Call for NASA Discovery Comet Mission Summaries
The NASA Discovery Program enables principal investigator-led space missions with a cost cap of $450 million (in 2015). Comets have been the target of two Discovery missions (Stardust, Deep Impact), which have made critical scientific advancements in our field. However, not since 2005 have we had a full mission dedicated to comet science. The last Discovery proposal due date was 2015 Feb 18, and the outer planets community has already advertised their missions during the last Outer Planets Assessment Group Meeting. The next Small-Bodies Assessment Group Meeting will be in July, but rather than wait, we can generate buzz for comet science now.
The Cometary Science Newsletter* solicits brief summaries (<1500 characters) of submitted Discovery 2014 proposals related to comets. Submit your mission's summary at the CSN submission page.
*Note, the CSN Editor is not involved in any Discovery 2014 proposal.
Announcements for cometary conferences or workshops. Limited to 2000 characters.
Stardust Workshop, 2015 Jul 24-25
I would like to draw your attention to a Stardust Workshop that we will be hosting in Berkeley on the Friday and Saturday before the Meteoritical Society meeting, Jul 24-25.
The principal goal of the workshop will be to summarize the current state of of analysis of Stardust Cometary and Interstellar collector samples, and to identify key areas of need for future analysis. We aim to produce a publication, either in the form of a book or group of papers in a special edition. We hope to generate a summary paper or chapter on the composition of Wild 2, as a reference and resource for the broader community of cosmochemists, comet spectroscopists and modelers, and astronomers.
Possible topics include: Wild 2 vs other comets vs meteorite classes vs IDPs, hydrous alteration products, sublimation vs melting, Wild 2 component ages — why so young?, presolar grain abundances in Wild 2, Rosetta findings vs Wild 2, noble gases in Wild 2, nature and abundance of metals in Wild 2, challenges and highest priority measurements for interstellar dust candidates, architecture of a new interstellar dust mission, and more.
Please consider attending, and please tell us if you are interested in giving a talk. We will not require a fee or an abstract for the workshop registration. For anyone staying in dormitory accommodation offered for the MetSoc 2015 meeting, there is an option to extend your stay for the workshop.
Abstracts of articles in press or recently published. Limited to 3000 characters.
Disappearance of Comet C/2010 X1 (Elenin): Gone with a Whimper, not a Bang
- Department of Earth, Planetary and Space Sciences, University of California at Los Angeles, CA, USA
- Department of Physics and Astronomy, University of California at Los Angeles, CA, USA
We examine the rise and sudden demise of comet C/2010 X1 (Elenin) on its approach to perihelion. Discovered inbound at 4.2 AU, this long-period comet was predicted to become very bright when near perihelion, at 0.48 AU on 2011 September 10. Observations starting 2011 February (heliocentric distance ~3.5 AU) indeed show the comet to brighten by about 11 magnitudes, with most of the increase occurring inside 1 AU from the Sun. The peak brightness reached mR = 6 on UT 2011 August 12.95±0.50, when at ~0.83 AU from the Sun. Thereafter, the comet faded even as the heliocentric distance continued to decrease. We find that most of the surge in brightness in mid-August resulted from dust particle forward-scattering, not from a sudden increase in the activity. A much smaller (~3 magnitudes) brightening began on UT 2011 August 18±1 (heliocentric distance 0.74 AU), reached a maximum on UT 2011 August 30±1 (at 0.56 AU), and reflects the true break-up of the nucleus. This second peak was matched by a change in the morphology from centrally condensed to diffuse. The estimated cross-section of the nucleus when at 1 AU inbound was ~1 km2, corresponding to an equal-area circle of radius 0.6 km. Observations were taken after the second peak using the Canada-France-Hawaii 3.6-m telescope to search for surviving fragments of the nucleus. None were found to a limiting red magnitude r' = 24.4, corresponding to radii ≲40 m (red geometric albedo = 0.04 assumed). The brightening, the progressive elongation of the debris cloud and the absence of a central condensation in data taken after UT 2011 August 30 are consistent with disintegration of the nucleus into a power law size distribution of fragments with index q = 3.3±0.2 combined with the action of radiation pressure. In such a distribution, the largest particles contain most of the mass while the smallest particles dominate the scattering cross-section and apparent brightness. We speculate about physical processes that might cause nucleus disruption in a comet when still 0.7 AU from the Sun. Tidal stresses and devolatilization of the nucleus by sublimation are both negligible at this distance. However, the torque caused by mass loss, even at the very low rates measured in comet Elenin, is potentially large enough to be responsible by driving the nucleus to rotational instability.
