Comets in UV

Comets in UV B. Shustov1 • M.Sachkov1 • Ana I. Gómez de Castro 2 • Juan C. Vallejo2 • E.Kanev1 • V.Dorofeeva3,1 Abstract Comets are important “eyewitnesses” of So- etary systems too. Comets are very interesting objects lar System formation and evolution. Important tests to in themselves. A wide variety of physical and chemical determine the chemical composition and to study the processes taking place in cometary coma make of them physical processes in cometary nuclei and coma need excellent space laboratories that help us understand- data in the UV range of the electromagnetic spectrum. ing many phenomena not only in space but also on the Comprehensive and complete studies require for ad- Earth. ditional ground-based observations and in-situ exper- We can learn about the origin and early stages of the iments. We briefly review observations of comets in evolution of the Solar System analogues, by watching the ultraviolet (UV) and discuss the prospects of UV circumstellar protoplanetary disks and planets around observations of comets and exocomets with space-born other stars. As to the Solar System itself comets are instruments. A special refer is made to the World Space considered to be the major “witnesses” of its forma- Observatory-Ultraviolet (WSO-UV) project. tion and early evolution. The chemical composition of cometary cores is believed to basically represent the Keywords comets: general, ultraviolet: general, ul- composition of the protoplanetary cloud from which traviolet: planetary systems the Solar System was formed approximately 4.5 billion years ago, i.e. over all this time the chemical composi- 1 Introduction tion of cores of comets (at least of the long period ones) has not undergone any significant changes. Comets are common and very diverse minor bodies of At present, in the outer part of the Solar System two the Solar System. They are common in extrasolar plan- populations of small bodies are considered to be sources of the observed comets: the trans-Neptunian region, B. Shustov consisting of the Kuiper belt and the scattered disc (30 M.Sachkov – 100AU) and the Oort cloud (up to 100000AU ). In arXiv:1802.06855v1 [astro-ph.EP] 19 Feb 2018 Institute of Astronomy of the RAS, Russia recent years, the Main asteroid belt is also considered Ana I. Gómez de Castro as a source for main-belt comets. In many models the Juan C. Vallejo origin of these populations is associated with processes AEGORA Research Group, Universidad Complutense, Spain that accompany the formation of the planetary system E.Kanev (see e.g. Dones et al. (2015); Meierhenrich (2015)). We believe that careful analysis of the chemical com- Institute of Astronomy of the RAS, Russia position of comets with adequate chemical model of V.Dorofeeva early Solar System (protoplanetary disk) could help to Institute of Geochemistry of the RAS, Russia restore information about area of the disk where these 1Institute of Astronomy, Russian Academy of Sciences, Pyatnit- skaya 48, 119017 Moscow, Russia bodies have been formed. By linking this information 2AEGORA Research Group, Fac. CC. Matematicas, Universidad with dynamic models of the evolution of Solar System, Complutense, Plaza de Ciencias 3, 28040 Madrid, Spain we can verify our understanding of the early Solar Sys- 3Vernadsky Institute of Geochemistry and Analytical Chemistry, tem. Russian Academy of Sciences, Kosygina 19, 119991 Moscow, Rus- At present, data on the comet composition of sia about twenty short-period and long-period comets can 2 be found in the literature (Bockelée-Morvan (2011); clues on the comets thermal history. But they have not Mumma and Charnley (2011); Ootsubo et al. (2012); yet been detected by any UV instrument in cometary Paganini et al. (2014); Cochran et al. (2015); Roth et al. comas. (2017)). This data are presented in Fig.1. Analysis of Delsemme (1980) reminded that the atmospheric this data shows that there are no fundamental differ- bands of ozone absorb strongly at wavelengths less than ences between the volatiles content in comets of differ- 300 nm and still absorb irregularly up to wavelengths ent dynamic types. Of course, we should bear in mind longer than 320nm. The 300 – 320nm range (where that only the very deep interiors (> 50m) of cometary the famous OH line at λ309 nm) lies is a very impor- nuclei would have information on the primary compo- tant “interface domain” typically covered by space-born sition of cometesimals. UV instruments and still reachable by groud-based tele- Observations of cometary interiors are challenging. scopes located in mountains. This makes possible vari- Since we are limited by the distant observations, a crit- ous tests and calibrations. ical problem is distinguishing between the coma and the In this paper we discuss findings from UV (far UV nucleus signatures. Chemical composition of the matter (FUV) and