The Procellarum region is a broad area on the nearside of the Moon that is characterized by low elevations, thin crust, and high surface concentrations of the heat-producing elements uranium, ...thorium, and potassium. The region has been interpreted as an ancient impact basin approximately 3,200 kilometres in diameter, although supporting evidence at the surface would have been largely obscured as a result of the great antiquity and poor preservation of any diagnostic features. Here we use data from the Gravity Recovery and Interior Laboratory (GRAIL) mission to examine the subsurface structure of Procellarum. The Bouguer gravity anomalies and gravity gradients reveal a pattern of narrow linear anomalies that border Procellarum and are interpreted to be the frozen remnants of lava-filled rifts and the underlying feeder dykes that served as the magma plumbing system for much of the nearside mare volcanism. The discontinuous surface structures that were earlier interpreted as remnants of an impact basin rim are shown in GRAIL data to be a part of this continuous set of border structures in a quasi-rectangular pattern with angular intersections, contrary to the expected circular or elliptical shape of an impact basin. The spatial pattern of magmatic-tectonic structures bounding Procellarum is consistent with their formation in response to thermal stresses produced by the differential cooling of the province relative to its surroundings, coupled with magmatic activity driven by the greater-than-average heat flux in the region.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
2.
The NASA Roadmap to Ocean Worlds Hendrix, Amanda R; Hurford, Terry A; Barge, Laura M ...
Astrobiology,
01/2019, Letnik:
19, Številka:
1
Journal Article
Recenzirano
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In this article, we summarize the work of the NASA Outer Planets Assessment Group (OPAG) Roadmaps to Ocean Worlds (ROW) group. The aim of this group is to assemble the scientific framework that will ...guide the exploration of ocean worlds, and to identify and prioritize science objectives for ocean worlds over the next several decades. The overarching goal of an Ocean Worlds exploration program as defined by ROW is to "identify ocean worlds, characterize their oceans, evaluate their habitability, search for life, and ultimately understand any life we find." The ROW team supports the creation of an exploration program that studies the full spectrum of ocean worlds, that is, not just the exploration of known ocean worlds such as Europa but candidate ocean worlds such as Triton as well. The ROW team finds that the confirmed ocean worlds Enceladus, Titan, and Europa are the highest priority bodies to target in the near term to address ROW goals. Triton is the highest priority candidate ocean world to target in the near term. A major finding of this study is that, to map out a coherent Ocean Worlds Program, significant input is required from studies here on Earth; rigorous Research and Analysis studies are called for to enable some future ocean worlds missions to be thoughtfully planned and undertaken. A second finding is that progress needs to be made in the area of collaborations between Earth ocean scientists and extraterrestrial ocean scientists.
A multiwavelength regionally dependent photometric analysis of Pluto's anti-Charon-facing hemisphere using images collected by New Horizons' Multispectral Visible Imaging Camera (MVIC) reveals large ...variations in the absolute value and spectral slope of the single-scattering albedo. Four regions of interest are analyzed: the dark equatorial belt, Pluto's north pole, nitrogen-rich regions, and the mid-latitude terrains. Regions dominated by volatile ices such as Lowell Regio and Sputnik Planitia present single-scattering albedos of ∼0.98 at 492 nm, almost neutral across MVIC's visible wavelength range (400-910 nm), indicating limited contributions from tholin materials. Pluto's dark equatorial regions, informally named Cthulhu and Krun Maculae, have single-scattering albedos of ∼0.16 at 492 nm and are the reddest regions. Applying the Hapke radiative transfer model to combined MVIC and Linear Etalon Imaging Spectral Array (LEISA) spectra (400-2500 nm) of Cthulhu Macula and Lowell Regio successfully reproduces the spectral properties of these two regions of dramatically disparate coloration, composition, and morphology. Since this model uses only a single coloring agent, very similar to the Titan-like tholin of Khare et al., to account for all of Pluto's colors, this result supports the Grundy et al. conclusion that Pluto's coloration is the result of photochemical products mostly produced in the atmosphere. Although cosmic rays and extreme ultraviolet photons reach Pluto's surface where they can drive chemical processing, observations of diverse surface colors do not require different chemical products produced in different environments. We report a correction scaling factor in the LEISA radiometric calibration of 0.74 0.05.
