arXiv subjects

P. H. Hasselmann

Publications and source records attributed to P. H. Hasselmann.

15 recordsLinked to original sources

arXiv2024

Phobos photometric properties from Mars Express HRSC observations

This study aims to analyze Phobos' photometric properties using Mars Express mission observations to support the Martian Moons eXploration mission. We analyzed resolved images of Phobos acquired between 2004 and 2022 by the HRSC and the SRC cameras on board the Mars Express spacecraft. We performed photometric analysis using the Hapke model for both integrated and disk-resolved data. The Phobos phase function has a strong opposition effect due to shadow hiding, with an amplitude and a half-width of the opposition surge of 2.28$\pm$0.03 and 0.0573$\pm$0.0001, respectively. Overall, the surface of Phobos is dark, with a geometric albedo of 6.8 % in the green filter and backscattering. We also found a surface porosity of 87\%, indicating the presence of a thick dust mantle or of fractal aggregates on the top surface. The SSA maps revealed high reflectance variability, with the blue unit area in the northeast Stickney rim being up to 65\% brighter than average, while the Stickney floor is among the darkest regions, with reflectance 10 to 20% lower than average. Photometric modeling of the regions of interest selected in the red and blue units indicates that red unit terrains have a stronger opposition effect and a smaller SSA value than the blue ones, but they have similar porosity and backscattering properties. The HRSC data provide a unique investigation of the Phobos phase function and opposition surge, which is valuable information for the MMX observational planning. The Phobos opposition surge, surface porosity, phase integral, and spectral slope are very similar to the values observed for the comet 67P and for Jupiter family comets in general. Based on these similarities, we formulate a hypothesis that the Mars satellites might be the results of a binary or bilobated comet captured by Mars.

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arXiv2024

Physical properties of asteroid Dimorphos as derived from the DART impact

On September 26, 2022, NASA's Double Asteroid Redirection Test (DART) mission successfully impacted Dimorphos, the natural satellite of the binary near-Earth asteroid (65803) Didymos. Numerical simulations of the impact provide a means to explore target surface material properties and structures, consistent with the observed momentum deflection efficiency, ejecta cone geometry, and ejected mass. Our simulation, which best matches observations, indicates that Dimorphos is weak, with a cohesive strength of less than a few pascals (Pa), similar to asteroids (162173) Ryugu and (101955) Bennu. We find that a bulk density of Dimorphos, rhoB, lower than 2400 kg/m3, and a low volume fraction of boulders (<40 vol%) on the surface and in the shallow subsurface, are consistent with measured data from the DART experiment. These findings suggest Dimorphos is a rubble pile that might have formed through rotational mass shedding and re-accumulation from Didymos. Our simulations indicate that the DART impact caused global deformation and resurfacing of Dimorphos. ESA's upcoming Hera mission may find a re-shaped asteroid, rather than a well-defined crater.

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arXiv2020

Phase reddening on asteroid Bennu from visible and near-infrared spectroscopy

The NASA mission OSIRIS-REx has been observing near-Earth asteroid (101955) Bennu in close proximity since December 2018. In this work, we investigate spectral phase reddening -- that is, the variation of spectral slope with phase angle -- on Bennu using spectra acquired by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) covering a phase angle range of 8-130$^{o}$. We investigate this process at the global scale and for some localized regions of interest (ROIs), including boulders, craters, and the designated sample collection sites of the OSIRIS-REx mission. Bennu has a globally negative spectra slope, which is typical of B-type asteroids. The spectral slope gently increases in a linear way up to a phase angle of 90$^{\circ}$, where it approaches zero. The spectral phase reddening is monotonic and wavelength-dependent with highest values in the visible range. Its coefficient is 0.00044 $μ$m$^{-1} ~deg^{-1}$ in the 0.55-2.5 $μ$m range. For observations of Bennu acquired at high phase angle (130$^{\circ}$), phase reddening increases exponentially. Similar behavior was reported in the literature for the carbonaceous chondrite Mukundpura in spectra acquired at extreme geometries. Some ROIs, including the sample collection site, Nightingale, have a steeper phase reddening coefficient than the global average, potentially indicating a surface covered by fine material with high micro-roughness. The gentle spectral phase reddening effect on Bennu is similar to that observed in ground-based measurements of other B-type asteroids, but much lower than that observed for other low-albedo bodies such as Ceres or comet 67P/Churyumov-Gerasimenko. Monotonic reddening may be associated with the presence of fine particles at micron scales and/or of particles with fractal structure that introduce micro- and sub-micro roughness across the surface of Bennu.

