5. Is there O2 or O3 in solar system comets?
An observational test of Noll et al.'s hypothesis is the presence of O2 and O3 in Solar System comets. At present neither O2 nor O3 has been detected, as parent species, in comets (Altwegg et al., 1993). An upper limit can be obtained as follows.
During its encounter with Halley's comet, The High Intensity Sensor (HIS) of the Giotto Ion-Mass Spectrometer has measured in situ ions from mass 12 amu/e to 56 amu/e from ca. 130 000 km from the nucleus up to 1 300 km (Altwegg et al., 1993). Details on the HIS instrument and on the data analysis have been given elsewhere (Balsiger et al., 1987; Meier, 1988; Altwegg et al., 1993). Inside the ionopause the ion density at mass channel 32 is relatively low, at least compared to the two neighbouring masses 31 and 33, where the densities of protonated formaldehyde ions (mass 31) and protonated methanol ions (mass 33) respectively, are a factor of 5 higher than at mass 32.
The chemistry in the cometary coma has to be taken into account, to deduce, from ion densities measured in the coma, the abundance of the parent molecules in the nucleus. We use two different chemical models to determine the molecules that are responsible for the ions at mass/charge = 32 amu/e, one inside the ionopause surface, one outside, because the plasma parameters differ vastly between these two regions.
Inside the ionopause there is no interaction between the cometary plasma and the solar wind, the temperatures of molecules, ions and electrons are low (a few hundred K at most) and the outflow velocity of the ions and the neutral particles are equal and nearly constant (0.9 km s-1, Krankowsky et al., 1986). Assuming steady state flow conditions, the chemical model takes into account the relevant ion-molecule reaction rates, the photo rates (photoionization and photodissociation) and the electron recombination rates. The model used has been described by Geiss et al., 1991. Outside the ionopause the interaction with the solar wind is important and has to be taken into account (Häberli et al., 1995).
The molecules included in our modelling are listed in Table 1. Apart from the most abundant molecule, H2O, we have also included extended sources (from grains in the coma) of formaldehyde and CO as described in Eberhardt et al.(1987), and point sources (comet nucleus) of CO2, methanol and ammonia. We did not include molecules with low abundances which are of minor importance for the masses considered here.
(a) extended sources taken from Meier et al., 1993
The major ions which could contribute to the mass channel 32 apart from O2 are methanol ions, , deute-rated formaldehyde ions , and formaldehyde ions with 13C. We know from an analysis of the mass range 32 - 35 amu/e that the contribution of inside the ionopause is minor (Altwegg et al., to be published).
To obtain an upper limit for O2 inside the ionopause, we neglect the contribution from and formaldehyde and take only methanol ions into account. The methanol abundance has been determined earlier from mass 33, where the protonated methanol ions is present (Geiss et al., 1991). The reaction rates for oxygen species that determine the O abundance are given in Table 2.
With these values, we obtain an upper limit , , for the abundance of O2 parent molecules relative to water which is representative of the cometary nucleus abundance. As mentioned, this value neglects sulphur and formaldehyde which are both present in the comet and may be the major contributors to mass 32. The actual value of O2 abundance is therefore probably much lowerthan the deduced here. A more detailed analysis of the deuterium content in formaldehyde and a comparison with the data from the Neutral Mass Spectrometer of Giotto should lead, in the future, to a more precise value for O2. However, the present upper limit is already lower than the value postulated by Noll et al.(1997) (a few percent).
Another piece of evidence of the very low content of comets in oxygen is provided by the study of Pluto and Triton. We do not find abundant O2 on the surface of these two objects that must have accreted from icy planetesimals. We DO find CH4. If there were large amounts of O2 in the icy planetesimals (comets) that formed these bodies and that continue to impact them, wouldn't we expect this CH4 to become CO2? CH 4 is a minor cons-tituent in comets and in the ISM. Thus its presence on Pluto and Triton is itself a bit of a puzzle. Perhaps it is made by impacts, but this again would be impossible if abundant O2 were present.
Whenever a telluric planet is young and large enough to still have volcanism, the O2 input by cometary impacts has to be high enough to overcome the efficient sink that results from the action of reducing rocks and gases from volcanoes, to build a O2 rich atmosphere. The low oxygen content of comets, if any, does not favour such a high input.
In summary, the hypothesis of O2 being present in comets has no observational support in the present Solar System.
© European Southern Observatory (ESO) 1999
Online publication: November 26, 1998