The Astronomical Journal (In press)
NASA ADS: 2015arXiv150300387L arXiv: 1503.00387
Peculiar Near-Nucleus Outgassing of Comet 17P/Holmes During Its 2007 Outburst
- Harvard-Smithsonian Center for Astrophysics, USA
- Leiden Observatory, Netherlands
- UCLA, USA
We present high angular resolution Submillimeter Array observations of the outbursting Jupiter family comet 17P/Holmes on 2007 October 26-29, achieving a spatial resolution of 2."5, or ~3000 km at the comet distance. The observations resulted in detections of the rotational lines CO 3-2, HCN 4-3, H13CN 4-3, CS 7-6, H2CO 31,2-21,1, H2S 22,0-21,1, and multiple CH3OH lines, along with the associated dust continuum at 221 and 349 GHz. The continuum has a spectral index of 2.7 +/- 0.3, slightly steeper than blackbody emission from large dust particles. From the imaging data, we identify two components in the molecular emission. One component is characterized by a relatively broad line width (~1 km s-1 FWHM) exhibiting a symmetric outgassing pattern with respect to the nucleus position. The second component has a narrower line width (< 0.5 km s-1 FWHM) with the line center redshifted by 0.1-0.2 km s-1 (cometocentric frame), and shows a velocity shift across the nucleus position with the position angle gradually changing from 66 deg; to 30 deg; within the four days of observations. We determine distinctly different CO/HCN ratios for each of the components. For the broad-line component we find CO/HCN < 7, while in the narrow-line component, CO/HCN = 40 +/- 5. We hypothesize that the narrow-line component originates from the ice grain halo found in near-nucleus photometry, believed to be created by sublimating recently released ice grains around the nucleus during the outburst. In this interpretation, the high CO/HCN ratio of this component reflects the more pristine volatile composition of nucleus material released in the outburst.
The Astrophysical Journal (Published)
DOI: 10.1088/0004-637X/799/1/110 NASA ADS: 2015ApJ...799..110Q arXiv: 1411.4632
Plasma Distribution of Comet ISON (C/2012 S1) Observed Using the Radio Scintillation Method
- Solar-Terrestrial Environment Laboratory, Nagoya University, Japan.
- Department of Aerospace Engineering, Nihon University, Japan.
We observed a plasma tail of Comet C/2012 S1 (ISON) using the interplanetary scintillation (IPS) method in November, 2013. We identified the radio source 1148-00 whose line-of-sight approached the ISON's plasma tail and obtained its IPS data using a radio telescope at 327 MHz. From our examinations, we confirmed three IPS enhancements of 1148-00, which could be attributed to the plasma tail of Comet ISON. IPS power spectra of 1148-00 had the steeper slope than normal ones during its occultation, which suggested turbulence with larger spatial scales in the plasma tail. For the electron density in the ISON's tail, we found 84 cm-3 around the tail axis at a distance of 3.74×107 km from the cometary nucleus and an unexpected variation of that in the vicinity of the tail boundary.
DOI: 10.1016/j.icarus.2015.02.007 NASA ADS: 2015Icar..252..301I arXiv: 1410.2119
A Significant Amount of Crystalline Silica in Returned Cometary Samples: Bridging the Gap Between Astrophysical and Meteoritical Observations
- Unité Matériaux et Transformations, Université Lille 1, CNRS, UMR 8207, F-59655 Villeneuve d’Ascq, France
Crystalline silica (SiO2) is recurrently identified at the percent level in the infrared spectra of protoplanetary disks. By contrast, reports of crystalline silica in primitive meteorites are very unusual. This dichotomy illustrates the typical gap existing between astrophysical observations and meteoritical records of the first solids formed around young stars. The cometary samples returned by the Stardust mission in 2006 offer an opportunity to have a closer look at a silicate dust that experienced a very limited reprocessing since the accretion of the dust. Here, we provide the first extended study of silica materials in a large range of Stardust samples. We show that cristobalite is the dominant form. It was detected in 5 out of 25 samples. Crystalline silica is thus a common minor phase in Stardust samples. Furthermore, olivine is generally associated with this cristobalite, which put constraints on possible formation mechanisms. A low-temperature subsolidus solid–solid transformation of an amorphous precursor is most likely. This crystallization route favors the formation of olivine (at the expense of pyroxenes), and crystalline silica is the natural byproduct of this transformation. Conversely, direct condensation and partial melting are not expected to produce the observed mineral assemblages. Silica is preserved in cometary materials because they were less affected by thermal and aqueous alterations than their chondritic counterparts. The common occurrence of crystalline silica therefore makes the cometary material an important bridge between the IR-based mineralogy of distant protoplanetary disks and the mineralogy of the early solar system.
