Weblecture
I distinguish two parts of it, which I call respectively the brighter and the darker. The brighter seems to surround and pervade the whole hemisphere; but the darker part, like a sort of cloud, discolors the Moon’s surface and makes it appear covered with spots. Now these spots, as they are somewhat dark and of considerable size, are plain to everyone and every age has seen them, wherefore I will call them great or ancient spots, to distinguish them from other spots, smaller in size, but so thickly scattered that they sprinkle the whole surface of the Moon, but especially the brighter portion of it. These spots have never been observed by anyone before me; and from my observations of them, often repeated, I have been led to the opinion which I have expressed, namely, that I feel sure that the surface of the Moon is not perfectly smooth, free from inequalities and exactly spherical… but that, on the contrary, it is full of inequalities, uneven, full of hollows and protuberances, just like the surface of the Earth itself, which is varied everywhere by lofty mountains and deep valleys.
— Galileo Galilei, The Starry Messenger, 1610
Observations: Seen from earth-based telescopes, each planet reflects a certain amount of the sunlight falling on it. The index of reflection, or the albedo of the planet, tells us something about the composition of its surface. Planets with high albedo tend to have highly-reflective atmospheres or water surfaces. Planets with lower albedo have hard rock surfaces which are rough and scatter light in different directions rather than reflecting it.
Individual features on a planet or moon can have different reflective properties. The dark and light areas on the Moon and Mars long puzzled earth-based observations. We now know from direct exploration and sampling that lunar "dark areas" or mare lowlands have lower albedo because the chemical composition of basalt flows in these areas absorbs more light than the more highly reflective breccia of the lunar highlands.
Moon at three-quarter full (gibbous waxing), August, 2009. Sunlight reflection is so bright that all details are washed out except for craters at the terminator line. |
Same photograph, but taken at a much faster shutter speed. Terminator details are missing, but maria and impact rays are now clearly visible. |
The size of objects we can see on the surface depends on the distance to the planet, governed by the small angle formula and the resolution of our telescopes. Earth-based telescopes can easily discern craters and mare on the moon, the polar ice caps on Mars and some of the changes due to windstorms re arranging dust layers, and the large storm called the Red Spot on Jupiter. To accurately observe surface features on Venus, we had to send in probes that could map through Venus' thick atmosphere. The moons of Jupiter and Saturn and the surface of Mercury remained mysteries until we were able to send photographic devices on unmanned probes.
Of course, we start first by looking for the geological structures with which we are already familiar. Highlands on earth can be formed by uplift, slow lava flow, and by both shield volcanoes and subduction volcanoes. Valleys formed by rift valleys, by glacial retreat, and by liquid water erosion have identifying characteristics.
We do not find subduction volcanoes or rift valleys caused by plate collision or withdrawal on other planets, an indication that Earth alone has multiple plates. We do find scarps and rift valleys that appear to be caused by the shrinking of the planet's single plate. We also find areas where seismic waves from large crater collisions have created hilly areas on the side of the planet opposite the impact -- more evidence of a single plate.
Most non-terrestrial volcanoes appear to be shield volcanoes, caused when the center of a plate is thinned to the point where molten material from beneath the crust can flow up and out. Venus and Jupiter's moon Io have active volcanoes; Mars' volcanic activity is perhaps over, but clearly in evidence in Mons Olympus.
We also find non-earthlike geological structures. Earth's atmosphere burns up most asteroids and meteors of small size, so that earth has fewer crater-causing impacts. The majority of these strike the oceans (3/4 of the earth's surface is water); those craters which have been created in the past have often been overrun by volcanic activity or by erosion of wind and water. Lacking a thick atmosphere and erosive forces, the surfaces of Mercury, the Moon, and Mars have small to enormous craters, and splash plains.
A major question is how these differences depend on the lack of plate tectonics, and in turn, what conditions that support plate tectonics on earth are missing from the other planets. One theory is that Earth's liquid water surface cools the crust enough that it is rigid and broke, where on the other terrestrial planets, the surface remained warmer and more plastic, able to bend and flex without cracking completely into separate "islands".
