H 3 + cooling in planetary atmospheres Steve Miller, * a Tom Stallard, b Henrik Melin b and Jonathan Tennyson a Received 10th March 2010, Accepted 8th April 2010 DOI: 10.1039/c004152c We review the role of H 3 + in planetary atmospheres, with a particular emphasis on its effect in cooling and stabilising, an effect that has been termed the ‘‘H 3 + thermostat’’ (see Miller et al., Philos. Trans. R. Soc. London, Ser. A, 2000, 58, 2485). In the course of our analysis of this effect, we found that cooling functions that make use of the partition function, Q(T) based on the calculated H 3 + energy levels of Neale and Tennyson (Astrophys. J., 1995, 454, L169) may underestimate just how much energy this ion is radiating to space. So we present a new fit to the calculated values of Q(T) that is accurate to within 2% for the range 100 K to 10 000 K, a very significant improvement on the fit originally provided by Neale and Tennyson themselves. We also present a fit to Q(T) calculated from only those values Neale and Tennyson computed from first principles, which may be more appropriate for planetary scientists wishing to calculate the amount of atmospheric cooling from the H 3 + ion. 1 Introduction Infrared emission from H 3 + has been detected in Jupiter, 1,2 Saturn 3 and Uranus, 4 but not—so far—in Neptune. For Jupiter, most of the emission comes from the auroral/ polar regions, although there is a planet-wide glow: at mid-to-low latitudes, this cannot be explained by EUV ionisation alone, 3 but the exact cause(s) and their rela- tive importance are not fully understood. Total H 3 + emission from Jupiter is 10 13 W. 4 Saturn’s emission is a few percent of that of Jupiter, and is—again—concen- trated around the poles as auroral activity. 5 For Uranus the situation is rather different: auroral emission is probably not more than 20% of the total, planetwide, and there is a glow that covers the entire disk. Again, uranian emission is a few percent that of Jupiter. 6 Taken together, however, these observations demonstrate that H 3 + is an important constituent of giant planet atmospheres and ionospheres. An outstanding problem for the Solar System’s giant planets is how to account for their high exospheric temperatures, all of which are hundreds of degrees hotter than can be accounted for by the absorption of sunlight alone. 7 Amongst the leading candidates to explain these temperatures are gravity wave heating from the lower atmosphere 8 and the distribution of energy from the auroral/polar regions. 9 Neither explanation is without serious problems, however. 10,11 The upper atmosphere of a planet such as Jupiter is an important interface between its space environment and the denser atmosphere below. In the Solar System, all planets are irradiated by the Sun and impacted upon by the Solar Wind, a stream of rarified plasma travelling at several hundred kilometres per a Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, U.K. E-mail: s.miller@ucl.ac.uk; j.tennyson@ucl.ac.uk; Fax: +44 20 7679 7155; Tel: +44 20 7679 2000 b Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, U.K. E-mail: tss@ion.le.ac.uk; hm@ion.le.ac.uk; Fax: +44 0116 252 2770; Tel: +44 0116 252 3575 PAPER www.rsc.org/faraday_d | Faraday Discussions This journal is ª The Royal Society of Chemistry 2010 Faraday Discuss., 2010, 147, 283–291 | 283