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Strong seasonal temperature variations in Neptune’s atmosphere

1 September 2007

For the first time, astrophysicists from JPL and CASS, California, and LESIA, Observatoire de Paris, have made resolved maps of the temperature on Neptune. Mid infrared images of Neptune, taken with the VISIR instrument on the VLT at ESO, reveal strong inhomogeneities in thermal emission, with high temperatures at the south pole, enhanced by seasonal variations. These high temperatures allow the tropospheric methane to escape into the stratosphere, solving the puzzle of the high abundance of CH4 there. In spite of its large distance to the Sun (30 times the Earth-Sun distance), Neptune is therefore the site of a high climatic activity.

Figure 1: Maps of the temperature on Neptune, taken on 1-2 september 2006, with north at top, and the south pole visible at the bottom of each image. Stars are also shown for comparison and calibration. The filter wavelength is indicated on each image. Click on the image to enlarge it
© VISIR/ESO. JPL. LESIA. CASS.

This is the first publication of spatially resolved thermal images of Neptune at wavelengths sensitive to temperatures near its tropopause (temperature minimum) which are substantially influenced by seasonally varying solar input. The images show the south pole of Neptune at this level (near 0.1 bars / 100 mbar pressure) is extremely warm, generally consistent with the fact that it is late southern summer and this region has been in sunlight for close to 40 years.

In fact the temperatures are so warm that methane (CH4) gas, which should be frozen ("cold trapped") out of the stratosphere above this level, can leak out through this region of warm temperatures. This solves a long-standing problem of identifying the source of Neptune’s high stratospheric methane abundances. It also means that methane could be migrating northward from the south polar leak if Neptune’s stratospheric circulation is vigorous enough. If the warm temperatures at the south pole are, indeed, seasonally driven, it means that methane will be leaking out of a warm *north* pole in about 80 years, during Neptune’s northern summer. This sort of migration has some similarities to the seasonally driven hemispherical migration of CO2 on Mars, although methane is not the primary constituent of Neptune’s atmosphere (those are molecular hydrogen and helium).

Figure 2: Latitudinal variations of the mean temperature at a pressure of 100mbar in the atmophere of Neptune. Click on the image to enlarge it

Another unexpected result was the detection of a discrete warm region near 65-70 degrees S latitude in emission from methane and ethane gas at higher levels in the stratosphere. This region was definitely not at the pole itself, unlike the temperature peak deeper down. Observations in 2005 at Keck and recently in 2007 at Gemini/North showed no such region.

Press release

Reference

  • Evidence for methane escape and strong seasonal and dynamical perturbations of Neptune’s atmospheric temperatures G. S. Orton (JPL, Caltech, USA), T. Encrenaz (LESIA, Obs-Paris), C. Leyrat (JPL, Caltech, USA), R. Puetter (CASS, California, USA), A.J. Friedson (JPL, Caltech, USA) Astronomy & Astrophysics, Letter published on 18 September 2007

Contact

  • Thérèse Encrenaz (Observatoire de Paris, LESIA, et CNRS)
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