Karen J. Meech

Beginning of activity in 67P/Churyumov-Gerasimenko and predictions for 2014–2015

Context . Comet 67P/Churyumov-Gerasimenko was selected in 2003 as the new target of the Rosetta mission. It has since been the subject of a detailed campaign of observations to characterise its nucleus and activity. Aims . We present previously unpublished data taken around the start of activity of the comet in 2007/8, before its last perihelion passage. We constrain the time of the start of activity, and combine this with other data taken throughout the comet’s orbit to make predictions for its likely behaviour during 2014/5 while Rosetta is operating. Methods . A considerable difficulty in observing 67P during the past years has been its position against crowded fields towards the Galactic centre for much of the time. The 2007/8 data presented here were particularly difficult, and the comet will once again be badly placed for Earth-based observations in 2014/5. We make use of the difference image analysis technique, which is commonly used in variable star and exoplanet research, to remove background sources and extract images of the comet. In addition, we reprocess a large quantity of archival images of 67P covering its full orbit, to produce a heliocentric lightcurve. By using consistent reduction, measurement and calibration techniques we generate a remarkably clean lightcurve, which can be used to measure a brightness-distance relationship and to predict the future brightness of the comet. Results . We determine that the comet was active around November 2007, at a pre-perihelion distance from the Sun of 4.3 AU. The comet will reach this distance, and probably become active again, in March 2014. We find that the dust brightness can be well described by A fρ ∝ r -3.2 pre-perihelion and ∝ r -3.4 post-perihelion, and that the comet has a higher dust-to-gas ratio than average, with log (A fρ /Q(H 2 O)) = − 24.94 ± 0.22 cm s molecule -1 at r < 2 AU. A model fit to the photometric data suggests that only a small fraction (1.4%) of the surface is active.

Evidence for primordial water in Earth's deep mantle.

Shaking out waters dusty origin Where did Earths water come from? Lavas erupting on Baffin Island, Canada, tap a part of Earths mantle isolated from convective mixing. Hallis et al. studied hydrogen isotopes in the lavas that help to “fingerprint” the origin of water from what could be a primordial reservoir. The isotope ratios for the Baffin Island basalt lavas suggest a pre-solar origin of water in Earth, probably delivered by adsorption onto dust grains. Science, this issue p. 795 Baffin Island basalt hydrogen isotopes suggest a protosolar origin for Earth’s water. The hydrogen-isotope [deuterium/hydrogen (D/H)] ratio of Earth can be used to constrain the origin of its water. However, the most accessible reservoir, Earth’s oceans, may no longer represent the original (primordial) D/H ratio, owing to changes caused by water cycling between the surface and the interior. Thus, a reservoir completely isolated from surface processes is required to define Earth’s original D/H signature. Here we present data for Baffin Island and Icelandic lavas, which suggest that the deep mantle has a low D/H ratio (δD more negative than –218 per mil). Such strongly negative values indicate the existence of a component within Earth’s interior that inherited its D/H ratio directly from the protosolar nebula.

Methane, ammonia, and their irradiation products at the surface of an intermediate-size KBO? - A portrait of Plutino (90482) Orcus

Orcus is an intermediate-size 1000 km-scale Kuiper belt object (KBO) in 3:2 mean-motion resonance with Neptune, in an orbit very similar to that of Pluto. It has a water-ice dominated surface with solar-like visible colors. We present visible and near-infrared photometry and spectroscopy obtained with the Keck 10 m-telescope (optical) and the Gemini 8 m-telescope (near-infrared). We confirm the unambiguous detection of crystalline water ice as well as absorption in the 2.2 μm region. These spectral properties are close to those observed for Pluto’s larger satellite Charon, and for Plutino (208996) 2003 AZ84. Both in the visible and near-infrared Orcus’ spectral properties appear to be homogeneous over time (and probably rotation) at the resolution available. From Hapke radiative transfer models involving intimate mixtures of various ices we find for the first time that ammonium (NH + ) and traces of ethane (C2H6), which are most probably solar irradiation products of ammonia and methane, and a mixture of methane and ammonia (diluted or not) are the best candidates to improve the description of the data with respect to a simple water ice mixture (Haumea type surface). The possible more subtle structure of the 2.2 μm band(s) should be investigated thoroughly in the future for Orcus and other intermediate size Plutinos to better understand the methane and ammonia chemistry at work, if any. We investigated the thermal history of Orcus with a new 3D thermal evolution model. Simulations over 4.5×10 9 yr with an input 10% porosity, bulk composition of 23% amorphous water ice and 77% dust (mass fraction), and cold accretion show that even with the action of long-lived radiogenic elements only, Orcus should have a melted core and most probably suffered a cryovolcanic event in its history which brought large amounts of crystalline ice to the surface. The presence of ammonia in the interior would strengthen the melting process. A surface layer of a few hundred meters to a few tens of kilometers of amorphous water ice survives, while most of the remaining volume underneath contains crystalline ice. The crystalline water ice possibly brought to the surface by a past cryovolcanic event should still be detectable after several billion years despite the irradiation effects, as demonstrated by recent laboratory experiments.