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arXiv2020

Spectrophotometric characterization of the Philae landing site and surroundings with the ROSETTA/OSIRIS cameras

We investigate Abydos, the final landing site of the Philae lander after its eventful landing from the Rosetta spacecraft on comet 67P/Churyumov-Gerasimenko on 12 November 2014. Over 1000 OSIRIS level 3B images were analysed, which cover the August 2014 to September 2016 timeframe, with spatial resolution ranging from 7.6 m/px to approximately 0.06 m/px. We found that the Abydos site is as dark as the global 67P nucleus and spectrally red, with an average albedo of 6.5% at 649 nm and a spectral slope value of about 17%/(100 nm) at 50$^\circ$ phase angle. Similar to the whole nucleus, the Abydos site also shows phase reddening but with lower coefficients than other regions of the comet which may imply a thinner cover of microscopically rough regolith compared to other areas. Seasonal variations, as already noticed for the whole nucleus, were also observed. We identified some potential morphological changes near the landing site implying a total mass loss of 4.7-7.0$\times$10$^5$ kg. Small spots ranging from 0.1 m$^2$ to 27 m$^2$ were observed close to Abydos before and after perihelion. Their estimated water ice abundance reaches 30-40% locally, indicating fresh exposures of volatiles. Their lifetime ranges from a few hours up to three months for two pre-perihelion spots. The Abydos surroundings showed low level of cometary activity compared to other regions of the nucleus. Only a few jets are reported originating nearby Abydos, including a bright outburst that lasted for about one hour.

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arXiv2019

Surface evolution of the Anhur region on comet 67P from high-resolution OSIRIS images

The southern hemisphere of comet 67P/Churyumov-Gerasimenko (67P) became observable by the Rosetta mission in March 2015, a few months before cometary southern vernal equinox. The Anhur region in the southern part of the comet's larger lobe was found to be highly eroded, enriched in volatiles, and highly active. We analyze high-resolution images of the Anhur region pre- and post-perihelion acquired by the OSIRIS imaging system on board the Rosetta mission. The Narrow Angle Camera is particularly useful for studying the evolution in Anhur in terms of morphological changes and color variations.}{Radiance factor images processed by the OSIRIS pipeline were coregistered, reprojected onto the 3D shape model of the comet, and corrected for the illumination conditions. We find a number of morphological changes in the Anhur region that are related to formation of new scarps; removal of dust coatings; localized resurfacing in some areas, including boulders displacements; and vanishing structures, which implies localized mass loss that we estimate to be higher than 50 million kg. The strongest changes took place in and nearby the Anhur canyon-like structure, where significant dust cover was removed, an entire structure vanished, and many boulders were rearranged. All such changes are potentially associated with one of the most intense outbursts registered by Rosetta during its observations, which occurred one day before perihelion passage. Moreover, in the niche at the foot of a new observed scarp, we also see evidence of water ice exposure that persisted for at least six months. The abundance of water ice, evaluated from a linear mixing model, is relatively high (> 20%). Our results confirm that the Anhur region is volatile-rich and probably is the area on 67P with the most pristine exposures near perihelion.

