JWST Observations Fo Rings and Small Satellites
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JWST Observations of Rings And Small Satellites Anne Verbiscer University of Virginia Planning Soar System Observations with JWST Workshop Space Telescope Science Institute 14 November 2017 Outline • Acknowledgements • Motivation • Observation Opportunities, Instrumentation • Rings – imaging, spectroscopy Occultations • Small Satellites – imaging, spectroscopy Motivation: Why observe rings and small satellites with JWST? 1. Discovering new rings and small satellites 2. Unprecedented spectroscopy 3. Time-domain observations Tiscareno et al. 2016 JWST Observation Opportunities Like Spitzer & WISE, JWST can only observe at quadrature. 20 NO Opposition observations or contributions to phase curves in the near-mid IR Observation windows lost due to launch slip (red) Keszthelyi et a. 2016 JWST Instrumentation Ring Observations with JWST Near, Mid IR Imaging will be challenged by scattered light from primary Spectroscopy will be challenged by scattered light from primary Occultations can distinguish fine structural details can determine precession rates of rings (puts constraints on planetary interiors) enhanced by Gaia catalog (occultations of 2014 MU69 brought us into the “Gaia era” of occultation prediction) Occultations of solid bodies (TNOs) have potential for ring discoveries (e.g. Chariklo, Haumea) Equinox Santos-Sanz et al. 2016 Cassini VIMS Spectra of Saturn’s Main Rings JWST will fill in the gap between Cassini VIMS and CIRS (5-7 µm) and at 1.65 µm where Cassini’s order sorting filters overlap 1.65 µm Hedman et al. 2013 Reflectance Spectrum of Saturn and Water Ice (at 40 K) and Bandpasses of NIRCam’s Medium Filters Santos-Sanz et al. 2016 Note the F300M filter is well matched to the 3 µm water ice absorption band in Saturn’s rings and Saturn is very dark in F335M and F360M filters. Every filter above F300M is a good choice. (F300M, F460M, and F480M have slightly larger amount of light from Saturn.) Saturn’s Ring Spokes Spokes in Saturn’s Main Rings as seen by HST trace the interactions between interplanetary impacts and Saturn’s magnetic field. Spokes are seen best near equinox and not at solstice. Next equinox is 2025, although Saturn will be at conjunction, observations ± 3 months will still have low Sun angles. JWST can continue to track seasonal behavior of spokes after Cassini. Spokes McGhee et al. 2005 Propellers in Saturn’s A Ring JWST may be able to track azimuthal arcs in Jupiter’s rings and propellers in Saturn’s A ring Tiscareno et al. 2010 Uranian Rings and Moons Faint rings and small satellites of Uranus discovered by HST in 2003 A. Raw image B. Filtered image shows Recovery of Perdita (Voyager 2) Discovery of Cupid C. 24 stacked images show two new rings: the µ and ν rings Showalter and Lissauer 2006) de Kleer et al. 2013 Example NIRSpec MSA configuration for Cupid Inner Uranian system for 60 min at a randomly Puck chosen time on Epsilon Ring Sept. 28, 2020. Mab Red boxes are object apertures; Cressida Uranus green boxes are Perdita empty nearby spectra for calibration. Desdemona Ariel will saturate, but Umbriel and Umbriel Miranda will not. Miranda Norwood et al. 2014 Voyager 2 Neptune’s Ring Arcs First images from Voyager 2, then reacquired by HST in 1996 and by Keck (bottom) Dumas et al. 1999 de Pater et al. 2005 WISE Band 4: 22 µm Saturn’s Phoebe Ring Will be a challenging observation for JWST due to the small FOV and background characterization Spitzer MIPS: 24 µm Hamilton et al. 2015 Haumea’s Ring At short wavelengths the diffraction limit of JWST is comparable to the radius of Haumea’s newly discovered ring. Haumea’s dimensions 6 2 ra=850 km rb=570 km (area = 1.5x10 km ) Ring characteristics semimajor axis: 2300 km width: 70 km Optical depth = 0.5 Inclination = 14 degrees Projected effective area 1.2x105 km Presuming 1/3 of the ring area corresponds to a single ring ansa, the ring ansa would represent a “point source” 4 magnitudes fainter than Haumea. Haumea is H=16.3, so the ring flux is equivalent to a 20 to 21st magnitude object in the near infrared. Ortiz et al. 2017 Haumea’s Ring At K-band (2.2 µm) the diffraction limit of JWST will be about 85 milliarcseconds 130mas Haumea is about 50 AU from the Sun at present. The long axis of Haumea is 50 milliarcseconds The ring semimajor axis is 65 milliarcseconds At right is the 2.2µm Airy pattern of a 650 cm circular aperture scaled to the diagram of the Haumea system. Given the expected tremendous PSF stability it may be possible to deconvolve the ring spectrum particularly because the flux lies near Haumea’s first Airy null. JWST Sensitivity So NIRSpec m = 20 NIRCam m = 25 Norwood et al. 2014 Stellar Occultations by Rings Saturn’s C Ring Maxwell Bond Dawes Uranian Rings Santos-Sanz et al. 2016 Uranus’ γ and δ rings are diffraction limited but model fits provide accurate estimates of ring widths, mean optical depths, and orbital radii. The highly eccentric ε ring is sufficiently broad to reveal non-uniform internal structure. Sky-plane view of Ring Occultations in 2019 viewed by JWST Tick marks are 30 minutes Red dot is ingress Santos-Sanz et al. 2016 Small Satellites – Imaging and Spectroscopy The Irregular Satellites of the giant planets are clustered in inclination and separation space. Compositional analysis enabled by JWST may directly connect the irregular satellites of the giant planets and TNOs. Nicholson et al. 2008 Read out in 3.3s Narrow-band filters Medium-band filters Wide-band filters Keszthelyi et al. 2016 Read out in 3.3s Satellite in Encke Gap (Main Rings) Narrow-band filters Medium-band filters Wide-band filters Keszthelyi et al. 2016 Read out in 0.1s Medium-band filters Wide-band filters Co-orbital, just outside Saturn’s Main Rings Keszthelyi et al. 2016 (No sub-array) Satellite in Encke Gap (Main Rings) Narrow-band filters Outer Irregular Saturnian Satellites Medium-band filters Wide-band filters Keszthelyi et al. 2016 Small Satellites: Nix and Hydra New Horizons LEISA spectra revealed a strong absorption at 2.21 µm which has been attributed to ammonia hydrate. The band is also seen on Haumea’s largest satellite, Hi’iaka (r ~ 150 km) (Barkume et al. 2006) as well as other icy satellites. Ammonia Nix and Hydra (and Kerberos and Hydrate Styx) are too faint to be observed with the MIRI imager in the mid- infrared. Nix and Hydra are accessible to NIRCam filters for photometry. Cook et al. 2017 Summary JWST observations of small inner satellites may be challenged by scattered light. Like HST, JWST has the potential to discover new rings and moons of the giant planets JWST will enable unprecedented spectroscopy of small satellites and rings – ices and possible organics With early baseline observations, time domain observations will track features, characterize orbit dynamics, etc. ERS Science on Jupiter System: will characterize Jovian ring structure, sources, sinks, and evolution (M. Showalter, Co-I) - also will characterize scattered light from Jupiter – coronography? .