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

[Ron Baalke]

http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_fuels_debate_on_origin_of_Earth_s_oceans

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 
years.

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 
well.

"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 
scientist.

"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