The Astrophysical Journal Letters (Published)
DOI: 10.1088/2041-8205/801/1/L7 NASA ADS: 2015ApJ...801L...7R
The Pre-perihelion Activity of Dynamically New Comet C/2013 A1 (SIDING SPRING) and Its Close Encounter With Mars
- Department of Astronomy, University Maryland, College Park, MD 20742, USA
- Planetary Science Institute, 1700 East Fort Lowell Road, Suite 106, Tucson, AZ 85719, USA
We used the UltraViolet-Optical Telescope on board Swift to systematically follow the dynamically new comet C/2013 A1 (Siding Spring) on its approach to the Sun. The comet was observed from a heliocentric distance of 4.5 AU pre-perihelion to its perihelion at 1.4 AU. From our observations, we estimate that the water production rate during closest approach to Mars was 1.5 ± 0.3 × 1028 molecules/s, that peak gas delivery rates were between 4.5 and 8.8 kg/s, and that in total between 3.1 and 5.4 × 104 kg cometary gas was delivered to the planet. Seasonal and evolutionary effects on the nucleus govern the pre-perihelion activity of comet Siding Spring. The sudden increase of its water production between 2.46 and 2.06 AU suggests the onset of the sublimation of icy grains in the coma, likely driven by CO2. As the comet got closer to the Sun, the relative contribution of the nucleus’ water production increased, while CO2 production rates decreased. The changes in the comet’s activity can be explained by a depletion of CO2, but the comet’s high mass loss rate suggests they may reflect primordial heterogeneities in the nucleus.
The Astrophysical Journal Letters (Published)
DOI: 10.1088/2041-8205/802/1/L6 NASA ADS: 2015arXiv150207216B arXiv: 1502.07216
Fast Rotation and Trailing Fragments of the Active Asteroid P/2012 F5 (Gibbs)
- Jagiellonian University, Poland
- California Institute of Technology, USA
- Max Planck Institute for Solar System Research, Germany
- University of California at Los Angeles, USA
- Carnegie Institution for Science, USA
While having a comet-like appearance, P/2012 F5 (Gibbs) has an orbit native to the Main Asteroid Belt, and physically is a km-sized asteroid which recently (mid 2011) experienced an impulsive mass ejection event. Here we report new observations of this object obtained with the Keck II telescope on UT 2014 August 26. The data show previously undetected 200 m scale fragments of the main nucleus, and reveal a rapid nucleus spin with a rotation period of 3.24 ± 0.01 hr. The existence of large fragments and the fast nucleus spin are both consistent with rotational instability and partial disruption of the object. To date, many fast rotators have been identified among the minor bodies, which, however, do not eject detectable fragments at the present-day epoch, and also fragmentation events have been observed, but with no rotation period measured. P/2012 F5 is unique in that for the first time we detected fragments and quantified the rotation rate of one and the same object. The rapid spin rate of P/2012 F5 is very close to the spin rates of two other active asteroids in the Main Belt, 133P/Elst-Pizarro and (62412), confirming the existence of a population of fast rotators among these objects. But while P/2012 F5 shows impulsive ejection of dust and fragments, the mass loss from 133P is prolonged and recurrent. We believe that these two types of activity observed in the rapidly rotating active asteroids have a common origin in the rotational instability of the nucleus.
The Astrophysical Journal Letters (Published)
DOI: 10.1088/2041-8205/802/1/L8 NASA ADS: 2015ApJ...802L...8D arXiv: 1503.05632
Considerations Regarding the Colors and Low Surface Albedo of Comets Using Hapke Methodology
- Lunar and Planetary Laboratory, University of Arizona, Tucson, Az, 85721
The single scattering albedos (SSA’s) determined for 9P/Tempel 1 are interpreted in terms of the Hapke model of irregular particle scattering efficiencies. Absorption coefficients versus wavelength from 0.31 to 2.5 μm are obtained. It is shown that the colors and exceedingly low reported SSA’s in the UV region of the spectrum below 0.4 μm cannot be reproduced with the geometric Hapke scattering model for irregular particles. However, by increasing the reported SSA’s by a small amount, absorption coefficients for particle radii of 10-100 μm vs. wavelength from 0.31 to 2.5 μm can be fitted. Several reasons are given for slightly increasing the SSA’s, such as neglect of the effects of porosity, having a more complex phase function for the particles, uncertainties in the absolute calibration and the uncertainties associated with the complex treatment of surface roughness. The absorption coefficients determined show good agreement with potential surface constituents Mg rich olivine and pyroxene with some amount of darkening iron or organic component.
DOI: 10.1016/j.icarus.2014.12.018 NASA ADS: 2015Icar..252...32F