Temporally varying ethylene emission on Jupiter

Ethylene (C ₂H ₄) emission has been measured in the poles and equator of Jupiter. The 949 cm ^-1 spectra were recorded with a high resolution spectrometer at the McMath-Pierce telescope at Kitt Peak in October-November 1998 and at the Infrared Telescope Facility at Mauna Kea in June 2000. C ₂H ₄ is an important product of methane chemistry in the outer planets. Knowledge of its abundance can help discriminate among the various proposed sets of CH ₄ photolysis branching ratios at Ly-α, and determine the relative importance of the reaction pathways that produce C ₂H ₂ and C ₂H ₆. In the equatorial region the C ₂H ₄ emission is weak, and we were only able to detect it at high air-mass, near the limb. We derive a peak equatorial molar abundance of C ₂H ₄ of 4.5 × 10 ^-7 - 1.7 × 10 ^-6 near 2.2 × 10 ^-3 mbar, with a total column of 5.7 × 10 ¹⁴ - 2.2 × 10 ¹⁵ molecules cm ^-2 above 10 mbar depending upon choice of thermal profile. We observed enhanced C ₂H ₄ emission from the poles in the regions where auroras are seen in X-ray, UV, and near infrared images. In 2000 we measured a short-term change in the distribution of polar C ₂H ₄ emission; the emission in the north IR auroral "hot spot" decreased by a factor of three over a two-day interval. This transient behavior and the sensitivity of C ₂H ₄ emission to temperature changes near its contribution peak at 5-10 microbar suggests that the polar enhancement is primarily a thermal effect coupled with vertical transport. Comparing our observations from Kitt Peak and Mauna Kea shows that the C ₂H ₄ emission of the northern non-"hot spot" auroral regions did not change over the three-year period while that in the southern polar regions decreased.