SBAG Science Goals DocumentComet science goals, input due by September 19. http://www.lpi.usra.edu/sbag/goals/
NASA SBAG Cometary Science Goals
The Small Bodies Assessment Group (SBAG) is developing a three-part Goals Document to serve as a concise collection of community-wide goals. Of particular interest to the comet community is the Science Goals section, which highlights major science questions, planetary mission priorities (New Frontiers or larger), research and analysis contributions, and key facilities and programs for each of the major sub-categories of small bodies, with a page specifically devoted to cometary science. This is intended to be a "live" document, with planned updates to take place roughly yearly.
Draft versions of each section are now on the SBAG website, and feedback from the community is encouraged. Please direct feedback to the lead for the appropriate goal by September 19, 2015.
- Emily Kramer (firstname.lastname@example.org)
Brief observational reports or other notes related to specific comets. Limited to 1000 characters. The CSN is not intended to replace telegram services or other breaking news outlets.
320P/McNaughtJupiter-family Comet 320P/McNaught passes earth 2015-Sep-05 at 0.1835 AU or 71.4 lunar distances.
Abstracts of articles in press or recently published. Limited to 3000 characters.
The Composition of Comets
- The University of Texas, McDonald Observatory, Austin, TX, USA
- UPMC, LATMOS, Paris, France
- NASA Goddard Space Flight Center, Greenbelt, MD, USA
- UPMC, LATMOS, Guyancourt, France
- UPS IRAP, Toulouse, France
- MPS, Gottingen, Germany
- Lowell Observatory, Flagstaff, AZ, USA
- LESIA, Observatoire de Paris, Meudon, France
- National Taiwan Normal University, Taiwan, ROC
This paper is the result of the International Cometary Workshop, held in Toulouse, France in April 2014, where the participants came together to assess our knowledge of comets prior to the ESA Rosetta Mission. In this paper, we look at the composition of the gas and dust from the comae of comets. With the gas, we cover the various taxonomic studies that have broken comets into groups and compare what is seen at all wavelengths. We also discuss what has been learned from mass spectrometers during flybys. A few caveats for our interpretation are discussed. With dust, much of our information comes from flybys. They include in situ analyses as well as samples returned to Earth for laboratory measurements. Remote sensing IR observations and polarimetry are also discussed. For both gas and dust, we discuss what instruments the Rosetta spacecraft and Philae lander will bring to bear to improve our understanding of comet 67P/Churyumov-Gerasimenko as "ground-truth" for our previous comprehensive studies. Finally, we summarize some of the initial Rosetta Mission findings.
Space Science Review (In press)
Dynamic Sublimation Pressure and the Catastrophic Breakup of Comet ISON
- Purdue University, Department of Physics and Astronomy, West Lafayette, IN 47907, USA
- Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, Cambridge, MA 02139, USA
- Purdue University, Department of Earth, Atmospheric, and Planetary Sciences, West Lafayette, IN 47907, USA
- Purdue University, Department of Aeronautical and Astronautical Engineering, West Lafayette, IN 47907, USA
- John Hopkins University – Applied Physics Laboratory, Laurel, MD, 20723, USA
- Naval Research Laboratory, Washington, D.C., 20375, USA
Previously proposed mechanisms have difficulty explaining the disruption of Comet C/2012 S1 (ISON) as it approached the Sun. We describe a novel cometary disruption mechanism whereby comet nuclei fragment and disperse through dynamic sublimation pressure, which induces differential stresses within the interior of the nucleus. When these differential stresses exceed its material strength, the nucleus breaks into fragments. We model the sublimation process thermodynamically and propose that it is responsible for the disruption of Comet ISON. We estimate the bulk unconfined crushing strength of Comet ISON’s nucleus and the resulting fragments to be 0.5 Pa and 1–9 Pa, respectively, assuming typical Jupiter Family Comet (JFC) albedos. However, if Comet ISON has an albedo similar to Pluto, this strength estimate drops to 0.2 Pa for the intact nucleus and 0.6-4 Pa for its fragments. Regardless of assumed albedo, these are similar to previous strength estimates of JFCs. This suggests that, if Comet ISON is representative of dynamically new comets, then low bulk strength is a primordial property of some comet nuclei, and not due to thermal processing during migration into the Jupiter Family.
