| 11. |
- Deshapriya, J. D. P., et al.
(författare)
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Exposed bright features on the comet 67P/Churyumov-Gerasimenko : distribution and evolution
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 613
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Tidskriftsartikel (refereegranskat)abstract
- Context. Since its arrival at the comet 67P/Churyumov-Gerasimenko in August 2014, the Rosetta spacecraft followed the comet as it went past the perihelion and beyond until September 2016. During this time there were many scientific instruments operating on board Rosetta to study the comet and its evolution in unprecedented detail. In this context, our study focusses on the distribution and evolution of exposed bright features that have been observed by OSIRIS, which is the scientific imaging instrument aboard Rosetta. Aims. We envisage investigating various morphologies of exposed bright features and the mechanisms that triggered their appearance. Methods. We co-registered multi-filter observations of OSIRIS images that are available in reflectance. The Lommel-Seeliger disk function was used to correct for the illumination conditions and the resulting colour cubes were used to perform spectrophotometric analyses on regions of interest. Results. We present a catalogue of 57 exposed bright features observed on the nucleus of the comet, all of which are attributed to the presence of H2O ice on the comet. Furthermore, we categorise these patches under four different morphologies and present geometric albedos for each category. Conclusions. Although the nucleus of 67P/Churyumov-Gerasimenko appears to be dark in general, there are localised H2O ice sources on the comet. Cometary activity escalates towards the perihelion passage and reveals such volatile ices. We propose that isolated H2O ice patches found in smooth terrains in regions, such as Imhotep, Bes, and Hapi, result from frost as an aftermath of the cessation of the diurnal water cycle on the comet as it recedes from perihelion. Upon the comet's return to perihelion, such patches are revealed when sublimation-driven erosion removes the thin dust layers that got deposited earlier. More powerful activity sources such as cometary outbursts are capable of revealing much fresher, less contaminated H2O ice that is preserved with consolidated cometary material, as observed on exposed patches resting on boulders. This is corroborated by our albedo calculations that attribute higher albedos for bright features with formations related to outbursts.
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| 12. |
- Fornasier, S., et al.
(författare)
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Rosetta's comet 67P/Churyumov-Gerasimenko sheds its dusty mantle to reveal its icy nature
- 2016
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 354:6319, s. 1566-1570
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Tidskriftsartikel (refereegranskat)abstract
- The Rosetta spacecraft has investigated comet 67P/Churyumov-Gerasimenko from large heliocentric distances to its perihelion passage and beyond. We trace the seasonal and diurnal evolution of the colors of the 67P nucleus, finding changes driven by sublimation and recondensation of water ice. The whole nucleus became relatively bluer near perihelion, as increasing activity removed the surface dust, implying that water ice is widespread underneath the surface. We identified large (1500 square meters) ice-rich patches appearing and then vanishing in about 10 days, indicating small-scale heterogeneities on the nucleus. Thin frosts sublimating in a few minutes are observed close to receding shadows, and rapid variations in color are seen on extended areas close to the terminator. These cyclic processes are widespread and lead to continuously, slightly varying surface properties.
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| 13. |
- Fornasier, S., et al.
(författare)
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The highly active Anhur-Bes regions in the 67P/Churyumov-Gerasimenko comet : results from OSIRIS/ROSETTA observations
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 469, s. S93-S107
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Tidskriftsartikel (refereegranskat)abstract
- The Southern hemisphere of the 67P/Churyumov-Gerasimenko comet has become visible from Rosetta only since 2015 March. It was illuminated during the perihelion passage and therefore it contains the regions that experienced the strongest heating and erosion rates, thus exposing the sub-surface 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 CO2 and H2O 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.
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| 14. |
- Giacomini, L., et al.
(författare)
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Geologic mapping of the Comet 67P/Churyumov-Gerasimenko's Northern hemisphere
- 2016
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 462, s. S352-S369
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Tidskriftsartikel (refereegranskat)abstract
- The Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS), the scientific imaging system onboard the Rosetta mission, has been acquiring images of the nucleus of the comet 67P/Churyumov-Gerasimenko since 2014 August with a resolution which allows a detailed analysis of its surface. Indeed, data reveal a complex surface morphology which is likely the expression of different processes which occurred at different times on the cometary nucleus. In order to characterize these different morphologies and better understand their distribution, we performed a geologic mapping of comet's 67P Northern hemisphere in which features have been distinguished based on their morphological, textural and stratigraphic characteristics. For this purpose, we used narrow-angle camera images acquired in 2014 August and September with a spatial scale ranging from 1.2 to 2.4 m pixel(-1). Several different geologic units have been identified on the basis of their different surface textures, granulometry and morphology. Some of these units are distinctive and localized, whereas others are more common and distributed all over the Northern hemisphere. Moreover, different types of linear features have been distinguished on the basis of their morphology. Some of these lineaments have never been observed before on a comet and can offer important clues on the internal structures of the nucleus itself. The geologic mapping results presented here will allow us to better understand the processes which affected the nucleus' surface and thus the origin and evolutionary history of comet 67P/Churyumov-Gerasimenko.
