scholarly journals Ceres observed at low phase angles by VIR-Dawn

2020 ◽  
Vol 634 ◽  
pp. A39 ◽  
Author(s):  
M. Ciarniello ◽  
M. C. De Sanctis ◽  
A. Raponi ◽  
B. Rousseau ◽  
A. Longobardo ◽  
...  
Keyword(s):  
Phase Angle ◽  
High Surface ◽  
Thermal Inertia ◽  
Reference Value ◽  
Phase Curve ◽  
Angular Width ◽  
Scattering Contribution ◽  
Grain Surface ◽  
Phase Angles ◽  
Angle Interval

Context. Particulate surfaces exhibit a surge of reflectance at low phase angles, a phenomenon referred to as the opposition effect (OE). Two mechanisms are recognized as responsible for the OE: shadow hiding (SH) and coherent backscattering. The latter is typically characterized by a small angular width of a few degrees at most and according to the theoretical prediction should exhibit wavelength and albedo dependence. Aims. We characterize the OE on the surface of Ceres using Dawn Visible InfraRed mapping spectrometer hyperspectral images at low phase angles. Furthermore, this dataset, coupled with previous observations, allows us to perform a complete spectrophotometric modeling at visual-to-infrared (VIS-IR) wavelengths (0.465–4.05 μm) in the broad phase angle range ≈0°−132°. Methods. We applied Hapke’s theory to the average phase curve for Ceres. Disk-resolved properties of the OE were investigated through an empirical model. Results. Across the investigated phase angle interval, Ceres’ average phase curve exhibits a smaller back-scattering contribution for increasing wavelengths. This determines a progressive spectral reddening at larger phase angles that we hypothesize as being related to the effect of submicron roughness on the grain surface. In the OE region, the shape of the phase curves is fairly constant across the VIS range and no sharp opposition surge at very small phase angles (α < 2°) can be recognized. This would suggest a major contribution from SH to Ceres’ OE. Assuming SH as the dominant mechanism, from the OE angular width we infer a high surface porosity (≈0.9), which appears in good qualitative agreement with Ceres’ low thermal inertia. Thanks to the OE observations we derive Ceres’ VIS-IR geometric albedo with a reference value at 0.55 μm of 0.098 ± 0.007. Mapping of the VIS normal albedo and OE angular width across a portion of the surface of Ceres does not reveal a spatial correlation between these quantities, consistent with SH dominating in the α = 0°−7° interval. The comparison of Ceres’ V -band magnitude curve with that of other asteroids indicates that Ceres’ OE is typical of a low-albedo object and compatible with the C-class type.

scholarly journals Ceres’ opposition effect observed by the Dawn framing camera

2018 ◽  
Vol 620 ◽  
pp. A201 ◽  
Author(s):  
Stefan E. Schröder ◽  
Jian-Yang Li ◽  
Marc D. Rayman ◽  
Steven P. Joy ◽  
Carol A. Polanskey ◽  
...  
Keyword(s):  
Wavelength Dependence ◽  
Phase Curve ◽  
Angular Width ◽  
Opposition Effect ◽  
Physically Based ◽  
Phase Angles ◽  
Coherent Backscatter ◽  
Zero Phase ◽  
Geometric Albedo ◽  
Framing Camera

