I. Toth

Large heterogeneities in comet 67P as revealed by active pits from sinkhole collapse

Pits have been observed on many cometary nuclei mapped by spacecraft. It has been argued that cometary pits are a signature of endogenic activity, rather than impact craters such as those on planetary and asteroid surfaces. Impact experiments and models cannot reproduce the shapes of most of the observed cometary pits, and the predicted collision rates imply that few of the pits are related to impacts. Alternative mechanisms like explosive activity have been suggested, but the driving process remains unknown. Here we report that pits on comet 67P/Churyumov–Gerasimenko are active, and probably created by a sinkhole process, possibly accompanied by outbursts. We argue that after formation, pits expand slowly in diameter, owing to sublimation-driven retreat of the walls. Therefore, pits characterize how eroded the surface is: a fresh cometary surface will have a ragged structure with many pits, while an evolved surface will look smoother. The size and spatial distribution of pits imply that large heterogeneities exist in the physical, structural or compositional properties of the first few hundred metres below the current nucleus surface.

Are fractured cliffs the source of cometary dust jets ? insights from OSIRIS/Rosetta at 67P/Churyumov-Gerasimenko

Dust jets (i.e., fuzzy collimated streams of cometary material arising from the nucleus) have been observed in situ on all comets since the Giotto mission flew by comet 1P/Halley in 1986, and yet their formation mechanism remains unknown. Several solutions have been proposed involving either specific properties of the active areas or the local topography to create and focus the gas and dust flows. While the nucleus morphology seems to be responsible for the larger features, high resolution imagery has shown that broad streams are composed of many smaller jets (a few meters wide) that connect directly to the nucleus surface. nAims. We monitored these jets at high resolution and over several months to understand what the physical processes are that drive their formation and how this affects the surface. nMethods. Using many images of the same areas with different viewing angles, we performed a 3-dimensional reconstruction of collimated jets and linked them precisely to their sources on the nucleus. nResults. We show here observational evidence that the northern hemisphere jets of comet 67P/Churyumov-Gerasimenko arise from areas with sharp topographic changes and describe the physical processes involved. We propose a model in which active cliffs are the main source of jet-like features and therefore of the regions eroding the fastest on comets. We suggest that this is a common mechanism taking place on all comets.

The rotation state of 67P/Churyumov-Gerasimenko from approach observations with the OSIRIS cameras on Rosetta

Aims: Approach observations with the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) experiment onboard Rosetta are used to determine the rotation period, the direction of the spin axis, and the state of rotation of comet 67Ps nucleus. Methods: Photometric time series of 67P have been acquired by OSIRIS since the post wake-up commissioning of the payload in March 2014. Fourier analysis and convex shape inversion methods have been applied to the Rosetta data as well to the available ground-based observations. Results: Evidence is found that the rotation rate of 67P has significantly changed near the time of its 2009 perihelion passage, probably due to sublimation-induced torque. We find that the sidereal rotation periods P1 = 12.76129 ± 0.00005 h and P2 = 12.4043 ± 0.0007 h for the apparitions before and after the 2009 perihelion, respectively, provide the best fit to the observations. No signs of multiple periodicity are found in the light curves down to the noise level, which implies that the comet is presently in a simple rotation state around its axis of largest moment of inertia. We derive a prograde rotation model with spin vector J2000 ecliptic coordinates λ = 65° ± 15°, β = + 59° ± 15°, corresponding to equatorial coordinates RA = 22°, Dec = + 76°. However, we find that the mirror solution, also prograde, at λ = 275° ± 15°, β = + 50° ± 15° (or RA = 274°, Dec = + 27°), is also possible at the same confidence level, due to the intrinsic ambiguity of the photometric problem for observations performed close to the ecliptic plane. Table 1 is available in electronic form at http://www.aanda.org

Gravitational slopes, geomorphology and material strengths of the nucleus of comet 67P/Churyumov-Gerasimenko from OSIRIS observations

