Comet ISON, as seen by Spitzer

[Note: this blog entry was written by Dr. Casey Lisse and posted on his behalf by Karl Battams.]

In April 2013 we1 realized that the Spitzer Space Telescope had yet to look at comet ISON and that the window to do so was closing fast. We searched our previous results with Spitzer observing comets, and realized it could do a good job of studying the gas and dust flowing from the comet, even far outside the water ice line where the comet was expected to be relatively inactive. HST observations in April-May 2013 told us that the comet was surprisingly small and very quietly behaved, with little variation due to rotation and a simple outgassing pattern, consistent with a brand new comet never before heat treated by passage through the inner solar system. In fact, we found that Spitzer could do a good job of potentially finding CO or CO2 gas escaping from the comet. Carbon monoxide and carbon dioxide are gasses commonly found in comets, and are believed to be responsible for “powering” their emission activity far from the Sun (the carbon monoxide ice line is out at ~40 AU, and the carbon dioxide ice line is at about 10 AU from the Sun). We could also expect to find any very heavy dust trail particles, and search for variations in the comet’s total brightness due to its rotation (as jets come in and out of sunlight over the course of a comet’s “day”, the solar powered emission from them can rise and fall, modulating the outflow of reflective dust and emissive gas in the surrounding coma).

Comet ISON, as seen in the 3.6 micron (left) and 4.5-micron (right) infrared wavelengths by the Spitzer Telescope.
So what did we find with Spitzer? We found a comet with an even more extended, but still apparently simple pattern of dust release in the 3.6 um image; a large cylindrically symmetric gas coma made of CO2 or possibly CO in the 4.5-3.6 um difference image; a comet that varied in brightness less than 2% over 24 hrs of constant monitoring at both wavelengths; and a comet nucleus that is likely between 0.2 and 20 km in size (the HST results show that it can't be bigger than 2 km in radius, so this translates to a nucleus between 0.2 and 2 km in radius after combining the two, a size range that still means it may or may not have enough ablative cooling power to survive its passage 1.7 Rsun above the solar photosphere on 28 Nov 2013).

It is important to note that we know that we are looking at CO2 or CO emissions, as these are the prominent sources of light in the 4.5 um Spitzer passband, along with scattering of sunlight by ISON’s dust. The shape of the difference image is very round, and looks just like the gas comae seen for other comets by Spitzer and other telescopes. Also, the spectrophotometry show a clear excess above the dust scattered light. We think that it is most likely CO2 emission we are seeing because the first recorded ISON detection was just a bit inside the CO2 ice line, and because the CO2 gas production rates we find with Spitzer are very reasonable compared to those found for 18 other comets by Ootsubo et al. (2012), while the gas production rates we derive assuming we are looking at 100% CO are very high, and we know from HST that the comet is not very big. The final proof of the CO2 pudding will come from a good CO2 spectroscopic detection, like the one the Deep Impact spacecraft will hopefully be getting in the first 2 weeks of August.

(And for those of you who can’t keep CO2 or CO straight, CO2 is what gives you the fizz in your soda pop, is what you breathe out after extracting energy from your food, is what causes a lot of the atmosphere’s greenhouse warming, and what plants eat to make leaves and twigs and branches and stuff; while CO is the poisonous gas that binds to your hemoglobin and doesn't let go, can kill you in large quantities, and is also used in the chemical industry to create many more complicated organic compounds we use in our daily lives. CO2 can also produce CO relatively easily by absorbing a solar UV photon in the reaction CO2 + hv -> CO + O)

The current Spitzer and HST findings, in my humble opinion, are consistent with an ISON that is similar to comet C/Kohoutek 1973, a comet that was brand new from the Oort Cloud, where it had resided for 4.5-Gyr in deep freeze since its formation. This also likely means that while we can expect great science from this comet as a piece of some of the most pristine material left from the first few M-yrs of our solar system’s formation, it is not at all clear what kind of show it is going to put on in the Earth’s night skies near perigee in December 2013. The dynamically new comet straight from the Oort cloud, Comet Kohoutek, is famous in cometary history for being a very overhyped “dud” Comet of the Century. :)

Casey Lisse, Aug 2013.
1. The Spitzer ISON observation team comprised the following scientists: C. M. Lisse, R. J. Vervack, and H. A. Weaver, Applied Physics Laboratory, Johns Hopkins University; J. M. Bauer, Jet Propulsion Laboratory, California Institute of Technology; Y. R. Fernandez, University of Central Florida; M.S.P. Kelley, University of Maryland; M. M. Knight, Lowell Observatory; D. Hines, Space Telescope Science Institute; J.-Y. Li, Planetary Science Institute; W. Reach, SOFIA, Universities Space Research Association; M. L. Sitko, University of Cincinnati; P. A. Yanamandra-Fisher, Space Science Institute; and K. J. Meech and J. Rayner, University of Hawaii