Survey of cometary CO2, CO, and particulate emissions using the Spitzer Space Telescope

Abstract

We surveyed 23 comets using the Infrared Array Camera on the Spitzer Space Telescope in wide filters centered at 3.6 and 4.5 µm. Emission in the 3.6 µm filter arises from sunlight scattered by dust grains; the 3.6 µm images generally have a coma near the nucleus and a tail in the antisolar direction due to dust grains swept back by solar radiation pressure. The 4.5 µm filter contains scattered sunlight by, and thermal emission from, the same dust grains, as well as strong emission lines from CO 2 and CO gas. The 4.5 µm images are often much brighter than could be explained by dust grains, and they show sometimes distinct morphologies, in which cases we infer they are dominated by gas. Based on the ratio of 4.5 to 3.6 µm brightness, we classify the survey comets as CO 2 +CO 'rich' and 'poor'. This classification is correlated with previous classifications by A'Hearn based on carbon-chain molecule abundance, in the sense that comets classified as 'depleted' in carbon-chain molecules are also 'poor' in CO 2 +CO. The gas emission in the IRAC 4.5 µm images is characterized by a smooth morphology, typically a fan in the sunward hemisphere with a radial profile that varies approximately as the inverse of projected distance from the nucleus, as would apply for constant production and free expansion. There are very significant radial and azimuthal enhancements in many of the comets, and these are often distinct between the gas and dust, indicating that ejection of solid material may be driven either by H 2 O or CO 2 . Notable features in the images include the following. There is a prominent loop of gas emission from 103P/Hartley 2, offset toward the sunward direction; the loop could be due to an outburst of 1 CO 2 before the Spitzer image. Prominent, double jets are present in the image of 88P/Howell, with one directed nearly toward the Sun and the other closer to the terminator (but still on the daytime hemisphere). A prominent single jet is evident for C/2002 T7 (LINEAR), 22P/Kopff and 81P/Wild 2. Spirals are apparent in 29P/Schwassmann-Wachmann 1 and C/2006 W3 (Christensen); we measure a rotation rate of 21 hr for the latter comet. Arcs (possibly parts of a spiral) are apparent in the images of 10P/Tempel 2, and 2P/Encke.

Colour variations of Comet C/2013 UQ4 (Catalina)

“…Using those data with the shortest frame times obtained as part of our HDR observations, we find a flux density ratio of 4.55.5 at a distance of 14,000 km to 30,000 km from the nucleus, barely constraining that ratio. Based on the dust emission model by Reach et al (2013) (see Figure 3 in that work), these flux ratios agree with emission from dust alone and do not suggest the existence of CO or CO 2 in the tail of the comet.…”

On the Formation of Amide Polymers via Carbonyl–amino Group Linkages in Energetically Processed Ices of Astrophysical Relevance

“…Both, carbon monoxide (CO) and ammonia (NH 3 ), were detected on ice-coated interstellar grains by the Spitzer observatory. Their respective abundances relative to water are typically 20% and 5%, respectively Bottinelli et al 2010;Reach et al 2013). Molecular clouds consist of nanometer-sized carbon-and/or silicatebearing grains with temperatures as low as 10 K. On these grains, icy layers, including carbon monoxide (CO) and ammonia (NH 3 ) can accrete (Allamandola et al 1999;Gibb et al 2004;Boogert et al 2008).…”

Space-Based Thermal Infrared Studies of Asteroids

“…Mid-IR spectra have also proven useful for constraining the abundance of volatiles on small bodies; for example, many species such as CO and CO 2 have been identified on comets photometrically through excess flux above their thermal emission (e.g. Bauer et al 2011;Reach et al 2013). Recently, Herschel heterodyne spectroscopy of the largest object in the Main Belt, (1) Ceres, has revealed the presence of water vapor as the dwarf planet approached perihelion (Küppers et al 2014).…”