Composition of cometary dust: The case against silicates

Mendis, D. A.; Wickramasinghe, N. C.

doi:10.1007/bf00640371

Cited by 15 publications

(4 citation statements)

References 14 publications

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“…(2006) mass composition [Silicates : organics : carbonaceous: ices ] = [0.26:0.23:0.086: 0.426] suggests refractory silicates to be the most important nonice component. Mendis (1975) give L = 2.30 × 10 10 erg/g for silicates, which is close to that of water ice, while organics will lower the mean slightly. Chyba et al (1993) adopted 2.3 × 10 10 erg/g for comets, 5 × 10 10 erg/g for carbonaceous and 8 × 10 10 erg/g for stone/iron bodies and just 10 11 erg/g even for solid iron.…”

Section: Latent Heat Of Sublimationsupporting

confidence: 51%

“…However, in situations of intense ablation and explosion, all components are eventually vaporized and the relevant value is the weighted mean over all mass components, including volatile and refractory ones. Sekanina (2003) Mendis (1975) give L = 2.30 × 10 10 erg/g for silicates, which is close to that of water ice, while organics will lower the mean slightly. Chyba et al (1993) adopted 2.3 × 10 10 erg/g for comets, 5 × 10 10 erg/g for carbonaceous and 8 × 10 10 erg/g for stone/iron bodies and just 10 11 erg/g even for solid iron.…”

Section: Latent Heat Of Sublimationmentioning

confidence: 68%

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Mass loss, destruction and detection of Sun-grazing and -impacting cometary nuclei

Brown

Potts

Porter

et al. 2011

A&A

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Context. Sun-grazing comets almost never re-emerge, but their sublimative destruction near the sun has only recently been observed directly, while chromospheric impacts have not yet been seen, nor impact theory developed. Aims. We seek simple analytic models of comet destruction processes near the sun, to enable estimation of observable signature dependence on original incident mass M o and perihelion distance q. Methods. Simple analytic solutions are found for M(r) versus q and distance r for insolation sublimation and, for the first time, for impact ablation and explosion. Results. Sun-grazers are found to fall into three (M o , q) regimes: sublimation-, ablation-, and explosion-dominated. Most sun-grazers have M o too small (<10 11 g) or q too large (>1.01 R ) to reach atmospheric densities (n > 2.5 × 10 11 /cm 3 ) where ablation exceeds sublimation. Our analytic results for sublimation are similar to numerical models. For q < 1.01 R , M o > 10 11 g, ablation initially dominates but results are sensitive to nucleus strength P c = 10 6 P 6 dyne/cm 2 and entry angle φ to the vertical. Nuclei with M o 10 10 (P 6 sec φ) 3 g are fully ablated before exploding, though the hot wake itself explodes. For most sun-impactors sec φ 1 (since q ∼ r * ), so for q very close to r * the ablation regime applies to moderate M o ∼ 10 13−16 P 3 6 g impactors unless P 6 0.1. For higher masses, or smaller q, nuclei reach densities n > 2.5 × 10 14 P 6 /cm 3 where ram pressure causes catastrophic explosion. Conclusions. Analytic descriptions define (M o , q) regimes where sublimation, ablation and explosion dominate sun-grazer/-impactor destruction. For q ≺ 1.01 R , M o 10 11 g nuclei are destroyed by ablation or explosion (depending on M o cos 3 φ/P c ) in the chromosphere, producing flare-like events with cometary abundance spectra. For all plausible M o , q and physical parameters, nuclei are destroyed above the photosphere.

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Section: Latent Heat Of Sublimationsupporting

confidence: 51%

Section: Latent Heat Of Sublimationmentioning

confidence: 68%

Mass loss, destruction and detection of Sun-grazing and -impacting cometary nuclei

Brown

Potts

Porter

et al. 2011

A&A

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“…Equations describing the energy balance at the nucleus surface and their solutions have been given by many authors dating back to Watson et al (1963) (see, e.g., Cowan and A 'Hearn 1979;Weissman and Kieffer 1981). The latent heat of sublimation of water ice is very large, about the same as for rock (Mendis and Wickramasinghe 1975;Brown et al 2011). While the equations generally used to compute vapour pressures may not be reliable at relatively high temperatures, volatile sublimation provides an effective cooling mechanism for keeping nucleus temperatures low, despite the extreme proximity to the Sun.…”

Section: Sublimation Processesmentioning

confidence: 99%

The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

et al. 2017

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This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all ob- jects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun's centre, equal to half of Mercury's perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground-and space-based facilities that will allow us to address those science topics.

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“…Comet Kobayashi-Berger-Milon did not show any 10 ym excess above a blackbody continuum [33]. The 10 ym and 18 ym peaks are generally attributed to silicate particles, although formaldehyde polymers have been suggested [34,35]. The presence of the peaks sets an upper limit of a few microns on the size of the emitting grains.…”

mentioning

confidence: 99%

Physical Characteristics of Cometary Dust from Optical Studies

Hanner

1980

Symp. - Int. Astron. Union

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Observations of the scattered sunlight and thermal emission from cometary dust provide information on the composition and dominant size of the dust. The observations will be summarized here and compared to theoretical models for dielectric and absorbing materials, with emphasis on the thermal emission. The compatibility of the optical data with the size distribution derived from dynamical studies is discussed.

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Composition of cometary dust: The case against silicates

Cited by 15 publications

References 14 publications

Mass loss, destruction and detection of Sun-grazing and -impacting cometary nuclei

Mass loss, destruction and detection of Sun-grazing and -impacting cometary nuclei

The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

Physical Characteristics of Cometary Dust from Optical Studies

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