2,914 research outputs found
Phase Angle Effects on 3-micron Absorption Band on Ceres: Implications for Dawn Mission
Phase angle-induced spectral effects are important to characterize since they
affect spectral band parameters such as band depth and band center, and
therefore skew mineralogical interpretations of planetary bodies via
reflectance spectroscopy. Dwarf planet (1) Ceres is the next target of NASA's
Dawn mission, which is expected to arrive in March 2015. The visible and
near-infrared mapping spectrometer (VIR) onboard Dawn has the spatial and
spectral range to characterize the surface between 0.25-5.0 microns. Ceres has
an absorption feature at 3.0 microns due to hydroxyl- and/or water-bearing
minerals (e.g. Lebofsky et al. 1981, Rivkin et al. 2003). We analyzed phase
angle-induced spectral effects on the 3-micron absorption band on Ceres using
spectra measured with the long-wavelength cross-dispersed (LXD: 1.9-4.2
microns) mode of the SpeX spectrograph/imager at the NASA Infrared Telescope
Facility (IRTF). Ceres LXD spectra were measured at different phase angles
ranging from 0.7o to 22o. We found that the band center slightly increases from
3.06 microns at lower phase angles (0.7o and 6o) to 3.07 microns at higher
phase angles (11 o and 22o), the band depth decreases by ~20% from lower phase
angles to higher phase angles, and the band area decreases by ~25% from lower
phase angles to higher phase angles. Our results will have implications for
constraining the abundance of OH on the surface of Ceres from VIR spectral
data, which will be acquired by Dawn starting spring 2015.Comment: 12 pages, 1 figure, 2 table
Composition of Jupiter irregular satellites sheds light on their origin
Irregular satellites of Jupiter with their highly eccentric, inclined and
distant orbits suggest that their capture took place just before the giant
planet migration. We aim to improve our understanding of the surface
composition of irregular satellites of Jupiter to gain insight into a narrow
time window when our Solar System was forming. We observed three Jovian
irregular satellites, Himalia, Elara, and Carme, using a medium-resolution
0.8-5.5 micro m spectrograph on the National Aeronautics and Space
Administration (NASA) Infrared Telescope Facility (IRTF). Using a linear
spectral unmixing model we have constrained the major mineral phases on the
surface of these three bodies. Our results confirm that the surface of Himalia,
Elara, and Carme are dominated by opaque materials such as those seen in
carbonaceous chondrite meteorites. Our spectral modeling of NIR spectra of
Himalia and Elara confirm that their surface composition is the same and
magnetite is the dominant mineral. A comparison of the spectral shape of
Himalia with the two large main C-type asteroids, Themis (D 176 km) and Europa
(D 352 km), suggests surface composition similar to Europa. The NIR spectrum of
Carme exhibits blue slope up to 1.5 microm and is spectrally distinct from
those of Himalia and Elara. Our model suggests that it is compositionally
similar to amorphous carbon. Himalia and Elara are compositionally similar but
differ significantly from Carme. These results support the hypotheses that the
Jupiter irregular satellites are captured bodies that were subject to further
breakup events and clustered as families based on their similar physical and
surface compositions
Exploring Exogenic Sources for the Olivine on Asteroid (4) Vesta
The detection of olivine on Vesta is interesting because it may provide
critical insights into planetary differentiation early in our Solar System's
history. Ground-based and Hubble Space Telescope (HST) observations of asteroid
(4) Vesta have suggested the presence of olivine on the surface. These
observations were reinforced by the discovery of olivine-rich HED meteorites
from Vesta in recent years. However, analysis of data from NASA's Dawn
spacecraft has shown that this olivine-bearing unit is actually impact melt in
the ejecta of Oppia crater. The lack of widespread mantle olivine, exposed
during the formation of the 19 km deep Rheasilvia basin on Vesta's South Pole,
further complicated this picture. Ammannito et al., (2013a) reported the
discovery of local scale olivine-rich units in the form of excavated material
from the mantle using the Visible and InfraRed spectrometer (VIR) on Dawn. Here
we explore alternative sources for the olivine in the northern hemisphere of
Vesta by reanalyzing the data from the VIR instrument using laboratory spectral
measurements of meteorites. We suggest that these olivine exposures could be
explained by the delivery of olivine-rich exogenic material. Based on our
spectral band parameters analysis, the lack of correlation between the location
of these olivine-rich terrains and possible mantle-excavating events, and
supported by observations of HED meteorites, we propose that a probable source
for olivine seen in the northern hemisphere are remnants of impactors made of
olivine-rich meteorites. Best match suggests these units are HED material mixed
with either ordinary chondrites, or with some olivine-dominated meteorites such
as R-chondrites.Comment: 62 pages, 12 figures, 4 tables; Icarus, Available online 30 January
2015, ISSN 0019-1035, http://dx.doi.org/10.1016/j.icarus.2015.01.01
Genetic distance predicts trait differentiation at the subpopulation but not the individual level in eelgrass, Zostera marina.
