STUDIES OF COMETS IN THE ULTRAVIOLET: THE PAST AND THE FUTURE S.A. STERN Southwest Research Institute, Department of Space Studies, 1050 Walnut St., No. 426, Boulder, CO 80302

Abstract. The remote sensing of comets in the ultraviolet bandpass has been a valuable tool for studying the structure, composition, variability, and physical processes at work in cometary comae. By extension, these studies of comae have revealed key insights into the composition of cometary nuclei. Here we briefly review the ultraviolet studies of comets, and then take a look toward the future of such work as anticipated by the advent of several key new instruments. Keywords: comets, ultraviolet instrumentation, Rosetta

1. Introduction The history of ultraviolet (UV) studies of comets stretches back to 1970, being dominated by results from sounding rocket missions (which opened up all of ˚ and 3000 A), ˚ and spacecraft orthe main UV bandpass regions between 250 A biting the Earth, including OGO-5, OAO-2, Skylab, IUE, HST, EUVE, SOHO, USSR/Astron, and various payloads aboard the Space Shuttle. Among these, the IUE workhorse was the greatest source of contributions (e.g., Festou and Feldman, 1987). To date, no UV instrument except the low-resolution TKS spectrometer aboard the USSR’s Vega mission to Halley has been flown to a comet, but that situation is changing (see x 4, below). The ultraviolet bandpass, like the radio, millimeter-wave, and IR, is a powerful tool for studying astrophysical objects and has been applied with dramatic success to the study of comets. In this brief review, we hope to give some perspective on the value of ultraviolet remote sensing of comets, the contributions UV work has made in the past, and the prospects for future advances. This review cannot, within its space limitations, cover this subject in detail, and the reader is referred to previous expository reviews on UV studies of comets, most particularly, P.D. Feldman’s (1982) review and his (1991) update and extension of that review, Bowyer and Malina’s excellent (1991) exposition on the value of EUV studies, the intensive, detailed discussion relevant to our topic in Festou, Rickman and West (1993) and Meier (1991), and the historically valuable early perspective provided by Keller (1976). Space Science Reviews 90: 355–361, 1999. Printed in the Netherlands.

c 1999 Kluwer Academic Publishers.

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2. The Value of the Ultraviolet

The UV region of the electromagnetic spectrum is loosely defined as that region be˚ with the blue end constrained by the (relatively arbitrarily tween 250 and 4000 A, defined) soft X-rays, and the red edge defined by the approximate blue boundary of human eyesight. Within the UV bandpass, commonly-used subdivisions include ˚ mid-UV (MUV; 2000–3000 A), ˚ far-UV (FUV; the near-UV (NUV; 3000–4000 A), ˚ ˚ 900–2000 A), and extreme-UV (EUV; 200–900 A). NUV observations can be carried out from the ground and high-altitude aircraft, but the increasing opacity of the atmosphere due to ozone absorption makes obser˚ and impossible below 3050 A. ˚ As such, space vations difficult below 3500 A, observations are required in the MUV, FUV, and EUV. A key characteristic of all solar system UV observations, including in the NUV, is the inherent weakness of UV signals compared to visible-bandpass signals (this is largely due to the UV being on the Wien side of the Sun’s G5V blackbody). Therefore UV detectors are particularly sensitive to visible light leaks (so-called “red-leaks”), which can easily pollute desired signals with an undesirable and potentially fatal source of noise. Red leaks can be controlled in various ways, most notably by suitable filtering, or more decisively, by employing detector photocathodes which are blind to long wavelength radiation due to the quantum mechanically-assured protection of photon work functions via the photoelectric effect. Once water production dominates the nuclear activity, UV spectra of comets are, to first order, similar, after adjustments are made for absolute water production rates and the removal of the dust continuum. (At greater heliocentric distances, emissions from CO and the high fluorescence-efficiency trace molecule CN’s 3883 ˚ 0-0 band emission dominate). Further, once water production dominates, a comA bination of the high relative abundance and the large resonance fluorescence efficiencies of H and OH, emissions from these two species completely dominate cometary UV spectra (although, owing to its high g-factor, CN emission can rival these two at times). Near 1 AU, for example, all of the other numerous (i.e., >50) UV features detected to date in comets are typically two or more orders of magnitude fainter than the main H (Ly) and OH (0–0) emissions. Despite this, however, the contributions to our understanding of coma processes and composition that these “minor species” have made far outweigh their contribution to the ultraviolet luminosity of comets. We now illustrate this point by identifying some of the key utilities of the various UV regions for cometary studies: The NUV: Trace metal species derived from the refractory and organic solids in the nucleus (e.g., Fe, Mn, Ni, Cu, etc. seen in sungrazers); daughter radicals OH, NH, and CN; CO+ , CO+2 , OH+ , N+2 (and C3 , CH, etc., but the brightest ˚ the small-particle emissions from these latter two are longward of 4000 A);

