The Solar System at Radio Wavelengths 699
FIGURE 4 A radio spectrum of Venus. At short
wavelengths, one probes approximately down to Venus’
cloud layers. The brightness temperature increases when
deeper warmer layers are probed and decreases again at
wavelengths long enough to probe down into Venus’
surface. (B. J. Butler and R. J. Sault, 2003, Long
wavelength observations of the surface of Venus,IAUSS,
1E, 17B.)clearly visible. These are modeled well, as exemplified by
the map in Fig. 3b, which shows the difference between
the observed map and a thermal model. The viewing ge-
ometry is superimposed on the latter image. The negative
temperatures near the poles and along the terminator are
indicative of areas colder than predicted in the model. This
is likely caused by surface topography, which causes per-
manent shadowing at high latitudes and transient effects in
the equatorial regions, where crater floors and hillsides are
alternately in shadow and sunlight as the day progresses.
Some crater floors near the poles are permanently shad-
owed, and radar observations have revealed evidence for
the existence of water ice in such crater floors.
Radio spectra and images, together withMariner 10in-
frared (IR) data show that Mercury’s surface properties are
quite similar to those of the Moon, except for the microwave
opacity, which is∼2–3 smaller than that of most lunar sam-
ples. This suggests a low ilmenite (FeTiO 3 ) content, the
mineral that is largely responsible for the dark appearance
of the lunar maria. The absence of iron (Fe) and titanium
(Ti) bearing minerals from Mercury’s surface suggests this
planet to be largely devoid of basalt, which, if true, contains
clues as to its volcanic past.
2.4.3 VENUS AND MARS
Venus and Mars have atmospheres which consist of over
95% carbon dioxide gas (CO 2 ). Other than having a similar
composition, the atmospheres are very different. The
surface pressure on Venus is approximately 90 times larger
than that on Earth, while that on Mars is∼140 times smaller.
The shear amount of CO 2 gas on Venus provides so much
opacity that Venus’ surface can only be probed at wave-
lengths longward of∼6 cm, whereas Mars’s atmosphere is
essentially transparent at most radio wavelengths. On both
planets, CO 2 gas is photodissociated (molecules are broken
up) by sunlight at high altitudes into carbon monoxide
(CO) and oxygen (O). CO gas has strong rotational tran-
sitions at millimeter wavelengths, which can be utilized to
determine the atmospheric temperature profile and the
CO abundance on Venus and Mars in the altitude regions
probed.
Radio astronomical observations of Venus go back to the
mid-1950s, when measurements at a wavelength of 3 cm in-
dicated a brightness temperature of over 560 K, well above
that expected (∼300 K) from a terrestrial analog. In the early
1960s, Carl Sagan postulated this high temperature to re-
sult from a strong greenhouse effect in Venus’ atmosphere.
A fullradio spectrum(Fig. 4) reveals that the planet’s sur-
face is probed at a wavelength of∼6 cm, with a surface
temperature close to∼700 K. At longer wavelengths, one
probes below the surface, where the observed brightness
temperatures are well below predicted values—by up to
200 K, an effect that is not (yet) understood.
The CO 1–0 (3 mm) and 2–1 (1 mm) rotational transi-
tions have been observed routinely. Since CO is formed in
the upper part of the atmosphere, the line is seen in absorp-
tion against the warm continuum background on both Venus
and Mars (Fig. 5). On Venus, one probes the so-called meso-
sphere in these transitions, a region between the massive
lower atmosphere (altitudes
̃<70 km), in which the radiativeFIGURE 5 Spectra of Venus in theJ= 1 −0 line. The upper
curve is for the day side hemisphere (when Venus is near superior
conjunction), the lower curve is for the night side hemisphere
(when Venus is near inferior conjunction). (F. P. Schloerb, 1985,
Millimeter-wave spectroscopy of solar system objects: Present
and future, “Proceedings of the ESO–IRAM–Onsala Workshop
on (Sub)millimeter Astronomy,” P. A. Shaver and K. Kjar, eds.,
Aspenas, Sweden, 17–20 June 1985, pp. 603–616.)