Here we present some preliminary results on surface variegation found on (21) Lutetia from ROSETTA-OSIRIS images acquired on 2010-07-10. The spectrophotometry obtained by means of the two cameras NAC ...and WAC (Narrow and Wide Angle Cameras) is consistent with ground based observations, and does not show surface diversity above the data error bars. The blue and UV images (shortward 500nm) may, however, indicate a variegation of the optical properties of the asteroid surface on the Baetica region (Sierks et al., 2011). We also speculate on the contribution due to different illumination and to different ground properties (composition or, more probably, grain size diversity). In particular a correlation with geologic units independently defined by Massironi et al. (2012) is evident, suggesting that the variegation of the ground optical properties is likely to be real.
► Preliminary analysis of surface variegation on Lutetia from OSIRIS images. ► NAC and WAC spectrophotometry is consistent with ground based observations. ► Indications of optical properties variegation in Blue and UV images. ► Illumination and composition contribution separated only at a zero level. ► Correlation between geological units and variegated regions found.
Context. We investigate the dust coma within the Hill sphere of comet 67P/Churyumov-Gerasimenko. Aims. We aim to determine osculating orbital elements for individual distinguishable but unresolved ...slow-moving grains in the vicinity of the nucleus. In addition, we perform photometry and constrain grain sizes. Methods. We performed astrometry and photometry using images acquired by the OSIRIS Wide Angle Camera on the European Space Agency spacecraft Rosetta. Based on these measurements, we employed standard orbit determination and orbit improvement techniques. Results. Orbital elements and effective diameters of four grains were constrained, but we were unable to uniquely determine them. Two of the grains have light curves that indicate grain rotation. Conclusions. The four grains have diameters nominally in the range 0.14–0.50 m. For three of the grains, we found elliptic orbits, which is consistent with a cloud of bound particles around the nucleus. However, hyperbolic escape trajectories cannot be excluded for any of the grains, and for one grain this is the only known option. One grain may have originated from the surface shortly before observation. These results have possible implications for the understanding of the dispersal of the cloud of bound debris around comet nuclei, as well as for understanding the ejection of large grains far from the Sun.
Abstract
The ASI cubesat LICIACube has been part of the first planetary defense mission DART, having among its scopes to complement the DRACO images to better constrain the Dimorphos shape. LICIACube ...had two different cameras, LEIA and LUKE, and to accomplish its goal, it exploited the unique possibility of acquiring images of the Dimorphos hemisphere not seen by DART from a vantage point of view, in both time and space. This work is indeed aimed at constraining the tridimensional shape of Dimorphos, starting from both LUKE images of the nonimpacted hemisphere of Dimorphos and the results obtained by DART looking at the impacted hemisphere. To this aim, we developed a semiautomatic Computer Vision algorithm, named VADER, able to identify objects of interest on the basis of physical characteristics, subsequently used as input to retrieve the shape of the ellipse projected in the LUKE images analyzed. Thanks to this shape, we then extracted information about the Dimorphos ellipsoid by applying a series of quantitative geometric considerations. Although the solution space coming from this analysis includes the triaxial ellipsoid found by using DART images, we cannot discard the possibility that Dimorphos has a more elongated shape, more similar to what is expected from previous theories and observations. The result of our work seems therefore to emphasize the unique value of the LICIACube mission and its images, making even clearer the need of having different points of view to accurately define the shape of an asteroid.
Context. The ESA cometary mission Rosetta was launched in 2004. In the past years and until the spacecraft hibernation in June 2011, the two cameras of the OSIRIS imaging system (Narrow Angle and ...Wide Angle Camera, NAC and WAC) observed many different sources. On 20 January 2014 the spacecraft successfully exited hibernation to start observing the primary scientific target of the mission, comet 67P/Churyumov-Gerasimenko. Aims. A study of the past performances of the cameras is now mandatory to be able to determine whether the system has been stable through the time and to derive, if necessary, additional analysis methods for the future precise calibration of the cometary data. Methods. The instrumental responses and filter passbands were used to estimate the efficiency of the system. A comparison with acquired images of specific calibration stars was made, and a refined photometric calibration was computed, both for the absolute flux and for the reflectivity of small bodies of the solar system. Results. We found a stability of the instrumental performances within ±1.5% from 2007 to 2010, with no evidence of an aging effect on the optics or detectors. The efficiency of the instrumentation is found to be as expected in the visible range, but lower than expected in the UV and IR range. A photometric calibration implementation was discussed for the two cameras. Conclusions. The calibration derived from pre-hibernation phases of the mission will be checked as soon as possible after the awakening of OSIRIS and will be continuously monitored until the end of the mission in December 2015. A list of additional calibration sources has been determined that are to be observed during the forthcoming phases of the mission to ensure a better coverage across the wavelength range of the cameras and to study the possible dust contamination of the optics.
