The Pluto System After New Horizons Stern, S. Alan; Grundy, William M; McKinnon, William B ...
Annual review of astronomy and astrophysics,
09/2018, Letnik:
56, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The
New Horizons
(NH) flyby of the Pluto-Charon binary planet and its system of four small surrounding satellites in mid-2015 revolutionized our knowledge of this distant planet and its moons. Beyond ...providing rich geo-logical, compositional, and atmospheric data sets, NH demonstrated that Pluto has been surprisingly geologically and climatologically active throughout the past 4+ Gyr and that the planet exhibits a remarkably complex range of atmospheric phenomenology and geological expressions that rival Mars in their richness. In contrast, Pluto's large, planet-sized satellite Charon, though also geologically complex, has no detected active surface volatiles, has no detectable atmosphere, has much more muted colors, has lower albedo, and exhibits only ancient terrains. Pluto's system of four small satellites orbiting outside of Charon is itself dynamically complex and geologically interesting. Here, we review both what was known about the Pluto system before NH and what it has taught us about the Pluto system specifically and, by inference, other small planets in the Kuiper Belt. We go on to examine the natural next steps in Kuiper Belt exploration.
Abstract
Pluto is covered by numerous deposits of methane, either diluted in nitrogen or as methane-rich ice. Within the dark equatorial region of Cthulhu, bright frost containing methane is observed ...coating crater rims and walls as well as mountain tops, providing spectacular resemblance to terrestrial snow-capped mountain chains. However, the origin of these deposits remained enigmatic. Here we report that they are composed of methane-rich ice. We use high-resolution numerical simulations of Pluto’s climate to show that the processes forming them are likely to be completely different to those forming high-altitude snowpack on Earth. The methane deposits may not result from adiabatic cooling in upwardly moving air like on our planet, but from a circulation-induced enrichment of gaseous methane a few kilometres above Pluto’s plains that favours methane condensation at mountain summits. This process could have shaped other methane reservoirs on Pluto and help explain the appearance of the bladed terrain of Tartarus Dorsa.
A recent examination of K2 lightcurves indicates that ∼15% of Jupiter Trojans have very slow rotation (spin periods Ps > 100 hr). Here we consider the possibility that these bodies formed as ...equal-size binaries in the massive outer disk at ∼20-30 au. Prior to their implantation as Jupiter Trojans, tight binaries tidally evolved toward a synchronous state with Ps ∼ Pb, where Pb is the binary orbit period. They may have been subsequently dissociated by impacts and planetary encounters with at least one binary component retaining its slow rotation. Surviving binaries on Trojan orbits would continue to evolve by tides and spin-changing impacts over 4.5 Gyr. To explain the observed fraction of slow rotators, we find that at least ∼15%-20% of outer disk bodies with diameters 15 < D < 50 km would have to form as equal-size binaries with 12 ab/R 30, where ab is the binary semimajor axis and R = D/2. The mechanism proposed here could also explain very slow rotators found in other small-body populations.
The topography of Neptune’s large icy moon Triton could reveal important clues to its internal evolution, but has been difficult to determine. New global digital color maps for Triton have been ...produced as well as topographic data for <40% of the surface using stereogrammetry and photoclinometry. Triton is most likely a captured Kuiper Belt dwarf planet, similar though slightly larger in size and density to Pluto, and a likely ocean moon that exhibited plume activity during Voyager 2′s visit in 1989. No surface features or regional deviations of greater than ±1 km amplitude are found. Volatile ices in the southern terrains may take the form of extended lobate deposits 300–500 km across as well as dispersed bright materials that appear to embay local topography. Limb hazes may correlate with these deposits, indicating possible surface–atmosphere exchange. Triton’s topography contrasts with high relief up to 6 km observed by New Horizons on Pluto. Low relief of (cryo)volcanic features on Triton contrasts with high-standing massifs on Pluto, implying different viscosity materials. Solid-state convection occurs on both and at similar horizontal scales but in very different materials. Triton’s low relief is consistent with evolution of an ice shell subjected to high heat flow levels and may strengthen the case of an internal ocean on this active body.
