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Register a new user2011 HM102: Discovery of a High-Inclination L5 Neptune Trojan in the Search for a post-Pluto New Horizons Target
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We present the discovery of a long-term stable L5 (trailing) Neptune Trojan in data acquired to search for candidate Trans-Neptunian objects for the New Horizons spacecraft to fly by during an extended post-Pluto mission. This Neptune Trojan, 2011 HM102, has the highest inclination (29.4 degrees) of any known member of this population. It is intrinsically brighter than any single L5 Jupiter Trojan at H~8.18. We have determined its gri colors (a first for any L5 Neptune Trojan), which we find to be similar to the moderately red colors of the L4 Neptune Trojans, suggesting similar surface properties for members of both Trojan clouds. We also present colors derived from archival data for two L4 Neptune Trojans (2006 RJ103 and 2007 VL305), better refining the overall color distribution of the population. In this document we describe the discovery circumstances, our physical characterization of 2011 HM102, and this objects implications for the Neptune Trojan population overall. Finally, we discuss the prospects for detecting 2011 HM102 from the New Horizons spacecraft during their close approach in mid- to late-2013.
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Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Plutos atmosphere. While the lower atmosphere (at altitudes <200 km) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N$_2$) dominates the atmosphere (at altitudes <1800 km or so), while methane (CH$_4$), acetylene (C$_2$H$_2$), ethylene (C$_2$H$_4$), and ethane (C$_2$H$_6$) are abundant minor species, and likely feed the production of an extensive haze which encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Plutos atmosphere to space. It is unclear whether the current state of Plutos atmosphere is representative of its average state--over seasonal or geologic time scales.
The New Horizons mission has provided resolved measurements of Plutos moons Styx, Nix, Kerberos, and Hydra. All four are small, with equivalent spherical diameters of $approx$40 km for Nix and Hydra and ~10 km for Styx and Kerberos. They are also highly elongated, with maximum to minimum axis ratios of $approx$2. All four moons have high albedos ( $approx$50-90 %) suggestive of a water-ice surface composition. Crater densities on Nix and Hydra imply surface ages $gtrsim$ 4 Ga. The small moons rotate much faster than synchronous, with rotational poles clustered nearly orthogonal to the common pole directions of Pluto and Charon. These results reinforce the hypothesis that the small moons formed in the aftermath of a collision that produced the Pluto-Charon binary.
We searched for dust or debris rings in the Pluto-Charon system before, during, and after the New Horizons encounter. Methodologies included searching for back-scattered light during the approach to Pluto (phase $sim15^circ$), in situ detection of impacting particles, a search for stellar occultations near the time of closest approach, and by forward-scattered light during departure (phase $sim165^circ$). A search using HST prior to the encounter also contributed to the results. No rings, debris, or dust features were observed, but our detection limits provide an improved picture of the environment throughout the Pluto-Charon system. Searches for rings in back-scattered light covered 35,000-250,000 km from the system barycenter, a zone that starts interior to the orbit of Styx, and extends to four times the orbital radius of Hydra. We obtained our firmest limits using the NH LORRI camera in the inner half of this region. Our limits on the normal $I/F$ of an unseen ring depends on the radial scale of the rings: $2times10^{-8}$ ($3sigma$) for 1500 km wide rings, $1times10^{-8}$ for 6000 km rings, and $7times10^{-9}$ for 12,000 km rings. Beyond $sim100,000$ km from Pluto, HST observations limit normal $I/F$ to $sim8times10^{-8}$. Searches for dust from forward-scattered light extended from the surface of Pluto to the Pluto-Charon Hill sphere ($r_{rm Hill}=6.4times10^6$ km). No evidence for rings or dust was detected to normal $I/F$ limits of $sim8.9times10^{-7}$ on $sim10^4$ km scales. Four occulation observations also probed the space interior to Hydra, but again no dust or debris was detected. Elsewhere in the solar system, small moons commonly share their orbits with faint dust rings. Our results suggest that small grains are quickly lost from the system due to solar radiation pressure, whereas larger particles are unstable due to perturbations by the known moons.
NASAs New Horizons spacecraft has revealed the complex geology of Pluto and Charon. Plutos encounter hemisphere shows ongoing surface geological activity centered on a vast basin containing a thick layer of volatile ices that appears to be involved in convection and advection, with a crater retention age no greater than $approx$10 Ma. Surrounding terrains show active glacial flow, apparent transport and rotation of large buoyant water-ice crustal blocks, and pitting, likely by sublimation erosion and/or collapse. More enigmatic features include tall mounds with central depressions that are conceivably cryovolcanic, and ridges with complex bladed textures. Pluto also has ancient cratered terrains up to ~4 Ga old that are extensionally fractured and extensively mantled and perhaps eroded by glacial or other processes. Charon does not appear to be currently active, but experienced major extensional tectonism and resurfacing (probably cryovolcanic) nearly 4 billion years ago. Impact crater populations on Pluto and Charon are not consistent with the steepest proposed impactor size-frequency distributions proposed for the Kuiper belt.
The Pluto system was recently explored by NASAs New Horizons spacecraft, making closest approach on 14 July 2015. Plutos surface displays diverse landforms, terrain ages, albedos, colors, and composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Plutos atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Plutos diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Plutos large moon Charon displays tectonics and evidence for a heterogeneous crustal composition, its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected.
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