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تسجيل مستخدم جديدROSETTA/OSIRIS observations of the 67P nucleus during the April 2016 flyby: high-resolution spectrophotometry
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In April 2016, the Rosetta spacecraft performed a low-altitude low-phase-angle flyby over the Imhotep-Khepry transition of 67P/Churyumov-Gerasimenkos nucleus. The OSIRIS/Narrow-Angle-Camera (NAC) acquired 112 images with mainly 3 broadband filters in the visible at a resolution of up to 0.53 m/px and for phase angles between 0.095{deg} and 62{deg}. Using those images, we have investigated the morphological and spectrophotometrical properties of this area. We assembled the images into coregistered color cubes. Using a 3D shape model, we produced the illumination conditions and georeference for each image. We projected the observations on a map to investigate its geomorphology. Observations were photometrically corrected using the Lommel-Seeliger disk law. Spectrophotometric analyses were performed on the coregistered color cubes. These data were used to estimate the local phase reddening. This region of the nucleus hosts numerous and varied types of terrains and features. We observe an association between a features nature, its reflectance, and its spectral slope. Fine material deposits exhibit an average reflectance and spectral slope, while terrains with diamictons, consolidated material, degraded outcrops, or features such as somber boulders, present a lower-than-average reflectance and higher-than-average spectral slope. Bright surfaces present here a spectral behavior consistent with terrains enriched in water-ice. We find a phase-reddening slope of 0.064{pm}0.001{%}/100nm/{deg} at 2.7 au outbound, similarly to the one obtained at 2.3 au inbound during the February 2015 flyby. Identified as the source region of multiple jets and a host of water-ice material, the Imhotep-Khepry transition appeared in April 2016, close to the frost line, to further harbor several potential locations with exposed water-ice material among its numerous different morphological terrain units.
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The Rosetta mission of the European Space Agency has been orbiting the comet 67P/Churyumov-Gerasimenko (67P) since August 2014 and is now in its escort phase. A large complement of scientific experiments designed to complete the most detailed study o
f a comet ever attempted are onboard Rosetta. We present results for the photometric and spectrophotometric properties of the nucleus of 67P derived from the OSIRIS imaging system, which consists of a Wide Angle Camera (WAC) and a Narrow Angle Camera (NAC). The disk-averaged phase function of the nucleus of 67P shows a strong opposition surge with a G parameter value of -0.13$pm$0.01 in the HG system formalism and an absolute magnitude $H_v(1,1,0)$ = 15.74$pm$0.02 mag. The integrated spectrophotometry in 20 filters covering the 250-1000 nm wavelength range shows a red spectral behavior, without clear absorption bands except for a potential absorption centered at $sim$ 290 nm that is possibly due to SO$_2$ ice. The nucleus shows strong phase reddening, with disk-averaged spectral slopes increasing from 11%/(100 nm) to 16%/(100 nm) in the 1.3$^{circ}$--54$^{circ}$ phase angle range. The geometric albedo of the comet is 6.5$pm$0.2% at 649 nm, with local variations of up to $sim$ 16% in the Hapi region. From the disk-resolved images we computed the spectral slope together with local spectrophotometry and identified three distinct groups of regions (blue, moderately red, and red). The Hapi region is the brightest, the bluest in term of spectral slope, and the most active surface on the comet. Local spectrophotometry shows an enhancement of the flux in the 700-750 nm that is associated with coma emissions.
We present observations of comet 67P/Churyumov-Gerasimenko acquired in support of the $Rosetta$ mission. We obtained usable data on 68 nights from 2014 September until 2016 May, with data acquired regularly whenever the comet was observable. We colle
cted an extensive set of near-IR $J$, $H$, and $Ks$ data throughout the apparition plus visible-light images in $g$, $r$, $i$, and $z$ when the comet was fainter. We also obtained broadband $R$ and narrowband $CN$ filter observations when the comet was brightest using telescopes at Lowell Observatory. The appearance was dominated by a central condensation and the tail until 2015 June. From 2015 August onwards there were clear asymmetries in the coma, which enhancements revealed to be due to the presence of up to three features (i.e., jets). The features were similar in all broadband filters; $CN$ images did not show these features but were instead broadly enhanced in the southeastern hemisphere. Modeling using the parameters from Vincent et al. (2013) replicated the dust morphology reasonably well, indicating that the pole orientation and locations of active areas have been relatively unchanged over at least the last three apparitions. The dust production, as measured by $A(0^{circ})f{rho}$ peaked $sim$30 days after perihelion and was consistent with predictions from previous apparitions. $A(0^{circ})f{rho}$ as a function of heliocentric distance was well fit by a power-law with slope $-$4.2 from 35-120 days post-perihelion. We detected photometric evidence of apparent outbursts on 2015 August 22 and 2015 September 19, although neither was discernible morphologically in this dataset.
