The Carbon-Rich Gas in the Beta Pictoris Circumstellar Disk

The edge-on disk surrounding the nearby young star Beta Pictoris is the archetype of the "debris disks", which are composed of dust and gas produced by collisions and evaporation of planetesimals, analogues of Solar System comets and asteroids. These disks provide a window on the formation and early evolution of terrestrial planets. Previous observations of Beta Pic concluded that the disk gas has roughly solar abundances of elements [1], but this poses a problem because such gas should be rapidly blown away from the star, contrary to observations of a stable gas disk in Keplerian rotation [1, 2]. Here we report the detection of singly and doubly ionized carbon (CII, CIII) and neutral atomic oxygen (OI) gas in the Beta Pic disk; measurement of these abundant volatile species permits a much more complete gas inventory. Carbon is extremely overabundant relative to every other measured element. This appears to solve the problem of the stable gas disk, since the carbon overabundance should keep the gas disk in Keplerian rotation [3]. New questions arise, however, since the overabundance may indicate the gas is produced from material more carbon-rich than the expected Solar System analogues.Comment: Accepted for publication in Nature. PDF document, 12 pages. Supplementary information may be found at http://www.dtm.ciw.edu/akir/Documents/roberge_supp.pdf *** Version 2 : Removed extraneous publication information, per instructions from the Nature editor. No other changes mad

The 2016 Feb 19 outburst of comet 67P/CG: an ESA Rosetta multi-instrument study

On 2016 Feb 19, nine Rosetta instruments serendipitously observed an outburst of gas and dust from the nucleus of comet 67P/Churyumov-Gerasimenko. Among these instruments were cameras and spectrometers ranging from UV over visible to microwave wavelengths, in situ gas, dust and plasma instruments, and one dust collector. At 09:40 a dust cloud developed at the edge of an image in the shadowed region of the nucleus. Over the next two hours the instruments recorded a signature of the outburst that significantly exceeded the background. The enhancement ranged from 50 per cent of the neutral gas density at Rosetta to factors >100 of the brightness of the coma near the nucleus. Dust related phenomena (dust counts or brightness due to illuminated dust) showed the strongest enhancements (factors >10). However, even the electron density at Rosetta increased by a factor 3 and consequently the spacecraft potential changed from ∼−16 V to −20 V during the outburst. A clear sequence of events was observed at the distance of Rosetta (34 km from the nucleus): within 15 min the Star Tracker camera detected fast particles (∼25 m s−1) while 100 μm radius particles were detected by the GIADA dust instrument ∼1 h later at a speed of 6 m s−1. The slowest were individual mm to cm sized grains observed by the OSIRIS cameras. Although the outburst originated just outside the FOV of the instruments, the source region and the magnitude of the outburst could be determined

A homogeneous nucleus for comet 67P/Churyumov-Gerasimenko from its gravity field

Cometary nuclei consist mostly of dust and water ice(1). Previous observations have found nuclei to be low-density and highly porous bodies(2-4), but have only moderately constrained the range of allowed densities because of the measurement uncertainties. Here we report the precise mass, bulk density, porosity and internal structure of the nucleus of comet 67P/Churyumov-Gerasimenko on the basis of its gravity field. The mass and gravity field are derived from measured spacecraft velocity perturbations at fly-by distances between 10 and 100 kilometres. The gravitational point mass is GM = 666.2 +/- 0.2 cubic metres per second squared, giving a mass M = (9,982 +/- 3) x 10(9) kilograms. Together with the current estimate of the volume of the nucleus(5), the average bulk density of the nucleus is 533 +/- 6 kilograms per cubic metre. The nucleus appears to be a low-density, highly porous (72-74 per cent) dusty body, similar to that of comet 9P/Tempel 1(2,3). The most likely composition mix has approximately four times more dust than ice by mass and two times more dust than ice by volume. We conclude that the interior of the nucleus is homogeneous and constant in density on a global scale without large voids. The high porosity seems to be an inherent property of the nucleus material

Comet 67P/Churyumov-Gerasimenko sheds dust coat accumulated over the past four years

International audienceComets are composed of dust and frozen gases. The ices are mixed with the refractory material either as an icy conglomerate1, or as an aggregate of pre-solar grains (grains that existed prior to the formation of the Solar System), mantled by an ice layer2, 3. The presence of water-ice grains in periodic comets is now well established4, 5, 6. Modelling of infrared spectra obtained about ten kilometres from the nucleus of comet Hartley 2 suggests that larger dust particles are being physically decoupled from fine-grained water-ice particles that may be aggregates7, which supports the icy-conglomerate model. It is known that comets build up crusts of dust that are subsequently shed as they approach perihelion8, 9, 10. Micrometre-sized interplanetary dust particles collected in the Earth’s stratosphere and certain micrometeorites are assumed to be of cometary origin11, 12, 13. Here we report that grains collected from the Jupiter-family comet 67P/Churyumov-Gerasimenko come from a dusty crust that quenches the material outflow activity at the comet surface14. The larger grains (exceeding 50 micrometres across) are fluffy (with porosity over 50 per cent), and many shattered when collected on the target plate, suggesting that they are agglomerates of entities in the size range of interplanetary dust particles. Their surfaces are generally rich in sodium, which explains the high sodium abundance in cometary meteoroids15. The particles collected to date therefore probably represent parent material of interplanetary dust particles. This argues against comet dust being composed of a silicate core mantled by organic refractory material and then by a mixture of water-dominated ices2, 3. At its previous recurrence (orbital period 6.5 years), the comet’s dust production doubled when it was between 2.7 and 2.5 astronomical units from the Sun14, indicating that this was when the nucleus shed its mantle. Once the mantle is shed, unprocessed material starts to supply the developing coma, radically changing its dust component, which then also contains icy grains, as detected during encounters with other comets closer to the Sun4, 5

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

Pits have been observed on many cometary nuclei mapped by spacecraft1,2,3,4. 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 experiments5,6 and models7,8 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 impacts8,9. Alternative mechanisms like explosive activity10 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

EPOXI: Comet 103P/Hartley 2 observations from a worldwide campaign

Earth- and space-based observations provide synergistic information for space mission encounters by providing data over longer timescales, at different wavelengths and using techniques that are impossible with an in situ flyby. We report here such observations in support of the EPOXI spacecraft flyby of comet 103P/Hartley 2. The nucleus is small and dark, and exhibited a very rapidly changing rotation period. Prior to the onset of activity, the period was 16.4hr. Starting in 2010 August the period changed from 16.6hr to near 19hr in December. With respect to dust composition, most volatiles and carbon and nitrogen isotope ratios, the comet is similar to other Jupiter-family comets. What is unusual is the dominance of CO 2 -driven activity near perihelion, which likely persists out to aphelion. Near perihelion the comet nucleus was surrounded by a large halo of water-ice grains that contributed significantly to the total water production. © 2011. The American Astronomical Society. All rights reserved.