Ecological studies often assume that genetically similar individuals will be more similar in phenotypic traits, such that genetic diversity can serve as a proxy for trait diversity. Here, we explicitly test the relationship between genetic relatedness and trait distance using 40 eelgrass (Zostera marina) genotypes from five sites within Bodega Harbor, CA. We measured traits related to nutrient uptake, morphology, biomass and growth, photosynthesis, and chemical deterrents for all genotypes. We used these trait measurements to calculate a multivariate pairwise trait distance for all possible genotype combinations. We then estimated pairwise relatedness from 11 microsatellite markers. We found significant trait variation among genotypes for nearly every measured trait; however, there was no evidence of a significant correlation between pairwise genetic relatedness and multivariate trait distance among individuals. However, at the subpopulation level (sites within a harbor), genetic (FST) and trait differentiation were positively correlated. Our work suggests that pairwise relatedness estimated from neutral marker loci is a poor proxy for trait differentiation between individual genotypes. It remains to be seen whether genomewide measures of genetic differentiation or easily measured "master" traits (like body size) might provide good predictions of overall trait differentiation
Ground-based Characterization of Hayabusa2 Mission Target Asteroid 162173 Ryugu: Constraining Mineralogical Composition in Preparation for Spacecraft Operations
Asteroids that are targets of spacecraft missions are interesting because
they present us with an opportunity to validate ground-based spectral
observations. One such object is near-Earth asteroid (NEA) (162173) Ryugu,
which is the target of the Japanese Space Agency's (JAXA) Hayabusa2 sample
return mission. We observed Ryugu using the 3-m NASA Infrared Telescope
Facility (IRTF) on Mauna Kea, Hawaii, on July 13, 2016 to constrain the
object's surface composition, meteorite analogs, and link to other asteroids in
the main belt and NEA populations. We also modeled its photometric properties
using archival data. Using the Lommel-Seeliger model we computed the predicted
flux for Ryugu at a wide range of viewing geometries as well as albedo
quantities such as geometric albedo, phase integral, and spherical Bond albedo.
Our computed albedo quantities are consistent with results from Ishiguro et al.
(2014). Our spectral analysis has found a near-perfect match between our
spectrum of Ryugu and those of NEA (85275) 1994 LY and Mars-crossing asteroid
(316720) 1998 BE7, suggesting that their surface regoliths have similar
composition. We compared Ryugu's spectrum with that of main belt asteroid (302)
Clarissa, the largest asteroid in the Clarissa asteroid family, suggested as a
possible source of Ryugu by Campins et al. (2013). We found that the spectrum
of Clarissa shows significant differences with our spectrum of Ryugu, but it is
similar to the spectrum obtained by Moskovitz et al. (2013). The best possible
meteorite analogs for our spectrum of Ryugu are two CM2 carbonaceous
chondrites, Mighei and ALH83100.Comment: 23 pages, 7 figures, 4 tables, accepted in Monthly Notices of the
Royal Astronomical Society Main Journa
Recommended from our members
Pre-clinical development of a lentiviral vector expressing the anti-sickling beta AS3 globin for gene therapy for sickle-cell disease
Movements, at-sea distribution and behaviour of a tropical pelagic seabird: the wedge-tailed shearwater in the western Indian Ocean
This is the first study using geolocators (global location sensing, GLS) to track the movements of a pelagic tropical seabird. We used GLS to describe at-sea distribution and activity patterns of wedge-tailed shearwaters Puffinus pacificus breeding on Aride Island, Seychelles, in the late chick-rearing, non-breeding and pre-breeding periods. During late chick-rearing and pre-breeding periods, shearwaters foraged relatively close to the colony. In the non-breeding period, shearwaters; were found on a west-east gradient along the equator, between 5 degrees N and 10 degrees S. Some of the tracked individuals showed little dispersion, staying as close as 1000 km to Aride Island, while others travelled 3500 km to the Central Indian Ocean Basin. Individual core areas of activity showed little overlap. Overall, wedge-tailed shearwaters showed short-distance movements and exploited relatively unproductive oceanic waters. At-sea distribution largely matched that of yellowfin and skipjack tunas, emphasising the importance of the association with subsurface predators rather than associations with physical oceanographic features that enhance primary productivity. During the non-breeding period, the feeding activity of, shearwaters was mainly concentrated in the daylight period, when tunas also forage. A different behaviour, characterised by a much lower proportion of the night sitting on the sea surface, was recorded in the pre-laying exodus of 1 female to more productive waters, suggesting a different feeding strategy and/or targeting of different prey. Knowledge of the at-sea distribution of wedge-tailed shearwaters allows quantification of the overlap with industrial fisheries, which will be crucial to devise fisheries policies for the Indian Ocean with important implications for the conservation of this species
Analysis of Thymocyte Development Reveals That the Gtpase Rhoa Is a Positive Regulator of T Cell Receptor Responses in Vivo
Loss of function of the guanine nucleotide binding protein RhoA blocks pre-T cell differentiation and survival indicating that this GTPase is a critical signaling molecule during early thymocyte development. Previous work has shown that the Rho family GTPase Rac-1 can initiate changes in actin dynamics necessary and sufficient for pre-T cell development. The present data now show that Rac-1 actions in pre-T cells require Rho function but that RhoA cannot substitute for Rac-1 and induce the actin cytoskeletal changes necessary for pre-T cell development. Activation of Rho is thus not sufficient to induce pre-T cell differentiation or survival in the absence of the pre-T cell receptor (TCR). The failure of RhoA activation to impact on pre-TCR–mediated signaling was in marked contrast to its actions on T cell responses mediated by the mature TCR α/β complex. Cells expressing active RhoA were thus hyperresponsive in the context of TCR-induced proliferation in vitro and in vivo showed augmented positive selection of thymocytes expressing defined TCR complexes. This reveals that RhoA function is not only important for pre-T cells but also plays a role in determining the fate of mature T cells
- …