Listing of Talk Abstracts

Observing Nix and Hydra at mm/submm Wavelengths with ALMA

Bryan Butler (NRAO)

The next generation millimeter to submillimeter wavelength synthesis telescope, the Atacama Large Millimeter Array (ALMA), will be an excellent instrument for solar and extrasolar system studies. With outstanding antennas, receivers, and site (at 5000m altitude in the northern Chilean Andes), the sensitivity of the instrument will be fantastic. Given expected physical properties of Nix and Hydra, ALMA should be able to detect them in a few to a few 10's of hours, depending on diameter, and discrimination between diameters of 50 and 100 km should be straightforward. Observing with ALMA should begin in 2011, and the Pluto system will be a prime candidate for early science observations.

Albedo, Color, and Spectral Prospects

Will Grundy (Lowell Observatory)

This talk will review what is known about the spectral properties of Nix and Hydra from Hubble Space Telescope observations in the visible and near infrared. Relevant observations were obtained through programs 10427, 10774, 10786, and 11556 (the latter is still ongoing). It will also look ahead to scientific questions which can be addressed by New Horizons spectral studies of Nix and Hydra.

The Pluto System in the Context of Kuiper Belt Formation & Evolution

Alessandro Morbidelli (Observatoire Cote d'Azur)

I will discuss our preferred scenario for the dynamical evolution of the trans-Neptunian population: this population was part of a massive planetesimal disk that was dynamically excited, depleted and transported during a phase of orbital instability of the giant planets that occurred at the time of the so-called Late Heavy Bombardment, 3.9 Gy ago. This scenario explains many properties of the solar system and is also relatively well consistent with the observed orbital and size distributions in the current Kuiper belt. Other scenarios, though, could be envisioned: the dynamical depletion of the trans-Neptunian disk might have occurred early, soon after giant planets formation; or the Kuiper belt might have lost its original mass by collisional grinding instead of dynamical processes. Each of these scenarios would imply a very different collisional history for the Pluto-system. The observation of the crater density and size distribution at the surfaces of Pluto and its satellites can therefore provide a unique and definitive discriminant among the proposed models.

KBO Multiples (Observations)

Dr.  Keith Noll (STScI)

Among the seeming oddities of Pluto was the discovery of Pluto's large companion, Charon. The Pluto-Charon binary remained unique in the solar system in terms of the relative sizes of the binary components until the discovery of many similar-sized systems in the Kuiper Belt and the Main Asteroid Belt. The 70 binary and multiple systems now known in the Kuiper Belt provide a new context in which to understand the formation of the Pluto system. Relative sizes, separations, angular momenta, albedo, densities and colors provide clues for understanding the origin and evolution of multiples and the Kuiper Belt as a whole. I will review the current state of knowledge of this important group of objects in the Kuiper Belt with a particular emphasis on what it tells us about the still-unique Pluto system.

Nix and Hydra Origin by Resonant transport???

Stan Peale (UCSB)

W. H. Cheng, and M. H. Lee (Univ. of Hong Kong) If Charon is created by a giant impact with Pluto along with debris left in orbit, the limited extent of the debris disk requires an additional process to account for the current distant orbits of the minor satellites, Nix and Hydra. The proximity of Nix to the 4:1 and Hydra to the 6:1 mean motion commensurabilities with Charon led Ward and Canup (2006) to propose that the two satellites were trapped in corotation resonances when Charon's semimajor axis aC was near 4RP and were thereby pushed to their current positions as Charon's orbit was expanded by tidal dissipation. Trapping in any other resonances at the commensurabilities leads to eccentricity growth and instability. The resonances were destroyed as Charon's eccentricity decreased upon approach to the current Pluto-Charon dual synchronous rotation state leaving the minor satellites near the above mean motion commensurabilities. However, Lithwick and Wu (2008) find that either Nix or Hydra could be so transported but not both simultaneously. Lithwick and Wu (private communication, 2010) have successfully evolved both Nix and Hydra in a (fragile) three body mean motion resonance that, however, requires careful control of Charon's eccentricity with Nix in the 4:1 corotation resonance. We have developed tidal models of the Pluto-Charon system from initial conditions with Charon at 4RP that exhibit every nuance of the tidal evolution for an eccentric orbit. With at least one tidal model, examples of the capture of Hydra in all six mean motion resonances at the 6:1 commensurability are found. While trapped in multi-resonances, Charon's orbit could be expanded without the eccentricity of Hydra growing significantly as long as Charon's eccentricity was maintained nearly constant by an appropriate ratio of tidal dissipation in Charon to that in Pluto. This is a new phenomenon that we think has never been seen before. Unfortunately, the drop in Charon's eccentricity at the approach to the current dual synchronous rotation causes Hydra's eccentricity to rise to the point where instability ensues immediately after the last resonance stops librating. We have been unsuccessful in trapping Nix into multi-resonances at the 4:1 commensurability, and we will show that conditions for trapping either Nix or Hydra into multi-resonances are not likely to prevail. We have explored parameter space with Charon's orbit expanding, starting with Charon at 4RP, and found evolution to instability in all cases. Probably the only convincing support of the resonant transport hypothesis will be robust numerical demonstration of the complete evolution to the current state, which so far has been elusive.