Carbon monoxide in comet 9P/Tempel 1 before and after the Deep Impact encounter

One of the goals of the Hubble Space Telescope program to observe periodic comet 9P/Tempel 1 in conjunction with NASAs Deep Impact mission was to study the generation and evolution of the gaseous coma resulting from the impact. For this purpose, the Solar Blind Channel of the Advanced Camera for Surveys was used with the F140LP filter, which is sensitive primarily to the ultraviolet emission (≥1400 A) from the CO fourth positive system. Following the impact we detected an increase in brightness, which if all due to CO corresponds to 1.5 × 1031 molecules, or a mass of 6.6 × 105 kg, an amount that would normally be produced by 7-10 hours of quiescent outgassing from the comet. This number is ≤10% of the number of water molecules excavated and suggests that the volatile content of the material excavated by the impact did not differ significantly from the surface or near-subsurface material responsible for the quiescent outgassing of the comet.

The nucleus of 103P/Hartley 2, target of the EPOXI mission

Context. 103P/Hartley 2 was selected as the target comet for the Deep Impact extended mission, EPOXI, in October 2007. There have been no direct optical observations of the nucleus of this comet, as it has always been highly active when previously observed. Aims. We aimed to recover the comet near to aphelion, to: a) confirm that it had not broken up and was in the predicted position; b) to provide astrometry and brightness information for mission planning; and c) to continue the characterisation of the nucleus. Methods. We observed the comet at heliocentric distances between 5.7 and 5.5 AU, using FORS2 at the VLT, at 4 epochs between May and July 2008. We performed VRI photometry on deep stacked images to look for activity and measure the absolute magnitude and therefore estimate the size of the nucleus. Results. We recovered the comet near the expected position, with a magnitude of mR = 23.74 ± 0.06 at the first epoch. The comet had no visible coma, although comparison of the profile with a stellar one showed that there was faint activity, or possibly a contribution to the flux from the dust trail from previous activity. This activity appears to fade at further epochs, implying that this is a continuation of activity past aphelion from the previous apparition rather than an early start to activity before the next perihelion. Our data imply a nucleus radius of ≤1 km for an assumed 4% albedo; we estimate a ~6% albedo. We measure a colour of (V-R) = 0. 26 ± 0.09.

Observations of Comet 9P/Tempel 1 with the Keck 1 HIRES instrument during Deep Impact ☆ ☆☆

Abstract We report high-spectral resolution observations of Comet 9P/Tempel 1 before, during and after the impact on 4 July 2005 UT of the Deep Impact spacecraft with the comet. These observations were obtained with the HIRES instrument on Keck 1. We observed brightening of both the dust and gas, but at different rates. We report the behavior of OH, NH, CN, C 3 , CH, NH 2 and C 2 gas. From our observations, we determined a CN outflow velocity of at least 0.51 km s −1 . The dust color did not change substantially. To date, we see no new species in our spectra, nor do we see any evidence of prompt emission. From our observations, the interior material released by the impact looks the same as the material released from the surface by ambient cometary activity. However, further processing of the data may uncover subtle differences in the material that is released as well as the time evolution of this material.

