THE SMALL SATELLITES of PLUTO. S. B. Porter1, M. R. Showalter2, H

47th Lunar and Planetary Science Conference (2016) 2390.pdf

THE SMALL SATELLITES OF PLUTO. S. B. Porter1, M. R. Showalter2, H. A. Weaver3, J. R. Spencer1, R. P. Binzel4, D. P. Hamilton5, T. R. Lauer6, T. Stryk7, M. W. Buie1, B. Buratti8, A. J. Verbiscer9, A. H. Parker1, K. Sing- er1, W. McKinnon10, S. Robbins1, J. Moore11, W. Grundy12, S. A. Stern1, L. A. Young1, C. B. Olkin1, K. Ennico11 and the New Horizons Geology and Geophysics Imaging Team 1Southwest Research Institute (por- [email protected]), 2SETI Institute, 3Johns Hopkins APL, 4MIT, 5U. Maryland, 6NOAO, 7Roane State Commu- nity College, 8JPL, 9U. Virginia, 10Washington U., 11NASA Ames, 12Lowell Observatory

Introduction: The Pluto dwarf planetary system respective orbital poles [10]. The poles of Styx and consists of six bodies orbiting their common center of Kerberos are inclined 80-90° to their orbits. The pole mass; the large inner binary Pluto (2374±4 km diame- of Nix is inclined 120-130° to its orbit. At this high ter) and Charon (1212±3 km diameter) [1] is surround- inclination, Nix’s rotation should be more perturbed by ed by four much smaller (~10-50 km diameter) satel- Charon than Styx or Kerberos, and this may be ex- lites, Styx, Nix, Kerberos, and Hydra [1]. All four of pressed in the variations seen in Nix’s rotation period these satellites are on near-circular orbits with low [11]. The pole of Hydra is harder to determine, as it inclinations to the orbital plane of the inner binary [2]. was pointing towards the spacecraft at the time the Discovery and Exploration: The brighter two of highest resolution images were taken, however it can Pluto’s small satellites, Nix and Hydra, were discov- be constrained to within 90-120° of its orbital pole. ered in 2005 [3]. Subsequent observations discovered Surfaces and Composition: The encounter face of Kerberos in 2011 [4] and Styx in 2012 [5]. NASA’s Nix appears to show a cratered surface of mostly New Horizons spacecraft was launched to the Pluto uniform composition. There is a large crater on the system in January 2006 and arrived in July 2015 [1]. encounter face, ~15 km diameter [12]. Around this Nix and Hydra were included in the science targeting crater is a region that is “redder” (larger Blue/NIR ra- schedule uploaded to the spacecraft shortly after tio) than the surrounding terrain. This may be evidence launch, but the late discovery of Styx and Kerberos of the crater excavating darker material from below, or only allowed limited imaging of those satellites. New of two compositionally distinct regions. A lower- Horizons flew closest to Nix, and on Nix’s day side, resolution color image of Hydra showed it to be a uni- allowing high quality imaging. The other three small form color similar to the light terrain on Nix. Numer- satellites were on the far side of the system (see Figure ous craters at the 1-2 km scale are apparent on both 1), and were imaged at lower resolution. Extensive Nix and Hydra [12]. Crater retention ages > 4 Gyr are searches for additional satellites by New Horizons on found for both Nix and Hydra. approach to Pluto did not detect any new satellites [1]. Relevance to the Proposed New Horizons Ex- Orbits and Stability: The Pluto system is a tended Mission: In October and November 2015, uniquely complex dynamical hierarchy in the solar New Horizons performed maneuvers to enable a pro- system. The small satellites orbit around the barycenter posed flyby of 2014 MU69 (formerly known as PT1). If of the central Pluto-Charon binary and are strongly NASA approves a New Horizons Extended Mission, perturbed by Charon [2]. Orbits interior to the 2:1 this flyby would occur on January 1, 2019 [13]. 2014 Charon Mean Motion Resonance (MMR) are generally MU69 is a cold classical Kuiper Belt object similar in unstable [6], except for highly inclined orbits between size to the small satellites of Pluto (30-50 km across). Pluto and Charon [8]. The satellites orbit close to (but If approved, New Horizons will perform a day-side not perfectly at) the 3:1, 4:1, 5:1, and 6:1 Charon flyby that will permit spatial resolutions similar to MMRs [2]. This very close packing and the strong those achieved on Pluto. perturbation by Charon severely limits where stable References: [1] Stern S.A. et al. (2015) Science, orbits lie between Styx and Hydra [7]. New Horizons 350, 292–300. [2] Brozović M. et al. (2015) Icarus, performed a deep search for new satellites in the Pluto 246, 317–329. [3] Weaver, H.A., et al. (2006) Nature, system, but none were detected [1]. 439, 943–945. [4] Showalter, M.R. et al. (2011) CBAT, Rotational Rates and Poles: All four small satel- 2769. [5] Showalter, M.R. et al. (2012) IAU Circ., lites exhibited brightness variations in pre-flyby obser- 9253. [6] Stern, S.A. et al. (1994) Icarus, 108, 2. [7] vations by the Hubble Space Telescope. This was in- Youdin, A.N. et al. (2012) ApJ 755, 17. [8] Giuliatti terpreted by [9] to mean that they were rotating chaoti- Winter et al. (2010) MNRAS 404, 1. [9] Showalter, cally at a rate close to their orbital period. However, M.R. and Hamilton, D.P. (2015) Nature, 522, 7554. photometry taken by New Horizons on approach to [10] Porter, S.B. et al. (2015) DPS 47, #102.10. [11] Pluto showed that all four small satellites were rotating Showalter, M.R. et al. (2015) DPS 47, #102.09. [12] much faster than their orbital periods. In addition, all Weaver, H.A. et al. 2015 DPS 47, #102.07. [13] Porter, four rotational poles are significantly inclined to their S.B. et al (2015) LPS XXXXVII, #1310. 47th Lunar and Planetary Science Conference (2016) 2390.pdf

Figure 1: Diagram of the Pluto system at the time of the New Horizons flyby. The spacecraft trajectory crosses the Pluto-Charon plane from above the page and right to below the page and left and inclined at 43° to the plane. The flyby geometry favored observations of Nix and Styx, but Styx was discovered so late (2012) that only a single resolved observation could be scheduled.

Figure 2: The highest-resolution images of the small satellites of Pluto, with the limb of Charon for scale.