The brightness temperature of Mercury at mm-wavelengths

¹ IRAM, 300 rue de la Piscine, 38406 Saint-Martin d`Hères, France e-mail: thum@iram.fr
² LESIA (LAM – bat. 18), 5 place Jules Janssen, 92195 Meudon Cedex, France
³ Service d'Aéronomie CNRS/IPSL, 91371 Verrières-le-Buisson, France

Received: 25 January 2007
Accepted: 24 November 2008

Abstract

We present observations of Mercury made with the IRAM 30-m telescope at 3, 2 and 1.3 mm wavelength (90, 150 and 230 GHz) during the years 1985-2005; we derive from these data the disk-averaged brightness temperatures. The observations at 3 mm combined with those by Epstein & Andrew allow a separation of the data into 40° wide longitude intervals and by this an investigation of the disk-averaged brightness temperature with Mercury's longitude. From the new mm-wavelength data, and data taken from the literature, we derive the disk-averaged brightness temperature as a function of wavelength. On Mercury's night side a significant decrease in brightness temperature occurs towards shorter wavelengths. 
We use the three surface models (A,B,C) discussed by Mitchell & de Pater and calculate for the cool and hot surface region the corrresponding diurnal variation of the disk-averaged brightness temperature at 90 GHz. For the same models we calculate the variation of the disk-averaged brightness temperature with wavelength between 1.3 mm and 37 mm, on Mercury's midnight side and noon side. Although the scatter in the observations is large, there seems to be a marginally better agreement with model B and A.