Near‐Earth asteroid (101955) Bennu is an active asteroid experiencing mass loss in the form of ejection events emitting up to hundreds of millimeter‐ to centimeter‐scale particles. The close ...proximity of the Origins, Spectral Interpretations, Resource Identification, and Security–Regolith Explorer spacecraft enabled monitoring of particles for a 10‐month period encompassing Bennu's perihelion and aphelion. We found 18 multiparticle ejection events, with masses ranging from near zero to hundreds of grams (or thousands with uncertainties) and translational kinetic energies ranging from near zero to tens of millijoules (or hundreds with uncertainties). We estimate that Bennu ejects ~104 g per orbit. The largest event took place on 6 January 2019 and consisted of ~200 particles. The observed mass and translational kinetic energy of the event were between 459 and 528 g and 62 and 77 mJ, respectively. Hundreds of particles not associated with the multiparticle ejections were also observed. Photometry of the best‐observed particles, measured at phase angles between ~70° and 120°, was used to derive a linear phase coefficient of 0.013 ± 0.005 magnitudes per degree of phase angle. Ground‐based data back to 1999 show no evidence of past activity for Bennu; however, the currently observed activity is orders of magnitude lower than observed at other active asteroids and too low be observed remotely. There appears to be a gentle decrease in activity with distance from the Sun, suggestive of ejection processes such as meteoroid impacts and thermal fracturing, although observational bias may be a factor.
Plain Language Summary
We measured the brightness of pebble‐sized particles in the vicinity of near‐Earth asteroid Bennu to better understand their physical characteristics and the events that launched them from Bennu's surface. Our measurements spanned 10 months, encompassing Bennu's closest and farthest distances from the Sun, so that we could assess how the level of ejection activity changes with solar distance. We observed 18 multiparticle ejection events containing anywhere from a few to 200+ particles. Individual particles ranged from millimeters to centimeters in diameter. The energy of the events and a possible decrease in activity with larger distances from the Sun suggest that meteoroid impacts, fracturing of surface boulders due to solar heating, or both may be responsible for ejecting the particles. We estimate that Bennu releases ~10,000 g of material over one orbit or 1.2 years. Although mass loss has been remotely observed for other asteroids, the comparatively low level of particle ejection activity at Bennu was only observable thanks to the close proximity of the Origins, Spectral Interpretations, Resource Identification, and Security–Regolith Explorer spacecraft.
Key Points
Asteroid (101955) Bennu is active from perihelion through aphelion with a possible decrease in activity further from the Sun
Bennu's activity is less than that detected by telescope for other active asteroids and is only observable up close
The particles' shallow phase functions resemble those of similarly sized individual rocks rather than those of ensemble asteroid surfaces
Context. Data derived from the reconstruction of the nucleus shape of comet 67P/Churyumov-Gerasimenko (67P) from images of the OSIRIS camera onboard ROSETTA show evidence that the nucleus rotates in ...complex mode. First, the orientation of the spin axis is not fixed in an inertial reference frame, which suggests a precessing motion around the angular momentum vector with a periodicity of approximately 257 h ± 12 h.Second, periodograms of the right ascension and declination (RA/Dec) coordinates of the body-frame Z axis show a very significant (higher than 99.99%) periodicity at 276 h ± 12 h, different from the rotational period of 12.40 h as previously determined from light-curve analysis. Aims. The main goal is to interpret the data and associated periodicities of the spin axis orientation in space. Methods. We analyzed the spin axis orientation in space and associated periodicities and compared them with solutions of Euler equations under the assumption that the body rotates in torque-free conditions. Statistical tests comparing the observationally derived spin axis orientation with the outcome from simulations were applied to determine the most likely inertia moments, excitation level, and periods. Results. Under the assumption that the body is solid-rigid and rotates in torque-free conditions, the most likely interpretation is that 67P is spinning around the principal axis with the highest inertia moment with a period of about 13 h. At the same time, the comet precesses around the angular momentum vector with a period of about 6.35 h. While the rotating period of such a body would be about 12.4 h, RA/Dec coordinates of the spin axis would have a periodicity of about 270 h as a result of the combination of the two aforementioned motions. Conclusions. The most direct and simple interpretation of the complex rotation of 67P requires a ratio of inertia moments significantly higher than that of a homogeneous body.