Visible and Near-Infrared Spectrophotometry of the Deep Impact Ejecta of Comet 9P/Tempel 1

Abstract We have obtained optical spectrophotometry of the evolution of Comet 9P/Tempel 1 after the impact of the Deep Impact probe, using the Supernova Integral Field Spectrograph (SNIFS) at the UH 2.2-m telescope, as well as simultaneous optical and infrared spectra using the Lick Visible-to-Near-Infrared Imaging Spectrograph (VNIRIS). The spatial distribution and temporal evolution of the “violet band” CN (0–0) emission and of the 630 nm [OI] emission was studied. We found that CN emission centered on the nucleus increased in the 2 h after impact, but that this CN emission was delayed compared to the light curve of dust-scattered sunlight. The CN emission also expanded faster than the cloud of scattering dust. The emission of [OI] at 630 nm rose similarly to the scattered light, but then remained nearly constant for several hours after impact. On the day following the impact, both CN and [OI] emission concentrated on the comet nucleus had returned nearly to pre-impact levels. We have also searched for differences in the scattering properties of the dust ejected by the impact compared to the dust released under normal conditions. Compared to the pre-impact state of the comet, we find evidence that the color of the comet was slightly bluer during the post-impact rise in brightness. Long after the impact, in the following nights, the comet colors returned to their pre-impact values. This can be explained by postulating a change to a smaller particle size distribution in the ejecta cloud, in agreement with the findings from mid-infrared observations, or by postulating a large fraction of clean ice particles, or by a combination of these two.

Results from the EPOXI and StardustNExT Missions – A Changing View of Comet Volatiles and Activity

Within a period of ~3 months there were two extended mission flybys of comets. Both encounters have provided an exciting new view of comet activity and volatile composition that is changing our paradigm of these small early solar system remnants. The EPOXI mission flew past the nucleus of comet 103P/Hartley 2 on 4 Nov. 2010. This small nucleus was known to be exceptionally active prior to the encounter, by virtue of a very large water production rate relative to its surface area. Both the encounter and ground-based data showed that comet Hartley 2fs perihelion activity was dominated by sub-surface CO2 outgassing rather than by water, suggesting our classic comet formation picture is not correct. The gas flow carried large grains (up to >10 cm in diameter) from the nucleus, and the icy grains contributed to the large observed water production. The CO2 abundance relative to water varies with rotation between 10-20% between the two lobes of the nucleus. The bi-lobed nucleus is rotating in an excited state, with a period that varied rapidly from ~16.5 hrs to longer than 18.5 hrs over 3 months. The nucleus morphology was different from that of other nuclei visited by space craft, with some regions of rough topography in which surface ice was visible. On 2011 Feb. 14 the Stardust-NExT spacecraft flew past the nucleus of comet 9P/Tempel 1, the target of the Deep Impact (DI) experiment in July 2005. The mission goal was to look at the nucleus after and intervening perihelion passage, extending the surface area imaged during the DI encounter and also image the 2005 impact site. The layering seen during the DI flyby was exhibited over the areas newly imaged in the NExT flyby, and it was found that 30% of the nucleus was covered by smooth deposits that were likely caused by eruption of subsurface materials. Although it has long been known that comets lose on average ~ a meter of their surface per perihelion passage, it was surprising to see that in the regions imaged by both DI and NExT there was little change in the surface photometric properties and morphology with the exception of the prominent smooth flow edges. As seen from both the spacecraft and ground-based campaign, the comet continued its trend of decreasing activity from previous perihelion passages. We will present highlights from both missions and discuss implications for formation scenarios.

DIVISION III: COMMITTEE ON SMALL BODY NOMENCLATURE

The CSBN meeting held in Rio de Janeiro on August 11 was attended by just six members, including Pam Kilmartin as the acting chair, and several visitors. Since there was not a quorum of members, it was not possible to make any decisions. But there was a good discussion on many topics, from which several points emerged that should be more fully discussed by the whole committee during the next few months:

Spectrophotometry of the Deep Impact Ejecta of Comet 9P/Tempel 1

We have obtained optical spectrophotometry of the evolution of comet 9P/Tempel 1 after the impact of the Deep Impact spacecraft [1], using the SNIFS Supernova Integral Field Spectrograph at the UH 2.2 m telescope. From the data-cubes, we extracted both continuum flux distributions as well as emission line fluxes of the violet CN system and of [OI].We found that the continuum brightness of the comet, i.e., scattered sunlight, started rising immediately after the impact, but that the ejecta were slightly bluer in color than the material normally released by the comet.The emission of [OI] at 630 nm, which is a tracer of water, rose similar to the scattered continuum light, but then remained nearly constant for several hours after impact.We found that CN emission at 388 nm centered on the nucleus was delayed compared to the rise of dust-scattered sunlight. This CN emission also expanded faster spatially than the cloud of scattering dust.