Icarus (In press)
DOI: 10.1016/j.icarus.2015.06.032 arXiv: 1507.02669
Nucleus and Mass Loss in Active Asteroid 313P/Gibbs
- UCLA, Los Angeles
- Max Planck Ins. for Solar System, Gottingen
- Johns Hopkins Applied Physics Laboratory, Maryland
- Space Telescope Science Institute, Baltimoe
- Lunar and Planetary Laboratory, Arizona
We present Hubble Space Telescope observations of active asteroid 313P/Gibbs (formerly P/2014 S4) taken over the five month interval from 2014 October to 2015 March. This object has been recurrently active near perihelion (at 2.4 AU) in two different orbits, a property that is naturally explained by the sublimation of near surface ice but which is difficult to reconcile with other activity mechanisms. We find that the mass loss peaks near 1 kg/s in October and then declines over the subsequent months by about a factor of five, at nearly constant heliocentric distance. This decrease is too large to be caused by the change in heliocentric distance during the period of observation. However, it is consistent with sublimation from an ice patch shadowed by local topography, for example in a pit like those observed on the nuclei of short-period comet 67P/Churyumov-Gerasimenko. While no unique interpretation is possible, a simple self shadowing model shows that sublimation from a pit with depth to diameter ratio near 1/2 matches the observed rate of decline of the activity, while deeper and shallower pits do not. We estimate the nucleus radius to be 700+/-100 m (geometric albedo 0.05 assumed). Measurements of the spatial distribution of the dust were obtained from different viewing geometries. They show that dust was ejected continuously not impulsively, that the effective particle size is large, about 50 microns, and that the ejection speed is about 2.5 m/s. The total dust mass ejected is 10^7 kg, corresponding to about 10^-5 of the nucleus mass. The observations are consistent with partially shadowed sublimation from about 10^4 m^2 of ice, corresponding to 0.2 percent of the nucleus surface. For ice to survive in 313P for billion-year timescales requires that the duty cycle for sublimation be less than 0.001.
The Astronomical Journal (In press)
Dynamics of High-Velocity Evanescent Clumps [HVECs] Emitted from Comet C/2011 L4 as Observed by STEREO
- The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA.
- SPACS, College of Science, George Mason University, Fairfax, VA, USA.
- UCL Mullard Space Science Laboratory, Dorking, Surrey, UK.
- Materials Science \& Engineering, University of Virginia, Charlottesville, VA, USA.
High-quality white-light images from the SECCHI/HI-1 telescope onboard STEREO-B reveal high-velocity evanescent clumps [HVECs] expelled from the coma of the C/2011 L4 [Pan-STARRS] comet. The observations were recorded around the comet's perihelion [i.e., ~0.3 AU] during the period 09–16 March 2013. Animated images provide evidence of highly dynamic ejecta moving near-radially in the anti-sunward direction. The bulk speed of the clumps at their initial detection in the HI1-B images range from 200–400 km s-1 followed by an appreciable acceleration up to speeds of 450–600 km s-1, which are typical of slow to intermediate solar wind speeds. The clump velocities do not exceed these limiting values and seem to reach a plateau. The images also show that the clumps do not expand as they propagate. The while-light images do not provide direct insight into the composition of the expelled clumps, which could potentially be composed of fine, sub-micron dust particles, neutral atoms and molecules, and/or ionized atomic/molecular cometary species. Although solar radiation pressure plays a role in accelerating and size sorting of small dust grains, it cannot accelerate them to velocities >200 km s-1 in the observed time interval of a few hours and distance of <106 km. Further, order of magnitude calculations show that ionized single atoms or molecules accelerate too quickly compared to observations, while dust grains micron sized or larger accelerate too slowly. We find that neutral Na, Li, K, or Ca atoms with β>50 could possibly fit the observations. Just as likely, we find that an interaction with the solar wind and the heliospheric magnetic field (HMF) can cause the observed clump dynamical evolution, accelerating them quickly up to solar wind velocities. We thus speculate that the HVECs are composed of charged particles (dust particles) or neutral atoms accelerated by radiation pressure at β>50 values. In addition, the data suggest that clump ejecta initially move along near-radial, bright structures, which then separate into HVECs and larger dust grains that steadily bend backwards relative to the comet's orbital motion due to the effects of solar radiation and gravity. These structures gradually form new striae in the dust tail. The near-periodic spacing of the striae may be indicative of outgassing activity modulation due to the comet nucleus' rotation. It is, however, unclear whether all striae are formed as a result of this process.
