The Solar System

458 PART 4^ |^ THE SOLAR SYSTEM

may have been produced by tidal stresses generated when the sun
slowed Mercury’s rotation.
Th e largest basin on Mercury is called Caloris Basin after the
Latin word for “heat,” recognition of its location at one of the
two “hot poles” that face the sun at alternate perihelions. At the
times of the Mariner encounters, the Caloris Basin was half in
shadow (■ Figure 21-15a). Although half cannot be seen, the low
angle of illumination was ideal for the study of the lighted half
because it produced dramatic shadows.
Caloris is a gigantic multiringed impact basin 1300 km
(800 mi) in diameter with concentric mountain rings up to 3 km
high. Th e impact threw ejecta 600 to 800 km across the planet, and
the focusing of seismic waves on the far side produced peculiar ter-
rain that looks much like the jumbled area on the moon’s surface
that lies opposite the Imbrium basin (Figure 21-15b and c). Th e
Caloris Basin is partially fi lled with lava fl ows. Some of this lava may
be material melted by the energy of the impact, but some may be
lava from below the crust that leaked up through cracks. Th e weight

of this lava and the sagging of the crust have produced deep cracks

in the central lava plains. Th e geophysics of such large, multiringed

crater basins is not well understood at present, but Caloris Basin

seems to be the same kind of structure as the Imbrium Basin on the

moon, although it has not been as deeply fl ooded with lava.

When the MESSENGER spacecraft begins orbiting Mercury

in 2011, it will photograph nearly all the planet’s surface at a

much higher resolution than did Mariner 10. Th ose new photo-

graphs will help planetary scientists build a much more complete

understanding of Mercury’s surface.

The Plains of Mercury

Th e most striking diff erence between Mercury and the moon is

that Mercury lacks the great dark lava plains so obvious on the

moon. Under careful examination, the Mariner 10 photographs

show that Mercury has plains, two diff erent kinds, in fact, but

they are diff erent from the moon’s. Understanding these diff er-

ences is the key to understanding the history of Mercury.

The impact that formed the

multiringed basin Caloris

pushed up mountain ranges

as high as 3 km.

Visual and

near-infrared wavelength images

The surface of Mercury

is heavily cratered.

Formation nicknamed

“the spider” on the

floor of Caloris basin.

b

a

c

■ Figure 21-13

(a) This 2008 MESSENGER spacecraft fl yby image in false-color highlights differences in composition between different

parts of Mercury’s surface. (NASA/JHU/APL) (b) The Caloris ringed basin, photographed by the Mariner 10 spacecraft as it

fl ew past the planet in 1975. (NASA) (c) The origin of the spider formation photographed by MESSENGER is a puzzle. (NASA/

JHU/APL)