Charon's exosphere may exhibit extreme seasonal dynamics, with centuries of quiescence punctuated by short lived (∼4 earth years) exospheric surges near the equinoxes, as spring sunrise bi‐annually ...drives frozen methane off the polar night zones. Charon's pole‐centric red spot has been proposed to be the product of Ly‐α photolysis of frozen methane into refractory hydrocarbon “tholins”, but the role of exospheric dynamics in the red material's formation has not been investigated. We show with exospheric modeling that methane “polar‐swap”, in which exospheric CH4 sublimated from the spring polar zone is rapidly re‐frozen onto the autumn hemisphere, deposits ∼30 μm polar frosts too thick for Ly‐α light to penetrate. Ethane, the primary methane photoproduct under these conditions, may unlike methane remain frozen decades after polar sunrise under solar wind exposure. Solar wind radiolysis of polar ethane frost synthesizes higher‐order refractories that may contribute to the coloration of Charon's polar zones.
Plain Language Summary
Charon's thin methane atmosphere undergoes “explosive” pulsations owing to the Pluto‐Charon's systems' near sideways tilt to the Sun, according to new computer simulations that we present here. Spring sunrise may drive polar methane frozen during the centuries long winter night back into Charon's atmosphere, causing the whole atmosphere to briefly and drastically surge in pressure by a factor of almost 1000 every equinox. During these exceedingly brief episodes, taking place just a few years out of the Pluto‐Charon system's 248 year orbit around the Sun, polar caps of methane frost tens of microns thick may be suddenly swapped between north and south, evaporated and then re‐frozen from the spring to the autumnal polar zones. Charon's polar red spot, seen by the New Horizons spacecraft, is suspected to be material synthesized from frozen methane by backscattered solar ultraviolet light. However we find that Charon's polar caps are frozen too fast and thick for synthesis of much material more complex than ethane. Nevertheless ethane, being less volatile than methane, stays frozen to Charon's surface for decades after spring sunrise, and may under exposure to solar wind be converted to permanent red‐colored surface deposits that contribute to the origins of the red spot.
Key Points
Surges in Charon's exosphere, driven by spring‐sunrise sublimation of polar methane, produce “flash frozen” CH4 frost at the autumn pole
Polar frost grows too thick and fast for efficient Ly‐α CH4 photolysis to hydrocarbons past C2H6 that may contribute to Charon's red spot
Origin of Charon's polar red material may be in part solar wind radiolysis of photolytic ethane remaining surface‐bound past spring sunrise
Surface Compositions of Trojan Asteroids Emery, Joshua P.; Binzel, Richard P.; Britt, Daniel T. ...
Space science reviews,
04/2024, Letnik:
220, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The Jupiter Trojan asteroids are a key population for understanding the chemical and dynamical evolution of the Solar System. Surface compositions of Trojans, in turn, provide crucial information for ...reconstructing their histories. NASA’s
Lucy
mission will soon complete the first spacecraft reconnaissance of this population. This review summarizes the current state of knowledge of Trojan surface compositions and looks ahead to expected advances in that knowledge from
Lucy
. Surface compositions of Trojans remain uncertain due to a relative lack of diagnostic absorption features, though dedicated observations have begun to provide some clues to compositions. Trojans have uniformly low albedos, with a population average of ∼5.3%, and red spectral slopes at ultraviolet, visible, and near-infrared wavelengths. A bimodality of spectral slopes has been detected and confirmed across all these wavelengths, and the ratio of “less-red” to “red” Trojans increases with decreasing size. A broad absorption at ∼3.1 μm in some less-red Trojans may indicate the presence of N-H bearing material. Mid-infrared emissivity spectra reveal the presence of fine-grained anhydrous silicates on the surfaces. The meteorite collection contains no identifiable analogs to Trojan asteroids. Among small body populations, some Main Belt asteroids, comets, irregular satellites, and Centaurs provide reasonable spectral matches, supporting some genetic relationships among some members of these groups. The cause of the observed spectral properties remains uncertain, but recent suggestions include a combination of volatile ice sublimation and space weathering or a combination of impact gardening and space weathering. The
Lucy
mission will provide detailed compositional analysis of (3548) Eurybates, (15094) Polymele, (11351) Leucus, (21900) Orus, and (617) Patroclus-Menoetius, a suite of targets that sample the diversity among the Trojan population along several dimensions. With these flybys, the
Lucy
mission is poised to resolve many of the outstanding questions regarding Trojan surface compositions, thereby revealing how the Trojans formed and evolved and providing a clearer view of Solar System history.