We study the link between gravitational slopes and the surface morphology on the nucleus of comet 67P/Churyumov- Gerasimenko and provide constraints on the mechanical properties of the cometary material (tensile, shear, and compressive strengths). nMethods. We computed the gravitational slopes for five regions on the nucleus that are representative of the different morpholo- gies observed on the surface (Imhotep, Ash, Seth, Hathor, and Agilkia), using two shape models computed from OSIRIS images by the stereo-photoclinometry (SPC) and stereo-photogrammetry (SPG) techniques. We estimated the tensile, shear, and compressive strengths using different surface morphologies (overhangs, collapsed structures, boulders, cliffs, and Philae’s footprint) and mechani- cal considerations. nResults. The different regions show a similar general pattern in terms of the relation between gravitational slopes and terrain mor- phology: i) low-slope terrains (0–20◦) are covered by a fine material and contain a few large (>10m) and isolated boulders, ii) intermediate-slope terrains (20–45◦) are mainly fallen consolidated materials and debris fields, with numerous intermediate-size boulders from <1 m to 10 m for the majority of them, and iii) high-slope terrains (45 – 90◦ ) are cliffs that expose a consolidated mate- rial and do not show boulders or fine materials. The best range for the tensile strength of overhangs is 3 – 15 Pa (upper limit of 150 Pa), 4 – 30 Pa for the shear strength of fine surface materials and boulders, and 30 – 150 Pa for the compressive strength of overhangs (up- per limit of 1500 Pa). The strength-to-gravity ratio is similar for 67P and weak rocks on Earth. As a result of the low compressive strength, the interior of the nucleus may have been compressed sufficiently to initiate diagenesis, which could have contributed to the formation of layers. Our value for the tensile strength is comparable to that of dust aggregates formed by gravitational instability and tends to favor a formation of comets by the accrection of pebbles at low velocities.

Hubble Space Telescope observations of the nucleus and inner coma of comet 67P/Churyumov-Gerasimenko

Context. Following the postponement of the launch of the Rosetta spacecraft scheduled in January 2003, comet 67P/Churyumov-Gerasimenko emerged as the most suitable new target. However a critical issue was the size, that is, the mass of its nucleus, as the surface module Philae was designed to land on a nucleus with a radius no larger than approximately 1.5 km. Aims. It was therefore crucial to the success of the mission to achieve a timely characterization of the nucleus of 67P/C-G so as to take any proper action on the design needed before the new launch. Methods. We used the Wide Field Planetary Camera 2 (WFPC2) of the Hubble Space Telescope (HST) whose superior spatial resolution enabled us to accurately separate the signal of the nucleus from that of the coma. The observations were performed on 12 and 13 March 2003, when the comet was 2.50 AU from the Sun, 1.52 AU from the Earth, and at a phase angle of 4.8°. Results. The nucleus of comet 67P/Churyumov-Gerasimenko was easily detected, and we measured its V and R magnitudes in the Johnson-Kron-Cousins photometric system. Assuming that the nucleus is spherical, with a geometric albedo of 0.04 and a phase coefficient of 0.04 mag/deg for the R band, we derived a radius of 1.98 ± 0.02 km. The spheroidal solution has semiaxes a = 2.41 km and b = c = 1.55 km and a rotational period of 12.41 ± 0.41 h. We inverted the light curve and solved it for a full three-dimensional nucleus having an overall size of 4.56 × 3.81 x 3.44 km viewed at an aspect angle of ∼80° (i.e., near equatorial view) at the time of our observations. When combined with other constraints, the direction of the pole is found at either RA = 40° +70° 20° and Dec = +70 ± 10° (prograde rotation) or RA = 250 ± 30° and Dec = -70 ± 10° (retrograde rotation). The color of the nucleus is moderately red with (V - R) = 0.52 ±0.05. From an analysis of the dust coma, we derived A fp = 40.2 ±0.3 cm, a dust production rate Q d ∼ 4 kg s -1 , and we characterized its color.

Thermal properties, sizes, and size distribution of Jupiter-family cometary nuclei

We present results from SEPPCoN, an on-going Survey of the Ensemble Physical Properties of Cometary Nuclei. In this report we discuss mid-infrared measurements of the thermal emission from 89 nuclei of Jupiter-family comets (JFCs). All data were obtained in 2006 and 2007 using imaging capabilities of the Spitzer Space Telescope. The comets were typically 4–5 AU from the Sun when observed and most showed only a point-source with little or no extended emission from dust. For those comets showing dust, we used image processing to photometrically extract the nuclei. For all 89 comets, we present new effective radii, and for 57 comets we present beaming parameters. Thus our survey provides the largest compilation of radiometrically-derived physical properties of nuclei to date. We have six main conclusions: (a) The average beaming parameter of the JFC population is 1.03 ± 0.11, consistent with unity; coupled with the large distance of the nuclei from the Sun, this indicates that most nuclei have Tempel 1-like thermal inertia. Only two of the 57 nuclei had outlying values (in a statistical sense) of infrared beaming. (b) The known JFC population is not complete even at 3 km radius, and even for comets that approach to ∼2 AU from the Sun and so ought to be more discoverable. Several recently-discovered comets in our survey have small perihelia and large (above ∼2 km) radii. (c) With our radii, we derive an independent estimate of the JFC nuclear cumulative size distribution (CSD), and we find that it has a power-law slope of around −1.9, with the exact value depending on the bounds in radius. (d) This power-law is close to that derived by others from visible-wavelength observations that assume a fixed geometric albedo, suggesting that there is no strong dependence of geometric albedo with radius. (e) The observed CSD shows a hint of structure with an excess of comets with radii 3–6 km. (f) Our CSD is consistent with the idea that the intrinsic size distribution of the JFC population is not a simple power-law and lacks many sub-kilometer objects.