near UV (NUV)) observations of comets evaporated from the surface of the comet (i.e. coma) is with space born instruments (Section 2). Special at- typicaly far from primordial. The upper layers of the tention is paid to the prospects of the World Space Ob- core of short period comets are intensively reproccessed servatory – Ultraviolet (WSO-UV) project (Section 3). during previous encounters with the Sun. This is con- Concluding remarks are given in Section 4. firmed by the data obtained for comet 67P/C-G in the Rosetta experiment (Le Roy et al. (2015); Rubin et al. (2015); Balsiger et al. (2016); Altwegg et al. (2016); 2 Half a century of UV-observations of comets Mall et al. (2016)). This data are also represented in from space Fig. 1: asterisks (before perihelion) and by encircled asterisks (after perihelium). All the volatile compo- In an interesting review by Delsemme (1980) of early nents are stronger after perihelion passage, when the UV observations of comets, all spectral first-time iden- upper shielding layer is destroyed in the southern hemi- tifications of comets are classified in three periods: sphere of the comet. From the analysis of the data and • From the early times until 1911 (named by Delsemme considering the propagation velocity of heat within the (1980) as Dark Ages of Cometary Spectra), when the nucleus, it can be concluded that observational data two major features of the neutral coma spectrum, obtained 2 – 3 weeks after the perihelion comet is of CN and C2, had been already identified as well as particular value, since they will most adequately corre- + the two major ions of the ion tail (CO+ and N ). spond to the composition of the cometary nucleus. 2 Metallic lines had already been seen in a sun-grazing Many problems of determining the chemical compo- comet, and the first UV features below 340 nm had sition of comets can only be solved by using observa- just been observed as a fuzzy unresolved band. Next tional data in the ultraviolet (UV) range of the electro- thirty years was a period with no bright achievements magnetic spectrum. Cometary spectra in the UV and in the field, visible range include lines formed by scattering of the • Period 1941 – 1970 started with the first definite de- sunlight on solid dust particles and emission lines from tection by Swings et al. (1941) of OH(0-0) at 309nm the gas in coma. UV spectroscopy provides important and NH(0-0) near 336nm in the spectrum of comet information about the composition of a cometary coma. Cunningham (1941I). The discovery of the UV line of In general, cometary coma are dominated by emission OH demonstrated that comets contain water. This features that originate from the dissociation products was strongly confirmed by the first rocket and satel- of water: OH, H and O, or correspond to secondary lite experiments running UV observations of comets. atomic, molecular, and ionic species, such as C, C+, The “space era” of UV studies of comets began about CO, CO+, CO+, S, and CS. Also, the spectra and im- 2 half a century ago. ages contain information on structure and dynamical • Since then, we are in the “modern” epoch in UV features of coma. The spatial structure and time varia- studies. A number of space-born instruments (few tions of the outflows inferred from the images and spec- to 240cm aperture, both spectroscopic and imaging tra help to reveal the structure and other properties of ones) were (and are) used for numerous UV observa- cores. There are ways to restore the thermal history of tions of comets. We think this period will continue cores too. For instance, N2 and noble gases are both up to the appearance of giant UV telescopes with chemically inert and highly volatile (evaporate at very aperture of 4 – 30m. This future epoch will come low temperatures), so they are particularly interesting not earlier than 20 years from now and currently, the 3 Fig. 1 Values of relative contents of gas components in the coma of long-period and short-period comets by percent of water mass. For 67P/Churyumov-Gerasimenko comet the gas contents are shown by black asterisks (before perihelion) and by encircled asterisks after perihelium passage. See text for references. most powerful tools are telescopes of the 2m class (1970)), comet Encke by the OAO-2 satellite and the (HST, WSO-UV). OGO-5 satellite (Bertaux et al. (1973)). Comet West (1976VI) was observed with an Aerobee rocket payload In the article by Delsemme (1980), the first two pe- (instruments): spectra (Feldman and Brune (1976)) riods are described in some detail. Here we briefly and imagery of the Lyα halo (Opal and Carruthers discuss the most important scientific and technological (1977)) were obtained. These are some of the first achievemets of the third epoch as well as some prospects observations of comets in UV.

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