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M. Roos-Serote (Lisbon Astron. Obs.), D. Stam, R. Fender (Univ. of Amsterdam)
Mars' radio spectrum at centimeter wavelengths is produced by thermal radiation from the surface and sub-surface. Observations at 2.8 cm made in the 1975 and 1978 show variations of its radio brightness as a function of longitude on the planet (Doherty et al. , ApJ 233, 1979). In addition, an overall increase in the radio brightness observed in 1978 just at the start of a global dust storm. Recent theoretical studies show that radio emission can be produced by corona discharges between dust particles, as these become charged through triboelectric processes in dust lifting/transportation events (Renno et al. GLR v.30, 2003). In this work we explore the hypothesis that changes in Mars' radio emission are be linked to changed in the atmospheric dust content.
We took profit of the very close approach of Mars during the summer of 2003 opposition, to observe Mars in the radio. We aimed at whole disk (unresolved) observations. We obtained 48 hours of regular observing time of the Westerbork Synthesis Radio Telescope (WSRT) facility in the Netherlands around the opposition date. We observed Mars on 6 occasions, for eight hour periods each, at 6 cm and 21 cm, covering the entire planet twice. We also obtained 55 hours of observing time at the Nançay Radio Telescope (NRT) facility in France (a transit instrument), between the beginning of July 2003 and the mid March 2004. We covered the planet several times at 11 cm and 21 cm.
At 6 cm (WSRT) no clear signal of the longitudinal variation is observed. There is one observation which shows a decrease in brightness temperature, which might be linked to an increase in the atmospheric dust content. At 11 cm and 21 cm (NRT) the longitudinal variation is clearly present. Changes in the brightness temperature are also seen as a function of time. At the time of writing, the analysis has not been fully concluded and it is not yet clear whether the time changes are linked to a change in dust content of the atmosphere. The absence of a radio curve at 6 cm, in contrast with 11 cm and 21 cm, needs an explanation.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.