The Solar System at Radio Wavelengths 705
FIGURE 10 A spectrum of Titan’s 12–11 transition of CH 3 CN,
taken in December 1999 with the IRAM 30-m telescope in
Spain. The upper panel shows the spectrum at 1 MHz
resolution; the lower panel shows it at 78 kHz. (A. T. Marten
et al., 2002, New millimeter heterodyne observations of Titan:
Vertical distributions of nitriles HCN, HC 3 N, CH 3 CN, and the
isotopic ratio^15 N/^14 N in its atmosphere,Icarus 158 , 532–544.)
on Titan is very similar to that on Earth (89), while^14 N/^15 N
was measured to be several times less than the terrestrial
value of 272. This has been explained by a large loss of
Titan’s primitive atmosphere over time, which would lead to
an isotopic fractionation in nitrogen. In contrast, the similar-
to-Earth value in^12 C/^13 C hints at a continuous or periodic
replenishment of methane gas into Titan’s atmosphere, such
as could happen, for example, through cryovolcanism or a
methane cycle akin to the hydrology cycle on Earth.
Radiometry maps of Titan obtained with theCassini
spacecraft can be used with radar and infrared measure-
ments to better constrain the surface composition and
compactness. Observations show variations in brightness
temperature up to∼10 K, which are more or less anticor-
related with infrared brightness (i.e., the infrared/optically
bright areas have a low radio brightness temperature). The
Cassiniradar team suggests the optically bright, radio-cold
areas perhaps are composed of fractured or porous ice (as
on Europa and Ganymede), and the optically dark, radio-
warm regions are composed of an organic sludge, or perhaps
more solid water ice (higher dielectric constant).2.6.3 ASTEROIDS AND TRANS-NEPTUNIAN OBJECTS
In analogy with the terrestrial planets, a comparison of
multiwavelength radio data of small airless bodies with
thermophysical models provides information on the
(sub)surface properties of the material, as composition and
compactness. Radio spectra of several main-belt asteroids
suggest that these bodies are typically overlain by a layer
of fluffy (highly porous) dust a few centimeters thick, as on
the Moon and Mercury.
It has been challenging to observe trans-Neptunian
(TNO) or Kuiper Belt objects (KBO) at radio wavelengths,
including Pluto, due to their small angular extent and low
surface temperature. Much progress has been made in the
past decades, however. For an object in radiative equi-
librium, with an albedo of∼0.6, the surface temperature
should be approximately 50 K, consistent with the 53–59 K
temperatures for Pluto as measured by IRAS at 60 and
100 μm. Since at radio wavelengths one probes approxi-
mately 10 wavelengths deep into the surface, a brightness
temperature of∼40 K is expected.
Observations of Pluto with the 30-m IRAM telescope at
millimeter wavelengths revealed brightness temperatures
closer to 30 K, indicative of a low radio emissivity (e≈
0.6–0.7), similar to that seen on Ganymede. Such a low
emissivity can be reconciled with a surface composed of icy
grains, and hence relatively high porosity.
Radio measurements of KBOs have been used to de-
termine the size and albedo of several of the largest ob-
jects (Quaoar, Ixion, Varuna, 2002 AW197), in concert with
optical measurements and the so-called Standard Thermal
Model (STM) to interpret the data. Although one has to be
aware of the assumptions made in the STM, which can lead
to over- or underestimates of the size and albedo, such mea-
surements are usually our only means to get a reasonable
size estimate for these objects.2.7 Comets
Radio observations of comets provide information that com-
plements studies at other wavelengths. Continuum mea-
surements are sensitive to the thermal emission from a
cometary nucleus and of large dust grains in its coma, while
spectroscopic observations provide information on the “par-
ent” molecules in a comet’s coma. Upper limits to the radio
continuum emission of a few comet nuclei suggest that the
temperature gradient in the nucleus may be very steep, or,
alternatively, that the emission is substantially suppressed
by subsurface scattering.
Quasi-continuum spectra reveal a spectral slope that is
steeper than that of blackbody thermal emission, yet smaller
than that expected from Rayleigh scattering from small