Compositional and physical results for Rosetta's new target Comet 67P/Churyumov–Gerasimenko from narrowband photometry and imaging

Abstract

We present compositional and physical results of Comet 67P/Churyumov–Gerasimenko, the new target of ESA's Rosetta mission. A total of 16 nights of narrowband photometry were obtained at Lowell Observatory during the 1982/83 and 1995/96 apparitions, along with one night of imaging near perihelion in 1996. These data encompass an interval of −61 to +118 days from perihelion, corresponding to a range of heliocentric distances before perihelion from 1.48 to 1.34 AU, and an outbound range from 1.30 to 1.86 AU. Production rates were determined for OH, NH, CN, C 3, and C 2, along with A ( θ ) f ρ , a proxy of the dust production. Water production, based on OH, has a steep ( ∼ − 5 ) power-law r H -dependence post-perihelion and the minor species are somewhat less steep ( ∼ − 3 to −4), while the dust is quite shallow ( ∼ − 1 ), possibly due to a lingering population of large, slow-moving grains. All species exhibit larger production rates after perihelion, with water having a ∼ 2 × pre/post -perihelion asymmetry, while minor species and dust have larger asymmetries. These asymmetries imply a strong seasonal effect and probable high obliquity of the rotational axis, along with one or more isolated source regions coming into sunlight near perihelion. Peak water production (which occurred about 1 month after perihelion) was ∼ 8 × 10 27 mol s −1 and, when combined with a standard water vaporization model, implies an effective active area on the surface of the nucleus of ∼1.5–2.2 km 2 or an active fraction of only about 3–4%. Abundances of carbon-chain molecules yield a classification of slightly “depleted” in the A'Hearn et al. [A'Hearn, M.F., Millis, R.L., Schleicher, D.G., Osip, D.J., Birch, P.V., 1995. Icarus 118, 223–270] database. The peak dust production (as measured by A ( θ ) f ρ , and uncorrected for phase angle) was ∼450 cm, while the color of the dust is moderately reddened, and the mean radial profile has a power-law slope of −1.3. Large night-to-night variability is also present, presumably due to the source region(s) rotating in and out of sunlight along with effects due to the use of differently sized apertures. A strong sunward radial feature was detected in images obtained near perihelion, along with a significant asymmetry between the two perpendicular directions from the Sun/tail line. These features may be the result of a mid-latitude source region sweeping out a cone with each rotation, which we are viewing from the side and where the sunward radial feature is one edge of the cone seen in projection. When combined with other constraints on the pole orientation, a possible pole solution is found having an obliquity of about 134° at an RA of about 223° and a Dec of −65°, with a source region located near +50° and in overall agreement with the photometric results. In comparison to the original Rosetta target Comet 46P/Wirtanen, Comet Churyumov–Gerasimenko has essentially the same peak water production but a peak dust production about 3 times greater than does Wirtanen based on A ( θ ) f ρ (i.e., if one assumes that the properties of the dust grains are similar) (cf. Farnham and Schleicher [1998. Astron. Astrophys. 335, L50–L55]).