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arXiv2019

VIRTIS-H observations of comet 67P's dust coma: spectral properties and color temperature variability with phase and elevation

We analyze 2-5 micrometre spectroscopic observations of the dust coma of comet 67P/Churyumov-Gerasimenko obtained with the VIRTIS-H instrument onboard Rosetta from 3 June to 29 October 2015 at heliocentric distances r_h = 1.24-1.55 AU. The 2-2.5 micrometre color, bolometric albedo, and color temperature are measured using spectral fitting. Data obtained at alpha = 90° solar phase angle show an increase of the bolometric albedo (0.05 to 0.14) with increasing altitude (0.5 to 8 km), accompanied by a possible marginal decrease of the color and color temperature. Possible explanations include the presence in the inner coma of dark particles on ballistic trajectories, and radial changes in particle composition. In the phase angle range 50-120°, phase reddening is significant (0.031 %/100 nm/°), for a mean color of 2 %/100 nm at alpha = 90°, that can be related to the roughness of the dust particles. Moreover, a decrease of the color temperature with decreasing phase angle is also observed at a rate of ~ 0.3 K/°, consistent with the presence of large porous particles, with low thermal inertia, and showing a significant day-to-night temperature contrast. Comparing data acquired at fixed phase angle (alpha = 90°), a 20% increase of the bolometric albedo is observed near perihelion. Heliocentric variations of the dust color are not significant in the analyzed time period. Measured color temperatures are varying from 260 to 320 K, and follow a r^0.6 variation in the r_h = 1.24-1.5 AU range, close to the expected r_h^0.5 value.

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arXiv2018

ROSETTA/OSIRIS observations of the 67P nucleus during the April 2016 flyby: high-resolution spectrophotometry

In April 2016, the Rosetta spacecraft performed a low-altitude low-phase-angle flyby over the Imhotep-Khepry transition of 67P/Churyumov-Gerasimenko's 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° and 62°. 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 feature's 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/° 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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arXiv2018

Linking surface morphology, composition, and activity on the nucleus of 67P/Churyumov-Gerasimenko

The Rosetta space probe accompanied comet 67P/Churyumov-Gerasimenko for more than two years, obtaining an unprecedented amount of unique data of the comet nucleus and inner coma. This work focuses identifying the source regions of faint jets and outbursts and on studying the spectrophotometric properties of some outbursts. We use observations acquired with the OSIRIS/NAC camera during July-October 2015, that is, close to perihelion. More than 200 jets of different intensities were identified directly on the nucleus. Some of the more intense outbursts appear spectrally bluer than the comet dark terrain in the vivible-to-near-infrared region. We attribute this spectral behavior to icy grains mixed with the ejected dust. Some of the jets have an extremely short lifetime. They appear on the cometary surface during the color sequence observations, and vanish in less than some few minutes after reaching their peak. We also report a resolved dust plume observed in May 2016 at a resolution of 55 cm/pixel, which allowed us to estimate an optical depth of $\sim$0.65 and an ejected mass of $\sim$ 2200 kg. We present the results on the location, duration, and colors of active sources on the nucleus of 67P from the medium-resolution (i.e., 6-10 m/pixel) images acquired close to perihelion passage. The observed jets are mainly located close to boundaries between different morphological regions. Jets depart not only from cliffs, but also from smooth and dust-covered areas, from fractures, pits, or cavities that cast shadows and favor the recondensation of volatiles. This study shows that faint jets or outbursts continuously contribute to the cometary activity close to perihelion passage, and that these events are triggered by illumination conditions. Faint jets or outbursts are not associated with a particular terrain type or morphology.