The Europa Thermal Emission Imaging System (E-THEMIS) on the Europa Clipper spacecraft will investigate the temperature and physical properties of Europa using thermal infrared (TIR) images in three ...wavelength bands centered from 7-14 μm, 14-28 μm and 28-80 μm. E-THEMIS will map >80% of the surface Europa at multiple times of day at a resolution of 8-km per pixel, ∼32% percent of the surface at ≤1 km/pixel resolution, and ∼6% percent at ≤100 m/pixel resolution. The specific objectives of the investigation are to 1) understand the formation of surface features, including sites of recent or current geologic activity, in order to understand regional and global processes and evolution and 2) to identify safe sites for future landed missions. E-THEMIS uses an uncooled microbolometer detector array for the IR focal plane. The E-THEMIS focal plane has 920 cross-track pixels (896 active) and 140 along-track pixels in each of the three spectral bands. The image data are collected at 14-bits per pixel at a frame rate of 60 Hz. The instrument can operate in framing mode, where full frame images are collected, and optionally co-added in time, in each band, or in time-delay-integration (TDI) mode where consecutive rows from each band are offset spatially to remove the spacecraft motion and then summed. In addition, the data in each band can be spatially aggregated from 2 × 2 to 5 × 5 pixels. These modes will be varied throughout each Europa flyby to optimize the data precision while fitting within the E-THEMIS data allocation. The expected temperature precision, measured as the noise equivalent spectral radiance, is 1.2 K at scene temperatures ≥90 K for a TDI of 16 with 4 × 4 pixel coaggregation in Band 2. The absolute accuracy at 90 K is 2−3 K in Band 2. E-THEMIS is an all-reflective, three-mirror anastigmat telescope with a 6.45-cm effective aperture and a speed of f/1.34 cross-track and 1.92 along-track. The mass of instrument Sensor Assembly, mounted on the spacecraft nadir deck, is 11.4 kg, the vault electronics are 1.8 kg, and the two are connected through a 3.1 kg harness. The Sensor volume is 23.7 cm x 31.8 cm x 29.8 cm. E-THEMIS consumes an average operation power of 34.8 W at 28 V. E-THEMIS was developed by Arizona State University with Raytheon Vision Systems developing the microbolometer focal plane assembly and Ball Aerospace developing the electronics. E-THEMIS was integrated, tested, and radiometrically calibrated on the Arizona State University campus in Tempe, AZ.
We use nighttime Cassini Composite Infrared Spectrometer (CIRS) data to look for discrete regions of elevated nighttime temperatures indicative of endogenic activity on Dione's surface. This is ...achieved by producing low latitude and midlatitude (less than 60°) maps of Dione's nighttime surface temperature, derived from 10 to 1,100‐cm−1 CIRS data. The surface temperatures observed do not show evidence of any small discrete regions of elevated nighttime temperatures and are comparable to temperatures predicted by a passive thermophysical model of Dione's surface. Thus, we conclude that no evidence for activity exists on Dione at midlatitude to low latitude. Using the derived surface temperature maps, we set upper limits for the temperature at which a 50‐, 100‐, or 200‐km2 hot spot would remain undetected by this study. We find the mean temperature of such a hot spot would be 117.1 ± 47.2 K (−249 F), 104.8 ± 27.7 K (−272 F), and 95.4 ± 19.5 K (−288 F) for a 50‐, 100‐, and 200‐km2 hot spot, respectively, corresponding to endogenic emission of 1.07, 0.68, and 0.47 GW.
Plain Language Summary
We use data from National Aeronautics and Space Administration's Cassini spacecraft to map the nighttime surface temperature of one of its large icy moons Dione. These maps show no evidence of small very hot regions, which (if present) would provide evidence of activity on Dione. Instead, the surface temperatures we do see are comparable with those we would expect to see on Dione if it were inactive. So we find no evidence for activity on Dione in this study. However, it is possible that very small regions of high surface temperatures would not be detected in this work. We use the observed surface temperatures to define how hot a small region (50, 100, or 200 km2) could be and remain undetected. The results show such a region would have a mean temperature of 117.1 ± 47.2 K (−249 F), 104.8 ± 27.7 K (−272 F), and 95.4 ± 19.5 K (−288 F) for a 50‐, 100‐, and 200‐km2 hot spot, respectively, corresponding to endogenic emission of 1.07, 0.68, and 0.47 GW.
Key Points
We use Cassini CIRS data to map Dione's nighttime temperature to look for evidence of surface activity
Observed temperatures are consistent with passive emission, and no evidence for hot spots (indicative of activity) is observed
We derive a temperature upper limits and limits of endogenic emission for 50‐, 100‐ and 200‐km2 hot spots
The Global Color of Pluto from New Horizons Olkin, Catherine B.; Spencer, John R.; Grundy, William M. ...
