The formation heritage of Jupiter Family Comet 10P/Tempel 2 as revealed by infrared spectroscopy

We present spectral and spatial information for major volatile species in Comet 10P/Tempel 2, based on high-dispersion infrared spectra acquired on UT 2010 July 26 (heliocentric distance Rh = 1.44 AU) and September 18 (Rh = 1.62 AU), following the comet’s perihelion passage on UT 2010 July 04. The total production rate for water on July 26 was (1.90 +/- 0.12) x 10^28 molecules s-1, and abundances of six trace gases (relative to water) were: CH3OH (1.58% +/- 0.23), C2H6 (0.39% +/- 0.04), NH3 (0.83% +/- 0.20), and HCN (0.13% +/- 0.02). A detailed analysis of intensities for water emission lines provided a rotational temperature of 35 +/- 3 K. The mean OPR is consistent with nuclear spin populations in statistical equilibrium (OPR = 3.01 +/- 0.18), and the (1-sigma) lower bound corresponds to a spin temperature > 38 K. Our measurements were contemporaneous with a jet-like feature observed at optical wavelengths. The spatial profiles of four primary volatiles display strong enhancements in the jet direction, which favors release from a localized vent on the nucleus. The measured IR continuum is much more sharply peaked and is consistent with a dominant contribution from the nucleus itself. The peak intensities for H2O, CH3OH, and C2H6 are offset by ~200 km in the jet direction, suggesting the possible existence of a distributed source, such as the release of icy grains that subsequently sublimed in the coma. On UT September 18, no obvious emission lines were present in our spectra, nevertheless we obtained a 3-sigma upper limit Q(H2O) < 2.86 x 10^27 molecules s-1.

💡 Research Summary

This paper presents a comprehensive infrared spectroscopic study of the Jupiter‑Family comet 10P/Tempel 2, focusing on its volatile composition, production rates, and spatial distribution of gases in the coma. High‑resolution spectra were obtained on 2010 July 26 (heliocentric distance Rh = 1.44 AU) shortly after perihelion, and again on 2010 September 18 (Rh = 1.62 AU). On the first date, the water production rate was measured as Q(H₂O) = (1.90 ± 0.12) × 10²⁸ molecules s⁻¹. Six trace species were simultaneously detected, with abundances relative to water of CH₃OH = 1.58 % ± 0.23 %, C₂H₆ = 0.39 % ± 0.04 %, NH₃ = 0.83 % ± 0.20 %, and HCN = 0.13 % ± 0.02 %. The rotational temperature derived from the water line intensities is 35 ± 3 K, indicating a cold coma environment. The ortho‑to‑para ratio (OPR) of water is 3.01 ± 0.18, essentially the statistical equilibrium value; the 1‑σ lower limit corresponds to a spin temperature > 38 K, suggesting that the nucleus material formed under relatively warm conditions (> 30 K) compared with some other comets that display lower OPRs.

Spatial profiles of the four primary volatiles (H₂O, CH₃OH, C₂H₆, NH₃) show pronounced enhancements in the direction of an optical jet observed contemporaneously. This strongly supports the presence of a localized active vent on the nucleus that preferentially releases gas in a collimated outflow. Moreover, the peak intensities of H₂O, CH₃OH, and C₂H₆ are displaced by roughly 200 km in the jet direction relative to the nucleus position. Such offsets are consistent with a distributed source scenario, where icy grains are ejected from the vent, travel a short distance, and then sublimate, adding a secondary gas component to the coma. In contrast, the infrared continuum is sharply peaked at the nucleus, indicating that thermal emission from the solid nucleus dominates the dust continuum and is not significantly contributed by the extended coma.

The September 18 observations yielded no detectable emission lines; a 3‑σ upper limit of Q(H₂O) < 2.86 × 10²⁷ molecules s⁻¹ was derived, reflecting a rapid decline in activity (to roughly 15 % of the July value) as the comet receded from the Sun.

Overall, the study demonstrates the power of high‑dispersion infrared spectroscopy to simultaneously constrain volatile abundances, excitation conditions, nuclear spin statistics, and spatial release patterns in a Jupiter‑Family comet. The combination of a near‑equilibrium OPR, relatively high methanol abundance, and evidence for both a localized vent and a distributed grain‑sublimation source provides a nuanced picture of Tempel 2’s formation heritage: the nucleus likely accreted in a region of the protoplanetary disk that experienced moderate temperatures, preserving a mixture of primitive ices while also allowing for localized processing that creates active vents. These findings have implications for interpreting cometary diversity, for modeling the thermal and chemical evolution of comet nuclei, and for planning future comet‑sample‑return or in‑situ missions that aim to link surface activity with interior composition.