Supplementary Material to:

An Introduction to Radio Astronomy

4th edition Cambridge University Press 2019   

Last updated 12/05/2019

 

Chapter 5:  The Nature of the Received Radio Signal

Early Microwave Measurements of the Surface Temperatures of Planets

 

An illustration of beam dilutions and flux density approaches

 

An interpretation of real-life measurements, important in the history of radio astronomy, makes use of concepts developed in the text thus far. We have adapted an original example in Chapter 3 of Kraus “Radio Astronomy” but have revised the numerical parameters using data from the original publications (Mayer, McCollough & Sloanaker (1958), ApJ, 127, 1 (Venus) and Mayer, McCollough & Sloanaker (1958), ApJ, 127, 11 (Mars and Jupiter). Note that these original measurements had significant uncertainties (10-20%) which we have ignored for clarity.  Mars is a thermal emitter and at the time of the observations (September 1956) the planet was in “opposition” i.e. closest to the Earth; it therefore subtended its largest solid angle and hence gave its strongest signal. The beam dilution method was used but the radio data alone were insufficient translate the measured antenna temperature to the surface brightness because the angular diameter of Mars was over 20 times smaller than the beam – the planet was essentially unresolved. The angular diameter of Mars was therefore obtained from an astronomical ephemeris based on optical telescope data (see http://spider.seds.org/spider/Mars/marsopps.html )

 

Measurements by the same team on Venus showed that its surface temperature is >600K. This was the first indication of a runaway greenhouse effect on Venus