Detection of large grains in the coma of Comet C/2001 A2 (LINEAR) from Arecibo radar observations

Abstract

Arecibo S-band (λ = 13 cm) radar observations of Comet C/2001 A2 (LINEAR) on 2001 July 7-9 showed a strong echo from large coma grains. This echo was significantly depolarized. This is the first firm detection of depolarization in a grain-coma radar echo and indicates that the largest grains are at least λ/2 or 2 cm in radius. The grains are moving at tens of m s −1 with respect to the nucleus. The non-detection of the nucleus places an upper limit of 3 km on its diameter. The broad, asymmetric echo power spectrum suggests a fan of grains that have a steep (differential number ∼ a −4 ) size distribution at cm-scales, though the observed fragmentation of this comet complicates that picture.

Radar observations of 8P/Tuttle: A contact-binary comet

“…Escape trajectories with this velocity are physically plausible, since a simple gas-drag model (see Harmon et al, 2004) for this comet, assuming the nominal sublimation gas mass flux of 5 Â 10 À5 g cm À2 , gives a terminal velocity of 6 m s À1 for 2-cm-radius grains and a maximum liftable grain radius of 4 cm. Finally, we note that this is the ninth of the 13 radar-detected comets to show a grain-coma echo, which further underscores the importance and prevalence of large-grain ejection by comets (Harmon et al, 2004;Nolan et al, 2006a).…”

“…The 60-Hz bandwidth (FWHM) of the spectrum corresponds to a characteristic velocity dispersion of 4 m s À1 for the grains. Although the SC echo is relatively weak, the measured SC/OC circular polarization ratio l c = 0.14 is sufficiently large to require the largest grains in the distribution to have radii larger than the Rayleigh-Mie transition size of k/2p, or 2 cm (see Harmon et al, 1989Harmon et al, , 2004Nolan et al, 2006a). Such large, slow-moving grains are precisely the type expected to populate the Ursid meteor stream known to be associated with this comet (Jenniskens et al, 2002).…”

Radar Observations of Comet 103p/Hartley 2

“…In Figure 3, we show the grain-coma CW spectra (with nucleus echo subtracted) for the OC and SC polarization senses. The presence of a substantial depolarized (SC) echo (µ c = 0.34) indicates that the largest grains have radii a exceeding the Rayleigh transition size a = λ/2π or 2 cm (Harmon et al 2004;Nolan et al 2006). In fact, Hartley exhibits the highest graincoma depolarization yet measured reliably for a comet, which suggests that the grain size distribution included a significant population at decimeter sizes or even larger.…”

“…velocity V(a) ∝ a −1/2 , where V = 30 m s −1 for a = 2 cm. Accelerating centimeter size grains to these velocities requires surface gas mass fluxes of 1 × 10 −3 g cm −2 s −1 , which is sufficient to lift meter size and larger boulders off the nucleus (Harmon et al 2004;Nolan et al 2006;Molina 2010). The preferential redshift of the grains indicates that their ejection was skewed toward the anti-Earthward direction.…”

Radar backscattering from a large-grain cometary coma: numerical simulation

“…The OC Doppler spectra of comets with a significant population of large grains show narrowband peaks from the nucleus, and a broadband signal is expected from moving particles in the coma. Using this technique, Doppler spectra measured for some of the comets show the presence of the so-called large-grain coma around cometary nuclei (Harmon et al 2011(Harmon et al , 2010Nolan et al 2006). Samecircular echoes are usually much weaker.…”