The Moon has fascinated mankind throughout the ages. By simply viewing with the naked eye, one can discern two major types of terrain: relatively bright highlands and darker plains. By the middle of the 17th century, Galileo and other early astronomers made telescopic observations, noting an almost endless overlapping of craters. It has also been known for more than a century that the Moon is less dense than the Earth. Although a certain amount of information was ascertained about the Moon before the space age, this new era has revealed many secrets barely imaginable before that time. Current knowledge of the Moon is greater than for any other solar system object except Earth. This lends to a greater understanding of geologic processes and further appreciation of the complexity of terrestrial planets.
The Moon is 384,403 kilometers (238,857 miles) distant from the Earth. Its diameter is 3,476 kilometers (2,160 miles). Both the rotation of the Moon and its revolution around Earth takes 27 days, 7 hours, and 43 minutes. This synchronous rotation is caused by an unsymmetrical distribution of mass in the Moon, which has allowed Earth's gravity to keep one lunar hemisphere permanently turned toward Earth. The above full disc of the Moon was photographed by the Apollo 17 crew during their trans-Earth coast homeward following a successful lunar landing mission in December 1972. Mare seen on this photo include Serentatis, Tranquillitatis, Nectaris, Foecunditatis and Crisium.
This image shows Earthrise over the Moon's limb.
Above is what the lunar surface was thought to look like in the 1800's.
Now we know it looks like the above image taking from the Apollo missions.
Many people have also seen imaginary faces on the Moon such as the Man on the Moon, the Lady on the Moon and the Bunny on the Moon.
This is a movie of Lunar rotation taken from the Clementine mission in 1994. Notice how the farside of the Moon does not have the large, smooth-looking maria that the nearside has.
The most obvious feature on the Moon is its many craters, due to impacts by space debris onto the surface. An Apollo 16 astronaut stands near the rim of Plum Crater (30m, or over 200 yards, in diameter).
Although Earth has experienced many meteorite impacts throughout its history, the action of wind and water quickly erases the resulting craters.
The Earth-Moon system forms one of the lowest ratios of primary to secondary diameters in the Solar System. The ratio of Earth to Lunar diameter is 3.6 to 1. For comparison, the next lowest is Saturn and its largest moon, Titan, with a ratio of 25 to 1. The Earth-Moon system is special in this regard and has an impact on the evolution of life.
- The Moon/Earth mass ratio is 1/80, also unprecedented in the Solar System
- The radius of the Moon is 1738 km with a mean distance from the Earth of 384,000 km.
- The average density of the Moon is 3.37 g/cm3 which is consistent with basaltic silicates and not consistent with a large iron (Fe)/nickel (Ni) core like the Earth.
- The lunar surface is grey-tan with a low albedo of 0.07.
- The combination of the Moon's lower mass and its smaller radius means that its surface gravity is 1/6 the Earth's surface gravity. Walking on the Moon is much different from walking on the Earth, as is falling on the Moon.
The major surface features on the Moon are craters, highlands and maria.
The minor features are:
- wrinkle ridges
The typical features to an impact crater are shown below:
Craters range in size from microscopic to large basins of order 1000's km (see Orientale Basin below).
Normal, round craters are due to impacts from objects up to a couple thousand meters. Object larger (asteroids) will typically crack the crust and form impact basins.
Erosion is slow on a world without an atmosphere and is caused by:
- slumping (gravity)
- other impacts
- temperature changes
The result is that young craters have sharp edges (usually less than 2x108 years old) and old craters are rounded, smoother (with ages of order a billion years old).
The lighter colored, heavily cratered regions are called the lunar highlands. The bare, chaotic terrain indicates that these regions are primordial and one would expect the oldest rocks in these regions. Mountains on the Moon are not due to tectonic activity, but rather are due to overlapping impact rims.
Dark-colored regions which turn out to be smooth plains of basaltic lava. They are the remnants of large impact events that cracked the crust and allowed the lava from the mantle to flow upward and erase early cratering. Note that the impacts must have occurred after the initial phase of cratering.
All the maria are on the nearside of the Moon, none on the farside. This is due to the fact that the nearside crust is thinner than the farside crust (easier to penetrate by impacts). The crust is thinner on the nearside due to tidal interaction with the Earth during the formation epoch.
The lunar soil is a fine grained, cohesive sand/gravel containing glass spheres (impact ejecta), igneous dust and coarse breccia (cemented material from impacts).
Soil from the maria are 3 to 4x109 years old. Soil from the highland is 4.6x109 years old (from the early Solar System).
The soil composition for the Moon is similar to Earth soil but with very different element ratios. In particular, the lunar soil is rich in refractory elements (i.e. ones with high boiling points) and low in volatile elements (i.e. ones with low boiling points). The conclusion is that the Moon was formed from hotter materials than the Earth.
Apollo Lunar Program:
The effort to put men on the Moon was modivated by a patriotic/panicked reaction to the USSR launch of Sputnik in 1957, i.e. it was initially a political goal. The early space program was unique in that it represented leadership from the highest levels of government on down, with unpresendented support from Congress.
While the motivation was political, careful attention to the science goals were maintained. Those were:
- To see if humankind could survive and operate outside the Earth's environment.
- To see if it was technologically possible to plan, construct and pilot a mission in space.
- To study the geological properties of the Moon and compare its history to the Earth's. 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.
- Establish a platform in orbit or the Moon for astronomical studies outside the interference of Earth's atmosphere.
The history of Lunar exploration is one of excellent planning and strategy on the part of NASA. The major components of this process were the following missions:
- Project Mercury: a single astronaut program designed at determining if successful Earth orbit could be achieved. This program lasted 2 years (1961--1963) and saw 7 flights.
- Project Gemini: a program for the launch of spacecraft 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 rendezvous and dock with another spacecraft. This program lasted 1.5 years (1965--1966) and saw 10 flights.
- Apollo Program: These spacecraft were manned by three astronauts with the aim of lunar orbit, then landing. This was achieved on July 19, 1969 when Apollo 11 reached the lunar surface.
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 buggy 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 0.2 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 rock, 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 occurred from the material that was left over from the accretion 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.
- 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 was the last geological process to occur on the Moon.
Origin of the Moon:
The Moon has long been an anamoly because its mass compared to the earth is 1/80 and there is very large for a planetary satellite
Possibilities for Lunar Origin:
- Formed in place by accretion with same planetesimals that formed the Earth (probably not, its composition is different from Earth's)
- Formed someplace else where the average density is 3.5 (less dense things formed in outer solar system, 3.5 implies formation near Mars ==> then how did it get here?)
- Fissioned from the earth's mantle (note the density of the earth's mantle is around 3.5) - but how? by what mechanism?
Note that the Moon forms from Earth mantle material, which is low in density around 3 or so.
Since the Moon formed from the condensation of a debris ring in relatively near-earth orbit, the Moon was initially quite close to the earth. At this time (4.6 billion years ago) the earth was rotating quite rapidly (about once every 5 hours). The nearby Moon exerted large tidal forces on the spinning earth causing it to slow down. This process continues to day. To conserve total system angular momentum, the response of the Moon is then to move farther away from the earth.