The New Horizons spacecraft returned images and compositional data showing that terrains on Pluto span a variety of ages, ranging from relatively ancient, heavily cratered areas to very young ...surfaces with few-to-no impact craters. One of the regions with very few impact craters is dominated by enormous rises with hummocky flanks. Similar features do not exist anywhere else in the imaged solar system. Here we analyze the geomorphology and composition of the features and conclude this region was resurfaced by cryovolcanic processes, of a type and scale so far unique to Pluto. Creation of this terrain requires multiple eruption sites and a large volume of material (>10
km
) to form what we propose are multiple, several-km-high domes, some of which merge to form more complex planforms. The existence of these massive features suggests Pluto's interior structure and evolution allows for either enhanced retention of heat or more heat overall than was anticipated before New Horizons, which permitted mobilization of water-ice-rich materials late in Pluto's history.
The Virgil Fossae region on Pluto exhibits three spatially coincident properties that are suggestive of recent cryovolcanic activity over an area approximately 300 by 200 km. Situated in the fossae ...troughs or channels and in the surrounding terrain are exposures of H2O ice in which there is entrained opaque red-colored matter of unknown composition. The H2O ice is also seen to carry spectral signatures at 1.65 and 2.2 μm of NH3 in some form, possibly as a hydrate, an ammoniated salt, or some other compound. Model calculations of NH3 destruction in H2O ice by galactic cosmic rays suggest that the maximum lifetime of NH3 in the uppermost meter of the exposed surface is ~109 years, while considerations of Lyman-α ultraviolet and solar wind charged particles suggest shorter timescales by a factor of 10 or 10000. Thus, 109 y is taken as an upper limit to the age of the emplacement event, and it could be substantially younger.
The red colorant in the ammoniated H2O in Virgil Fossae and surroundings may be a macromolecular organic material (tholin) thought to give color to much of Pluto's surface, but probably different in composition and age. Owing to the limited spectral range of the New Horizons imaging spectrometer and the signal precision of the data, apart from the H2O and NH3 signatures there are no direct spectroscopic clues to the chemistry of the strongly colored deposit on Pluto. We suggest that the colored material was a component of the fluid reservoir from which the material now on the surface in this region was erupted. Although other compositions are possible, if it is indeed a complex organic material it may incorporate organics inherited from the solar nebula, further processed in a warm aqueous environment inside Pluto.
A planet-scale stress pattern in Pluto's lithosphere induced by true polar wander, freezing of a putative interior ocean, and surface loading has caused fracturing in a broad arc west of Sputnik Planitia, consistent with the structure of Virgil Fossae and similar extensional features. This faulting may have facilitated the ascent of fluid in subsurface reservoirs to reach the surface as flows and as fountains of cryoclastic materials, consistent with the appearance of colored, ammoniated H2O ice deposits in and around Virgil Fossae. Models of a cryoflow emerging from sources in Virgil Fossae indicate that the lateral extent of the flow can be several km (Umurhan et al., 2019). The deposit over the full length (>200 km) of the main trough in the Virgil Fossae complex and extending through the north rim of Elliot crater and varying in elevation over a range of ~2.5 km, suggests that it debouched from multiple sources, probably along the length of the strike direction of the normal faults defining the graben. The source or sources of the ammoniated H2O are one or more subsurface reservoirs that may or may not connect to the global ocean postulated for Pluto's interior. Alternatives to cryovolcanism in producing the observed characteristics of the region around Virgil Fossae are explored in the discussion section of the paper.
•A tectonic structure (Virgil Fossae) on Pluto may be a source of a cryolava that has been erupted onto the planet's surface.•The cryolava consists of H2O and some form of ammonia, as well as a (NH3) signature, and a colored component thought to be complex organic matter.•Ammonia in its various forms is susceptible to destruction and its presence suggests emplacement on Pluto's surface sometime in the past billion years.•In addition to the debouchment of cryolava along fault lines in Virgil Fossae, fountaining from one or more associated sites appears to have distributed a mantling layer covering a few thousand square kilometers.•Large-scale fractures in Pluto's crust appear to have facilitated the emergence of a cryolava from one or more reservoirs in the subsurface.