Journal of Geophysical Research: Space Physics (In press)
The Composition of the Protosolar Disk and the Formation Conditions for Comets
- NASA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena CA 91109
- Institut UTINAM, CNRS/INSU, UMR 6213, Universite de Franche-Comte, 25010 Besancon, France
- IPAG Univ. Grenoble Alpes, 3800 Grenoble, France
- Astrochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771
- Laboratoire de Chimie Theorique, Sorbonne Universite, UPMC Univ Paris 06, CNRS UMR 7616, 75252 Paris, CEDEX 05, France
- Institute of Planetary Research, Rutherfordstrasse 2, 12489 Berlin, Germany
- Department of Physics and Astronomy, University of Missouri - St Louis, St Louis, MO 63121
- Laboratoire d'Astrophysique de Marseille, UMR 726, Aix-Marseille-Universite, CNRS, 13388 Marseille, France
- Onsala Space Observatory, Department of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
Conditions in the protosolar nebula have left their mark in the composition of cometary volatiles, thought to be some of the most pristine material in the solar system. Cometary compositions represent the end point of processing that began in the parent molecular cloud core and continued through the collapse of that core to form the protosun and the solar nebula, and finally during the evolution of the solar nebula itself as the cometary bodies were accreting. Disentangling the effects of the various epochs on the final composition of a comet is complicated. But comets are not the only source of information about the solar nebula. Protostellar disks around young stars similar to the protosun provide a way of investigating the evolution of disks similar to the solar nebula while they are in the process of evolving to form their own solar systems. In this way we can learn about the physical and chemical conditions under which comets formed, and about the types of dynamical processing that shaped the solar system we see today.
This paper summarizes some recent contributions to our understanding of both cometary volatiles and the composition, structure and evolution of protostellar disks.
Space Science Review (In press)
DOI: 10.1007/s11214-015-0167-6 arXiv: 1507.02328
Effect of Stellar Encounters on Comet Cloud Formation
- Tokyo Institute of Technology
- National Astronomical Observatory of Japan
We have investigated the effect of stellar encounters on the formation and disruption of the Oort cloud using the classical impulse approximation. We calculate the evolution of a planetesimal disk into a spherical Oort cloud due to the perturbation from passing stars for 10 Gyr. We obtain the empirical fits of the e-folding time for the number of Oort cloud comets using the standard exponential and Kohlrausch formulae as functions of the stellar parameters and the initial semimajor axes of planetesimals. The e-folding time and the evolution timescales of the orbital elements are also analytically derived. In some calculations, the effect of the Galactic tide is additionally considered. We also show the radial variations of the e-folding times to the Oort cloud. From these timescales, we show that if the initial planetesimal disk has the semimajor axes distribution dn/da∝a−2, which is produced by planetary scattering (Higuchi et al. 2006), the e-folding time for planetesimals in the Oort cloud is ∼10 Gyr at any heliocentric distance r. This uniform e-folding time over the Oort cloud means that the supply of comets from the inner Oort cloud to the outer Oort cloud is sufficiently effective to keep the comet distribution as dn/dr∝r−2. We also show that the final distribution of the semimajor axes in the Oort cloud is approximately proportional to a−2 for any initial distribution.