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dust continuum; and the dust/ice ratio of both grains and the cometary surface itself. The MUV: Daughter molecules, most particularly including the neutrals C2 , CS, S2 , CO, and OH (again), and the ions CO+ and CO+2 ; SO2 and SO (not yet detected in the UV); and the small-particle dust continuum. We note that the CO Cameron bands in this region are produced by the prompt emission from CO2 dissociation or electron-excitation of CO, and therefore can be used as a neutral CO2 tracer. The FUV: The parent molecules CO and CO2 ; the neutral atomic daughter products of molecular dissociation: C, H, O, N, and S; H/D ratios; the C+ ion; the Kr and Xe noble gases (for which only upper limits have as yet been obtained); the H2 daughter (for which, again, only upper limits are available); absorption studies of the parent molecule H2 O in both its gaseous and solidstate phases (which requires very high spatial resolution indeed, and has as yet only been planned for spacecraft missions to comets); and the small-particle dust continuum which can be studied using the light source at Lyman-. The lack of detected signals from H2 , and N and N2 constitutes a pair of major objectives for future research. The EUV: Noble gas emissions including He, He+ , Ne, and Ar; the H Lyman series below Ly- (which avoids the optical depth problems often encountered at Ly-); numerous ions including C+ , S+ , O++ , S++ , and O++ ; absorption studies involving the parent molecules CO, CO2 , as well as N2 ; and the so-called X-ray emission of comets. Though certainly not exclusively true, most of the emissions referred to above are due to solar-stimulated fluorescence. Key secondary mechanisms include electron impact excitation, dissociative recombination, and photodissociative excitation. Ultraviolet studies of comets have been dominated by spectroscopic investigations, but UV imaging have also been successfully employed. In the next section we briefly review some of the major contributions that such UV observations have made to our understanding of comets.

3. Notable Results from the UV UV observations of cometary comae have revealed a rich variety of features and processes in cometary comae. Here we touch only on a few items of notable historic import: OH and H: The Chemical Clue to Water’s Dominant Role in Generating Cometary Activity Near 1 AU. Perhaps the single most fundamental contribution of UV work to cometary science was the almost inescapable chemical evidence that the detections of large OH (Swings et al., 1941) and H (Code et al., 1972; Bertaux et al., 1973) abundances in comets secured the case for H2 O as the dominant cometary volatile and the source of activity in comets

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inside 2.0–2.5 AU. Over the past 30 years, a considerable database of H and OH abundance measurements have been built up (see Feldman, 1991), which have yielded a database on the production rate, Q, of H2 O for many comets as a function of heliocentric distance. These data have revealed that individual comets display a considerable variety of H2 O production curves, and have (along with dust continuum and direct H2 O measurements in the IR and additional OH data in the radio) allowed researchers to discover that comets actually display a wide variation of active surface fractions (e.g., A’Hearn et al., 1995). The Lack of Sulfur and Nitrogen Molecule Detections. Although sulfur is cosmogonically abundant, only S, CS, and S2 have been detected in UV spectra of comets. (And it remains true that, as Feldman (1991) and Festou et al. (1993) also point out, that other sulfur-bearing species, notably including SO and SO2 have never been detected in the UV, no doubt owing in part to a recent downward revision in oscillator strengths.) S2 is difficult to detect because it is short-lived against photodissociation, and has only been seen in the Earthapproaching comets IRAS-Iraki-Alcock (A’Hearn et al., 1983; and at abundance levels near that of CS (i.e., 10 3 H2 O’s abundance), and then much later, in 1996, in Hyakutake (Weaver, 1998; see also A’Hearn and Livengood, 1996). Regarding nitrogen: The difficulty of detecting N2 , a homonuclear species with forbidden electric dipole transitions, is well known. However, the complete lack of N2 molecules (and N atoms themselves) remains puzzling on cosmogonic grounds (e.g., Mumma et al., 1993). The Unexpected Discovery of EUV/X-ray Emission. Without a doubt, one of the most unexpected, and most intriguing cometary discoveries in recent years has been the discovery of unexpectedly bright cometary X-ray emission. Cometary X-rays were first detected in Hyakutake in early 1996, using the ROSAT observatory in Earth orbit (Lisse et al., 1996). Rapidly thereafter, comet C/Tabur 1996 Q1 and then four ROSAT all-sky survey comets (Dennerl et al., 1997) were found to also emit X-rays, indicating that the X-ray emission phenomenon is common. The observed X-ray emission is quite soft (kT0.20.1 KeV). Although it is not fully clear as yet whether the emission is dominated by a continuum process such as thick-target bremsstrahlung, or is simply a dense forest of line emission due to solar wind charge exchange processes, the charge exchange mechanism seems the early favorite. The detection of EUV emissions apparently related to the X-ray emission (e.g., Krasnopolsky et al., 1997) provides additional clues to and leverage on the problems posed in understanding the root cause of this emission. Ne and Ar : Witnesses to Cometary Thermal History. Noble gases, owing to their disaffinity for chemical reactions and their (related) high volatility, have long been known to provide powerful probes of the thermal history of comets. Although the interpretation of noble gas abundances in cometary coma is complicated by the details of their trapping and release efficiencies (Owen et

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al., 1991), their detection has nonetheless been highly anticipated. Significant upper limits revealing depletions of order 104 in He (Stern et al., 1992) and of order 25 in Ne (Krasnopolsky et al., 1997) have been reported. Very recently, Stern et al. (1999) have reported the detection of Ar at near-cosmogonic levels in Hale-Bopp. Combining this detection with the Ne depletion reported in this same comet (Krasnopolsky et al., 1997), it is possible to conclude that the cometary ices formed at temperatures between 20 and perhaps 35 K, indicating an origin for Hale-Bopp in the Uranus-Neptune region.