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arXiv2017

Asteroid (21) Lutetia: Disk-resolved Photometric Analysis of Baetica Region

(21) Lutetia has been visited by Rosetta mission on July 2010 and observed with a phase angle ranging from 0.15 to 156.8 degrees. The Baetica region, located at the north pole has been extensively observed by OSIRIS cameras system. Baetica encompass a region called North Pole Crater Cluster (NPCC), shows a cluster of superposed craters which presents signs of variegation at the small phase angle images. For studying the location, we used 187 images distributed throughout 14 filter recorded by the NAC (Narrow Angle Camera) and WAC (Wide Angle Camera) taken during the fly-by. We photometrically modeled the region using Minnaert disk-function and Akimov phase function to finally reconstruct a resolved spectral slope map at 5 and 20 degrees of phase angle. We observed a dichotomy between Gallicum and Danuvius-Sarnus Labes in the NPCC, but no significant phase reddening. In the next step, we applied the Hapke (2008, 2012) model for the NAC F82+F22 (649.2 nm), WAC F13 (375 nm) and WAC F17 (631.6 nm), enabling us to compose the normal albedo and Hapke parameter maps for NAC F82+F22. On Baetica, the 649 nm global properties are: geometric albedo of 0.205+-0.005, the average single-scattering albedo of 0.181+-0.005, the average asymmetric factor of -0.342+-0.003, the average shadow-hiding opposition effect amplitude and width respectivelly of 0.824+-0.002 and 0.040+-0.0007, the average roughness slope of 11.45+-3 deg. and the average porosity is 85+-0.2%. In the NPCC, the normal albedo variegation among the craters walls reach 8% brighter for Gallicum Labes and 2% fainter for Danuvius Labes. The Hapke parameter maps also show a dichotomy at the opposition effect coefficients, single-scattering albedo and asymmetric factor, that may be attributed to the maturation degree of the regolith or to compositonal variation.

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arXiv2017

Characterizing spectral continuity in SDSS u'g'r'i'z' asteroid photometry

Context. The 4th release of the SDSS Moving Object Catalog (SDSSMOC) is presently the largest photometric dataset of asteroids. Up to this point, the release of large asteroid datasets has always been followed by a redefinition of asteroid taxonomy. In the years that followed the release of the first SDSSMOC, several classification schemes using its data were proposed, all using the taxonomic classes from previous taxonomies. However, no successful attempt has been made to derive a new taxonomic system directly from the SDSS dataset. Aims. The scope of the work is to propose a different interpretation scheme for gauging u0g0r0i0z0 asteroid observations based on the continuity of spectral features. The scheme is integrated into previous taxonomic labeling, but is not dependent on them. Methods. We analyzed the behavior of asteroid sampling through principal components analysis to understand the role of uncertainties in the SDSSMOC. We identified that asteroids in this space follow two separate linear trends using reflectances in the visible, which is characteristic of their spectrophotometric features. Results. Introducing taxonomic classes, we are able to interpret both trends as representative of featured and featureless spectra. The evolution within the trend is connected mainly to the band depth for featured asteroids and to the spectral slope for featureless ones. We defined a different taxonomic system that allowed us to only classify asteroids by two labels. Conclusions. We have classified 69% of all SDSSMOC sample, which is a robustness higher than reached by previous SDSS classifications. Furthermore, as an example, we present the behavior of asteroid (5129) Groom, whose taxonomic labeling changes according to one of the trends owing to phase reddening. Now, such behavior can be characterized by the variation of one single parameter, its position in the trend.

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arXiv2017

The highly active Anhur-Bes regions in the 67P/Churyumov - Gerasimenko comet: results from OSIRIS/ROSETTA observations

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 erosion 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.

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arXiv2016

Decimetre-scaled spectrophotometric properties of the nucleus of comet 67P/Churyumov-Gerasimenko from OSIRIS observations

We present the results of the photometric and spectrophotometric properties of the 67P/Churyumov-Gerasimenko nucleus derived with the OSIRIS instrument during the closest fly-by over the comet, which took place on 14 th February 2015 at a distance of {\~} 6 km from the surface. Several images covering the 0°-33° phase angle range were acquired, and the spatial resolution achieved was 11 cm/pxl. The flown-by region is located on the big lobe of the comet, near the borders of the Ash, Apis and Imhotep regions. Our analysis shows that this region features local heterogeneities at the decimetre scale. We observed difference of reflectance up to 40{\%} between bright spots and sombre regions, and spectral slope variations up to 50{\%}. The spectral reddening effect observed globally on the comet surface by Fornasier et al. (2015) is also observed locally on this region, but with a less steep behaviour. We note that numerous metre-sized boulders, which exhibit a smaller opposition effect, also appear spectrally redder than their surroundings. In this region, we found no evidence linking observed bright spots to exposed water-ice-rich material. We fitted our dataset using the Hapke 2008 photometric model. The region overflown is globally as dark as the whole nucleus (geometric albedo of 6.8{\%}) and it has a high porosity value in the uppermost-layers (86{\%}). These results of the photometric analysis at a decimetre scale indicate that the photometric properties of the flown-by region are similar to those previously found for the whole nucleus.