The first billion years of Martian geologic history consisted of surface environments and landscapes dramatically different from those seen today, with flowing liquid water sculpting river channels ...and ponding to form bodies of water. However, the hydro‐climatic context, the frequency, and the duration under which these systems existed remain uncertain. Addressing these fundamental questions may improve our understanding of early Mars climate. Here, we reconstruct a long‐lived archive consisting of an array of fluvial systems inside the Antoniadi crater––one of the largest lake basins on Mars (9.58 × 104 km2). We found that the fluvial activity occurred throughout four major intermittent active intervals during the Late Noachian to Early Amazonian (∼3.7 to >2.4 Ga). This resulted in at least two major lakes, which formed during periods of markedly increased surface runoff production. The record of these four riverine phases is preserved in fluvial ridges, valley networks, back‐stepping or down‐stepping fan‐shaped landforms, and terrace‐like formations within an outlet canyon. These morphologies point to lake‐controlled base‐level fluctuations suggestive of episodic precipitation‐fed surface runoff punctuated by intermittent catastrophic floods that were capable of breaching crater‐lake rims and incising outlet canyons. Fluvial‐deposit thickness, junction angles of channels, and lake morphometry suggest that riverine systems lasted at least 103–106 years and episodically occurred under primarily arid and semi‐arid climates. These findings place new regional constraints on the fluvial frequency, longevity, and climatic regime of one of the largest Martian lakes, thereby bolstering the hypothesis that episodic warming likely punctuated the planet's early history.
Plain Language Summary
The planet Mars is now a vast desert. However, geologic evidence points to radically different kinds of landscapes in the past, with precipitation‐fed ancient rivers and lakes. As a consequence, questions have been raised about the climatic and environmental contexts that persisted during the formation of these hydrological records. Here, we have used high‐resolution remotely sensed data to constrain the volumes, frequency, and periodicity of an array of water‐formed landforms inside one of the largest lake systems on Mars that occupy the Antoniadi crater. We demonstrate that the Antoniadi crater was intermittently wet, hosting multiple rivers and at least two main bodies of standing water between 3.7 and 2.4 Ga. The morphometries of the lake and river systems imply that they episodically survived between a few thousand and 1 million years under arid climates. These findings make Antoniadi an interesting site for future Mars exploration dedicated to the potential ancient habitability of Mars because of such long‐lived fluvial history.
Key Points
Antoniadi crater is the site of an ancient lake that was punctuated by locally and regionally wet conditions between 3.7 and 2.4 Ga
Antoniadi crater likely records at least four episodes of surface runoff
The river and lake systems at Antoniadi were probably active for 103–106 years, supporting long‐lived fluvial activity under arid climates
Abstract
On 2022 September 26, the DART spacecraft will impact the surface of Dimorphos, the ∼160 m size satellite of the binary near-Earth asteroid (NEA) (65803) Didymos. What will be observed on ...the surfaces of both asteroids and at the DART impact site is largely unknown, beyond the details of Didymos revealed by previous Arecibo and Goldstone radar observations. We present here the expected DART and LICIACube observations of the Didymos system and discuss the planned mapping strategies. By searching similar geological features and processes identified on other NEAs, we constrain the impact conditions that DART might encounter at Dimorphos, assessing both the asteroid’s surface and interior structure.
