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42nd Lunar and Planetary Science Conference (2011) 1201.pdf

The Effect of and Hydra on the Putative - Dust Cloud An- drew R. Poppe1,2 and Mih´aly Hor´anyi1,2, 1Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, 80309, 2Dept. of Physics, University of Colorado, Boulder, CO, 80309 ([email protected]; [email protected]) the Pluto-Charon dust cloud than do Pluto-ejected grains [9]. The optical depth for the cloud was predicted to be τ ≈ 10−11, far too low to be observed from the ground. Following the discovery of two additional satellites in the Pluto system, Nix and Hydra [11, 12], observationsof the Pluto system with the were analyzed and did not detect any dust rings or clouds [10]. Correspondingly, an upper limit was placed on the opti- cal density of any such cloud at τ ≈ 5 × 10−5. An ad- ditional theoretical estimate for the optical depth of any rings or dust clouds, including the presence of Nix and Hydra, was derived at, τ ≈ 10−6, with the assumption that the grain lifetimes from Nix and Hydra were on the order of 105 yr [13]. In order to investigate the role that Nix and Hydra play in the generation of a dust cloud in the Pluto-Charon Figure 1: The predicted net optical depth for the Pluto- system, we have extended the earlier simulation model Charon system. to include these small as both potential sinks and sources of dust grains [9]. Dynamical Simulation Introduction The equation of motion for a dust grain of , md, is Airless bodies throughout the are exposed given by:

to continual bombardment by interplanetary micromete- 2 orites. These micron and sub-micron sized grains pro- Γ⊙ πa mdr¨ = Fp + Fc + Fn + Fh + 2 , (1) duce large amounts of impact-generated ejecta, forming dP c debris clouds or rings. Examples of this process are ex- r F pected to exist at or have been found at ’s [1], where ¨ is the grain’s net acceleration and i is the grav- i p i c the Martian moons [2], the minor and Galilean satellites itational force from Pluto ( = ), Charon ( = ), Nix i n i h of [3, 4, 5, 6], and the Saturnian satellites [7, 8]. ( = ) and Hydra ( = ), respectively. Additionally, These rings or tori are often extremely sparse, with op- solar radiation pressure is included, where Γ⊙ is the solar −5 c d tical densities, τ < 10 , making ground-based optical f ux at 1 AU, is the speed of light, P is the heliocen- a detection diff cult. tric distance of Pluto (in AU), and is the grain radius. Given the ubiquity of impact-generated dust clouds Radiation pressure shortens grain lifetimes by increasing and rings throughout the solar system, it is natural to ex- the eccentricity of the grains and raising the likelihood pect that a similar effect is present in the Pluto-Charon of a close encounter and subsequent ejection from the Pluto-Charon system [14]. system [9]. Previous work has investigated the possibil- × −16 ity of an impact-generated dust cloud around Pluto and For each body, the trajectories of 2500, 5 10 . µ Charon via both simulation and observation [9, 10]. By kg (0 5 m, assuming a density of ice) grains were in- assuming typical values for the interplanetary microme- tegrated until they either impacted Pluto or one of its ≈ R teorite f ux, the impact ejecta yield, mass and speed dis- moons, or reached the Pluto Hill radius ( 6000 p). . µ tributions, the Pluto-Charon system can be modeled via Only 0 5 m grains were simulated as they represent the a particle-tracing code. Previous simulations, completed most eff cient scatterers at visible wavelengths. Dust before the discovery of Pluto’s additional small satellites, grains were launched from random positions on Pluto Nix and Hydra, found that between Pluto and Charon, and its moons, with a random launch angle inside a cone θ < ◦ dust grains launched from Charon have much longer life- of 45 , from the surface normal. The launch ve- times and therefore, represent a greater contribution to locity of each grain was randomly assigned between 0.7 and 2.0 times the relevant escape velocity, representing 42nd Lunar and Planetary Science Conference (2011) 1201.pdf

