Heading For The Hills
< Driving
on the Moon
The search for fragments of the Moon’s ancient crust was
continued by Apollo 16, which landed in a rugged highland area near the
crater Descartes. Geologists expected this area to consist of lava
flows more ancient than those of the lowland plains, but they were
wrong. Astronauts John Young and Charles Duke soon realised that the
rocks around them were broken and shattered ejecta from ancient impacts
that excavated some of the Moon’s great lowland basins,
possibly both Mare Imbrium and Mare Nectaris. The Descartes highlands
were therefore similar in nature and age to the Fra Mauro area sampled
by Apollo 14. However, Apollo 16 did
collect one particularly ancient rock (sample
60025, shown in situ at the centre of this
photograph)dating back nearly 4.5 billion years, older even
than the so-called Genesis Rock found by the Apollo 15 crew.
Apollo 17, the final mission of the series, landed on the eastern side
of Mare Serenitatis, between the Taurus Mountains and the crater
Littrow. Here, dark lava met bright highlands and offered many different
types of rock samples, including plenty of large
boulders. Astronauts Gene Cernan and geologist Harrison
(“Jack”) Schmitt set new records by spending three
days on the Moon, driving 35 km and collecting 110 kg of rocks.
Study of these rocks back on Earth established that the impact that
formed the Mare Serenitatis basin occurred 3.9 billion years ago, while
the lavas that filled the basin flooded out between 3.7 and 3.8 billion
years ago. Lunar observers can see that the Apollo 17 landing site is
crossed by a bright ray from the prominent impact crater Tycho, 2,000
km away. Arrival of this ejecta apparently caused landslides on the
surrounding mountains about 100 million years ago, when Tycho was
formed.
A highlight of the Apollo 17 explorations was the discovery of orange
soil, consisting of microscopic glass beads, near a crater
110 metres wide and 20 metres deep called Shorty. At first it was
thought that the soil was young, like the crater, but it turned out to
be 3.6 billion years old. Geologists think that the tiny beads were
caused by a volcanic eruption akin to fire fountains on Earth. Soon
after formation, they were buried beneath a subsequent lava flow until
being exposed by the impact that caused Shorty, perhaps 30 million
years ago.
When Apollo 17 splashed down on 1972 December 19 it brought the first
era of manned lunar exploration to a close. No humans have been to the
Moon since. Exploring text by Ian Ridpath 
The site where
orange soil was found (left)  |