Photometry of dust grains of comet 67P and connection with nucleus regions

Abstract

Aims. Multiple pairs of high-resolution images of the dust coma of comet 67P/Churyumov-Gerasimenko have been collected by OSIRIS onboard Rosetta allowing extraction and analysis of dust grain tracks. Methods. We developed a quasi automatic method to recognize and to extract dust tracks in the Osiris images providing size, FWHM and photometric data. The dust tracks characterized by a low signal-to-noise ratio were checked manually. We performed the photometric analysis of 70 dust grain tracks observed on two different Narrow Angle Camera images in the two filters F24 and F28, centered at λ = 480.7 nm and at λ = 743.7 nm, respectively, deriving the color and the reddening of each one. We then extracted several images of the nucleus observed with the same filters and with the same phase angle to be compared with the dust grain reddening. Results. Most of the dust grain reddening is very similar to the nucleus values, confirming they come from the surface or subsurface layer. The histogram of the dust grain reddening has a secondary peak at negative values and shows some grains with values higher than the nucleus, suggesting a different composition from the surface grains. One hypothesis comes from the negative values point at the presence of hydrated minerals in the comet.

Mt. Wendelstein imaging of the post-perihelion dust coma of 67P/Churyumov–Gerasimenko in 2015/2016

“…Within the measurement uncertainties these values also agree with the colors and spectral gradients determined for the nucleus of 67P by Tubiana et al (2008Tubiana et al ( , 2011 and Lowry (2006). It is slightly below the spectral slope determined from in-situ measurements of dust grains in the coma of 67P (Cremonese et al 2015). No clear increase of the reddening with phase angle is seen in our data.…”

Colour variations of Comet C/2013 UQ4 (Catalina)

“…Other examples of blue color of cometary dust can be found in Jewitt et al (1982), when comparing the high-resolution spectrum of Comet C/1980 E1 (Bowell) with the solar spectrum; the spectrum of Comet Bowell appears to be almost featureless and, therefore, Jewitt et al concluded that it is completely devoid of gaseous emission. Finally, it is worth noting the recent Rosetta measurements of the color of dust in Comet 67P/Churyumov-Gerasimenko (Cremonese et al 2016), in which 16 out of 70 individual dust grains detected in the vicinity of the nucleus have blue photometric color. Thus, the blue photometric color also can be a feature of some comets or, at least, some part of their dust.…”

The Rosetta mission orbiter science overview: the comet phase

“…The compact particles are correlated with lower phase angle observations (between 30° and 40°) and with the neck region of the comet, with velocities of 2.5 ± 0.8 m s −1 at 10 km from comet centre and 4.3 ± 0.9 m s −1 at 30 km [ 61 ]. Photometry of coma dust grains shows agreement with surface values, although some differences in composition from the surface could potentially indicate the presence of hydrated minerals [ 64 ]. A more detailed characterization of the dust environment was made following the collection of over 10 000 particles of scale of a few 10 µm to several 100 µm during the period 11 August 2014–3 April 2015 (3.57–1.95 AU) [ 65 ].…”

“…These regions are characterized by bluer spectra, indicating ice exposure or possibly hydrated minerals [64,76], although comparisons with laboratory samples suggest this is not the case [135]. In the case of ice, erosion rates would be 1–2 orders of magnitude lower than that observed, with speculation of this enhanced erosion being driven by the low tensile strength of the surface matrix, or clathrate destabilization/amorphous ice crystallization, although observation of sulfur-bearing species in the comet have been used to argue against the existence of clathrates [136].…”

“…Although there were indications of surface changes in terms of mass wasting near cliffs and granular flows near pits [ 121 ], the first major temporal changes appeared in the smooth areas of the Imhotep region in the form of rounded features growing in size at a rate of 5.6–8.1 × 10 −5 m s −1 during the period 24 May–11 July 2015 [ 134 ]. These regions are characterized by bluer spectra, indicating ice exposure or possibly hydrated minerals [ 64 , 76 ], although comparisons with laboratory samples suggest this is not the case [ 135 ]. In the case of ice, erosion rates would be 1–2 orders of magnitude lower than that observed, with speculation of this enhanced erosion being driven by the low tensile strength of the surface matrix, or clathrate destabilization/amorphous ice crystallization, although observation of sulfur-bearing species in the comet have been used to argue against the existence of clathrates [ 136 ].…”