Climate zones on Pluto and Charon Binzel, Richard P.; Earle, Alissa M.; Buie, Marc W. ...
Icarus (New York, N.Y. 1962),
05/2017, Letnik:
287
Journal Article
Recenzirano
•Pluto's large axis tilt creates “seasons” that are very different from Earth's.•Most of Pluto's surface is actually “tropical” in experiencing direct sunlight.•Much of Pluto's surface experiences ...both tropical AND arctic seasons.•The unusual seasons experienced by Pluto may shape what we see.
We give an explanatory description of the unusual “climate zones” on Pluto that arise from its high obliquity (mean 115°) and high amplitude (±12°) of obliquity oscillation over a 2.8 million year period. The zones we describe have astronomically defined boundaries and do not incorporate atmospheric circulation. For such a high mean obliquity, the lines of tropics (greatest latitudes where the Sun can be overhead) cycle closer to each pole than does each arctic circle, which in turn cycle nearly to the equator. As a consequence in an astronomical context, Pluto is more predominantly “tropical” than “arctic.” Up to 97% of Pluto's surface area can experience overhead Sun when the obliquity cycle is at its minimum of 103°. At this same obliquity phase (most recently occurring 0.8Myr ago), 78% of Pluto's surface experienced prolonged intervals without sunlight or “arctic winter” (and corresponding “arctic summer”). The intersection of these climate zones implies that a very broad range of Pluto's latitudes (spanning 13–77° in each hemisphere; 75% of the total surface area) are both tropical and arctic. While some possible correlations to these climate zones are suggested by comparison with published maps of Pluto and Charon yielded by the New Horizons mission, in this work we present a non-physical descriptive analysis only. For example, the planet-wide dark equatorial band presented by Stern et al. (2015; Science, 350, 292–299) corresponds to Pluto's permanent “diurnal zone.” In this zone spanning latitudes within ±13° of the equator, day-night cycles occur each Pluto rotation (6.4 days) such that neither “arctic winter” nor “arctic summer” has been experienced in this zone for at least 20 million years. The stability of this and other climate zones may extend over several Gyr. Temperature modeling shows that the continuity of diurnal cycles in this region may be the key factor enabling a long-term stability for the high albedo contrast between Tombaugh Regio adjacent to the dark Cthulhu Regio (Earle et al. (2017) Icarus, special issue, submitted). (All names are informal.) Charon's synchronous alignment with Pluto dictates that both bodies in the binary pair have the same climate zone structure, but any effects on Charon's morphology may be limited if volatile transport there is minimal or absent. Cold-trapped methane-rich volatiles on top of its water ice surface may be responsible for forming Charon's dark red north polar cap (Grundy et al., 2016b), and we note the most concentrated area of this feature resides almost entirely within the permanent “polar zone” (above 77° latitude) where the Sun never reaches the overhead point and arctic seasons have been most consistently experienced over at least tens of millions of years. Pluto is not alone among bodies in the Kuiper belt (and uranian satellites) in having high obliquities, overlapping tropical and arctic zones, and latitude bands that remain in a continuous diurnal cycle over long terms.
•Pluto has undergone thousands of cycles of obliquity change and polar precession.•Such changes could produce dramatic increases in surface pressure.•Such changes may also explain geomorphologic ...features suggesting paleo-liquids.•This paper motivates future climate modeling/geologic interpretation in this area.
Pluto is known to have undergone thousands of cycles of obliquity change and polar precession. These variations have a large and corresponding impact on the total average solar insolation reaching various places on Pluto's surface as a function of time. Such changes could produce dramatic increases in surface pressure and may explain certain features observed by New Horizons on Pluto's surface, including some that indicate the possibility of surface paleo-liquids. This paper is the first to discuss multiple lines of geomorphological evidence consistent with higher pressure epochs in Pluto's geologic past, and it also the first to provide a mechanism for potentially producing the requisite high pressure conditions needed for an environment that could support liquids on Pluto. The presence of such liquids and such conditions, if borne out by future work, would fundamentally affect our view of Pluto's past climate, volatile transport, and geological evolution. This paper motivates future, more detailed climate modeling and geologic interpretation efforts in this area.
•Bladed Terrain are deposits of CH4, which occur at low latitudes and high elevations.•CH4 preferentially precipitates at low latitudes where net solar energy is lowest.•CH4 and N2 will both ...precipitate at low elevations.•At high elevations atmospheric warmth limits precipitation to CH4 only.•Excursions in Pluto's climate have partially eroded these deposits into the blade
Bladed Terrain on Pluto consists of deposits of massive CH4, which are observed to occur within latitudes 30° of the equator and are found almost exclusively at the highest elevations (> 2 km above the mean radius). Our analysis indicates that these deposits of CH4 preferentially precipitate at low latitudes where net annual solar energy input is lowest. CH4 and N2 will both precipitate at low elevations. However, since there is much more N2 in the atmosphere than CH4, the N2 ice will dominate at these low elevations. At high elevations the atmosphere is too warm for N2 to precipitate so only CH4 can do so. We conclude that following the time of massive CH4 emplacement; there have been sufficient excursions in Pluto's climate to partially erode these deposits via sublimation into the blades we see today. Blades composed of massive CH4 ice implies that the mechanical behavior of CH4 can support at least several hundred meters of relief at Pluto surface conditions. Bladed Terrain deposits may be widespread in the low latitudes of the poorly seen sub-Charon hemisphere, based on spectral observations. If these locations are indeed Bladed Terrain deposits, they may mark heretofore unrecognized regions of high elevation.
