans.
The reason why it remains unclear how our oceans were created is because our planet formed at such high temperatures that any original water must have evaporated.
Yet today, two-thirds of the surface is covered in water and this must have been delivered from space after Earth cooled down.
Comets seem a natural explanation: they are giant icebergs travelling through space with orbits that take them across the paths of the
planets, making collisions possible.
The impact of comet Shoemaker-Levy 9 on Jupiter in 1994 was one such event.
But in the early Solar System, when there were larger numbers of comets around, collisions would have been much more common.
However, until now, astronomers' observations have failed to back up the idea that comets provided Earth's water.
The key measurement they make is the level of deuterium – a heavier form of hydrogen – found in water.
All the deuterium and hydrogen in the Universe was made just after the Big Bang, about 13.7 billion years ago, fixing the overall
ratio between the two kinds of atoms.
Measurements from the Herschel Space Observatory show that comet Hartley 2, which comes from the distant Kuiper Belt,
contains water with the same chemical signature as water in Earth's oceans. The findings may help explain how Earth’s surface ended up covered in water.
CREDIT: NASA/JPL-Caltech/R. Hurt
However, the ratio seen in water can vary from location to location. The chemical reactions involved in making ice in space lead to a higher or
lower chance of a deuterium atom replacing one of the two hydrogen
atoms in a water molecule, depending on the particular environmental conditions.
This illustration shows the orbit of comet Hartley 2 in relation to those of the five innermost planets of the Solar System.
The comet made its latest close pass of Earth on 20 October, coming to 19.45 million km. On this occasion, Herschel observed the comet.
The inset on the right side shows the image obtained with Herschel’s PACS instrument. The two lines are the water data from HIFI instrument.
Credits: ESA/AOES Medialab; Herschel/HssO Consortium
Thus, by comparing the deuterium to hydrogen ratio found in the water in Earth's oceans with that in extraterrestrial objects,
astronomers can aim to identify the origin of our water.
The nucleus of Comet Halley as seen from the Giotto spacecraft by the Halley Multicolour Camera.
Credits: MPAe, Lindau
All comets previously studied have shown deuterium levels around twice that of Earth's oceans.
If comets of this kind had collided with Earth, they could not have contributed more than a few percent of Earth's water.
In fact, astronomers had begun to think that meteorites had to be responsible, even though their water content is much lower.
Now, however, Herschel has studied comet Hartley 2 using HIFI, the most sensitive instrument so far for detecting
water in space, and has shown that at least this one comet does have ocean-like water.
The Heterodyne Instrument
for the Far Infrared (HIFI)
is a high-resolution heterodyne
spectrometer. It works by mixing
the incoming signal with a stable
monochromatic signal, generated
by a local oscillator, and extracting
the frequency difference for further
processing in a spectrometer. HIFI
will have seven separate local
oscillators covering two bands from
480-1250 gigaHertz and 1410–1910 gigaHertz.
HIFI was developed by a consortium
led by SRON (Groningen, The Netherlands).
Credits: ESA (image by C. Carreau)
The key to why comet Hartley 2 is different may be because of where it was born: far beyond Pluto, in a frigid region of
the Solar System known as the Kuiper Belt.
The other comets previously studied by astronomers are all thought to have formed near to Jupiter and Saturn before being
thrown out by the gravity of those giant planets, only to return much later from great distances.
Thus the new observations suggest that perhaps Earth's oceans came from comets after all – but only a specific family of
them, born in the outer Solar System.
Out there in the deep cold, the deuterium to hydrogen ratio imprinted into water ice might have been quite different from
that which arose in the warmer inner Solar System.
Herschel is now looking at other comets to see whether this picture can be backed up.
"Thanks to this detection made possible by Herschel, an old, very interesting discussion will be revived and invigorated,"
says Göran Pilbratt, ESA Herschel Project Scientist.
"It will be exciting to see where this discovery will take us."
MessageToEagle.com based on material provided by ESA
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