One of the most important features we seek is presence or evidence of surface water, generally in the form of ice. Water is necessary for all known forms of life, and wherever we find water on earth, we find life. Water appears to exist as ice at the poles of the Mars; as ice in crater shadows near the poles of the earth's Moon and possibly as surface ice on the moons of Jupiter and Saturn. If the planet was warm enough at some point in the past, it may also have had liquid surface water, which would have eroded terrain and left behind river valleys, as it does on earth.
All planets share certain characteristics of motion, atmosphere, and core structure; each planet also has unique and often puzzling features. As we look at each of the major solar system bodies in turn, look for patterns in the shared characteristics. There is an abundance of detail at the NASA websites, among others, so these lectures will concentrate on providing some highlight information in a standard format.
The Moon is easily the most observable celestial body after the sun, with major surface portions of the moon visible from near first quarter to after third quarter. The moon rises 51 minutes later each night. Binoculars with 8X magnification are sufficient to show craters and major surface phenomena. Over the course of the moon's revolution around earth, we can see slightly more than 50% of the lunar surface.
The first unmanned exploration of the moon began with the Soviet Luna series, which mapped the far side of the moon for the first time in 1959. Numerous Luna and US-built Ranger missions provided extensive mapping of 99.5% of the lunar surface by 1970. Manned Apollo missions orbited the moon in 1968, and first landed in 1969; the last of these landed in 1974. These missions delivered payloads of equipment to monitor moonquakes and returned with hundreds of pounds of lunar rocks. Subsequent unmanned missions have continued to monitor the lunar surface and near surface "atmosphere". Most recently, the Chinese Lunar Exploration program launched the Chang'e lunar orbiter in October, 2007. NASA's current exploration plans aim to return humans to the moon by 2024, with the goal of establishing a permanent human colony there. Take a look at NASA's Lunar Space Exploration plans.
The moon's orbit is slightly inclined to the ecliptic (the plane of the sun's orbit); the moon is slightly tilted on its axis (which allows us to see more than 50% of the lunar surface -- the phenomena called libration). The orbit is slightly eccentric, about 0.055 (meaning that it is nearly perfectly circular).
Some planets and moons have a synchronized rotation, which means that the number of times the body turns on its axis per revolutions around its primary is a whole number ratio. For the moon, the ratio is 1:1 -- that is, it rotates once on its axis per each revolution around the earth. This ratio conserves angular momentum and results in the moon keeping the same side facing toward earth at all times, so that the "far side" of the moon is never seen from earth.
The moon has no discernible magnetic field, so it does not distort the solar wind flow.
Planets retain an atmosphere only if there is a balance between the mass and temperature of the planet and the gases in the atmosphere. The temperature of the atmosphere determines the average velocity of the gas particles. The mass of the planet determines the escape velocity, which is how fast an object of any mass must be moving to get out of the gravitational pull of the planet. If the average velocity is at least 1/6 of the escape velocity, the atmosphere will gradually dissipate.
The moon is not very massive, so its escape velocity is relatively low. Most of its surface is exposed to the sun over the course of a month; the "daytime" temperature on the moon can rise above 250 °F, so most gases will achieve escape velocity. The lack of atmosphere explains the sharpness of the terrain: no erosion forces exist to wear them down.
From its lack of magnetic field and data from moonquake sensors, paleontologists estimate that the moon has a very small, solid, iron-rich core; a plastic layer around the core, a solid rock sphere, and a very thin crust. It is primarily basaltic, much like the surface of the Pacific Ocean's floor.
Because of the difference in density between the moon and the earth, and the difference in composition indicated in the moon rocks astronauts have returned to earth, planetologists are forced to assume that the moon was formed by some very different process from the process to form the earth, possibly from material thrown into space by a large body striking the Earth while the earth was still forming.
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