The nuclei of comets 126P/IRAS and 103P/Hartley 2

We report the detection of the nucleus of 126P/IRAS and 103P/Hartley 2 with the Infrared Camera of the Infrared Space Observatory (ISOCAM). 126P/IRAS was observed on 12 November 1996, when it was at rh = 1.71 AU from the Sun and ∆= 1.32 AU from the Earth. 103P/Hartley 2 was observed on 5 February 1998, when it was at rh = 1.21 AU from the Sun and ∆= 0.91 AU from the Earth. The observations were performed with the broadband LW10 filter centered at 11.5 µm. The spatial resolution was adequate to separate the thermal emission of the nuclei from that of their respective comae. We combined the ISOCAM observations with measured water production rates, using a model that considers a spherical nucleus with a macroscopic mosaic of small and numerous active and inactive regions, and we derived a radius of 1.57 ± 0.14 km and an active fraction at perihelion of 0.11 ± 0.03 for 126P/IRAS, and a radius of 0.71 ± 0.13 km and an active fraction of ∼ 1a t perihelion and 0.30 ± 0.11 at 1.11 AU post-perihelion for 103P/Hartley 2. These two examples illustrate the large diversity of activity pattern that exists among cometary nuclei.

Rosetta’s comet 67P/Churyumov-Gerasimenko sheds its dusty mantle to reveal its icy nature

Rosetta observes sublimating surface ices Comets are “dirty snowballs” made of ice and dust, but they are dark because the ice sublimates away, leaving some of the dust behind on the surface. The Rosetta spacecraft has provided a close-up view of the comet 67P/Churyumov-Gerasimenko as it passes through its closest point to the Sun (see the Perspective by Dello Russo). Filacchione et al. detected the spectral signature of solid CO2 (dry ice) in small patches on the surface of the nucleus as they emerged from local winter. By modeling how the CO2 sublimates, they constrain the composition of comets and how ices generate the gaseous coma and tail. Fornasier et al. studied images of the comet and discovered bright patches on the surface where ice was exposed, which disappeared as the ice sublimated. They also saw frost emerging from receding shadows. The surface of the comet was noticeably less red just after local dawn, indicating that icy material is removed by sunlight during the local day. Science, this issue p. 1563, p. 1566; see also p. 1536 Rosetta spotted patches of ice on the surface of a comet, which quickly sublimate in sunlight. 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.

Temporal morphological changes in the Imhotep region of comet 67P/Churyumov-Gerasimenko

We report on the first major temporal morphological changes observed on the surface of the nucleus of comet 67P/Churyumov-Gerasimenko in the smooth terrains of the Imhotep region. We used images of the OSIRIS cameras onboard Rosetta to follow the temporal changes from 24 May 2015 to 11 July 2015. The morphological changes observed on the surface are visible in the form of roundish features that are growing in size from a given location in a preferential direction at a rate of 5.6-8.1 x 10 -5 m s -1 during the observational period. The location where the changes started and the contours of the expanding features are bluer than the surroundings, which suggests that ices (H 2 O and/or CO 2 are exposed on the surface. However, sublimation of ices alone is not sufficient to explain the observed expanding features. No significant variations in the dust activity pattern are observed during the period of changes.

Variegation of comet 67P/Churyumov-Gerasimenko in regions showing activity

We carried out an investigation of the surface variegation of comet 67P/Churyumov-Gerasimenko, the detection of regions showing activity, the determination of active and inactive surface regions of the comet with spectral methods, and the detection of fallback material. nMethods. We analyzed multispectral data generated with Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) narrow angle camera (NAC) observations via spectral techniques, reflectance ratios, and spectral slopes in order to study active regions. We applied clustering analysis to the results of the reflectance ratios, and introduced the new technique of activity thresholds to detect areas potentially enriched in volatiles. nResults. Local color inhomogeneities are detected over the investigated surface regions. Active regions, such as Hapi, the active pits of Seth and Ma’at, the clustered and isolated bright features in Imhotep, the alcoves in Seth and Ma’at, and the large alcove in Anuket, have bluer spectra than the overall surface. The spectra generated with OSIRIS NAC observations are dominated by cometary emissions of around 700 nm to 750 nm as a result of the coma between the comet’s surface and the camera. One of the two isolated bright features in the Imhotep region displays an absorption band of around 700 nm, which probably indicates the existence of hydrated silicates. An absorption band with a center between 800–900 nm is tentatively observed in some regions of the nucleus surface. This absorption band can be explained by the crystal field absorption of Fe2+, which is a common spectral feature seen in silicates.