[meteorite-list] Rosetta Instrument Detects Argon at Comet 67P/Churyumov-Gerasimenko

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 30 Sep 2015 14:03:37 -0700 (PDT)
Message-ID: <201509302103.t8UL3bxb016944_at_zagami.jpl.nasa.gov>

http://blogs.esa.int/rosetta/2015/09/25/rosina-detects-argon-at-comet-67pc-g/

ROSINA detects argon at Comet 67P/C-G
European Space Agency
September 25, 2015

The noble gas argon has been detected in the coma of Comet 67P/Churyumov-Gerasimenko
for the first time, thanks to the ROSINA mass spectrometer on-board Rosetta.
Its detection is helping scientists to understand the processes at work
during the comet's formation, and adds to the debate about the role of
comets in delivering various "ingredients" to Earth.

The new results are reported in Science Advances today and describe data
collected on 19, 20, 22, and 23 October 2014, when the comet was around
465 million km (3.1 AU) from the Sun, and Rosetta was in a 10 km orbit
around the comet.

[Images]
Four image NAVCAM montage of Comet 67P/C-G comprising images taken on
20 October 2014, during the timeframe of the ROSINA measurements reported
today. The images were taken about 7.4 km from the comet surface. Credits:
ESA/Rosetta/NAVCAM

During the time spent close to the comet, the ROSINA instrument was able
to take an inventory of the key constituents of the comet's coma, with
many ingredients already reported (see links at end of article). Determining
the chemical make-up of comets is a necessary step to understanding their
role in bringing water and other ingredients to the inner planets during
the Solar System's early history.

The so-called noble gases (helium, neon, argon, krypton, xenon, and radon)
rarely react chemically with other elements to form molecules, mostly
remaining in a stable atomic state, representative of the environment
around a young star in which planets, comets, and asteroids are born.

In addition, their abundance and isotopic compositions can be compared
to the values known for Earth and Mars, and for the solar wind and meteorites,
for example. The relative abundance of noble gases in the atmospheres
of terrestrial planets is largely controlled by the early evolution of
the planets, including outgassing via geological processes, atmospheric
loss, and/or delivery by asteroid or cometary bombardment. Thus the study
of noble gases in comets can also provide information on these processes.

However, noble gases are very easily lost from comets through sublimation,
and so this first detection of argon at Comet 67P/C-G is a key discovery.
Not only that, but it is also an important step in determining if comets
of this type played any significant role in the noble gas inventory of
the terrestrial planets.

[Graph]
ROSINA-DFMS mass spectra identifying the two isotopes of 36Ar and 38Ar
in October 2014, along with other gases. The extreme high mass-resolution
of DFMS is a prerequisite for separating and identifying the two argon
isotopes. The spacecraft background spectrum was obtained on 2 August
2014, before the comet signal became apparent. (m/z) = mass/charge. Data
from Balsiger et al (2015).

Scientists analysing data from ROSINA's high-resolution Double Focusing
Mass Spectrometer (DFMS) identified argon, along with other gases, in
the coma spectra of Comet 67P/C-G in October 2014. They identified 36Ar
and 38Ar, yielding an isotopic ratio for 36Ar/38Ar of 5.4 ? 1.4, which
is compatible with Solar System values: for Earth, this isotopic ratio
is 5.3, while for the solar wind it is 5.5.

The relative abundance of argon to other gases was also investigated.
For example, the abundance of argon relative to water vapour was determined
to be between 0.1 x 10^?5 and 2.3 x 10^?5, the range of values measured
being due to variable solar illumination, which influences the rate of
water sublimation on different parts of the comet nucleus.

"Even though the argon signal is very low overall, this unambiguous first
in-situ detection of a noble gas at the comet demonstrates the impressive
sensitivity of our instrument," says Professor Kathrin Altwegg, principal
investigator of the ROSINA instrument at the University of Bern.

"The argon-to-water ratio varied by more than a factor of 20. While the
very volatile argon can escape under any conditions, water sublimation
depends strongly on the amount of sunlight being received, and so with
it the argon-to-water ratio,' explains Professor Hans Balsiger, also from
the University of Bern, and lead author of the paper reporting the discovery.

"In contrast, the relative abundance of argon to molecular nitrogen is
quite stable - explained by the fact that argon and nitrogen have similar
high volatilities."

Although the measured abundance of argon-to-water spans a wide range,
it still has implications for the question of whether comets brought water
to Earth. That is because the argon-to-water ratio at Earth is only 6.5
x 10^?8, several orders of magnitude lower than observed for 67P/C-G.

"The relatively high argon content of Comet 67P/C-G compared with Earth
again argues against a cometary origin for terrestrial water, in an independent
way to the similar finding indicated by the earlier ROSINA result on the
deuterium-to-hydrogen ratio for 67P/C-G," comments Hans.

The argon detection can also be used to learn about the conditions in
which the comet formed.

"The argon we detected comes from inside the icy nucleus of the comet;
the nature of that ice - how, when, and where it formed - determines how
it captured and subsequently released the gases we are measuring", says
Kathrin.


[Image]
Single frame enhanced NAVCAM image of Comet 67P/C-G taken on 21 September
2015. Credits: ESA/Rosetta/NAVCAM ? CC BY-SA IGO 3.0

The two simplest forms of ice are crystalline and amorphous. These form
at different temperatures and pressures, capturing and releasing gases
in different ways. Argon, nitrogen, carbon monoxide, along with the heavier
noble gases krypton and xenon are particularly useful for distinguishing
between the various possibilities, because they remain in the same condition
as when they were first incorporated into the comet.

Models can be used to predict how readily highly-volatile gases were incorporated
into the icy grains that grew at low temperature in the protosolar nebula.
These models show that the high abundance of argon at Comet 67P/C-G and
the good correlation with nitrogen are both consistent with the comet
forming in the cold outer reaches of the Solar System.

Almost a year has passed since these argon data were collected. Now that
the comet has passed perihelion, its closest point to the Sun along its
orbit, the density of the coma has increased greatly, implying that searches
for even rarer gases should be possible.

However, the increased activity of 67P/C-G means that Rosetta cannot fly
close to the comet without running into navigation issues, and therefore
it is currently operating at distances greater than 350 km from the comet's
nucleus: this week, it has embarked on a trajectory taking it 1500 km
from the comet in order to study the wider coma and plasma environment.

The ROSINA team are therefore eagerly waiting for Rosetta to return to
closer distances as activity dies down in the coming months, in order
to continue their investigation of the noble gases - including searching
for krypton and xenon ? to add further insights into the part played by
comets in the delivery of these ingredients to Earth.

The paper "Detection of argon in the coma of comet 67P/Churyumov-Gerasimenko,"
by H. Balsiger et al is published online in Science Advances.

About ROSINA

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.
Received on Wed 30 Sep 2015 05:03:37 PM PDT