Planetary geomorphology methods - Introduction to Planetary Geomorphology - page 25

Table 2.1. Radar bands

Designation Wavelength Frequency Examples

Ku, K, Ka 0.8 - 2.4 cm 37.5 - 12.5 GHz Aircraft - Earth

X 2.4 - 3.8 cm 12.5 - 8 GHz SIR - Earth

C 3.8 - 7.5 cm 8 - 4 GHz SIR - Earth

S 7.5 - 15 cm 4 - 2 GHz Magellan - Venus

L 15 - 30 cm 2 - 1 GHz Seasat - Earth

P 30 - 100 cm 1 GHz - 300 MHz MARSIS - Mars

Different parts of the radio wavelength EM spectrum
are used in SAR imaging, designated with letters of the
alphabet ( Table 2.1 ). These were de ned mostly during
the Second World War as classi ed information, and the
letters were arbitrarily assigned. In addition, radar data are
typically polarized, by which means the waveform ener-
gies both in the

atmospheres and surfaces that lack atmospheres. UV
spectra provide useful information on some of the phys-
ical properties of the surface, such as grain size and the
presence of frost. X-ray spectrometers detect energy
generated by the Sun, in which the X-ray spectra are
diagnostic for elements such as aluminum, silicon, and
magnesium. Gamma rays are produced from radioactive
decay and from the bombardment of surfaces by cosmic
rays from deep space. Gamma-ray spectrometers meas-
urethisenergyandcanbeusedtomapthedistributions
of some elements, such as titanium. For example, X-ray
and gamma-ray spectrometers were flown on Apollos 15
and 16 to map parts of the Moon.

phases are
filtered into either a horizontal plane or a vertical plane.
Thus, the data can occur in one of four combinations: (a)
HH for horizontal send, horizontal receive; (b) VV for
vertical send, vertical receive; (c) HV for horizontal send,
vertical receive, or (d) VH for vertical send, horizontal
receive. Each mode has advantages and disadvantages
depending on the application and the nature of the surface.
The Soviet Venera 15 and 16 and the US Magellan
missions carried radar imaging systems to Venus to obtain
the first detailed views of the surface from orbit. NASA ' s
Cassini orbiter, sent to Saturn, carried a radar system to
obtain images of the moon Titan.
Very-long-wavelength - low-frequency radar systems
are capable of penetrating into the subsurface, depending
on the nature of the surface materials. In general, some of
the radar energy is re ected from subsurface boundaries,
such as contacts between rock units, and is recorded; from
the geometry of the received signal, the depth to the
boundary can be calculated. Instruments using this prin-
ciple were used on the Moon during Apollo and were
flown on the European Space Agency Mars Express mis-
sion and on NASA ' s Mars Reconnaissance Orbiter.
Penetrating radar is likely to fly on missions to the outer
Solar System as a means of investigating the ice structure
of some satellites, such as Jupiter

“ send ” and in the “ receive ”

2.6 Geophysical data

Various techniques are employed to obtain geophysical
data relevant for planetary geomorphology, including the
use of instruments to measure altimetry, gravity, and mag-
netic fields. Topographic data are fundamental for most
studies and are derived from images (see Section 2.10 )or
from altimeters that measure the distance from the space-
craft to the ground. For example, topographic data from
the Mars Orbiter Laser Altimeter (MOLA) are commonly
used to make images that have the appearance of photo-
graphs in which the illumination direction and angle, as
well as the vertical exaggeration, can be controlled.
Both radar systems and lasers have been used in plan-
etary exploration for altimetry, in which the time between
energy emission from the spacecraft and energy receipt
from the surface yields the altitude. The spacecraft posi-
tion referenced to the overall shape of the planet then
enables the surface topography to be derived for compar-
ison with the geomorphology.
Mapping variations in local gravity is a common tech-
nique in geophysical studies on Earth and for mineral
prospecting. Areas containing high-density materials

' s moon Europa.

2.5.5 Ultraviolet, X-ray, and gamma-ray data

At very short wavelengths ( Fig. 2.14 ), EM energy is
strongly in uenced by gasses, and the ultraviolet (UV)
part of the EM spectrum is used to study planetary