46th Lunar and Planetary Science Conference (2015)
COMETARY DUST TAILS IN NEOWISE E. A. Kramer1, J. M. Bauer1, Y. R. Fernández2, A. K. Mainzer1, J. R.
Masiero1, T. Grav3, C. R. Nugent1, S. Sonnett1, C. M. Lisse4, K. J. Meech5, and the WISE Team. 1Jet Propulsion
Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109; 2University of Central
Florida, 4000 Central Florida Blvd., Orlando, FL 32816; 3Planetary Science Institute, 1700 East Fort Lowell, Suite
106, Tucson, AZ 85719-2395; 4Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road,
Laurel, MD 20723; 5Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822.
Introduction: Comets are among the most primitive
objects in our Solar System, having undergone little
alteration since their formation in the protoplanetary
disk. Yet the comets have still experienced some
changes due to different heating processes such as impacts and insolation [1]. These processes may have
caused significant structural changes to the comet, possibly depleting the comet of volatiles at the same time.
As a comet approaches perihelion, the thermal energy
from the Sun warms the comet, causing volatiles to be
released, and intermixed dust particles to be carried
along with the escaping gas. The dust and gas form the
characteristic tails for which comets are so wellknown. Since the comets themselves are relatively
pristine, the dust particles that comprise their tails are
thought to be similar to particles in the protoplanetary
disk. By studying the dust tails of comets in different
populations (long-period comets [LPCs] and shortperiod comets [SPCs]), we can investigate the effect of
heating on the structure of comets.
Figure 1: Montage of six comets observed by
NEOWISE during the cryogenic mission, showing the
variety of activity levels present in the data. The images are in 22-micron, and each image is 10’ across.
NEOWISE is the planetary-funded mission that utilizes data from the Wide-field Infrared Survey Explorer (WISE) spacecraft to detect and characterize moving
objects. The WISE mission surveyed the sky in four
infrared wavelength bands (3.4, 4.6, 12 and 22-micron)
between January 2010 and February 2011 [2, 3]. During the course of the prime mission, over 160 comets
were serendipitously observed, including 22 newly
discovered comets. About 89 of the comets observed
by NEOWISE displayed a significant dust tail in the 12
and 22-micron (thermal emission) bands, showing a
wide range of activity levels and dust morphology. A
sample of six comets are shown in Figure 1. Since the
observed objects are a mix of about 1/3 LPCs and 2/3
SPCs, differences in their activity can be used to better
understand the thermal evolution that each of these
populations has undergone.
Methods: Each comet was serendipitously observed multiple times by NEOWISE, and the individual images were stacked to increase the signal-to-noise
ratio. For the comets that displayed a significant dust
tail, we have estimated the sizes and ages of the particles using dynamical models based on the FinsonProbstein method [4, 5]. The Finson-Probstein method
assumes that the motion of cometary dust particles is
controlled only by solar radiation pressure and solar
gravity, which can be parameterized by the ratio β =
Frad/Fgrav. β can be thought of as a proxy for particle
size, with larger β corresponding to larger particles. β
is incorporated into the equation of motion that can be
integrated to track the motion of particles with different β values.
For a selection of 40 comets, we have then compared these models to the data using a novel tail-fitting
method that allows the best-fit model to be chosen
analytically rather than subjectively [6]. For comets
that were observed multiple times by WISE, the dust
tail particle properties were estimated separately, and
then compared.
Results: We find that the dust tails of both LPCs
and SPCs are primarily comprised of ~mm to cm sized
particles, which were the result of emission that occurred several months to several years prior to the observations. The LPCs nearly all have strong dust emission close to the comet's perihelion distance, and the
SPCs mostly have strong dust emission close to perihelion, but some have strong dust emission well before
perihelion. When comparing the two populations as a
whole, we find that there is no statistical difference in
the size of the particles between the two populations.
Similar sized particles suggest similar internal structure and possibly a similar origin for the two popula-
46th Lunar and Planetary Science Conference (2015)
tions. Further implications of these results will also be
NEOWISE Restart: Since the restart of
NEOWISE in late 2013, over 60 comets have been
observed in the 3.4 and 4.6-micron bands, including a
roughly even mix of both SPCs and LPCs and three (as
of early January 2015) newly discovered comets. As
during the prime mission, the comets seen by
NEOWISE have a wide range of activity levels, dust
morphology, and gas morphology over a wide range of
heliocentric distances. These data can be used to provide estimates of CO/CO2 production rates, nucleus
size, dust temperature, dust production rates, and dust
particle size and age. We will showcase some preliminary results from the new data, highlighting several
interesting cases.
References: [1] Prialnik, D. and Bar-Nun, A.
(1987) ApJ, 313:893-905; [2] Wright, E.L. et al. (2010)
AJ, 140; [3] Mainzer, A. et al. (2011) ApJ, 731:1; [4]
Finson, M. and Probstein, R. (1968) ApJ, 154; [5]
Lisse, C.M. et al. (1998) ApJ, 496; [6] Kramer, E.A.
(2014) PhD diss.
Acknowledgments: This publication makes use of
data products from (1) WISE, which is a joint project
of UCLA and JPL/Caltech, funded by NASA; and (2)
NEOWISE, which is a project of JPL/Caltech, funded
by the Planetary Science Division of NASA. EK was
supported by a NASA Postdoctoral Program Fellowship.