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

[Ron Baalke]

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.