•The geology of Sputnik Planitia on Pluto is mapped at 1:2M scale.•All mapped units are presently being affected by the action of flowing N2 ice.•Sputnik Planitia is experiencing convection, glacial ...flow, and sublimation.•Condensation of N2 onto much of Sputnik Planitia creates a bright mantle.•Blocky H2O ice mountains and hills have been mobilized by flow of N2 ice.
The geology and stratigraphy of the feature on Pluto informally named Sputnik Planitia is documented through geologic mapping at 1:2,000,000 scale. All units that have been mapped are presently being affected to some degree by the action of flowing N2 ice. The N2 ice plains of Sputnik Planitia display no impact craters, and are undergoing constant resurfacing via convection, glacial flow and sublimation. Condensation of atmospheric N2 onto the surface to form a bright mantle has occurred across broad swathes of Sputnik Planitia, and appears to be partly controlled by Pluto's obliquity cycles. The action of N2 ice has been instrumental in affecting uplands terrain surrounding Sputnik Planitia, and has played a key role in the disruption of Sputnik Planitia's western margin to form chains of blocky mountain ranges, as well in the extensive erosion by glacial flow of the uplands to the east of Sputnik Planitia.
•Present results of simple volatile sublimation and deposition model to explore the relationship between albedo variations, latitudes, and volatile sublimation and deposition.•Consider the current ...epoch as well as past epochs during which Pluto experienced “super seasons” due to variations in its orbit over million-year timescales.•Shows that Plutos geometry creates the potential for runaway albedo and volatile variations, particularly in the equatorial region, which can sustain stark longitudinal contrasts.•Considers how other hypothetical obliquities would effect that latitudinal extent of the “runaway zone”.
The data returned from NASA’s New Horizons reconnaissance of the Pluto system show striking albedo variations from polar to equatorial latitudes as well as sharp longitudinal boundaries. Pluto has a high obliquity (currently 119°) that varies by 23° over a period of less than 3 million years. This variation, combined with its regressing longitude of perihelion (360° over 3.7 million years), creates epochs of “Super Seasons” where one pole is pointed at the Sun at perihelion, thereby experiencing a short, relatively warm summer followed by its longest possible period of winter darkness. In contrast, the other pole experiences a much longer, less intense summer and a short winter season. We use a simple volatile sublimation and deposition model to explore the relationship between albedo variations, latitude, and volatile sublimation and deposition for the current epoch as well as historical epochs during which Pluto experienced these “Super Seasons.” Our investigation quantitatively shows that Pluto’s geometry creates the potential for runaway albedo and volatile variations, particularly in the equatorial region, which can sustain stark longitudinal contrasts like the ones we see between Tombaugh Regio and the informally named Cthulhu Regio.
The July 2015 encounter of the Pluto system by the NASA New Horizons spacecraft has facilitated the study of Pluto’s origin, surface processes, volatile transport cycles, and the energetics and ...chemistry of its atmosphere in an unprecedented level of detail. Earle et al. (2018b) presented the highest spatial resolution composition maps of Pluto using data from the Ralph/MVIC instrument and provided a global interpretation of the maps. Here we build upon that work and leverage MVIC’s high spatial resolution to study the volatile distribution in and around craters to better understand how small scale topography affects volatile transport. We find that the compositional morphology in and around craters in our study can be divided into four different latitudinal bands, where differences are found for distribution trends in nitrogen, methane, and organic signatures in crater floors, walls, and surrounding slopes. We summarize the compositional characteristics of a “typical” crater in each latitude band, provide some possible explanation for the distribution based on current volatile transport models, and highlight some questions to be addressed by ongoing models.
•Use data from New Horizon’s Ralph/MVIC instrument to study volatile distribution.•Leverage high spatial resolution to study how small-scale topography effects volatile transport.•Identify latitudinal trends in volatile distribution in and around craters.
Much of Pluto’s surface and atmosphere can be understood by considering its volatile and climate cycles (e.g., Spencer et al., 1997). Pluto has three volatile ices on its surface and in its ...atmosphere: N2, CH4, and CO. N2 is the main constituent in Pluto’s atmosphere, and is present on large areas of its surface. Unlike Earth, Venus, or Titan, Pluto’s main atmospheric constituent is able to condense on its surface. This leads to important interactions between the atmosphere and surface via relaxation to solid-gas equilibrium. There are similarities with other atmospheres whose main constituent is also solid on the surface:
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages ...169-186).