Spectroscopic measurements are a powerful tool to investigate the surface composition of airless bodies and provide clues of their origin. The composition and origin of Phobos and Deimos are still ...unknown and are currently widely debated. We present spectroscopic measurements of Phobos and Deimos at ultraviolet and visible wavelengths (250–650 nm) made by the NOMAD‐Ultraviolet and Visible Spectrometer (UVIS) on the ExoMars TGO mission. These new spectra cover multiple areas on Phobos and Deimos, and are of generally higher spectral resolution and signal‐to‐noise than previous spectra, and extend to lower wavelengths than most previous measurements. The UVIS spectra confirm a red‐sloped spectrum lacking any strong absorption features; however, we confirm the presence of a previously identified absorption feature near 0.65 μm and tentative absorption near 0.45 μm. The observed Phobos and Deimos spectra are similar to D‐ and T‐type asteroids, adding weight to the captured asteroid hypothesis for the moons' origins. We also find, however, that the UVIS Phobos reflectance spectra of Phobos' red unit is a relatively close match to the olivine‐rich, highly shocked Mars meteorite NWA 2737, with a low overall reflectance, a red‐sloped spectrum, and lack of olivine‐associated absorption bands in the UVIS spectral range. This meteorite, however, exhibits spectral features at longer wavelengths that not observed in the Martian moon spectra, indicating a need for further investigation at longer wavelengths to interpret whether this material could inform our understanding of Phobos' origin.
Plain Language Summary
Whether the Martian moons Phobos and Deimos are captured asteroids that originated outside the Mars system or formed from a debris disk around Mars during its formation or following a large impact on Mars is widely debated. Measurements of reflected sunlight from the surfaces of Phobos and Deimos can provide pivotal information on their composition as well as clues to their origins. In this study, we present a comparison between observations of Phobos and Deimos made with the NOMAD Ultraviolet and VIsible Spectrometer (UVIS) on the ExoMars Trace Gas Orbiter (TGO) and previous spectral measurements from other missions. The UVIS spectra show a good agreement with previous observations, with the Phobos and Deimos spectra showing a red‐sloped spectrum similar to primitive carbonaceous chondrite asteroids. A change in the spectral slope at wavelengths longer than 0.53 μm is shown to be consistent with the previously identified 0.65 μm absorption feature. The similarity between the measured Phobos ultraviolet and visible spectrum and highly‐shocked olivine from the Mars meteorite NWA 2737 entices the question of whether Phobos and Deimos surfaces could be composed of a similar dark olivine matrix and formed within the accretion disk created from a giant impact on Mars.
Key Points
Phobos and Deimos reflectance spectra have been obtained from the ExoMars NOMAD spectrometer covering 250–650 nm
UVIS measurements confirm the spectral similarity of Phobos and Deimos to primitive carbonaceous chondrite asteroids
The spectrum of Phobos is analogous to some features of shock‐produced dark olivine which could support an alternative planetary origin
Abstract
We modeled the geometry and the three-dimensional orientation of the ejecta cone triggered by the impact of the DART spacecraft on the asteroid Dimorphos. We used eight LUKE images of the ...impact acquired by the CubeSat LICIACube that flew by the Didymos system shortly after the impact. These images, which show the ejecta cone in both face-on and side-on profiles, enabled us to reconstruct the ejecta cone in inertial space. We started our model as a simple cone with a circular base and developed it to a rotated cone with an elliptical base that best fit the data. The cone axis points to R.A., decl. (in J2000):
147
−
10
°
+
1
°
, +
16
−
6
°
+
4
°
. The cone is characterized by two perpendicular half-angles of
η
=
69
−
3
°
+
1
°
,
γ
=
51
−
11
°
+
1
°
and a rotation of
ω
= 12° around its axis. The apex of the cone is located near the center of Dimorphos within 15 m. The intersection of the cone and the surface of Dimorphos (surface enclosed by the cone) would correspond to a crater with a maximum radius of about 65 m. The characterization of the cone axis is directly related to the computation of the momentum enhancement factor (
β
) of the impact, and it hence proves the crucial need of studying impacts in the context of planetary defence scenarios. The results of this work could potentially be used to constrain whether the impact took place in a strength-dominated or a gravity-dominated regime. This work shows the important scientific return of the LICIACube CubeSat in the context of planetary defence.
PDF
