Publications of the Astronomical Society of the Pacific, 128:018008 (6pp), 2016 January doi:10.1088/1538-3873/128/959/018008 © 2016. The Astronomical Society of the Pacific. All rights reserved. Printed in the U.S.A. Observing Planetary Rings and Small Satellites with the James Webb Space Telescope: Science Justification and Observation Requirements Matthew S. Tiscareno1,2, Mark R. Showalter2, Richard G. French3, Joseph A. Burns1, Jeffrey N. Cuzzi4, Imke de Pater5, Douglas P. Hamilton6, Matthew M. Hedman7, Philip D. Nicholson1, Daniel Tamayo8, Anne J. Verbiscer9, Stefanie N. Milam10, and John A. Stansberry11 1 Cornell University, Ithaca, NY, USA 2 SETI Institute, Mountain View, CA, USA 3 Wellesley College, Wellesley, MA, USA 4 NASA Ames Research Center, Moffett Field, CA, USA 5 University of California at Berkeley, Berkeley, CA, USA 6 University of Maryland, College Park, MD, USA 7 University of Idaho, Moscow, ID, USA 8 Canadian Institute for Theoretical Astrophysics, Toronto, ON, Canada 9 University of Virginia, Charlottesville, CA, USA 10 NASA Goddard Space Flight Center, Greenbelt, MD, USA 11 Space Telescope Science Institute, Baltimore, MD, USA Received 2015 July 24; accepted 2015 October 10; published 2016 January 4 Abstract The James Webb Space Telescope (JWST) will provide unprecedented opportunities to observe the rings and small satellites in our Solar System, accomplishing three primary objectives: (1)discovering new rings and moons, (2)unprecedented spectroscopy, and (3)time-domain observations. We give details on these science objectives and describe requirements that JWST must fulfill in order to accomplish the science objectives. Key words: methods: observational – planets and satellites: general – planets and satellites: rings – telescopes Online material: color figures 1. Introduction The James Webb Space Telescope(JWST) is being prepared for launch in 2018 to begin a planned five-year mission. JWST The rings that adorn the four giant planets are of prime will have the capability to observe Solar System objects as importanceasaccessiblenaturallaboratoriesfordiskprocesses,as close as Mars (Milam et al. 2016). Although most of the clues to the origin and evolution of planetary systems and as hardware is already designed and under construction, if not shapers as well as detectors of their planetary environments completed, work continues on the development of operations (Tiscareno 2013; Hedman 2015). The retinue of small moons guidelines and software and the completion of calibration tasks. accompanyingallknownringsystemsareintimatelyconnectedas The purpose of this paper is to identify observations of both sources and products, as well as shepherds and perturbers, of planetary rings that might be undertaken by JWST and to the rings. Leading sources of data on ring systems include describe what is required for JWST to accomplish those goals. spacecraft such as Cassini and Voyager, but also space telescopes The three primary motivations for observing rings and small such as Hubble(HST) and Spitzer, as well as ground-based moons with JWST are (1) discovering new rings and moons, (2) telescopes. unprecedented spectroscopy, and (3) time-domain observa- Additionally, the newly discovered rings around the minor tions. Section 2 gives details on these science objectives. planet Chariklo (Braga-Ribas et al. 2014) confirm for the first Section 3 describes requirements that JWST must fulfill in order time that small objects and solid objects can host rings. Due to to accomplish the science objectives, and Section 4 gives our several similarities with the known giant-planet rings (e.g., conclusions. orbit rate, radial structure), more detailed observations of the Chariklo rings are likely to shed light on the general workings 2. Observation Types of ring systems (Tiscareno 2014b). Furthermore, the discovery of the Chariklo rings—along with possible rings around 2.1. Imaging of Faint Objects Chiron (Ruprecht et al. 2015; Ortiz et al. 2015)—raises the In the context of rings, observations of faint targets are question of whether rings can also be observed around other complicated by the nearby presence of the bright planet. minor planets. Strategies are needed to enhance the apparent brightness of 1 Publications of the Astronomical Society of the Pacific, 128:018008 (6pp), 2016 January Tiscareno et al. discovered using Hubble in 2013, has a Vmagnitude of26.5 (Showalter et al. 2013), corresponding to a diameter of 16–20km if its albedo is 0.07–0.1, as is typical for Neptune’s inner moons. While the actual scattered-light contribution for JWST is not known, simple scaling would suggest that at the shortest photometric wavelength (∼0.7 μm), JWST is ∼2orders of magnitude more sensitive than Hubble. At Neptune, this would correspond to discovering moons as small as 2km across. For targets that are closer and/or brighter, the size may be even smaller. JWST will have new sensitivity to yet-undiscovered small moons or faint rings, including the predicted rings of Mars (Showalter et al. 2006) and Pluto (Steffl & Stern 2007). The NewHorizons spacecraft, whose flyby of Pluto will pre-date JWST, will likely not have the last word on Pluto’s possible rings due to its flyby speed and limited range of viewing geometries. JWST will be ideal for follow-up observations, possibly with greater sensitivity, and can also search for rings around other trans-Neptunian dwarf planets. Imaging of faint objects with JWST may be further enhanced by coronagraphy (see Section 3.3). 