The Astronomical journal,
12/2017, Letnik:
154, Številka:
6
Journal Article
Recenzirano
Odprti dostop
The New Horizons flyby provided the first high-resolution color maps of Pluto. We present here, for the first time, an analysis of the color of the entire sunlit surface of Pluto and the first ...quantitative analysis of color and elevation on the encounter hemisphere. These maps show the color variation across the surface from the very red terrain in the equatorial region, to the more neutral colors of the volatile ices in Sputnik Planitia, the blue terrain of East Tombaugh Regio, and the yellow hue on Pluto's North Pole. There are two distinct color mixing lines in the color-color diagrams derived from images of Pluto. Both mixing lines have an apparent starting point in common: the relatively neutral-color volatile-ice covered terrain. One line extends to the dark red terrain exemplified by Cthulhu Regio and the other extends to the yellow hue in the northern latitudes. There is a latitudinal dependence of the predominant color mixing line with the most red terrain located near the equator, less red distributed at mid-latitudes and more neutral terrain at the North Pole. This is consistent with the seasonal cycle controlling the distribution of colors on Pluto. Additionally, the red color is consistent with tholins. The yellow terrain (in the false color images) located at the northern latitudes occurs at higher elevations.
Rhea's exosphere is thought to originate from sources of carbon, water ice and other volatiles that arrived at Rhea by bombardment. Its seasonal variability is directly driven by polar surface ...temperatures allowing surface adsorption, and the persistence of source volatiles require that seasonal temperatures remain sufficiently cold to retain them. Cassini CIRS detected temperatures in Rhea's winter polar region of 23 K, amongst the coldest measured in the Solar System, but with a relatively large footprint we seek to add value to these observations by modeling sub field-of-view (FOV) temperature distribution and examining how the coldest scenes evolve on a seasonal basis.
A simple, rough surface 1-dimensional thermal model is developed using a digital elevation map as input to a thermal model. We compared averaged rough and flat modeled FOVs to CIRS temperatures for a set of case study observations in the south polar region in winter darkness and found they both agree within expected CIRS error in all cases. We develop an asymmetric estimate of CIRS FOV temperature uncertainty, which is particularly important for very cold spectra to accurately represent the sensitivity of the instrument. This approach provides a conservative upper temperature limit to spectral fits whilst highlighting there is often little information to constrain the lower bound of temperature uncertainty in these cases.
The distribution of modeled sub-FOV facet temperatures is explored for the full range of azimuth angles and slope gradients. More extensive and cooler temperatures were found in the rough model, complemented by fewer but warmer areas than the flat model. We find temperature contrasts of individual model facets of up to 15 K warmer and 11 K colder within some CIRS FOV when the rough model was compared flat, in a case study of scenes in winter darkness. This is due to the persistence of the seasonal thermal wave beneath the surface. We tested model sensitivity to thermal input parameters (thermal inertia and bolometric Bond albedo). These values are challenging to constrain due to limited observations and measurement noise and are expected to vary with subtle changes in surface characteristics. We found that within 20° of the pole, temperatures do not rise significantly above 80 K all year, implying that a variety of simple organic molecules, linear amides and other carbon-containing compounds would remain stable on a quasi-permanent basis, potentially until photolytic or radiolytic mechanisms liberate by-products which provide a source for exospheric constituents. The simple rough model indicates that some facets experience very different summer/winter season lengths than a flat model can represent, which is important in terms of the exospheric sequestration process.
We use threshold temperatures of 55 K for CO2 and 30 K for O2 ice as proxies for their relative stability at the surface. The surface coverage of these temperatures with time was compared to modeled exospheric abundance by Teolis and Waite (2016). They are anti-correlated which is expected, but with slight differences in timings of minima and the equinoctial maxima. We find that the rough model suggests larger areas able to adsorb these species than the flat model, and that the cumulative influence of topography would have a direct relationship on the timing and abundance of the exosphere.
We place Rhea's polar environment in context with other icy moons in the Saturnian system using the flat model. The satellites Tethys, Dione and Enceladus experience similar polar thermal regimes to Rhea (with exceptionally cold winters) and would potentially benefit from the consideration of topography in relation to exospheric modeling.
•We characterize new asymmetrical temperature errors for the coldest CIRS measurements (< 30 K).•Rhea’s polar regions remain < 80 K, preserving many volatile species that could interact with the seasonal exosphere.•Several of Saturn’s icy satellites are as cold as Rhea.
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