4. Key Upcoming UV Opportunities for Comet Science Many future advances in UV remote sensing will be driven by the advent of new observational capabilities. Major new observing opportunities include: The Far Ultraviolet Spectroscopic Explorer (FUSE) mission set to launch in 1999. FUSE (Moos, et al., 1997) is a general purpose, Earth orbital observatory with high-resolution (/ =24,000–30,000) spectroscopic capabilities ˚ and 1195 A. ˚ The instrument will achieve an effective area of between 905 A 2 20–80 cm (varying across the bandpass) and a 1.5 arcsecond resolution for a Strehl ratio of 0.90. The MICAS UV Spectrometer Comet flybys aboard New Millennium/Deep Space 1 (NMP/DS1) set for 2000–2001. NMP/DS1 (Nelson, 1998) was launched in October 1998, and is en route to a flyby with near-Earth asteroid 1992KD on 29 July 1999. If all goes well, DS1 will also execute a flyby with either the comet/asteroid transition object Wilson-Harrington (October 2000), or comet 19P/Borrelly (January 2001). The DS1 UVS offers a characteristic ˚ across its 800–1850 A ˚ bandpass, a 0:1  6:0 deg slit, and resolution of 12 A ˚ a characteristic effective area of 0.5 cm2 — all vastly superior to the 70 A resolution TKS instrument aboard Vega. DS1 also includes a visible imager, a low-resolution IR spectrograph, and the PEPE plasma sensor package. The installation of the Advanced Camera for Surveys (ACS) aboard HST in 2000. The ACS (http://www.stsci.edu/instruments/acs/index2.html) offers to provide HST with a photon-counting, high-resolution (0.05”), high-through˚ imaging capabilities with a FOV about twice that put, near-UV (>2000 A) of the present Wide Field (WF) Camera 2. ACS also includes coronagraphic and polarization capabilities. The UV sensitivity of the ACS is comparable to HST/STIS and about 10that of WFPC2, and is also expected to contribute to imaging surveys of cometary nuclei (H.A. Weaver, pers. comm.). The installation of the Cosmic Origins Spectrograph (COS) aboard HST in 2003. COS (Morse et al., 1998) is a major new, high-throughput UV facility planned for installation aboard the Hubble Space Telescope. COS will cover ˚ bandpass, achieve spectral resolutions up to 30,000, and the 1150–3200 A ˚ As a offer effective areas exceeding 1000 cm2 at peak performance (2600 A).




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figure of merit, COS is expected to achieve S/N=10 at R=20,000 for flux levels ˚ 1 in a 10,000 second integration across much of 1–210 15 ergs cm 2 s 1 A ˚ bandpass. of its 1150–3200 A The rendezvous of the Alice UV Spectrometer, aboard Rosetta, at comet 46P/Wirtanen in 2011. The Rosetta/Comet Wirtanen rendezvous is the most ambitious comet mission now in development. It will include a lander and some 12 orbiter investigations. Alice (Stern et al., 1998) will obtain 700–2050 ˚ spectra with a characteristic resolution of 6 A, ˚ and a characteristic effective A ˚ are area of 0.1–0.5 cm2 through its 6 deg long slit. Resolutions as high as 1 A possible in occultation mode. Rosetta will make detailed studies of Wirtanen beginning in November 2011, when the comet is near 3.3 AU, until the comet reaches perihelion in 2013.

5. Future Needs Despite the enormous potential afforded by the FUSE, NMP/DS1, HST/COS, and Rosetta/Alice opportunities, a number of important desires remain unfulfilled. These include: Extremely High UV Spectral Resolution (R>3105 ) to directly probe the velocity fields of species revealed through UV emissions, to study hyperfine structure, and to measure OH/OD abundances. Low Solar Elongation Angle (SEA) Observing Capability to permit the routine observation of perihelion passages substantially inside 1 AU, as well as those comets with poor apparition geometry. This capability will, for example, allow many trace species that either have low g-factors, or are still bound in grains at 1 AU, to reveal themselves. Flyby Reconnaissance of Multiple Comets with UV Spectroscopic Capabilities to sample the diversity of noble gas and other species abundances seen only in the UV with high spatial resolution, which is particularly important both for studies of nuclear heterogeneity and those species with short scale lengths. One looks forward to these and other aspirations being achieved as well in the years to come.

Acknowledgements I thank my colleagues Will Colwell, Joel Parker, and Dave Slater for their comments on the initial draft of this manuscript, and Mike A’Hearn, Dan Boice, Michel Festou, Hal Weaver, and the ISSI reviewer for comments on the subsequent draft.

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Address for correspondence: Alan Stern, Southwest Research Institute, Space Studies Dept., 1050 Walnut St., No. 426, Boulder, CO 80302, USA, [email protected]