2.2. Spectroscopy of Faint Objects The compositional diversity of solid objects in the outer Solar System is apparent from the near-infrared spectra of Figure 1. Faint rings and moons of Uranus were discovered by Hubble in these bodies such as Triton, Pluto, and Charon, which show 2003 images. (A) Unprocessed image. (B) Filtered image showing two absorption features of varying strengths due to varying discoveries: Perdita (red circle), recovered 14years after its discovery by amounts of methane, water, and other ices on their surfaces Voyager, and Cupid (green circle), observed for the first time. (C) Summation (de Bergh et al. 2013). The smaller moons and rings of Neptune of 24images showing the newly discovered rings R1 andR2, now respectively might have originally been made of the same stuff as these named the μ and νrings. Figure from Showalter & Lissauer (2006). larger objects, but they also would have had much different (A color version of this figure is available in the online journal.) evolutionary histories (perhaps less thermal processing, more desired targets and/or to suppress the apparent brightness of pollution from infalling matter, etc.). Comparing the surface the planet (Figure 1). composition of these smaller objects to their larger neighbors JWST will be equipped with filters (see Section 3.2; Milam et should therefore help clarify the origins and histories of both, fi al. 2016) that allow it to image giant-planet systems at but it is dif cult to obtain good-quality spectra of these very wavelength bands in which the planet is greatly darkened by small and/or faint objects from ground-based observatories. absorption due to methane and other atmospheric constituents. With its large mirror and high-quality spectrometer (Milam et al. 2016), JWST will be able to take spectra of very faint For observations of faint moons or rings that are close to bright objects. Potential targets include the rings and small moons of giant planets, this will lead to greatly improved signal to noise Uranus and Neptune, which have never been the subjects of a and spatial resolution compared to HST and other observatories high-fidelity spectroscopic study, as Voyager2 did not carry a operating in the same wavelength bands (put another way, spectrometer capable of detecting them. Characterizing their JWST will operate within the infrared methane bands at a chemical compositions is of considerable interest for addres- spatial resolution comparable to that at which HST operates in sing the origins of the Uranus and Neptune systems as well as visible bands, with vastly improved signal to noise when for addressing the question of why the Uranian and Neptunian suppression of glare from the planet is an important factor). rings are so qualitatively different from those of Saturn As a result, JWST will provide major advances in resolving (Tiscareno et al. 2013). and separating the main rings of Uranus and Neptune, By the same token, JWST will be able to acquire very improving upon HST and ground-based observations of their sensitive spectra of all objects over a broad range of fine structure (de Pater et al. 2005, 2006, 2007; Showalter & wavelengths. It will be able to fill in the gap between Lissauer 2006). For example, the faintest moon of Neptune, CassiniVIMS and CassiniCIRS (from 5 to 8 μm) and will 2 Publications of the Astronomical Society of the Pacific, 128:018008 (6pp), 2016 January Tiscareno et al. be able to map Saturn’s rings in the 1.65 μm water absorption feature, which falls in an internal gap in VIMS’ spectral coverage and is unusual in that its depth is useful for mapping temperature variations (Grundy et al. 1999). Its spatial resolution will be a few hundred kilometers, comparable to CIRS, and its sensitivity will be greater, so it should be capable of improving current maps of Saturn’s rings in the thermal infrared (though over a very limited range of phase angles) and may achieve the first detection of the faint silicate absorption features at 10 μm (Crovisier et al. 1997; Stansberry et al. 2004; Emery et al. 2006), yielding information about the little-understood non-water-ice components of Jupiter’s and Saturn’s rings. What we do know about the spectra of giant-planet rings, as they are likely to be seen by JWST, is shown in Figure 2. Only Saturn’s rings have detailed spectra at low phase angles, taken by CassiniVIMS (Hedman et al.