An official website of the United States government
Abstract We present a thermal observation of Callisto's leading hemisphere obtained using the Atacama Large Millimeter/submillimeter Array at 0.87 mm (343 GHz). The angular resolution achieved for this observation was ∼0.″16, which for Callisto at the time of this observation (D∼ 1.″05) was equivalent to ∼six elements across the surface. Our disk-integrated brightness temperature of 116 ± 5 K (8.03 ± 0.40 Jy) is consistent with prior disk-integrated observations. Global surface properties were derived from the observation using a thermophysical model constrained by spacecraft data. We find that models parameterized by two thermal inertia components more accurately fit the data than single thermal inertia models. Our best-fit global parameters adopt a lower thermal inertia of 15–50 J m−2K−1s−1/2and a higher thermal inertia component of 1200–2000 J m−2K−1s−1/2, with retrieved millimeter emissivities of 0.89–0.91. We identify several thermally anomalous regions, including spots ∼3 K colder than model predictions colocated with the Valhalla impact basin and a complex of craters in the southern hemisphere; this indicates the presence of materials possessing either a higher thermal inertia or a lower emissivity. A warm region confined to the midlatitudes in these leading hemisphere data may be indicative of regolith property changes due to exogenic sculpting.
more »
« less
Subsurface Thermophysical Properties of Europa’s Leading and Trailing Hemispheres as Revealed by ALMA
Abstract We present best-fit values of porosity—and the corresponding effective thermal inertiae—determined from three different depths in Europa’s near-subsurface (∼1–20 cm). The porosity of the upper ∼20 cm of Europa’s subsurface varies between 75% and 50% (Γeff≈ 50–140 J m−2K−1s−1/2) on the leading hemisphere and 50%–40% (Γeff≈ 140–180 J m−2K−1s−1/2) on the trailing hemisphere. Residual maps produced by comparison with these models reveal thermally anomalous features that cannot be reproduced by globally homogeneous porosity models. These regions are compared to Europa’s surface terrain and known compositional variations. We find that some instances of warm thermal anomalies are co-located with known geographical or compositional features on both the leading and trailing hemisphere; cool temperature anomalies are well correlated with surfaces previously observed to contain pure, crystalline water ice and the expansive rays of Pwyll crater. Anomalous regions correspond to locations with subsurface properties different from those of our best-fit models, such as potentially elevated thermal inertia, decreased emissivity, or more porous regolith. We also find that ALMA observations at ∼3 mm sound below the thermal skin depth of Europa (∼10–15 cm) for a range of porosity values, and thus do not exhibit features indicative of diurnal variability or residuals similar to other frequency bands. Future observations of Europa at higher angular resolution may reveal additional locations of variable subsurface thermophysical properties, while those at other wavelengths will inform our understanding of the regolith compaction length and the effects of external processes on the shallow subsurface.
more »
« less
- Award ID(s):
- 2308280
- PAR ID:
- 10493239
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Planetary Science Journal
- Volume:
- 5
- Issue:
- 2
- ISSN:
- 2632-3338
- Format(s):
- Medium: X Size: Article No. 56
- Size(s):
- Article No. 56
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
ABSTRACT This work investigates effects of poly(γ‐butyrolactone) (PγBL) with different initiation and termination chain ends on five types of materials properties, including thermal stability, thermal transitions, thermal recyclability, hydrolytic degradation, and dynamic mechanical behavior. Four different chain‐end‐capped polymers with similar molecular weights, BnO‐[C(=O)(CH2)3O]n‐R, R = C(=O)Me, C(=O)CH=CH2, C(=O)Ph, and SiMe2CMe3, along with a series of uncapped polymers R′O‐[C(=O)(CH2)3O]n‐H (R′ = Bn, Ph2CHCH2) withMnranging from low (4.95 kg mol−1) to high (83.2 kg mol−1), have been synthesized. The termination chain end R showed a large effect on polymer decomposition temperature and hydrolytic degradation, relative to H. Overall, for those properties sensitive to the chain ends, chain‐end capping renders R‐protected linear PγBL behaving much like cyclic PγBL. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem.2018,56, 2271–2279more » « less
-