Context. The surface reflectance of planetary regoliths may increase dramatically towards zero phase angle, a phenomenon known as the opposition effect (OE). Two physical processes that are thought to be the dominant contributors to the brightness surge are shadow hiding (SH) and coherent backscatter (CB). The occurrence of shadow hiding in planetary regoliths is self-evident, but it has proved difficult to unambiguously demonstrate CB from remote sensing observations. One prediction of CB theory is the wavelength dependence of the OE angular width. Aims. The Dawn spacecraft observed the OE on the surface of dwarf planet Ceres. We aim to characterize the OE over the resolved surface, including the bright Cerealia Facula, and to find evidence for SH and/or CB. It is presently not clear if the latter can contribute substantially to the OE for surfaces as dark as that of Ceres. Methods. We analyze images of the Dawn framing camera by means of photometric modeling of the phase curve. Results. We find that the OE of most of the investigated surface has very similar characteristics, with an enhancement factor of 1.4 and a full width at half maximum of 3° (“broad OE”). A notable exception are the fresh ejecta of the Azacca crater, which display a very narrow brightness enhancement that is restricted to phase angles <0.5° (“narrow OE”); suggestively, this is in the range in which CB is thought to dominate. We do not find a wavelength dependence for the width of the broad OE, and lack the data to investigate the dependence for the narrow OE. The prediction of a wavelength-dependent CB width is rather ambiguous, and we suggest that dedicated modeling of the Dawn observations with a physically based theory is necessary to better understand the Ceres OE. The zero-phase observations allow us to determine Ceres’ visible geometric albedo as pV = 0.094 ± 0.005. A comparison with other asteroids suggests that Ceres’ broad OE is typical for an asteroid of its spectral type, with characteristics that are primarily linked to surface albedo. Conclusions. Our analysis suggests that CB may occur on the dark surface of Ceres in a highly localized fashion. While the results are inconclusive, they provide a piece to the puzzle that is the OE of planetary surfaces.

scholarly journals Phase-curve analysis of comet 67P/Churyumov-Gerasimenko at small phase angles

2019 ◽  
Vol 630 ◽  
pp. A11
Author(s):  
N Masoumzadeh ◽  
L Kolokolova ◽  
C Tubiana ◽  
M. R. El-Maarry ◽  
S Mottola ◽  
...  
Keyword(s):  
Phase Angle ◽  
Textural Properties ◽  
Phase Ratio ◽  
Phase Curve ◽  
Model Parameters ◽  
Study Phase ◽  
Opposition Effect ◽  
Small Phase ◽  
Phase Angles ◽  
Comet 67P

Aims. The Rosetta-OSIRIS images acquired at small phase angles in three wavelengths during the fly-by of the spacecraft on 9–10 April 2016 provided a unique opportunity to study the opposition effect on the surface of comet 67P/Churyumov-Gerasimenko (67P). Our goal is to study phase curves of the nucleus at small phase angles for a variety of surface structures to show the differences in their opposition effect and to determine which surface properties cause the differences. Methods. We used OSIRIS NAC images that cover the Ash-Khepry-Imhotep region to extract the phase curve, that is, the reflectance of the surface as a function of phase angle. We selected six regions of interest (ROIs) and derived the phase curves for each ROI. We fit a linear-exponential function to the phase curves. The resulting model parameters were then interpreted by spectrophotometric, geomorphological, and phase-ratio analyses, and by investigating the influence of structural and textural properties of the surface. Results. We find evidence for the opposition effect (deviation of the phase curve from linear behavior) in phase curves for all areas. We found an anticorrelation between the phase ratio and reflectance in a small phase angle range. This provides evidence for the shadow-hiding effect. We conclude that the decrease in the slope of the phase ratio versus reflectance indicates a decrease in the proportion of shadowed regions and reduces the contribution of the shadow-hiding effect. Large uncertainties in the determination of the opposition effect parameters with respect to wavelength do not allow us to conclusively claim coherent backscattering in the opposition effect phenomenon. Based on the two analyses, we conclude that the opposition effect of comet 67P in the Ash-Khepry-Imhotep region is mainly affected by shadow-hiding.

scholarly journals Geometric correction for thermographic images of asteroid 162173 Ryugu by TIR (thermal infrared imager) onboard Hayabusa2

2021 ◽  
Vol 73 (1) ◽  
Author(s):  
Takehiko Arai ◽  
Tatsuaki Okada ◽  
Satoshi Tanaka ◽  
Tetsuya Fukuhara ◽  
Hirohide Demura ◽  
...  
Keyword(s):  
Surface Roughness ◽  
High Surface ◽  
Thermal Inertia ◽  
Thermal Infrared ◽  
Shape Model ◽  
Infrared Imager ◽  
Least Squares Fit ◽  
Temperature Maps ◽  
Pixel Scale ◽  
Thermal Infrared Imager