Once perceived as distant, cold, dark, and seemingly unknowable,
Pluto had long been marked as the farthest and most unreachable
frontier for solar system exploration. After Voyager accomplished
its ...final planetary reconnaissance at Neptune in 1989, Pluto and
its cohort in the Kuiper Belt beckoned as the missing puzzle piece
for completing the first reconnaissance of our solar system. In the
decades following Voyager, a mission to the Pluto system was not
only imagined but also achieved, culminating with the historic 2015
flyby by the New Horizons spacecraft. Pluto and its
satellite system ("the Pluto system"), including its largest moon,
Charon, have been revealed to be worlds of enormous complexity that
fantastically exceed preconceptions. The Pluto System After New
Horizons seeks to become the benchmark for synthesizing our
understanding of the Pluto system. The volume's lead editor is S.
Alan Stern, who also serves as NASA's New Horizons Principal
Investigator; co-editors Richard P. Binzel, William M. Grundy,
Jeffrey M. Moore, and Leslie A. Young are all co-investigators on
New Horizons . Leading researchers from around the globe
have spent the last five years assimilating Pluto system flyby data
returned from New Horizons. The chapters in this volume form an
enduring foundation for ongoing study and understanding of the
Pluto system. The volume also advances insights into the nature of
dwarf planets and Kuiper Belt objects, providing a cornerstone for
planning new missions that may return to the Pluto system and
explore others of the myriad important worlds beyond Neptune.
► KCTF can significantly transform the orbits of trans-neptunian binaries. ► A large fraction of random TNB systems decay to close, circular orbits. ► Shape effects can halt KCTF, explaining some ...observed systems.
Recent observational surveys of trans-neptunian binary (TNB) systems have dramatically increased the number of known mutual orbits. Our Kozai Cycle Tidal Friction (KCTF) simulations of synthetic trans-neptunian binaries show that tidal dissipation in these systems can completely reshape their original orbits. Specifically, solar torques should have dramatically accelerated the semimajor axis decay and circularization timescales of primordial (or recently excited) TNBs. As a result, our initially random distribution of TNBs in our simulations evolved to have a large population of tight circular orbits. This tight circular population appears for a range of TNO physical properties, though a strong gravitational quadrupole can prevent some from fully circularizing. We introduce a stability parameter to predict the effectiveness of KCTF on a TNB orbit, and show that a number of known TNBs must have a large gravitational quadrupole to be stable.
We investigated whether ammonia-rich constituents are present on the surface of the Uranian moon Ariel by analyzing 32 near-infrared reflectance spectra collected over a wide range of sub-observer ...longitudes and latitudes. We measured the band areas and depths of a 2.2 m feature in these spectra, which has been attributed to ammonia-bearing species on other icy bodies. Ten spectra display prominent 2.2 m features with band areas and depths >2 . We determined the longitudinal distribution of the 2.2 m band, finding no statistically meaningful differences between Ariel's leading and trailing hemispheres, indicating that this band is distributed across Ariel's surface. We compared the band centers and shapes of the five Ariel spectra displaying the strongest 2.2 m bands to laboratory spectra of various ammonia-bearing and ammonium-bearing species, finding that the spectral signatures of the Ariel spectra are best matched by ammonia-hydrates and flash frozen ammonia-water solutions. Our analysis also revealed that four Ariel spectra display 2.24 m bands (>2 band areas and depths), with band centers and shapes that are best matched by ammonia ice. Because ammonia should be efficiently removed over short timescales by ultraviolet photons, cosmic rays, and charged particles trapped in Uranus' magnetosphere, the possible presence of this constituent supports geologic activity in the recent past, such as emplacement of ammonia-rich cryolavas and exposure of ammonia-rich deposits by tectonism, impact events, and mass wasting.
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