Explosion of Comet 17P/Holmes as revealed by the Spitzer Space Telescope

Abstract

An explosion on comet 17P/Holmes occurred on 2007 Oct 23, projecting particulate debris of a wide range of sizes into the interplanetary medium. We observed the comet using the midinfrared spectrograph (5-40 µm), on 2007 Nov 10 and 2008 Feb 27, and the imaging photometer (24 and 70 µm), on 2008 Mar 13, on board the Spitzer Space Telescope. The 2007 Nov 10 spectral mapping revealed spatially diffuse emission with detailed mineralogical features, primarily from small crystalline olivine grains. The 2008 Feb 27 spectra, and the central core of the 2007 Nov 10 spectral map, reveal nearly featureless spectra, due to much larger grains that were ejected from the nucleus more slowly. Optical images were obtained on multiple dates spanning 2007 Oct 27 to 2008 Mar 10 at the Holloway Comet Observatory and 1.5-m telescope at Palomar Observatory. The images and spectra can be segmented into three components: (1) a hemispherical shell fully 28 on the sky in 2008 Mar, due to the fastest (262 m s −1 ), smallest (2 µm) debris, with a mass 1.7×10 12 g; (2) a 'blob' or 'pseudonucleus' offset from the true nucleus and subtending some 10 on the sky, due to intermediate speed (93 m s −1 ) and size (8 µm) particles, with a total mass 2.7 × 10 12 g; and (3) a 'core' centered on the nucleus due to slower (9 m s −1 ), larger (200 µm) ejecta, with a total mass 3.9 × 10 12 g. This decomposition of the mid-infrared observations can also explain the temporal evolution of the mm-wave flux. The orientation of the leading edge of the ejecta shell and the ejecta 'blob,' relative to the nucleus, do not change as the orientation of the Sun changes; instead, the configuration was imprinted by the orientation of the initial explosion. The distribution and speed of ejecta implies an explosion in a conical pattern directed approximately in the solar direction on the date of explosion. The kinetic energy of the ejecta > 10 21 erg is greater than the gravitational binding energy of the nucleus. We model the explosion as being due to crystallization and release of volatiles from interior amorphous ice within a subsurface cavity; once the pressure in the cavity exceeded the surface strength, the material above the cavity was propelled from the comet. The size of the cavity and the tensile strength of the upper layer of the nucleus are constrained by the observed properties of the ejecta; tensile strengths on > 10 m scale must be greater than 10 kPa (or else the ejecta energy exceeds the binding energy of the nucleus) and they are plausibly 200 kPa. The appearance of the 2007 outburst is similar to that witnessed in 1892, but the 1892 explosion was less energetic by a factor of about 20.

Interferometric mapping of the 3.3-mm continuum emission of comet 17P/Holmes after its 2007 outburst

“…Constraints obtained on the size index and maximum dust size show that the core is dominated by large (a max > 2 mm) particles with a shallow size index |q| < 3.4. The same conclusion was obtained by Reach et al (2010) from the analysis of the Spitzer images. In addition, the core revealed featureless 10-μm spectra in mid-November 2007, in contrast to the shell, consistent with much larger grains populating the core (Reach et al 2010).…”

“…The ratio M core dust /M shell dust is in the range 0.1−1, lower than the value ∼2 determined by Reach et al (2010). The large mass in the shell component and its steep size distribution indicate that the disruption of the nucleus was accompanied or followed by a massive production of small particles, indicative of material with small cohesive strength.…”

“…Table 4 presents dust opacities at 3.3 mm (k 3.3 mm ) for different ice/silicate mixtures, porosities and size distributions. The maximum size considered in the table is a max = 1000 mm, as evidence was found for cm-sized particles and even largesize fragments in 17P coma (Reach et al 2010;Stevenson et al 2010). When ice is included in the aggregates, the opacity ranges from 0.005 and 0.037 m 2 kg −1 in the range of considered size indexes.…”

“…The inconsistency between these concurrent measurements reflects different assumptions on the size distribution, especially on the maximum particle size in the ejecta. For example, Reach et al (2010) made their estimations using characteristic sizes in the core, blob and shell components of 200, 8 and 2 μm, respectively, so that their values are lower limits to the dust masses.…”

“…An (O−C) offset of ∼+0.3 at the 1-σ confidence level is measured in Declination for both dates when considering the whole data set. On optical images, the inner coma of 17P/Holmes presents an asymmetric distribution, mainly characterized by the presence at the position angle PA = 220 • (corresponding to the anti-Sun direction) of a bright elongated cloud (so-called "blob"), quickly separating at a rate of 9−10 /day from the (much brighter) condensation of material surrounding the nucleus (Montalto et al 2008;Watanabe et al 2009;Reach et al 2010). The (O−C) marginal offsets measured in the PdBI maps (PA within 0−60 • ) are not towards the direction of this expanding blob: besides the uncertainty of the measurements and of the comet ephemeris, they can be due to spatial asymmetries of the brightness distribution in either direction.…”