Abstract A novel transition metal chalcohalide [Cr7S8(en)8Cl2]Cl3 ⋅ 2H2O, with [Cr7S8]5+dicubane cationic clusters, has been synthesized by a low temperature solvothermal method, using dimethyl sulfoxide (DMSO) and ethylenediamine (en) solvents. Ethylenediamine ligand exhibits bi‐ and monodentate coordination modes; in the latter case ethylenediamine coordinates to Cr atoms of adjacent clusters, giving rise to a 2D polymeric structure. Although magnetic susceptibility shows no magnetic ordering down to 1.8 K, a highly negative Weiss constant,θ=−224(2) K, obtained from Curie‐Weiss fit of inverse susceptibility, suggests strong antiferromagnetic (AFM) interactions betweenS=3/2 Cr(III) centers. Due to the complexity of the system with (2S+1)7=16384 microstates from seven Cr3+centers, a simplified model with only two exchange constants was used for simulations. Density‐functional theory (DFT) calculations yielded the two exchange constants to beJ1=−21.4 cm−1andJ2=−30.2 cm−1, confirming competing AFM coupling between the shared Cr3+center and the peripheral Cr3+ions of the dicubane cluster. The best simulation of the experimental data was obtained withJ1=−20.0 cm−1andJ2=−21.0 cm−1, in agreement with the slightly stronger AFM exchange within the triangles of the peripheral Cr3+ions as compared to the AFM exchange between the central and peripheral Cr3+ions. This compound is proposed as a synthon towards magnetically frustrated systems assembled by linking dicubane transition metal‐chalcogenide clusters into polymeric networks.more » « less
-
Abstract Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008TC3). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008TC3was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008TC3. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondrules (~130–600 μm) and chondrule fragments. The C1 material consists of fine‐grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28‐42), an unidentified Ca‐rich silicate phase, Fe,Ni sulfides, and minor Ca‐phosphate and ilmenite. It has similarities toCI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ18O = 13.53‰, δ17O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of severalCC‐like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75‐88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal‐sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3OCand/orCC, based on mineral and oxygen isotope compositions. Some of the metal‐sulfide clasts are derived fromEC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04–0.05) and relatively featureless inVNIR, and have an ~2.7 μm absorption band due toOH−in phyllosilicates. Spectral modeling, using mixtures of laboratoryVNIRreflectance spectra of AhS stones to fit the F‐type spectrum of the asteroid, suggests that 2008TC3consisted mainly of ureilitic and AhS 91A‐like materials, with as much as 40–70% of the latter, and <10% ofOC,EC, and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g cm−3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7–2.2 g cm−3). Its porosity (36%) is near the low end of estimates for the asteroid (33–50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008TC3in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when aCC‐like body impacted into already well‐gardened ureilitic + impactor‐derived debris. AhS 91A bulk samples do not show a solar wind component, so they represent subsurface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5–9 Ma) than previously studied AhS stones (11–22 Ma). The spread inCREages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands.more » « less
-
Abstract Bis‐porphyrin nanocages (M2BiCage, M = FeCl, Co, Zn) and their host‐guest complexes with C60and C70were used to examine how molecular porosity and interactions with carbon nanomaterials affect the CO2reduction activity of metalloporphyrin electrocatalysts. The cages were found to adsorb on carbon black to provide electrocatalytic inks with excellent accessibilities of the metal sites (≈50%) even at high metal loadings (2500 nmol cm−2), enabling good activity for reducing CO2to CO. A complex of C70bound inside(FeCl)2BiCageachieves high current densities for CO formation at low overpotentials (|jCO| >7 mA cm−2,η= 320 mV; >13.5 mA cm−2,η= 520 mV) with ≥95% Faradaic efficiency (FECO), andCo2BiCageachieves high turnover frequencies (≈1300 h−1,η= 520 mV) with 90% FECO. In general, blocking the pore with C60or C70improves the catalytic performance of(FeCl)2BiCageand has only small effects onCo2BiCage, indicating that the good catalytic properties of the cages cannot be attributed to their internal pores. Neither enhanced electron transfer rates nor metal‐fullerene interactions appear to underlie the ability of C60/C70to improve the performance of(FeCl)2BiCage, in contrast to effects often proposed for other carbon nanosupports.more » « less
