ROSINA detects argon at Comet 67P/C-G

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.

Four image NAVCAM montage of Comet 67P/C-G comprising images taken on 20 October 2014, during the timeframe of the ROSINA measurements when the spacecraft was less than 10 km from the centre of the comet – these images were taken at about 7.4 km from the surface. Credits: ESA/Rosetta/NAVCAM

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.

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).

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.

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

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.

Related ROSINA blog articles:
The perfume of 67P/C-G 
Rosetta fuels debate on origin of Earth's oceans
Rosetta makes first detection of molecular nitrogen at a comet
Comet's coma composition varies significantly over time
Rosina tastes the comet's gases 

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.



  • Ramcomet says:

    Just had a thought.

    Wondering if H. Balsiger and team (and other teams) not only look for results of processes of comet's being formed "in the cold outer reaches of the Solar System", but also factor in possible brief periods of hot temperatures affecting comets, such as when our solar system travels through galaxy arms over eons, likely travelling near exploding supernovas at times?

    Also, when the solar system travels into another heliosphere of a different galaxy group, such as we are apparently due to travel through in about a hundred years according to a recent paper I read. there is supposedly million degree temperatures, although granted, the molecules are so far apart, probably has no bearing on the comet.

    And could these events have any bearing on the minerals, ices, out gassing ratios, even clathrates within the comet?

    I am not a scientist, just a space and science enthusiast, so go easy on me. Thanks!
    Just curious!

  • Harvey says:

    On earth, some 99.6% of argon is 40Ar, which is not mentioned here. For an explanation of the origin of argon isotopes, why 36, 38, there is a good brief summary here:

    It might perhaps have been helpful to the generally interested non specialist reader to include a brief summary of that, if I can gently criticise the 'press release' - but applaud it's technical content!

    It's possible any 40Ar signal is masked by some other species of very closely similar mass, but given the excellent resolution of ROSINA it has to be very close and I've not figured out what it might be, if there is something. Perhaps the team could comment?

    One is again forced to conclude from some of the species present that 67P is a rather smelly comet - if you have a very sensitive nose 🙂

    • Kathrin Altwegg says:

      40Ar is very close in mass to 40K (39..962 vs.39.964). At that time we detected sputtered refractories (Na, Si, K; see Solar wind sputtering of dust on the surface of 67P/Churyumov-Gerasimenko, Wurz, et al.,2015, A&A,
      DOI: As the total signal is very low there is no chance to separate them.

      • originalJohnn says:

        There you are Harvey, sputtered silicon in answer to your often repeated question where is the silicon. No doubt then sputtered oxygen too.

        • ianw16 says:

          And the point being? These results were released on the blog some months ago.
          The main features to note are: the total anti-correlation, location-wise, of the sputtering and the sublimation products. If one were trying to say that sputtering were the reason for the observed H2O, then one would expect to see the H2O and the Si in the same locations. They aren't, due to the solar wind scarcely, if at all, reaching the surface where the outgassing is most intense.
          Secondly, if one were suggesting sputtering or some form of electric woo was responsible for creating the O from which the water is formed (by combining with the solar wind H+ at 400 km/s; not sure that will happen), then if the O comes from SiO or SiO2, then there should be roughly the same amount of Si knocking about as H2O. There isn't. As fig. 3 in the paper shows, it is about 1/20 000th of the H2O value.
          By the way, if the solar wind is somehow combining with O at the near surface, then it would need to be O-. To the best of my knowledge we only see + ions near the surface.

        • Harvey says:

          The two species differ by many, many orders of magnitude in quantity. Also sputtered O does not explain molecular H2O.

      • Harvey says:

        Katherin - many thanks, I assumed there must be something very close in mass.

  • logan says:

    Genesis of proto-planetary disk is also too much simplified. When a young star 'appropriates' of a disk, that matter should contain smaller siblings, in all the size spectra. The mother inter-stellar cloud also has its own noble gases dynamics.

    This is cometary fiction.

    • logan says:

      Maybe this is wrong. Maybe proto-planetary disk is far from representative sample of mother nebula. Require super-computers...

    • logan says:

      Imagining of two mother nebula colliding, no frontal, sliding one over other, or one being a lot more massive than the other, or one being more gravitationally collapsed than the other.

      Imagining of what kind of turbulence could be present at the frontier.

  • logan says:

    Savage speculation is that proto-planetary disks contains cometary material, even contains comets, but comets are not formed there.

  • Bill Harris says:

    Very informative paper. I've been an advocate of considering the effects of "cold traps" in the colder un-sunlit sides of the comet surface, but thought more of the long-time cold sinks on the "winter hemisphere" of the comet. But the diurnal cold traps can have significant impacts on the mobilization, immobilization and remobilization of volatiles in the comatic (?) atmosphere.

    I'm also mulling over the effects of the fractionalization of gases sublimating from the primordial "cores" of the comet interior and making their way out through many temperature levels, as well as recondensing and resublimating with temperature fluctuations.

    And look at the nonvolatile body of the comet-- a "sintered" silicate matrix with various organic nonvolatiles, as well as water-ices (and don't get me started on clathrates!). THe comet body is a very sophisticated gas chromatograph column.

    What an interesting mission...


  • Ramcomet says:

    Erratum: I should have said star group, not galaxy group!

    • Gerald says:

      A (very) close encounter of our solar system to another star would have heavily disturbed our solar system by gravity.
      Encounters of our sun closer than about one light year are rare.
      At those distances the heat of the other star is negligible for most objects in the solar system.
      One light year is 63241 astronomical units
      or more than 1000-fold the distance Sun-Pluto.