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arXiv2016

Detection of exposed H$_2$O ice on the nucleus of comet 67P/Churyumov-Gerasimenko

Since the orbital insertion of the Rosetta spacecraft, comet 67P/Churyumov-Gerasimenko (67P/C-G) has been mapped by OSIRIS camera and VIRTIS spectro-imager, producing a huge quantity of images and spectra of the comet's nucleus. The aim of this work is to search for the presence of H$_2$O on the nucleus which, in general, appears very dark and rich in dehydrated organic material. After selecting images of the bright spots which could be good candidates to search for H$_2$O ice, taken at high resolution by OSIRIS, we check for spectral cubes of the selected coordinates to identify these spots observed by VIRTIS. The selected OSIRIS images were processed with the OSIRIS standard pipeline and corrected for the illumination conditions for each pixel using the Lommel-Seeliger disk law. The spots with higher I/F were selected and then analysed spectrophotometrically and compared with the surrounding area. We selected 13 spots as good targets to be analysed by VIRTIS to search for the 2 micron absorption band of water ice in the VIRTIS spectral cubes. Out of the 13 selected bright spots, eight of them present positive H$_2$O ice detection on the VIRTIS data. A spectral analysis was performed and the approximate temperature of each spot was computed. The H$_2$O ice content was confirmed by modeling the spectra with mixing (areal and intimate) of H$_2$O ice and dark terrain, using Hapke's radiative transfer modeling. We also present a detailed analysis of the detected spots.

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arXiv2015

Spectrophotometric properties of the nucleus of comet 67P/Churyumov-Gerasimenko from the OSIRIS instrument onboard the ROSETTA spacecraft

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 of 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.

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arXiv2013

Surface composition and dynamical evolution of two retrograde objects in the outer solar system: 2008 YB3 and 2005 VD

Most of the objects in the trans-Neptunian belt (TNb) and related populations move in prograde orbits with low eccentricity and inclination. However, the list of icy minor bodies moving in orbits with an inclination above 40 deg. has increased in recent years. The origin of these bodies, and in particular of those objects in retrograde orbits, is not well determined, and different scenarios are considered. In this paper, we present new observational and dynamical data of two objects in retrograde orbits, 2008 YB3 and 2005 VD. We find that the surface of these extreme objects is depleted of ices and does not contain the 'ultra-red' matter typical of some Centaurs. Despite small differences, these objects share common colors and spectral characteristics with the Trojans, comet nuclei, and the group of grey Centaurs. All of these populations are supposed to be covered by a mantle of dust responsible for their reddish- to neutral-color. To investigate if the surface properties and dynamical evolution of these bodies are related, we integrate their orbits for 10^(8) years to the past. We find a remarkable difference in their dynamical evolutions: 2005 VD' s evolution is dominated by a Kozai resonance with planet Jupiter while that of 2008 YB3 is dominated by close encounters with planets Jupiter and Saturn. Our models suggest that the immediate site of provenance of 2005 VD is the in the Oort cloud, whereas for 2008 YB3 it is in the trans-Neptunian region. Additionally, the study of their residence time shows that 2005 VD has spent a larger lapse of time moving in orbits in the region of the giant planets than 2008 YB3. Together with the small differences in color between these two objects, with 2005 VD being more neutral than 2008 YB3, this fact suggests that the surface of 2005 VD has suffered a higher degree of processing, probably related to cometary activity episodes.

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