The Earth's Moon
The moon is the nearest extraterrestrial object to the earth and
is obvious. Hence there has been natural curiousity about it for
thousands of years.
The history of Lunar Exploration is one of excellent planning
and strategy on the part of NASA. The chief Components of this
process were the following missions:
- The effort to put men on the moon started with a patriotic/panicked
reaction to the USSR launch of Sputnik in 1957.
- Going to the moon was clearly a political goal (but big deal
it got us there!)
- It represented leadership from the highest levels on down
that is no one stood up in congress in the mid-60's and said
"Let's not go the Moon". That would surely be the case today.
- While the motivation was political careful attention to the science
to be accomplished also occurred.
- We landed on the moon more than 25 years ago!
Mercury Program This was a single astronaut program
designed at determining if sucessful earth orbit could be
acheived. This program lasted 2 years (1961--1963) and saw 7 flights.
Gemini Program These spacecraft were manned by two
astronauts. The program was designed to test the endurance
of astronauts in space (the longest Gemini mission was 14 days)
as well as the ability to rendevous and dock with another spacecraft.
This program lasted 1.5 years (1965--1966) and saw 10 flights.
It remains NASA's most aggressive mission.
The Apollo Program These spacecraft were manned by
three astronauts. This was the lunar landing program. The
initial launch of Apollo 7 in October 1968 was to check out the
spacecraft over a 10 day mission. Two months later, during
Christmas of 1968, Apollo 8 went to the moon and acheived
a lunar orbit and returned to the Earth. Apollo 8 remains the
most symbolic of all our trips into space as, for the first
times, photographs of the blue earth rising about another
object were seen. In that one moment, everyone was made to
realize that the earth is a finite resource. On July 20,
1969 Apollo 11 reached the lunar surface.
Prior to actually retrieving lunar samples, little was known about the
geological history of the Moon.
Theories for this history were based on examination of various photographics
of the lunar surface. So let's take a look at our history of
lunar exploration which have established the
properties of the moon
From the retrieval of lunar rocks a fairly precise geological history
of the moon has now been established. This history consists of the
Altogether there were 6 landings on the moon. The first three
(Apollo's 11,12,14) returned lunar samples that were extremely
local to the landing site (no one wanted to wander too far away
from the lander vehicle). The last three (Apollos 15,16,17)
took a dune bugy with them and drove around the lunar surface
in order to perform more extensive sampling.
From the analysis of returned lunar samples, the following sequence
of events regarding the geological history of the moon have been
- The oldest rocks on the moon, found in the lunar highlands,
are 4.4 billion years old. Since the oldest rocks in the solar system
are 4.6 billion years old, this would seem to imply the accretion formation
timescale is .2 billion years (200 million years)
- The rocks found in the ejecta blanket near craters are 4.2 billion
- The rocks near the lunar maria are 3.9 billion years old
- The basalt that comprises the lunar maria is 3.7 billion years old
- No rocks younger than 3.5 billion years have been found -->
the moon has been geologically dead for quite some time
The age dating of the lunar rocks have allowed us to identify four
distinct periods in its geological history:
- The moon solidified and cooled 4.4 billion years ago
- Between 4.4 and 4.2 billion years and intense period of bombardment
occured from the material that was left over from the acretion process.
Most of this material was chunks of rock less than 10 km in size.
- By now the moon is tidally locked to the earth. At 3.9 billion years
there was another period of bombardment which involved a few large
(> 100 km) pieces of debris. The earth acted as a gravitational
focussing mechanism which caused most of these impacts to occur on
the side of the moon always facing the earth (there are no lunar maria
on the far side of the moon).
- These large impacts produced the lunar maria. This was a severe
shock to the crust of the moon and over time, molten basalt would flow
out of the deep fissures and cracks in the crust that were caused by the
initial impact. This took 200 million years to occur and is the last
geological process which occurred on the moon.