AbstractThe thermal infrared imager (TIR) onboard the Hayabusa2 spacecraft performed thermographic observations of the asteroid 162173 Ryugu (1999 JU$$_3$$ 3 ) from June 2018 to November 2019. Our previous reports revealed that the surface of Ryugu was globally filled with porous materials and had high surface roughness. These results were derived from making the observed temperature maps of TIR using a projection method onto the shape model of Ryugu as geometric corrections. The pointing directions of TIR were calculated using an interpolation of data from the SPICE kernels (NASA/NAIF) during the periods when the optical navigation camera (ONC) and the light detection and ranging (LIDAR) observations were performed. However, the mapping accuracy of the observed TIR images was degraded when the ONC and LIDAR were not performed with TIR. Also, the orbital and attitudinal fluctuations of Hayabusa2 increased the error of the temperature maps. In this paper, to solve the temperature image mapping problems, we improved the correction method by fitting all of the observed TIR images with the surface coordinate addressed on the high-definition shape model of Ryugu (SFM 800k v20180804). This correction adjusted the pointing direction of TIR by rotating the TIR frame relative to the Hayabusa2 frame using a least squares fit. As a result, the temperature maps spatially spreading areas were converged within high-resolved $$0.5^\circ$$ 0 . 5 ∘ by $$0.5^\circ$$ 0 . 5 ∘ maps. The estimated thermal inertia, for instance, was approximately 300$$\sim$$ ∼ 350 Jm$$^{-2}$$ - 2 s$$^{-0.5}$$ - 0.5 K$$^{-1}$$ - 1 at the hot area of the Ejima Saxum. This estimation was succeeded in case that the surface topographic features were larger than the pixel scale of TIR. However, the thermal inertia estimation of smooth terrains, such as the Urashima crater, was difficult because of surface roughness effects, where roughness was probably much smaller than the pixel scale of TIR.

LTCC-3D Coaxial Flow Focusing Microfluidic Reactor for Micro and Nanoparticle Fabrication and Production Scale-Out

2013 ◽  
Vol 10 (3) ◽  
pp. 102-108 ◽  
Author(s):  
Mario Ricardo Gongora-Rubio ◽  
Juliana de Novais Schianti ◽  
Houari Cobas Gomez ◽  
Andre da Costa Teves
Keyword(s):  
Particle Production ◽  
High Surface ◽  
Thermal Inertia ◽  
Volume Ratio ◽  
Chemical Processes ◽  
Nano Particles ◽  
Batch Reactors ◽  
Thermal Coefficient ◽  
Flow Focusing ◽  
Production Scale

Miniaturization of chemical processes is becoming a must for green chemistry and sustainable industry processes, so technological research in this direction is well received. Continuous microreactor systems hold many potential benefits over batch reactors, in that they allow: high surface-to-volume ratio, fine adjustment of chemical reaction residence times, small thermal inertia, and fast changes in temperature. Advantages of multilayer green ceramics for microprocess applications include: that the LTCC substrate is chemically inert to most solvents, that it has a high contact angle, that it presents low thermal coefficient of expansion, and that it can withstand high operational temperatures and high internal pressures. For these reasons, LTCC-based microsystem technologies allow the implementation of different unitary operations for chemical processes, making it an enabling technology for the miniaturization of chemical processes. In fact, recently, LTCC microfluidic reactors have been used to produce microparticles and nanoparticles with excellent control of size distribution and morphology. The present work provides a report on the performance of a 3D LTCC coaxial flow focusing micro-fluidic reactor designed to fabricate microparticles and nano-particles using nanoprecipitation through an antisolvent; with electric potential size tuning. We also implement an approach to particle production scale-out.

scholarly journals Attention and Temporal Expectations Modulate Power, Not Phase, of Ongoing Alpha Oscillations

