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 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!

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:

• 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 basic 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 following events:

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 determined:

• 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 years old

• 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.

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