Astron. Astrophys. 358, L83-L87 (2000)

Letter to the Editor

The ISO spectra of Uranus and Neptune between 2.5 and 4.2 µm: constraints on albedos and H₃⁺

Th. Encrenaz ¹, B. Schulz ², P. Drossart ¹, E. Lellouch ¹, H. Feuchtgruber ³ and S. K. Atreya <sup>4

1</sup> DESPA, Observatoire de Paris, 92195 Meudon, France
² ISO Data Center, ESA, PO box 50727, 28080 Madrid, Spain
³ MPI, Postfach 1603, 85740 Garching, Germany
⁴ The University of Michigan, Ann Arbor, MI 48109-1243, USA

Received 31 March 2000 / Accepted 24 May 2000

Abstract

Spectra of Uranus and Neptune were recorded in May 1997, between 2.5 and 4.2 µm, using the ISOPHOT-S instrument. The two planets were detected at 2.75 µm, with geometric albedos of 0.0015 and 0.0052 for Uranus and Neptune respectively. In the case of Uranus, the shape of the CH₄ absorption is consistent with a reflection over the 3 bar-level cloud level, presumably due to H₂S; in the case of Neptune, the reflection seems to take place above the CH₄ cloud at about 0.3 bar. Two ISO-SWS spectra of Uranus, recorded around 3.3 µm in April and May 1998, show the presence of four emission lines. From the May 1998 data, the inferred column density is cm^-2, and the rotational temperature is  K. The observed emission is significantly stronger than previous ground-based determinations, obtained in 1992 and 1995.

Key words: Planets and satellites: Uranus Neptune Infrared: solar system

Send offprint requests to: Th. Encrenaz

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Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. Interpretation
  • 3.1. Uranus and Neptune: the 2.75 µm window
  • 3.2. The emission spectrum of Uranus
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: June 10, 2000
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