H-implantation in SO2 and CO2 ices

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Abstract

Ices in the solar system are observed on the surface of planets, satellites, comets and asteroids where they are continuously subordinate at particle fluxes (cosmic ions, solar wind and charged particles caught in the magnetosphere of the planets) that deeply modify their physical and structural properties. Each incoming ion destroys molecular bonds producing fragments that, by recombination, form new molecules also different from the original ones. Moreover, if the incoming ion is reactive (H+, On+, Sn+, etc.), it can concur to the formation of new molecules.

Those effects can be studied by laboratory experiments where, with some limitation, it is possible to reproduce the astrophysical environments of planetary ices.

In this work, we describe some experiments of 15–100 keV H+ and He+ implantation in pure sulfur dioxide (SO2) at 16 and 80 K and carbon dioxide (CO2) at 16 K ices aimed to search for the formation of new molecules. Among other results we confirm that carbonic acid (H2CO3) is formed after H-implantation in CO2, vice versa H-implantation in SO2 at both temperatures does not produce measurable quantity of sulfurous acid (H2SO3). The results are discussed in the light of their relevance to the chemistry of some solar system objects, particularly of Io, the innermost of Jupiter's Galilean satellites, that exhibits a surface very rich in frost SO2 and it is continuously bombarded with H+ ions caught in Jupiter's magnetosphere.

Introduction

Observations from earth and from space have shown that ices are widely present in the solar system: in the surface of asteroids, comets, planets and their satellites (e.g., Schimitt et al., 1998).

By interaction with cosmic ions (mainly H+ and He2+), solar wind and charged particles caught in the magnetosphere of the planets (Jupiter and Saturn particularly) these ices are physically and structurally modified. Each incoming ion breaks molecular bonds and a large number of new molecules can be formed by recombination of fragments of the irradiated species. If the incident ions are reactive (H+, On+, Sn+, etc.), they can concur to the formation of new molecules.

Those effects can be studied by laboratory simulations, where the relevant targets are bombarded with fast charged particles under physical conditions as close as possible to the astrophysical ones (e.g. Johnson, 1998; Strazzulla, 1998). Ion bombardment experiments have demonstrated the ability of cosmic particles to change the spectral behavior of irradiated surfaces (e. g. Mennella et al., 1997; Brunetto et al., 2006) as well as the chemical composition of planetary (e.g. Strazzulla et al., 1991) or interstellar (e.g. Palumbo et al., 2000) ices.

Ion bombardment has been suggested to explain the presence, on planetary surfaces, of some molecules not necessarily native of that object but formed by the implantation of incoming reactive ions. For example, it has been suggested that sulfur dioxide (SO2) on the surface of Europa and Callisto is due to Sn+ implantation coming from Jupiter's magnetosphere on the water ices on their surfaces (Lane et al., 1981; Sack et al., 1992). Recently such a suggestion has been challenged on the basis of new experiments (Strazzulla et al., 2007) of sulfur implantation in water ice. The experiments demonstrated the efficient formation of hydrate sulfuric acid but only an upper limit has been found for the formation of SO2.

Another example (Strazzulla et al., 2003; Dawes et al., 2007) concerns the results on C implantation into water ice to study the production of carbon dioxide (CO2). It has been shown that although a relevant quantity of CO2 can be synthesized by C implantation into water ice on the Galilean moons, this is not the dominant formation mechanism of the CO2 molecule. In a subsequent work, Gomis and Strazzulla (2005) presented laboratory results on experiments of irradiation of water ice deposited on top of carbonaceous materials. Those authors found that CO2 is produced efficiently after irradiation, and their results show that radiolysis of mixtures of frost water and carbonaceous compounds could account for the quantity of CO2 ice present on the surfaces of the Galilean satellites.

Analysis of the data collected by the Galileo spacecraft during its mission around Jupiter and its satellites has allowed a better understanding of the Jovian system but, as usual, it also raised new questions.

One of them concerns the possible presence of sulfurous acid (H2SO3) on Io, the innermost of Jupiter's Galilean satellites.

The formation and stability of H2SO3 is possible only under opportune conditions. The acid is formed by the reactionSO2+H2O→H2SO3but there must not be oxidants otherwise for SO2 it is energetically convenient to oxidize it into SO3 (Loerting and Liedl, 2000) by the following reaction:SO2+1/2O2→SO3

Water concentration must not be greater than that of SO2 otherwise exceeding water would destroy H2SO3 by the reactionH2SO3+nH2O→SO2+(n+1) H2O

This process becomes dominant with an increasing number of water molecules (Li and McKee, 1997).

H2SO3 must stay in a low-temperature environment yet. In fact its half-life is 1 day at 300 K and almost 3 billion years at 100 K (Voegele et al., 2004).

Io satisfies such conditions; in fact, it exhibits a particular and unique surface composition rich in sulfur compounds, mainly frozen SO2, a temperature around 100 K and low water content.

However, if Io seems to be adapted to accommodate H2SO3, it remains to understand how this acid is formed in the absence of water.

Voegele et al. (2004) proposed that the formation of H2SO3 on Io may be because of H+ implantation in SO2 ices, taking cue for this affirmation from Brucato et al. (1997) who synthesized carbonic acid (H2CO3) by 1.5 keV H+ implantation on pure CO2 ice. H2SO3 and H2CO3, in fact, show several common characteristics like the high half-life at low temperatures and low humidity environments.

This hypothesis is an interesting cue for a laboratory experiment. In this article we present results obtained by implanting 30 keV H+ and He+ in SO2 ice at 16 K and 50 keV H+ in SO2 ice at 80 K. We also discuss the results obtained after experiments of implantation, 50–100 keV H+ and He+ in pure CO2 ice aimed to verify and extend the results of Brucato et al. (1997). Possible astrophysical implications of the obtained results are also presented.

Section snippets

Experimental apparatus

The infrared spectra shown here have been obtained by infrared transmission spectroscopy performed in a high-vacuum chamber (P<10−7 mbar) faced, through IR-transparent windows, to an FTIR spectrophotometer (Bruker Equinox 55). Frosts are accreted onto a silicon substrate, placed in thermal contact with a cold finger, by admitting the chosen gas into the chamber, through a needle valve. The cold finger temperature can be varied between 10 and 300 K.

The vacuum chamber is interfaced to an ion

Results and discussion

The deposited SO2 ice film in the two cases treated (16 and 80 K) had a thickness, respectively, of about 1.5 and 2.1 μm, larger than the penetration depth of the incoming ions calculated by SRIM 2000 software (Ziegler et al., 1996), ∼0.32 μm for 30 keV H+ and ∼0.45 μm for 50 keV H+. Normalized spectra of SO2 ices as deposited at 16 and 80 K are shown in Fig. 1. It is evident that the profile of the main absorption bands (see Table 1 for their identification) strongly depends on the temperature at

Acknowledgments

We thank G.A. Baratta for useful discussions and F. Spinella for his help in the laboratory.

This research has been supported by Italian Space Agency Contract no. I/015/07/0 (Studi di Esplorazione del Sistema Solare).

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