      The planetary equilibrium temperature is proportional to the inverse of the square root of the distance to the star.
      For a black body near the Earth the equilibrium temperature would be near 273 K, at 63241 a.u. hence 273K/sqrt(63241) = 1.08K. That's below the cosmic background, hence negligible.

      Only encounters as close as a few hundred a.u. would be relevant in terms of temperature.
      The statistical probability of an encouter decreases with the 3rd power of the distance.
      Assuming a 3 ly encounter each 10,000 years would mean a 0.3 ly encounter each 10,000,000 years, and a 0.03 ly encouter each 10,000,000,000 years, meaning about one during the lifetime of our solar system.
      0.03 ly is about 1900 a.u., corresponding to a black-body planetary equilibrium temperature of 6.3 K for a sun-like star.
      That's unlikely to be relevant for the surface temperature of comets.
      "Barnard's Star will make its closest approach to the Sun around AD 11,800, when it approaches to within about 3.75 light-years."
      " In 2001, J. García-Sánchez et al. predicted that Proxima will make its closest approach to the Sun, coming within 3.11 ly of the latter, in approximately 26,700 years. A 2010 study by V. V. Bobylev predicted a closest approach distance of 2.90 ly in about 27,400 years."

      Supernovae are hard to predict. The peaks are short-term events, and would only affect the surface of a comet, if nearby.
      But supernovae are relevant in another way, meaning that they provide much or even most of the interstellar dust comets and planets are made of.

  • originalJohn says:

    The investigators identified 36Ar and 38Ar with a ratio 36/38 of 5.4 with an error of + or - 1.4. So the ratio could lie anywhere within the range 4.0 to 6.8. The solar wind they inform us has a 36Ar/38Ar ratio of 5.5 so how do they know they were measuring the argon isotope ratio for the comet coma and not the solar wind.

    • dave says:


      I had the exact same question.


    • Kathrin Altwegg says:

      Solar wind is heavily ionized, ROSINA was measuring neutrals coming from the direction of the comet, not the Sun.

      • originalJohnn says:

        In which case Katherine why mention the solar wind isotope ratio if the two cannot be confused. However the reliance on the characteristic neutral or ionised state presumes that that state can never change under any conditions encountered in the
        inter planetary medium. A rather hopeful presumption.
        I would be interested to know also how the the direction of origin and motion of the ions or neutrals is determined. Surely particles from any direction can enter the detector.

        • originalJohn says:

          Sorry, I misspelled your name Kathrin.

        • Harvey says:

          The direction of entry is immaterial. A neutral will not be detected in ion mode for obvious reasons. Personally I have enough faith in the team to assume they have ensured the reverse is true too, though its perhaps slightly less obvious.

    • ianw16 says:

      I assume it has something to do with the fact that they were measuring neutral, rather than ionised argon. As it says in the paper, "DFMS is a mass spectrometer that measures the neutral composition of the coma at the position of the spacecraft with unprecedented mass resolution."
      Solar wind argon would be ionised, coma argon would be neutral, certainly at 10km.

  • Harvey says:

    Good point, but there is an answer.

    ROSINA has a 'gas' mode in which it detects neutrals, ionizing them in the conventional way. It also has an 'ion' mode in which already ionized atoms are guided into the spectrometer. So they should be able to tell which is which without difficulty.

  • Dave says:

    Thanks Harvey re guides to spectrometer, there had to be a simple solutioregards

  • logan says:

    "...One of the prime goals of the Rosetta mission (2) to comet 67P/Churyumov-Gerasimenko (hereafter 67P/CG), a Jupiter family comet, is the in situ measurement of the volatile inventory of 67P/CG with high sensitivity and high mass resolution".

    No doubt ROSINA Team is full filling mission goals.

    No doubt also about the indispensability of noble gases studies. They are the 'whisperers' among gases and will help to elucidate history of 67P, more than any other substances.

    Also commenting that this an area of science totally unknown to me up to this moment. But not surrendering 🙂

    Congratulations to Hans Balsiger, Kathrin Altwegg et al!

  • logan says:

    "...The good correlation between 36 Ar and N 2 (Fig. 2B) due to their very similar volatility is noteworthy."

    Does this data suggest that nitrogen chemical activity has been irrelevant? At least as long as the 2 gases has been part of 67P?

    • logan says:

      Could be that M. Rubin et al. are telling a lot more of CO than about N2.

  • logan says:

    "... These are currently “level 2” data, meaning more or less as received from the spacecraft, followed by decompression and the addition of metadata including the distance to the comet, direction of the Sun, and the spacecraft pointing. Future releases will be at a higher, more directly usable level containing mass spectra with physical units (e.g. detected particles vs. mass)",

    ROSINA's published data set has been complete and direct, as well as signaling a future release of more consolidated data. Can't be said more open than that.

  • logan says:

    "Have comets left any traces of their undeniable impacts on the inner planets?"

    Before proto-planetary disk 'appropriation'? Before Sun ignition? Before going out of Mother nebula?

  • logan says:

    Hi Hans, Kathrin and Friends: Clear on my imagination that almost all planetary water has beginnings as cometary material [with possible exception of Jupiter].

    Comets witness of that. But the few ones visiting us don't have but scarce fragments of the complete history. Hardly will come as supreme judges.

Comments are closed.