2015 ◽  
Vol 27 (8) ◽  
pp. 1573-1586 ◽  
Author(s):  
Rosanne M. van Diepen ◽  
Michael X Cohen ◽  
Damiaan Denys ◽  
Ali Mazaheri
Keyword(s):  
Phase Angle ◽  
Phase Locking ◽  
Auditory Target ◽  
Alpha Power ◽  
Visual Condition ◽  
Top Down ◽  
Alpha Oscillations ◽  
Alpha Oscillation ◽  
Sensory Cortices ◽  
Phase Angles

The perception of near-threshold visual stimuli has been shown to depend in part on the phase (i.e., time in the cycle) of ongoing alpha (8–13 Hz) oscillations in the visual cortex relative to the onset of that stimulus. However, it is currently unknown whether the phase of the ongoing alpha activity can be manipulated by top–down factors such as attention or expectancy. Using three variants of a cross-modal attention paradigm with constant predictable stimulus onsets, we examined if cues signaling to attend to either the visual or the auditory domain influenced the phase of alpha oscillations in the associated sensory cortices. Importantly, intermixed in all three experiments, we included trials without a target to estimate the phase at target presentation without contamination from the early evoked responses. For these blank trials, at the time of expected target and distractor onset, we examined (1) the degree of the uniformity in phase angles across trials, (2) differences in phase angle uniformity compared with a pretarget baseline, and (3) phase angle differences between visual and auditory target conditions. Across all three experiments, we found that, although the cues induced a modulation in alpha power in occipital electrodes, neither the visual condition nor the auditory cue condition induced any significant phase-locking across trials during expected target or distractor presentation. These results suggest that, although alpha power can be modulated by top–down factors such as attention and expectation, the phase of the ongoing alpha oscillation is not under such control.

Research on Small Dimension Precision Turning Technology for Internal Spherical Surface

2008 ◽  
Vol 53-54 ◽  
pp. 381-386
Author(s):  
Ya Dong Gong ◽  
Jian Yu Yang ◽  
Yan Cheng Zhang ◽  
Wan Shan Wang
Keyword(s):  
High Efficiency ◽  
Spherical Surface ◽  
High Surface ◽  
Reference Value ◽  
Small Dimension ◽  
High Surface Quality ◽  
Technological Method ◽  
Precision Turning ◽  
Processing Cost ◽  
Dimension Precision

To the processing requirements of high precision small dimension and high surface quality for internal spherical surface with through hole processing, a new technological method of precision turning was proposed in this paper, the processing equipment and control system were designed and developed, the parameters of precision turning technology were given. Through precision turning experiment’s verification, this technological method of precision turning can satisfy processing requirement, and has features of simple, high efficiency, and low processing cost. Some comparative research in various small dimension precision turning technology for internal spherical surface were also made in this paper, and conclusions full of research and processing reference value were drawn.

scholarly journals Terrestrial Planet Optical Phase Curves. I. Direct Measurements of the Earth

2021 ◽  
Vol 163 (1) ◽  
pp. 5
Author(s):  
Roderick De Cock ◽  
Timothy A. Livengood ◽  
Daphne M. Stam ◽  
Carey M. Lisse ◽  
Tilak Hewagama ◽  
...  
Keyword(s):  
Phase Angle ◽  
Terrestrial Planet ◽  
Wavelength Dependence ◽  
Deep Impact ◽  
Spectral Filters ◽  
Optical Phase ◽  
The Earth ◽  
Direct Measurements ◽  
Phase Angles ◽  
Geometric Albedo

