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In-Depth Information
According to Wu and Doyle (
1990
), ground-based radar observations of Mercury,
Venus, and Mars were performed for the first time in the early 1970s, providing
surface cross sections with vertical accuracies of 100-150 m. From orbit, the three-
dimensional surface profile of Venus has been explored by radar measurements of
the Pioneer 10 Venus Orbiter (Pettengill et al.,
1980
; Wu and Doyle,
1990
). Later
radar-based topographic measurements of Mercury are described by Harmon and
Campbell (
1988
).
The laser altimeters on the Apollo 15-17 spacecrafts provided lunar topographic
data for surface points at a high accuracy of 2 m (Wu and Doyle,
1990
). The lunar
surface has been globally mapped at a lateral resolution of 0
.
25
◦
in longitude and
latitude, i.e. better than 7
.
5 km, by laser altimetry from the Clementine spacecraft
(Bussey and Spudis,
2004
). According to Araki et al. (
2009
), a global topographic
map of the Moon with a standard deviation of the elevation values of 4
.
1 m has been
constructed using the Laser Altimeter (LALT) instrument on board the Kaguya (SE-
LENE) spacecraft. The Lunar Reconnaissance Orbiter (LRO) carries the Lunar Or-
biter Laser Altimeter (LOLA), which provides elevation data with a standard devia-
tion of 90 mm (Riris et al.,
2010
). The LOLA elevation measurements have been re-
sampled into global topographic maps ('gridded data record') of 4, 16, 64, 128, and
256 pixels per degree nominal lateral resolution (Neumann,
2009
). With increasing
number of elevation measurements, topographic maps of 512 and 1024 pixel per de-
gree nominal lateral resolution have also been made available.
1
However, in many
lunar regions the true lateral resolution of these topographic maps is much lower
than the nominal one, as they tend to contain a significant number of interpolation
artefacts.
The surface of Mars has been mapped almost entirely at a nominal lateral res-
olution of 463 m at the equator and an elevation accuracy of about 10 m by the
Mars Orbiter Laser Altimeter (MOLA) carried by the Mars Global Surveyor Or-
biter spacecraft (Gwinner et al.,
2010
).
8.1.1.2 Shadow Length Measurements
The classical passive approach to determine height differences on planetary surfaces
is the measurement of shadow lengths. This method dates back to Herschel's visual
telescopic observations in the year 1787 (Wu and Doyle,
1990
). Modern shadow
length measurements have been systematically performed using spacecraft imagery
of the Moon (Wood,
1973
; Wood and Andersson,
1978
), Mercury (Pike,
1988
), and
Mars (Cintala et al.,
1976
) to determine the depths and rim heights of craters and
the heights of their central peaks.
1
The LOLA topographic maps are accessible on the NASA Planetary Data System at
http://pds-
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