[meteorite-list] Rosetta Makes First Detection of Molecular Nitrogen at a Comet

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 20 Mar 2015 16:26:33 -0700 (PDT)
Message-ID: <201503202326.t2KNQXCs010030_at_zagami.jpl.nasa.gov>

http://sci.esa.int/rosetta/55620-rosetta-makes-first-detection-of-molecular-nitrogen-at-a-comet/
Rosetta makes first detection of molecular nitrogen at a comet
European Space Agency
19 March 2015

ESA's Rosetta spacecraft has made the first measurement of
molecular nitrogen at a comet, providing clues about the
temperature environment in which Comet 67P/Churyumov-Gerasimenko
formed.

Rosetta arrived last August, and has since been collecting
extensive data on the comet and its environment with its suite of
11 science instruments.

The in situ detection of molecular nitrogen has long been sought
at a comet. Nitrogen had only previously been detected bound up in
other compounds, including hydrogen cyanide and ammonia, for example.

Its detection is particularly important since molecular nitrogen
is thought to have been the most common type of nitrogen available
when the Solar System was forming. In the colder outer regions, it
likely provided the main source of nitrogen that was incorporated
into the gas planets. It also dominates the dense atmosphere of
Saturn's moon, Titan, and is present in the atmospheres and
surface ices on Pluto and Neptune's moon Triton.

It is in these cold outer reaches of our Solar System in which the
family of comets that includes Rosetta's comet is believed to have
formed.

The new results are based on 138 measurements collected by the
Rosetta Orbiter Spectrometer for Ion and Neutral Analysis
instrument, ROSINA, during 17???23 October 2014, when Rosetta was
about 10 km from the centre of the comet.

"Identifying molecular nitrogen places important constraints on
the conditions in which the comet formed, because it requires very
low temperatures to become trapped in ice," says Martin Rubin of
the University of Bern, lead author of the paper presenting the
results published today in the journal Science.

The trapping of molecular nitrogen in ice in the protosolar nebula
is thought to take place at temperatures similar to those required
to trap carbon monoxide. So in order to put constraints on comet
formation models, the scientists compared the ratio of molecular
nitrogen to carbon monoxide measured at the comet to that of the
protosolar nebula, as calculated from the measured nitrogen to
carbon ratio in Jupiter and the solar wind.

That ratio for Comet 67P/Churyumov-Gerasimenko turns out to be
about 25 times less than that of the expected protosolar value.
The scientists think that this depletion may be a consequence of
the ice forming at very low temperatures in the protosolar nebula.

One scenario involves temperatures of between roughly -250??C and
perhaps -220??C, with relatively inefficient trapping of molecular
nitrogen in either amorphous water ice or cage-like water ice
known as a clathrate, in both cases yielding a low ratio directly.

Alternatively, the molecular nitrogen could have been trapped more
efficiently at even lower temperatures of around -253??C in the
same region as Pluto and Triton, resulting in relatively
nitrogen-rich ices as seen on them.

Subsequent heating of the comet through the decay of radioactive
nuclides, or as Rosetta's comet moved into orbits closer to the
Sun, could have been sufficient to trigger outgassing of the
nitrogen and thus a reduction of the ratio over time.

"This very low-temperature process is similar to how we think
Pluto and Triton have developed their nitrogen-rich ice and is
consistent with the comet originating from the Kuiper Belt," says
Martin.

The only other body in the Solar System with a nitrogen-dominated
atmosphere is Earth. The current best guess at its origin is via
plate tectonics, with volcanoes releasing nitrogen locked in
silicate rocks in the mantle.

However, the question remains as to the role played by comets in
delivering this important ingredient.

"Just as we wanted to learn more about the role of comets in
bringing water to Earth, we would also
like to place constraints on the delivery of other ingredients,
especially those that are needed for the building blocks of life,
like nitrogen," says Kathrin Altwegg, also at the University of
Bern, and principal investigator for ROSINA.

To assess the possible contribution of comets like Rosetta's to
the nitrogen in Earth's atmosphere, the scientists assumed that
the isotopic ratio of ^14 N to ^15 N in the comet is the same as
that measured for Jupiter and solar wind, which reflects the
composition of the protosolar nebula.

However, this isotopic ratio is much higher than measured for
other nitrogen-bearing species present in comets, such as hydrogen
cyanide and ammonia.

Earth's ^14 N/^15 N ratio lies roughly between these two values,
and thus if there was an equal mix of the molecular form on the
one hand, and in hydrogen cyanide and ammonia on the other in
comets, it would be at least conceivable that Earth's nitrogen
could have come from comets.

"However, the amount of nitrogen found in
67P/Churyumov-Gerasimenko is not an equal mix between
molecular nitrogen and the other nitrogen-bearing molecules.
Rather, there is 15 times too little molecular nitrogen, and
therefore Earth's ^14 N/^15 N ratio cannot be reproduced through
delivery of Jupiter family comets like Rosetta's," says Martin.

"It's another piece of the puzzle in terms of the role of
Jupiter-family comets in the evolution of the Solar System, but
the puzzle is by no means finished yet," says ESA's Rosetta
project scientist, Matt Taylor.

"Rosetta is about five months away from perihelion now, and we'll
be watching how the composition of the gases changes over this
period, and trying to decipher what that tells us about the past
life of this comet."


Notes for Editors

"Molecular nitrogen in comet 67P/Churyumov-Gerasimenko indicates
a low formation temperature, " by M. Rubin et
al. is published in the 20 March issue of the journal Science.

ROSINA is the Rosetta Orbiter Spectrometer 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 ROSINA team is led by Kathrin Altwegg of the University
of Bern, Switzerland.

An average ratio of N_2 /CO = (5.70 ?? 0.66) ?? 10^-3 was determined
for the period 17???23 October 2014. The minimum and maximum values
measured were 1.7 ?? 10^-3 and 1.6 ?? 10^-2 , respectively. Because
the amount and composition of the gases change with comet rotation
and position of the spacecraft with respect to the comet's
surface, an average value is used.

The ^14 N/^15 N ratio for the N_2 in Comet
67P/Churyumov-Gerasimenko is assumed to be 441, the value for the
protosolar nebula as measured from Jupiter and the solar wind,
while the corresponding value for nitrogen in hydrogen cyanide and
ammonia is 130, as measured at other comets. The value for the
Earth's nitrogen is 272.


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:

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

Martin Rubin
University of Bern, Switzerland
Email: martin.rubin at space.unibe.ch

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

Matt Taylor
ESA Rosetta project scientist
Email: matthew.taylor at esa.int
Received on Fri 20 Mar 2015 07:26:33 PM PDT