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Register a new userThe Geology of Pluto and Charon Through the Eyes of New Horizons
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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.
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The exploration of the Pluto-Charon system by the New Horizons spacecraft represents the first opportunity to understand the distribution of albedo and other photometric properties of the surfaces of objects in the Solar Systems Third Zone of distant ice-rich bodies. Images of the entire illuminated surface of Pluto and Charon obtained by the Long Range Reconnaissance Imager (LORRI) camera provide a global map of Pluto that reveals surface albedo variegations larger than any other Solar System world except for Saturns moon Iapetus. Normal reflectances on Pluto range from 0.08-1.0, and the low-albedo areas of Pluto are darker than any region of Charon. Charon exhibits a much blander surface with normal reflectances ranging from 0.20-0.73. Plutos albedo features are well-correlated with geologic features, although some exogenous low-albedo dust may be responsible for features seen to the west of the area informally named Tombaugh Regio. The albedo patterns of both Pluto and Charon are latitudinally organized, with the exception of Tombaugh Regio, with darker regions concentrated at the Plutos equator and Charons northern pole The phase curve of Pluto is similar to that of Triton, the large moon of Neptune believed to be a captured Kuiper Belt Object (KBO), while Charons is similar to that of the Moon. Preliminary Bond albedos are 0.25+/-0.03 for Charon and 0.72+/-0.07 for Pluto. Maps of an approximation to the Bond albedo for both Pluto and Charon are presented for the first time. Our work shows a connection between very high albedo (near unity) and planetary activity, a result that suggests the KBO Eris may be currently active.
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.
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.
New Horizons mission observations show that the small satellites Styx, Nix, Kerberos and Hydra, of the Pluto-Charon system, have not tidally spun-down to near synchronous spin states and have high obliquities with respect to their orbit about the Pluto-Charon binary (Weaver et al. 2016). We use a damped mass-spring model within an N-body simulation to study spin and obliquity evolution for single spinning non-round bodies in circumbinary orbit. Simulations with tidal dissipation alone do not show strong obliquity variations from tidally induced spin-orbit resonance crossing and this we attribute to the high satellite spin rates and low orbital eccentricities. However, a tidally evolving Styx exhibits intermittent obliquity variations and episodes of tumbling. During a previous epoch where Charon migrated away from Pluto, the minor satellites could have been trapped in orbital mean motion inclination resonances. An outward migrating Charon induces large variations in Nix and Styxs obliquities. The cause is a commensurability between the mean motion resonance frequency and the spin precession rate of the spinning body. As the minor satellites are near mean motion resonances, this mechanism could have lifted the obliquities of all four minor satellites. The high obliquities need not be primordial if the minor satellites were at one time captured into mean motion resonances.
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.
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