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