Comet 19P/Borrelly at multiple apparitions: seasonal variations in gas production and dust morphology

We present analysis and results from both narrowband photometry and CCD imaging of Comet 19P/Borrelly from multiple apparitions. Production rates for Borrelly a few days prior to the Deep Space 1 spacecraft encounter were Q(OH) = 2.1 X 10(28) molecule s(-1), Q(CN) = 5.1 X 10(25) molecule s(-1), and A(phi)fp = 400-500 cm. The equivalent Q(water; vectorial) = 2.5 X 10(28) molecule s(-1). We also find that the radial fall-off of the dust is significantly steeper than the canonical 1/rho for aperture sizes larger than rho = 2x10(4) km. In the near-UV, a strong trend in dust colors with aperture size is present. Imaging of Borrelly revealed a strong radial jet in the near-sunward direction that turns off late in the apparition. For the jet to appear radial, it must originate at or very close to the nucleus' pole. Modeling the measured position angle of this jet as a function of time during the 1994 and 2001 apparitions yields a nucleus in a simple, rather than complex, rotational state with a pole orientation having an obliquity of 102.7degrees +/- 0.5degrees and an orbital longitude of the pole of 146degrees +/- 1degrees, corresponding to an RA of 214.1degrees and a Declination of -5.7degrees (J2000). There is also evidence for a small (similar to8degrees) precession of the pole over the past century, based on our preferred model solution for jet measurements obtained during the 1911-1932 apparitions. Our solution for the orientation of the rotation axis implies a very strong seasonal effect as the source region for the jet moves from summer to winter. This change in solar illumination quantitatively explains both the nearly level water production measured in the seven weeks preceding perihelion and the extremely large decrease in water production (25 X) as Borrelly moved from perihelion to 1.9 AU. A much smaller fall-off inapparent dust production after perihelion can be explained by a population of old, very slowly moving large grains released near peak water production, and therefore not indicative of the actual ongoing release of dust grains late in the apparition. Based on the water vaporization rate, the source region has an area of approximately 3.5 km(2) or 4% of the total surface area of the nucleus, and water ice having an effective depth of 3-10 m is released each apparition from this source region. (C) 2003 Elsevier Science (USA). All rights reserved.