Abstract NASA’s EPOXI mission used the Deep Impact spacecraft to observe the disk-integrated Earth as an analog to terrestial exoplanets’ appearance. The mission took five 24 hr observations in 2008–2009 at various phase angles (57.°7–86.°4) and ranges (0.11–0.34 au), of which three equatorial (E1, E4, E5) and two polar (P1, North and P2, South). The visible data taken by the HRIV instrument ranges from 0.3 to 1.0 μm, taken trough seven spectral filters that have spectral widths of about 100 nm, and which are centered about 100 nm apart, from 350 to 950 nm. The disk-integrated, 24 hr averaged signal is used in a phase angle analysis. A Lambertian-reflecting, spherical planet model is used to estimate geometric albedo for every observation and wavelength. The geometric albedos range from 0.143 (E1, 950 nm) to 0.353 (P2, 350 nm) and show wavelength dependence. The equatorial observations have similar values, while the polar observations have higher values due to the ice in view. Therefore, equatorial observations can be predicted for other phase angles, but (Earth-like) polar views (with ice) would be underestimated.

scholarly journals Blue, white, and red ocean planets

2019 ◽  
Vol 626 ◽  
pp. A129 ◽  
Author(s):  
V. J. H. Trees ◽  
D. M. Stam
Keyword(s):  
Phase Angle ◽  
Surface Albedo ◽  
Color Change ◽  
Water Flux ◽  
Angular Distance ◽  
Total Flux ◽  
Degree Of Polarization ◽  
Reflected Light ◽  
Phase Functions ◽  
Phase Angles

Context. An exoplanet’s habitability will depend strongly on the presence of liquid water. Flux and/or polarization measurements of starlight that is reflected by exoplanets could help to identify exo-oceans. Aims. We investigate which broadband spectral features in flux and polarization phase functions of reflected starlight uniquely identify exo-oceans. Methods. With an adding-doubling algorithm, we computed total fluxes F and polarized fluxes Q of starlight that is reflected by cloud-free and (partly) cloudy exoplanets, for wavelengths from 350 to 865 nm. The ocean surface has waves composed of Fresnel reflecting wave facets and whitecaps, and scattering within the water body is included. Results. Total flux F, polarized flux Q, and degree of polarization P of ocean planets change color from blue, through white, to red at phase angles α ranging from ~134° to ~108° for F, and from ~123° to ~157° for Q, with cloud coverage fraction fc increasing from 0.0 (cloud-free) to 1.0 (completely cloudy) for F, and to 0.98 for Q. The color change in P only occurs for fc ranging from 0.03 to 0.98, with the color crossing angle α ranging from ~88° to ~161°. The total flux F of a cloudy, zero surface albedo planet can also change color, and for fc = 0.0, an ocean planet’s F will not change color for surface pressures ps ≿ 8 bars. Polarized flux Q of a zero surface albedo planet does not change color for any fc. Conclusions. The color change of P of starlight reflected by an exoplanet, from blue, through white, to red with increasing α above 88°, appears to identify a (partly) cloudy exo-ocean. The color change of polarized flux Q with increasing α above 123° appears to uniquely identify an exo-ocean, independent of surface pressure or cloud fraction. At the color changing phase angle, the angular distance between a star and its planet is much larger than at the phase angle where the glint appears in reflected light. The color change in polarization thus offers better prospects for detecting an exo-ocean.

Saturn rings: divisions, gaps and ringlets from ground-based telescopes

1984 ◽  
Vol 75 ◽  
pp. 147-154 ◽  
Author(s):  
Audouin Dollfus
Keyword(s):  
Phase Angle ◽  
Volume Density ◽  
Light Curves ◽  
High Magnification ◽  
Density Dependent ◽  
Opposition Effect ◽  
Small Phase ◽  
Saturn's Rings ◽  
Phase Angles ◽  
Zero Phase

ABSTRACTThe high magnification visual telescopic observation of Saturn’s rings exhibits divisions, gaps and bright sub-rings. B. Lyot gave a first description of these features. Later, with still more resolving telescopes, we improved the analysis of the ring features. Some gaps and concentric bright or dark sub-rings are phase angle dependent; the steep luminance peaks of their light curves around zero phase angle are volume-density dependent (opposition effect); the overall result produces changes in the shapes and intensities of these features at small phase angles, which are analysed.

Sign in / Sign up

Export Citation Format

Share Document