The Solar System

460 PART 4^ |^ THE SOLAR SYSTEM

It is also diffi cult to explain the proportionally large size of the

metallic core inside Mercury. In Chapter 19, you saw that planets

forming near the sun should incorporate more metals, but some

models suggest that this eff ect alone cannot account for the size of

Mercury’s very large metallic core. One hypothesis is that heat

from the sun vaporized and drove away some of the rock-forming

elements in the inner solar nebula. Another theory involves a giant

impact when Mercury was young, an impact much like the planet-

shattering impact proposed to explain the origin of Earth’s moon.

If the forming planet had diff erentiated and was then struck by a

large planetesimal, the impact could have shattered the crust and

mantle and blasted much of the lower-density material into space.

Th e denser core could have survived, re-formed, and then swept

up some of the lower-density debris to form a thin mantle and

crust. Th is scenario would leave Mercury with a defi ciency of low-

density crustal rock. It is possible that both Mercury and the moon

may be products of giant impacts.

Th e MESSENGER spacecraft will be able to test this

hypothesis when it goes into orbit around Mercury in 2011. Th e

probe will analyze the composition of the crust by remote mea-

surements from orbit, and the chemical composition of the rock

will reveal its history.

A History of Mercury

Can you combine evidence and theory to tell the story of

Mercury? It formed in the innermost part of the solar nebula,

and, as you have seen, a giant impact may have robbed it of some

■ Figure 21-15

The huge impact that formed the Caloris Basin on Mercury sent seismic
waves through the planet. Where the waves came together on the far side,
they produced this lineated terrain, which resembles the jumbled terrain on
Earth’s moon opposite the Imbrium impact. (NASA)

cc

Caloris

impact

Pressure

wave

Path of seismic energy

Lineated

terrain

b

a

Visual

Outer rim of basinOuter rim of basinOuter rim of basin

VisualVisual

Center of Caloris

Basin lies in darkness

to lower left.

Lineated terrain

appears to have

been disturbed.

MESSENGER spacecraft will make measurements of Mercury’s
gravitational and magnetic fi elds that will help planetary astron-
omers understand its core.

■ Figure 21-16

Study this region of Mercury carefully, and you will notice the plains between

the craters. This surface is not saturated as are the lunar highlands, but it

contains more craters than the surface of the maria on Earth’s moon. This

shows that these plains formed after the heaviest of the cratering in the

early solar system. (NASA)