Multiple Systems in the Kuiper Belt

Dr.  Hilke Schlichting (CITA)

A wide variety of multiple systems have been discovered in the Kuiper Belt to date. Among these multiple systems we can identify, broadly speaking, two classes of binaries. The first class consists of collisionally formed satellites around the largest Kuiper Belt objects, like the Pluto-Charon system. The second class consists of roughly equal-mass binaries with wide separations, which most likely formed by a dynamical process like three body scattering or dynamical friction. We show that the distribution of comparable-mass binaries occupying low-inclination, resonant orbits can constrain models of Neptune’s migration history. In addition, I will discuss the formation of the first collisional family in the Kuiper Belt and the implications of our formation scenario for the occurrence of additional collisional families in the Kuiper Belt.

Potential for Rings at Pluto

Dr.  Mark Showalter (SETI Institute)

Many of the small moons orbiting the outer planets are associated with faint rings of dust. Such rings are understood to arise as ejecta from meteoroid impacts into the moons' surfaces. Once freed from the moon, the dust grains spread out to encircle the planet. The ring particles subsequently evolve due to a variety of on-gravitational forces (solar radiation pressure and electromagnetism) and can also be destroyed or swept up by subsequent impacts. Such rings persist in a steady state, where the rates of production and removal roughly balance. The discoveries of Hydra and Nix give new credence to the possibility of faint rings encircling Pluto. The dynamical details of hypothetical Plutonian rings have not yet been widely studied in detail. However, this problem becomes more timely with the impending New Horizons flyby in 2015. Understanding the most likely ring configurations will substantially improve our likelihood of detecting them from Earth, and will enable New Horizons to focus its time on the set of observations likely to be most informative. In this talk we will discuss the techniques that one can use to make such predictions, and the unknowns inherent in such investigations. Reasoning by analogy to the known rings, we will identify the common traits of such rings and consider how they might scale to Pluto's orbital environment. We will also introduce some new dynamical models for how such rings might arise and evolve.

Nix and Hydra: What lies in wait?

Dr.  Peter Thomas (Cornell University)

The second and third moons of Pluto are likely less than 100 km across. Previous studies of other solar system objects below 100 km diameters have shown a variety of surface forms, environmental effects on colors and regoliths, and abundant evidence they meet expectations in that they are too small to have suffered viscous relaxation. Although such objects may lack some of the complications of internal activity, other satellites show large ranges in evidence of possible fracturing, styles of regolith transport, depth of ejecta coverage, and mean densities that the properties of Nix and Hydra will be valuable in generalizing processes on small objects. This talk will review the range of forms on other small objects and discuss puzzles as to what are the controls of specific forms and how the moderate resolution views of these two satellites can be related to other bodies.

A Retrospective on the Discovery of Nix and Hydra

Hal Weaver (JHU Applied Physics Laboratory)

The Pluto system has never easily surrendered its secrets. After Pluto’s discovery in 1930 by Clyde Tombaugh, 48 years passed before James Christy and Robert Harrington announced the discovery of Charon in 1978, which enabled the first accurate estimate for the mass of Pluto. Subsequent searches during the next 17 years for other objects in the Pluto system were unsuccessful, including deep searches using large ground-based telescopes. After the selection by NASA in November 2001 of the New Horizons (NH) mission to perform the first in situ reconnaissance of the Pluto system, and that program’s successful Preliminary Design Review in the fall of 2002, a contingent of NH Science Team members embarked on a plan to take inventory of the Pluto system using the Hubble Space Telescope (HST). A request for two orbits of HST Director’s Discretionary Time to search for Pluto companions was rejected in April 2003, as was a follow-up General Observer request in cycle 13 one year later. But the unfortunate failure of a power supply on the Space Telescope Imaging Spectrograph (STIS) in August 2004 created a hole in HST’s planned observing program, and our Pluto companion search program was selected in September 2004 to help fill the schedule gap. The observations were successfully performed on 2005 May 15 and 18, and the analysis of those data led to the announcement on 2005 October 31 (Weaver & Stern, et al.; IAUC 8625) of the discovery of two new satellites of Pluto. Here we describe the long and sometimes torturous journey that eventually led to the discovery of Nix and Hydra.