NASA's New Horizons mission has provided a wealth of new data about the Pluto system, including detailed surface geology and volatile distribution maps revealing striking latitudinal and longitudinal variations. We begin by studying the methane distribution and surface colors using data from New Horizons' Ralph/MVIC instrument. From this study we find that Pluto's equatorial region shows a broader diversity of terrains and more stark longitudinal contrasts than the more homogeneous north polar region. Pluto's south polar region is currently in constant shadow and thus was not observed by New Horizons. We consider how this diversity formed and survived in the context of Pluto's extreme Milancovid cycles and resultant "super seasons". Over timescales of roughly 3 million years Pluto's obliquity varies by 23 degrees (between 103 degrees and 126 degrees) while its longitude of perihelion regresses. This pair of cycles create "super season" epochs where one pole experiences a short intense summer and long winter in constant darkness, while the other experiences a short winter and much longer, but less intense summer. Through thermal modeling and volatile sublimation and deposition modeling we determined that Pluto's high obliquity creates conditions at its equator that favor albedo contrast and can support them on million year timescales more effectively than Pluto's polar regions can. Finally, we look ahead to a possible next step in small body spacecraft exploration, a study of Apophis during its 2029 close approach to Earth. Since the earlier portion of this thesis focused on the encounter, data collection, and scientific analysis portion of a spacecraft mission (New Horizons), we go full circle by exploring the early stage of the
by Alissa M. Earle.
Ph. D.
Evidence is seen for young, fresh surfaces among Near-Earth and Main-Belt asteroids even though space-weathering timescales are shorter than the age of the surfaces. A number of mechanisms have been ...proposed to refresh asteroid surfaces on short timescales, such as planetary encounters, YORP spinup, thermal degradation, and collisions. Additionally, other factors such as grain size effects have been proposed to explain the existence of these “fresh-looking” spectra. To investigate the role each of these mechanisms may play, we collected a sample of visible and near-infrared spectra of 477 near-Earth and Mars Crosser asteroids with similar sizes and compositions — all with absolute magnitude H > 16 and within the S-complex and having olivine to pyroxene (ol/(ol+opx)) ratios >0.65. We taxonomically classify these objects in the Q (fresh) and S (weathered) classes. We find four trends in the Q/S ratio: (1) previous work demonstrated the Q/S ratio increases at smaller sizes down to H ≲16, but we find a sharp increase near H∼19 after which the ratio decreases monotonically. (2) in agreement with many previous studies, the Q/S ratio increases with decreasing perihelion distance, and we find it is non-zero for larger perihelia >1.2AU, (3) as a new finding our work reveals the Q/S ratio has a sharp, significant peak near ∼5° orbital inclination, and (4) we confirm previous findings that the Q/S ratio is higher for objects that have the possibility of encounter with Earth and Venus versus those that do not, however this finding cannot be distinguished from the perihelion trend. No single resurfacing mechanism can explain all of these trends, so multiple mechanisms are required. YORP spin-up scales with size, thermal degradation is dependent on perihelion, planetary encounters trend with inclination, perihelion and MOID, noting that asteroid–asteroid collisions are also dependent on inclination. It is likely that a combination of all four resurfacing mechanisms are needed to account for all observational trends.
•First spectral analysis of Nix, Hydra and Kerberos.•Crystalline water ice found on all three.•2.21 µm band seen on Nix and Hydra indicating an ammoniated species.•Disk resolved spectroscopy of ...Nix.•Temperature and crystalline H2O-ice fraction estimated for Nix and Hydra.
On July 14, 2015, NASA’s New Horizons spacecraft encountered the Pluto-system. Using the near-infrared spectral imager, New Horizons obtained the first spectra of Nix, Hydra, and Kerberos and detected the 1.5 and 2.0 µm bands of H2O-ice on all three satellites. On Nix and Hydra, New Horizons also detected bands at 1.65 and 2.21 µm that indicate crystalline H2O-ice and an ammoniated species, respectively. A similar band linked to NH3-hydrate has been detected on Charon previously. However, we do not detect the 1.99 µm band of NH3-hydrate. We consider NH4Cl (ammonium chloride), NH4NO3 (ammonium nitrate) and (NH4)2CO3 (ammonium carbonate) as potential candidates, but lack sufficient laboratory measurements of these and other ammoniated species to make a definitive conclusion. We use the observations of Nix and Hydra to estimate the surface temperature and crystalline H2O-ice fraction. We find surface temperatures < 20 K ( <70 K with 1-σ error) and 23 K ( < 150 K with 1-σ error) for Nix and Hydra, respectively. We find crystalline H2O-ice fractions of 78−22+12% and > 30% for Nix an Hydra, respectively. New Horizons observed Nix and Hydra twice, about 2–3 hours apart, or 5 and 25% of their respective rotation periods. We find no evidence for rotational differences in the disk-averaged spectra between the two observations of Nix or Hydra. We perform a pixel-by-pixel analysis of Nix’s disk-resolved spectra and find that the surface is consistent with a uniform crystalline H2O-ice fraction, and a ∼ 50% variation in the normalized band area of the 2.21 µm band with a minimum associated with the red blotch seen in color images of Nix. Finally, we find evidence for bands on Nix and Hydra at 2.42 and possibly 2.45 µm, which we cannot identify, and, if real, do not appear to be associated with the ammoniated species. We do not detect other ices, such as CO2, CH3OH and HCN.
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