The phase function of the dust coma of comet 67P has been determined from Rosetta/OSIRIS images citep{Bertini17}. This function show a deep minimum at phase angles near 100$^circ$, and a strong backscattering enhancement. These two properties cannot
be reproduced by regular models of cometary dust, most of them based on wavelength-sized and randomly-oriented aggregate particles. We show, however, that an ensamble of oriented elongated particles of a wide variety of aspect ratios, with radii $r gtrsim$10 $mu$m, and whose long axes are perpendicular to the direction of the solar radiation, are capable of reproducing the observed phase function. These particles must be absorbing, with an imaginary part of the refractive index of about 0.1 to match the expected geometric albedo, and with porosity in the 60-70% range.
The Southern hemisphere of the 67P/Churyumov-Gerasimenko comet has become visible from Rosetta only since March 2015. It was illuminated during the perihelion passage and therefore it contains the regions that experienced the strongest heating and er
osion rate, thus exposing the subsurface most pristine material. In this work we investigate, thanks to the OSIRIS images, the geomorphology, the spectrophotometry and some transient events of two Southern hemisphere regions: Anhur and part of Bes. Bes is dominated by outcropping consolidated terrain covered with fine particle deposits, while Anhur appears strongly eroded with elongated canyon-like structures, scarp retreats, different kinds of deposits, and degraded sequences of strata indicating a pervasive layering. We discovered a new 140 m long and 10 m high scarp formed in the Anhur/Bes boundary during/after the perihelion passage, close to the area where exposed CO$_2$ and H$_2$O ices were previously detected. Several jets have been observed originating from these regions, including the strong perihelion outburst, an active pit, and a faint optically thick dust plume. We identify several areas with a relatively bluer slope (i.e. a lower spectral slope value) than their surroundings, indicating a surface composition enriched with some water ice. These spectrally bluer areas are observed especially in talus and gravitational accumulation deposits where freshly exposed material had fallen from nearby scarps and cliffs. The investigated regions become spectrally redder beyond 2 au outbound when the dust mantle became thicker, masking the underlying ice-rich layers.
We investigated Wosret, a region located on the small lobe of the 67P/Churyumov-Gerasimenko comet subject to strong heating during the perihelion passage. This region includes Abydos, the final landing site of the Philae lander. We analyzed high-reso
lution images of the Wosret region acquired between 2015 and 2016 by the OSIRIS instrument on board the Rosetta spacecraft. We observed a few morphological changes in Wosret, related to local dust coating removal with an estimated depth of $sim$ 1 m, along with the formation of a cavity measuring 30 m in length and 6.5 m in depth, for a total estimated mass loss of 1.2 $times$ 10$^6$ kg. The spectrophotometry of the region is typical of medium-red regions of comet 67P, with spectral slope values of 15-16 %/(100 nm) in pre-perihelion data acquired at phase angle 60$^o$. Wosret has a spectral phase reddening of 0.0546 $times 10^{-4}$ nm$^{-1} deg^{-1}$, which is about a factor of 2 lower than what was determined for the nucleus northern hemisphere regions, possibly indicating a reduced surface micro-roughness due to the lack of widespread dust coating. A few tiny bright spots are observed. Morphological features such as goosebumps or clods are widely present and larger in size than similar features located in the big lobe. Compared to Anhur and Khonsu, two southern hemisphere regions in the big lobe which are also exposed to high insolation during perihelion, Wosret exhibits fewer exposed volatiles and less morphological variations due to activity events. Our analysis indicates that the small lobe has different physical and mechanical properties than the big one and a lower volatile content, at least in its uppermost layers. These results support the hypothesis that comet 67P originated from the merging of two distinct bodies in the early Solar System.
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