320 Encyclopedia of the Solar System
planet’s history so that the bulge had largely formed at the
end of heavy bombardment. A much smaller bulge, cen-
tered in Elysium at 25◦N, 3213◦W, has also been a center
of volcanic, tectonic, and fluvial activity. Other prominent
topographic features are large impact basins; the largest are
Hellas (2600 km diameter), Isidis (1600 km), and Argyre
(1500 km).
The physiography of the poles is distinctively different
from that of the rest of the planet. At each pole, extending
out to the 80◦latitude circle, is a stack of finely layered
deposits a few kilometers thick. In the north, they rest on
plains; in the south, they rest on cratered uplands. The small
number of superimposed impact craters suggests that they
are only a few tens of millions of years old.
3. Impact Cratering
3.1 Cratering Rates
All solid bodies in the solar system are subject to impact by
asteroidal and cometary debris. (See Fig. 2.) The cratering
rates are low. On Earth, in an area the size of the United
States, a crater larger than 10 km across is expected to form
FIGURE 2 Impact craters in Lunae Planum. The ejecta are
distributed around the craters in lobes, each surrounded by a low
ridge or rampart. The largest crater is 35 km across. Thermal
Emission Imaging System (THEMIS.)
every 10–20 million years and one larger than 100 km across,
every billion years. The rates on the other terrestrial planets
are likely to be within a factor of 2 or 3 of these rates. As a
consequence, any surface that has a large number of craters
several tens of kilometers across or larger must date back
to a time when cratering rates were higher. On the Moon,
surfaces are either densely covered by large craters (lunar
highlands) or sparsely affected by large craters (maria) with
no surfaces of intermediate crater densities. This contrast
arises because of the Moon’s cratering history. Very early on,
cratering rates were high. Around 3.8 billion years ago they
declined rapidly to roughly the present rate. Accordingly,
surfaces that formed prior to 3.8 billion years ago are heavily
cratered, and those that formed afterward are much less
cratered. Mars has had a similar cratering history, hence
the contrast between the heavily cratered uplands and the
sparsely cratered plains.
Craters provide a means of estimating the ages of sur-
faces. As we just saw, the most densely cratered surfaces
formed prior to 3.8 billion years ago, and the cratering rate
has been roughly constant since that time. Consequently, a
3-billion-year-old surface will have three times more craters
on it than a 1-billion-year-old surface. There is considerable
uncertainty in estimating absolute ages this way because we
do not know exactly what the cratering rate on Mars has
been for the past few billion years. Nevertheless, by count-
ing craters, we can put surfaces in a time-ordered sequence
and make rough estimates of their absolute ages.3.2 Crater Morphology
Impact craters have similar morphologies on different plan-
ets. Small craters are simply bowl-shaped depressions with
constant depth-to-diameter ratios. With increasing size, the
craters become more complex as central peaks appear, ter-
races form on the walls, and the depth-to-diameter ratio
decreases. At very large diameters, the craters become mul-
tiringed, and it is not clear which ring is the equivalent of
the crater rim of smaller craters. On Mars the transition
from simple to complex takes place at 8–10 km, and the
transition from complex craters to multiringed basins takes
place at 130–150 km diameter.
Although impact craters on Mars resemble those on the
Moon, the patterns of ejecta are quite different. Lunar
craters generally have continuous hummocky ejecta near
the rim crest, outside of which is a zone of radial or concen-
tric ridges, which merge outward into strings or loops of sec-
ondary craters, formed by material thrown out of the main
crater. In contrast, the ejecta around most fresh-appearing
martian craters, especially those in the 5–100 km size range,
are disposed in discrete, clearly outlined lobes. Various pat-
terns are observed. The ejecta around craters smaller than
15 km in diameter are enclosed in a single, continuous lo-
bate ridge, or rampart, situated about one crater diameter
from the rim. Around larger craters, there may be many