Long-term observations of Uranus and Neptune at 90 GHz with the IRAM 30 m telescope. (1985-2005)

International audienceContext: The planets Uranus and Neptune with small apparent diameters are primary calibration standards. Aims: We investigate their variability at ~90 GHz using archived data taken with the IRAM 30 m telescope during the 20 year period 1985 to 2005. Methods: We calibrate the planetary observations against non-variable secondary standards (NGC 7027, NGC 7538, W3OH, K3-50A) observed almost simultaneously. Results: Between 1985 and 2005, the viewing angle of Uranus changed from south-pole to equatorial. We find that the disk brightness temperature declines by almost 10% (~2sigma) over this time span indicating that the south-pole region is significantly brighter than average. Our finding is consistent with recent long-term radio observations at 8.6 GHz. Both data sets show a rapid decrease of the Uranus brightness temperature during 1993, indicating a temporal, planetary scale change. We do not find indications for a variation of Neptune's brightness temperature at the 8% level. Conclusions: If Uranus is to be used as a calibration source, and if accuracies better than 10% are required, the Uranus sub-earth point latitude needs to be taken into account