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| 15. |
- Gicquel, A., et al.
(författare)
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Modelling of the outburst on 2015 July 29 observed with OSIRIS cameras in the Southern hemisphere of comet 67P/Churyumov-Gerasimenko
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 469, s. S178-S185
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Tidskriftsartikel (refereegranskat)abstract
- Images of the nucleus and the coma (gas and dust) of comet 67P/Churyumov-Gerasimenko have been acquired by the OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) cameras since 2014 March using both the wide-angle camera and the narrow-angle camera (NAC). We use images from the NAC camera to study a bright outburst observed in the Southern hemisphere on 2015 July 29. The high spatial resolution of the NAC is needed to localize the source point of the outburst on the surface of the nucleus. The heliocentric distance is 1.25 au and the spacecraft-comet distance is 186 km. Aiming to better understand the physics that led to the outgassing, we used the Direct Simulation Monte Carlo method to study the gas flow close to the nucleus and the dust trajectories. The goal is to understand the mechanisms producing the outburst. We reproduce the opening angle of the outburst in the model and constrain the outgassing ratio between the outburst source and the local region. The outburst is in fact a combination of both gas and dust, in which the active surface is approximately 10 times more active than the average rate found in the surrounding areas. We need a number of dust particles 7.83 x 10(11) to 6.90 x 10(15) (radius 1.97-185 mu m), which correspond to a mass of dust (220-21) x 10(3) kg.
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| 16. |
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| 17. |
- Hasselmann, P. H., et al.
(författare)
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The opposition effect of 67P/Churyumov-Gerasimenko on post-perihelion Rosetta images
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 469, s. S550-S567
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution OSIRIS/Rosetta images of 67P/Churyumov-Gerasimenko acquired on the night run of 2016 April 9-10 show, at large scale, an opposition effect (OE) spot sweeping across Imhotep as the phase angle ranges from 0 degrees to 17 degrees. In this work, we fitted the phase curve of the whole surface imaged as well as three particular features using both the linear-exponential and Hapke models. These features encompass different types of spectral behaviour: a circular mesa, one venous structure and an assemblage of bright spots, going from red to blue colours. Both the Hapke and linear-exponential parameters indicate a stepwise sharpening of the OE from bright spots to circular mesa. Yet a very broad nonlinear phase curve is verified and no sign of sharp OE associated with a coherent-backscattering mechanism is observed. We estimate that the 67P surface is dominated by opaque, desiccated and larger-than-wavelength irregular grains. Veins and bright spots display photometric properties consistent with surfaces becoming slightly brighter as they are enriched by high-albedo ice grains. We also report the estimation of normal albedo for all cometary regions observed throughout the image sequence. Comparison to pre-perihelion results indicates that far better insolation of northern brighter regions, i.e. Hapi, Hathor and Seth, is sufficient to explain mismatches on the photometric parameters. However, metre-scale photometric analysis of the Imhotep-Ash boundary area advocates for mild darkening (<7 per cent) of the surface at local scale.
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| 18. |
- Hu, X., et al.