Astronomical Journal (Published)
DOI: 10.1088/0004-6256/150/1/26 NASA ADS: 2015AJ....150...26H arXiv: 1507.00502
CCD Polarimetry of Distant Comets C/2010 S1 (LINEAR) and C/2010 R1 (LINEAR) at the 6-m Telescope of the SAO RAS
- Main Astronomical Observatory of NASU, Ukraine
- Special Astrophysical Observatory of RAS, Russia
- Taras Shevchenko National University, Ukraine
We present first measurements of the degree of linear polarization of distant comets C/2010 S1 (LINEAR) and C/2010 R1 (LINEAR) at heliocentric distances r=5.9–7.0 AU. Observations were carried out with the SCORPIO-2 focal reducer at the 6-m telescope of the SAO RAS. Both comets showed considerable level of activity beyond a zone where water ice sublimation is negligible (up to 5 AU). Significant spatial variations both in the intensity and polarization are found in both comets. The slope of radial profiles of intensity changes gradually with the distance from the photocenter: from -0.7 near the nucleus up to about -1.3 for larger distances (up to 100000 km). The variation in polarization profiles indicates the non uniformity in the polarization distribution over the coma. The polarization degree over the coma gradually increases (in absolute value) with increasing the photocentric distance from of about -1.9% up to -3% for comet C/2010 S1 (LINEAR), and from of about -2.5% up to -3.5% for comet C/2010 R1 (LINEAR). These polarization values are significantly higher than typical value of the whole coma polarization (-1.5%) for comets at heliocentric distances less than 5 AU. The obtained photometric and polarimetric data are compared with those derived early for other comets at smaller heliocentric distances. Numerical modeling of light scattering characteristics was performed for media composed of particles with different refractive index, shape, and size. The computations were made by using the superposition T-matrix method. We obtained that for comet C/2010 S1 (LINEAR), the dust in the form of aggregates of overall radius R~1.3 μm composed of N=1000 spherical monomers with radius a=0.1 μm, refractive index m=1.65 + i0.05, allows to obtain a satisfactory agreement between the results of polarimetric observations of comet C/2010 S1 and computations.
Planetary and Space Science (In press)
DOI: 10.1016/j.pss.2015.05.009 arXiv: 1506.07986
What Drives the Dust Activity of Comet 67P/Churyumov-Gerasimenko?
- Technische Universität Braunschweig, Institut für Geophysik und extraterrestrische Physik, Mendelssohnstraße 3, D-38106 Braunschweig, Germany
- Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany
The gas-driven dust activity of comets is still an unsolved question in cometary physics. Homogeneous dust layers composed of micrometer-sized grains possess tensile strengths ~1 kPa, which is far higher than typical gas pressures caused by the sublimation of the ices beneath the covering dust layer. This implies that the dust grains cannot be detached from the surface by the gas pressure of the sublimating ices. One possibility to avoid this problem is that the nucleus formed through the gravitational collapse of an ensemble of millimeter- to centimeter-sized aggregates. In this case, an aggregate layer with a tensile strength on the order of ~1 Pa is formed on the surface of the nucleus, which allows for the release of the aggregates from the surface by the gas pressure build up at the ice-dust interface.
We use the gravitational instability formation scenario of cometesimals to derive the aggregate size that can be released by the gas pressure from the nucleus of comet 67P/Churyumov-Gerasimenko for different heliocentric distances and different volatile ices. To derive the ejected aggregate sizes, we developed a model based on the assumption that the entire heat absorbed by the surface is consumed by the sublimation process of one volatile species. The calculations were performed for the three most prominent volatile materials in comets, namely, H2O ice, CO2 ice, and CO ice.
We find that the size range of the dust aggregates able to escape from the nucleus into space widens when the comet approaches the Sun and narrows with increasing heliocentric distance, because the tensile strength of the aggregates decreases with increasing aggregate size. The activity of CO ice in comparison to H2O ice is capable to detach aggregates smaller by approximately one order of magnitude from the surface. As a result of the higher sublimation rate of CO ice, larger aggregates are additionally able to escape from the gravity field of the nucleus.
Our model can explain the large grains (ranging from 2 cm to 1 m in radius) in the inner coma of comet 67P/Churyumov-Gerasimenko that have been observed by the OSIRIS camera at heliocentric distances between 3.4 AU and 3.7 AU. Furthermore, the model predicts the release of decimeter-sized aggregates (trail particles) close to the heliocentric distance at which the gas-driven dust activity vanishes. However, the gas-driven dust activity cannot explain the presence of particles smaller than ~1 mm in the coma because the high tensile strength required to detach these particles from the surface cannot be provided by evaporation of volatile ices. These smaller particles can be produced for instance by spin-up and centrifugal mass loss of ejected larger aggregates.
Astronomy & Astrophysics (In press)
NASA ADS: 2015arXiv150608545G arXiv: 1506.08545