the portion of the ejecta velocity distribution that con- craft, which will cross through the Pluto-Charon system tributes most to the orbiting density of grains (ie., slower in mid-2015, will not be able to image the dust environ- grains remain gravitationally bound to the body while ment. Any detection by to the contrary faster grains rapidly leave the Pluto system). could suggest active geologic processes from Pluto or its moons, such as cryo-volcanism [15]. Predictions for New Horizons The optical depth of the Pluto-Charon dust cloud can References be estimated using the trajectories from the simulation, [1] H. A. Zook and J. E. McCoy. Large scale lunar which are reported regularly. The density for grains from horizon glow and a high altitude lunar dust each Moon, ni(x, y, z), are f rst calculated and the geo- exosphere. Geophys. Res. Lett., 18(11): metric optical depth for an individual body, τi, is then 2117–2120,Nov.1991. [2]W.-H.Ipand given by: M. Banaszkiewicz. On the dust/gas tori of

ℓ+ and . Geophys. Res. Lett., 17(6):857–860, 2 1990. [3]J. A. Burns,et al. TheFormationof τi = πa ℓ X ni(x, yj , z), (2) Science j=ℓ− Jupiter’s Faint Rings. , 284:1146–1150, 1999. [4]H.Kr¨uger,etal. Detectionofan where a is the grain radius, ℓ+ and ℓ− are the simula- impact-generated dust cloud around Ganymede. tion bounds in y, and ℓ ≡ ℓ+ − ℓ−. Figure 1 shows the Nature, 399,June 1999. [5] H. Kr¨uger,et al. A model prediction of the net optical depth after appropri- dust cloud of ganymede maintained by ately normalizing and combining all four bodies. Also hypervelocity impacts of interplanetary marked are the orbits of all four bodies (dotted lines) as micrometeoroids. . Space Sci., 48(5): well as the location at which New Horizons will cross the 1457–1471,2000. [6]A.V.Krivov,etal. A Pluto-Charon orbital plane (X). The peak optical depth is tenuous dust ring of Jupiter formed by escaping −11 predicted to be, τ ≈ 4 × 10 , much lower than both ejecta from the Galilean satellites. J. Geophys. the limit set with previous Hubble Space Telescope ob- Res.,107(E1),2002. [7]M.M. Hedman,et al. servations [10] as well as an earlier theoretical estimate Three tenuous rings/arcs for three tiny moons. −6 of the optical depth of τ ≈ 5 × 10 [13]. It should be Icarus, 199:378–386, 2009. [8] A. J. Verbischer, et noted that this calculation is dependent upon a number al. ’s largest ring. Nature, 461, 2009. of parameters that are uncertain and in sum, we suggest [9] K.-U. Theissenhusen, et al. A dust cloud that the optical depth estimate quoted above is accurate around Pluto and Charon. Planet. Space Sci., 50: to an order-of-magnitude. 79–87,2002. [10] A. J. Steff and S. A. Stern. First Conclusion constraints on rings in the pluto system. Astro. Jour., 133:1485–1489,2007. [11] H. A. Weaver, et We have addressed the Pluto-Charon dust environment al. Discovery of two new satellites of Pluto. with the inclusion of two newly-discovered moons, Nix Nature, 439:943–945,2006. [12] A. J. Steff , et al. and Hydra [11]. To model the full dynamics and life- New constraints on additional satellites of the times of the ejecta from all four moons, a particle-tracing Pluto system. Astro. Jour., 123:614–619, 2006. code was developed and applied to all four bodies. The [13] S. A. Stern, et al. A giant impact origin for results showed that Nix and Hydra-generated grains have Pluto’s small moons and satellite multiplicity in average lifetimes between one to ten years, with maxi- 3 the . Nature,439,2006. [14]J.A. mum lifetimes up to 10 years. The optical depth from Burns, et al. Dusty Rings and Circumplanetary τ ≈ × −11 all four bodies combined, estimated at 4 10 , is Dust. In E. Gr¨un, B. A. S. Gustafson, S. Dermott, signif cantly lower than an estimate obtained via a differ- and H. Fechtig, editors, Interplanetary Dust. τ ≈ × −6 ent method of 5 10 [13]. The discrepancy be- Springer,2001. [15] C. C. Porco et al. Cassini tween these two models is due to the difference in grain Observes the Active South Pole of Enceladus. lifetimes for Nix and Hydra, which our model predicts to Science, 311:1393–1401, March 2006. be approximately four orders of magnitude shorter. With our predicted optical depths, the New Horizons space-