(författare)
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Seasonal erosion and restoration of the dust cover on comet 67P/Churyumov-Gerasimenko as observed by OSIRIS onboard Rosetta
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 604
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Tidskriftsartikel (refereegranskat)abstract
- Context: Dust deposits or dust cover are a prevalent morphology in the northern hemi-nucleus of comet 67P /Churyumov-Gerasimenko (67P). The evolution of the dust deposits was captured by the OSIRIS camera system onboard the Rosetta spacecraft having escorted the comet for over two years. The observations shed light on the fundamental role of cometary activity in shaping and transforming the surface morphology.Aims: We aim to present OSIRIS observations of surface changes over the dust deposits before and after perihelion. The distribution of changes and a timeline of their occurrence are provided. We perform a data analysis to quantify the surface changes and investigate their correlation to water activity from the dust deposits. We further discuss how the results of our investigation are related to other findings from the Rosetta mission.Methods: Surface changes were detected via systematic comparison of images, and quantified using shape-from-shading technique. Thermal models were applied to estimate the erosion of water ice in response to the increasing insolation over the areas where surface changes occurred. Modeling results were used for the interpretation of the observed surface changes.Results: Surface changes discussed here were concentrated at mid-latitudes, between about 20 degrees N and 40 degrees N, marking a global transition from the dust-covered to rugged terrains. The changes were distributed in open areas exposed to ample solar illumination and likely subject to enhanced surface erosion before perihelion. The occurrence of changes followed the southward migration of the sub-solar point across the latitudes of their distribution. The erosion at locations of most changes was at least about 0 : 5 m, but most likely did not exceed several meters. The erosive features before perihelion had given way to a fresh, smooth cover of dust deposits after perihelion, suggesting that the dust deposits had been globally restored by at least about 1 m with ejecta from the intensely illuminated southern hemi-nucleus around perihelion, when the north was inactive during polar night.Conclusions: The erosion and restoration of the northern dust deposits are morphological expressions of seasonality on 67P. Based on observations and thermal modeling results, it is inferred that the dust deposits contained a few percent of water ice in mass on average. Local inhomogeneity in water abundance at spatial scales below tens of meters is likely. We suspect that dust ejected from the deposits may not have escaped the comet in bulk. That is, at least half of the ejected mass was afloat in the inner-coma or /and redeposited over other areas of the nucleus.
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| 19. |
- Ip, W. -H, et al.
(författare)
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Physical properties and dynamical relation of the circular depressions on comet 67P/Churyumov-Gerasimenko
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 591
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We aim to characterize the circular depressions of comet 67P/Churyumov-Gerasimenko and investigate whether such surface morphology of a comet nucleus is related to the cumulative sublimation effect since becoming a Jupiter family comet (JFC). Methods. The images from the Rosetta/OSIRIS science camera experiment are used to construct size frequency distributions of the circular depression structures on comet 67P and they are compared with those of the JFCs 81P/Wild 2, 9P/Tempel 1, and 103P/Hartley 2. The orbital evolutionary histories of these comets over the past 100 000 yr are analyzed statistically and compared with each other. Results. The global distribution of the circular depressions over the surface of 67P is charted and classified. Descriptions are given to the characteristics and cumulative size frequency distribution of the identified features. Orbital statistics of the JFCs visited by spacecraft are derived. Conclusions. The size frequency distribution of the circular depressions is found to have a similar power law distribution to those of 9P/Tempel 1 and 81P/Wild 2. This might imply that they could have been generated by the same process. Orbital integration calculation shows that the surface erosion histories of 81P/Wild 2, and 9P/Tempel 1 could be shorter than those of 67P, 103 P/Hartley 2 and 19P/Borrelly. From this point of view, the circular depressions could be dated back to the pre-JFC phase or the transneptunian phase of these comets. The north-south asymmetry in the distribution of the circular depressions could be associated with the heterogeneous structure of the nucleus of comet 67P and/or the solar insolation history.
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| 20. |
- Masoumzadeh, N., et al.
(författare)
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Opposition effect on comet 67P/Churyumov-Gerasimenko using Rosetta-OSIRIS images
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 599
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We aim to explore the behavior of the opposition effect as an important tool in optical remote sensing on the nucleus of comet 67P/Churyumov-Gerasimenko (67P), using Rosetta-OSIRIS images acquired in different filters during the approach phase, July-August 2014 and the close flyby images on 14 of February 2015, which contain the spacecraft shadow. Methods. We based our investigation on the global and local brightness from the surface of 67P with respect to the phase angle, also known as phase curve. The local phase curve corresponds to a region that is located at the Imhotep-Ash boundary of 67P. Assuming that the region at the Imhotep-Ash boundary and the entire nucleus have similar albedo, we combined the global and local phase curves to study the opposition-surge morphology and constrain the structure and properties of 67P. The model parameters were furthermore compared with other bodies in the solar system and existing laboratory study. Results. We found that the morphological parameters of the opposition surge decrease monotonically with wavelength, whereas in the case of coherent backscattering this behavior should be the reverse. The results from comparative analysis place 67P in the same category as the two Mars satellites, Phobos and Deimos, which are notably different from all airless bodies in the solar system. The similarity between the surface phase function of 67P and a carbon soot sample at extremely small angles is identified, introducing regolith at the boundary of the Imhotep-Ash region of 67P as a very dark and fluffy layer.
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