Philips Atlas of the Universe

Features of Mercury

ATLAS OF THE UNIVERSE

▲ Brahmsis a large crater

north of the Caloris Basin. It

has a central peak complex,

terraced walls, and displays

ejecta deposits.

M

ercury is above all a cratered world. The formations

range from small pits up to colossal structures larger

than anything comparable on the Moon. (Beethoven,

Mercury’s largest crater, is well over 600 kilometres

[370 miles] across.) Small craters below 20 kilometres

(12 miles) in diameter are, in general, bowl-shaped; larger

craters have flatter floors, often with terraced walls and

central peaks. As with the Moon, the distribution is non-

random. There are lines, chains and groups, and where one

formation breaks into another it is virtually always the

smaller crater which is the intruder. Between the heavily

cratered areas are what are termed intercrater plains, with

few large structures but many craterlets in the 5 to 10 kilo-

metres (3 to 6 miles) range; these are not found on the

Moon or Mars. Neither are the lobate scarps, cliffs from

20 to 500 kilometres (12 to 300 miles) long and up to

3 kilometres (almost 2 miles) high; they seem to be thrust

faults, cutting through features and displacing older ones.

Of the basins, much the most imposing is Caloris,

which has some points in common with the lunar Mare

Imbrium. It is 1500 kilometres (over 900 miles) in diam-

eter and surrounded by mountains rising to between 2000

and 3000 metres (6600 and 10,000 feet) above the floor.

Antipodal to the Caloris Basin is the ‘hilly and lin-

eated terrain’, often called ‘weird terrain’. It covers

360,000 square kilometres (139,000 square miles), and

consists of hills, depressions and valleys which have

destroyed older features. Evidently the formation of this

terrain is linked with the origin of Caloris.

Presumably the ages of the surface features are much

the same as those of the Moon. It has been estimated

that the Caloris Basin is about 4000 million years old,

and that extensive vulcanism ended about 3900 million

years ago. Certainly there can be virtually no activity there

now. It has been suggested, on the basis of radar observa-

tions, that there may be ice inside some of the polar

craters, whose floors are always shadowed and are there-

fore intensely cold, but the idea of ice on a world such as

Mercury does not seem very plausible.

It is a pity that our coverage of Mercury is so incom-

plete; for example, only half the Caloris Basin was in

sunlight during the three active passes of Mariner 10. For

more detailed information we must await the results from

a new spacecraft. The next mission, Messenger (Mercury

Surface, Space Environment, Geochemistry and Ranging

mission), will be an orbiter, scheduled for launch in 2004.

▲ Crater morphology

is similar to the Moon’s.

Craters are generally circular,

have ejecta rim deposits,

fields of secondary craters,

terraced inner walls and

central peaks, or even

concentric inner rings.

The smallest craters are

bowl-shaped; with increasing

size there may be a central

peak, then inner terracing of

the walls. Still larger craters

have more frequent central

peaks, and in the very largest

structures complete or

partial concentric inner rings

may develop. The change

from one type to the next

occurs at lower diameters on

Mercury than on the Moon.

▼ Degasis a bright ray-

crater. Craters such as this

are thought to be relatively

young, for rays emanating

from them cross all other

formations. These high

albedo, wispy filaments

consist of fine particles

of ejecta. The black streak

towards the left indicates a

portion of the surface not

imaged by Mariner 10.

Typical craters with central peaks

Multiple inner rings

100 km

90

80

70

60

50

40

30

20

No central peaks

Inner ring diameter

Multiple central peaks

Overalldiameter

15

10

5 95

115

130

130

145

180

190

225 km

0 100 200 300 km

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