(4a) The Moon: A Closer Look

When Galileo became the first human to view the Moon through a telescope, our understanding of the Moon changed forever. No longer a mysterious object in the sky, but a sister-world full of ring-shaped mountains and other formations!

Giovanni Riccioli in 1651 named the more prominent features after famous astronomers, while the large dark and smooth areas he called "seas" or "maria" (singular "mare," mah-reh). Some of the names he used for the Moon's crater are of persons discussed in "Stargazers"--Tycho (distinguished by bright streaks that radiate from it), Ptolemy ("Ptolemaeus"), Copernicus, Kepler, Aristarchus, Hipparchus, Erathosthenes; Meton and Pythagoras are on the edge, near the northern pole.

Late-comers who lived after the 17th century had to make do with left-overs: the craters Newton and Cavendish are at the southern edge of the visible disk, Goddard and Lagrange too are near the edge. Also, "Galilaei" is a small undistinguished crater (because of Galileo's banishment?). However, since the Russians were the first to observe the rear side of the Moon, a prominent crater there bears the name of Tsiolkovsky, who at the end of the 19th century promoted the idea of spaceflight.

Note: Strictly for moon junkies--all you ever wanted to know, perhaps even more. From Cambridge University Press (1999), Mapping and naming the moon: A history of lunar cartography and nomenclature by Ewen A. Whitaker, xix+242 pp., $59.95.

Part of the evidence has come from the nicely rounded impact remnants found on Earth, e.g. Meteor Crater (Canyon Diablo) in Arizona and Manicougan lake in Canada, in northern Quebec (picture on left), which is about 100 km (60 miles) wide and 214 million years old. Note that rather than having a pit in its center, the Manicougan lake has a round island. After the impact, the land rose again to the level of its surroundings, pushed by fluid pressure of the material below it, which acts like a viscous fluid and tries to establish equilibrium between the different loads which it supports. (For another picture of Lake Manicougan, and more about it, click here.)

Other solid bodies of the solar system also diplay round craters. On the large ice-covered moons of Jupiter, the return to equilibrium is much more pronounced, because ice sags and flows much more readily than rock. Those moons display "palimpsest" craters which are merely surface markings, because as time passed, the walls which originally existed sagged back onto the flat surface.

The Airless Moon

Why? By the laws of motion, the Moon orbits not the center of the Earth, but the center of gravity of the Earth and Moon (this will be discussed in section #11a, and the center of gravity is defined in section #25). The location of that point allows astronomers to deduce the mass of the Moon, and from that, the pull of the Moon's gravity. At the surface of the Moon, it turned out, gravity is only 1/6 as strong as at the surface of the Earth.

Gravity is important for the retention of an atmosphere. It holds an atmosphere down, while heat is what can make it escape.

Heat is atomic or molecular motion. In a hot solid or liquid, it can be viewed as a shaking motion of atoms or molecules around their average position, like the rustling of leaves in a wind. The higher the temperature, the more vigorous the motion, until the material boils or evaporates, at which point its particles shake loose altogether. In a gas atoms and molecules fly around randomly, colliding constantly (if the gas is as dense as it is in the atmosphere), and their collisions lead to a very good explanation ("the kinetic theory of gases") of the observed properties of a gas.

The average velocity of a gas molecule depends on the temperature of the gas, and at room temperature it is comparable to that of the speeding bullet, quite below the "escape velocity" needed for escaping Earth's gravity. However, that is just an average: actual velocities are expected to be distributed around that average, following the "Maxwellian distribution" first derived by James Clerk Maxwell, whom we meet again in the discovery of the three color theory of light (section #S-4) and the prediction of electromagnetic waves (section #S-5). According to that distribution, a few molecules always move fast enough to escape, and if they happen to be near the top of the atmosphere, moving upwards and and avoiding any further collisions, such molecules would be lost.

For Earth, their number is too small to matter, but with the Moon, having only 1/6 of the surface gravity, it can be shown that any atmosphere would be lost within geological time. The planet Mercury, only slightly larger, also lacks any atmosphere, while Mars, with 1/3 the Earth's surface gravity, only retains a very thin atmosphere.

Water evaporates easily and once in gas form, is quickly lost by the same process. That suggested the "maria" could not possibly be oceans, though their name remained. They actually turned out to be basaltic flows, hardened lava which long ago flowed out of fissures on the Moon; no present-day volcanism on the Moon has been reliably identified. The vast majority of craters probably date back to the early days of the solar system, because the lava of the maria has very few craters on it, suggesting it flooded and obliterated older ones.

The picture of a dry Moon was reinforced by Moon rocks brought back by US astronauts. Earth rocks may contain water bound chemically ("water of hydration"), but not these. Water, of course, would be essential to any human outpost on the Moon. Yet small amounts of water may still exist, brought by comets which occasionally hit the Moon. All this water is sure to evaporate in the heat of the collision, but some of it may re-condense in deep craters near the Moon's pole, which are permanently in the shade and therefore extremely cold. Observations by the "Clementine" spacecraft suggest that one such crater may indeed contain a layer of ice.