[meteorite-list] Rosetta Fuels Debate on Origin of Earth's Oceans

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 10 Dec 2014 15:10:42 -0800 (PST)
Message-ID: <201412102310.sBANAgJG003158_at_zagami.jpl.nasa.gov>


Rosetta Fuels Debate on Origin of Earth's Oceans
European Space Agency
10 December 2014

ESA's Rosetta spacecraft has found the water vapour from its target comet
to be significantly different to that found on Earth. The discovery fuels
the debate on the origin of our planet's oceans.

The measurements were made in the month following the spacecraft's arrival
at Comet 67P/Churyumov-Gerasimenko on 6 August. It is one of the most
anticipated early results of the mission, because the origin of Earth's
water is still an open question.

One of the leading hypotheses on Earth's formation is that it was so hot
when it formed 4.6 billion years ago that any original water content should
have boiled off. But, today, two thirds of the surface is covered in water,
so where did it come from?

In this scenario, it should have been delivered after our planet had cooled
down, most likely from collisions with comets and asteroids. The relative
contribution of each class of object to our planet's water supply is,
however, still debated.

The key to determining where the water originated is in its "flavour",
in this case the proportion of deuterium - a form of hydrogen with an
additional neutron - to normal hydrogen.

This proportion is an important indicator of the formation and early evolution
of the Solar System, with theoretical simulations showing that it should
change with distance from the Sun and with time in the first few million

One key goal is to compare the value for different kinds of object with
that measured for Earth's oceans, in order to determine how much each
type of object may have contributed to Earth's water.

Comets in particular are unique tools for probing the early Solar System:
they harbour material left over from the protoplanetary disc out of which
the planets formed, and therefore should reflect the primordial composition
of their places of origin.

But thanks to the dynamics of the early Solar System, this is not a straightforward
process. Long-period comets that hail from the distant Oort cloud originally
formed in Uranus-Neptune region, far enough from the Sun that water ice
could survive.

They were later scattered to the Solar System's far outer reaches as a
result of gravitational interactions with the gas giant planets as they
settled in their orbits.

Conversely, Jupiter-family comets like Rosetta's comet were thought to
have formed further out, in the Kuiper Belt beyond Neptune. Occasionally
these bodies are disrupted from this location and sent towards the inner
Solar System, where their orbits become controlled by the gravitational
influence of Jupiter.

Indeed, Rosetta's comet now travels around the Sun between the orbits
of Earth and Mars at its closest and just beyond Jupiter at its furthest,
with a period of about 6.5 years.

Deuterium-to-hydrogen in the Solar System

Previous measurements of the deuterium/hydrogen (D/H) ratio in other comets
have shown a wide range of values. Of the 11 comets for which measurements
have been made, it is only the Jupiter-family Comet 103P/Hartley 2 that
was found to match the composition of Earth's water, in observations made
by ESA's Herschel mission in 2011.

By contrast, meteorites originally hailing from asteroids in the Asteroid
Belt also match the composition of Earth's water. Thus, despite the fact
that asteroids have a much lower overall water content, impacts by a large
number of them could still have resulted in Earth's oceans.

It is against this backdrop that Rosetta's investigations are important.
Interestingly, the D/H ratio measured by the Rosetta Orbiter Spectrometer
for Ion and Neutral Analysis, or ROSINA, is more than three times greater
than for Earth's oceans and for its Jupiter-family companion, Comet Hartley 2.
Indeed, it is even higher than measured for any Oort cloud comet as

"This surprising finding could indicate a diverse origin for the Jupiter-family
comets - perhaps they formed over a wider range of distances in the young
Solar System than we previously thought," says Kathrin Altwegg, principal
investigator for ROSINA and lead author of the paper reporting the results
in the journal Science this week.

"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."

"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

"As Rosetta continues to follow the comet on its orbit around the Sun
throughout next year, we'll be keeping a close watch on how it evolves
and behaves, which will give us unique insight into the mysterious world
of comets and their contribution to our understanding of the evolution
of the Solar System."

Notes for Editors

"67P/Churyumov-Gerasimenko, a Jupiter Family Comet with a high D/H ratio"
by K. Altwegg et al., is published in the 10 December 2014 issue of Science.

ROSINA is the Rosetta Orbiter Sensor for Ion and Neutral Analysis instrument
and comprises two mass spectrometers: the double focusing mass spectrometer
(DFMS) and the reflectron time of flight mass spectrometer (RTOF) - and
the cometary pressure sensor (COPS). The measurements reported here were
conducted with DFMS.

The analysis is based on the results of over 50 spectra collected between
8 August and 5 September 2014, and the D/H ratio was derived from measurements
of HD16O/H2 16O.

The ROSINA team is led by Kathrin Altwegg of the University of Bern, Switzerland.


More about Rosetta

Rosetta is an ESA mission with contributions from its Member States and
NASA. Rosetta's Philae lander was provided by a consortium led by DLR,
MPS, CNES and ASI. Rosetta is the first mission in history to rendezvous
with a comet. It is escorting the comet as they orbit the Sun together.
Philae landed on the comet on 12 November 2014. Comets are time capsules
containing primitive material left over from the epoch when the Sun and
its planets formed. By studying the gas, dust and structure of the nucleus
and organic materials associated with the comet, via both remote and in
situ observations, the Rosetta mission should become the key to unlocking
the history and evolution of our Solar System.


For further information, please contact:

Kathrin Altwegg
Principal investigator for ROSINA
University of Bern, Switzerland
Email: kathrin.altwegg at space.unibe.ch

Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer at esa.int

Matt Taylor
ESA Rosetta project scientist
Email: matthew.taylor at esa.int
Received on Wed 10 Dec 2014 06:10:42 PM PST

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