Structure and Features of the Moon

Overview of this page

On this page, you will learn to identify the major features of the Moon.

You will learn that the Moon is geologically dead. Its core is cooled and solid. There is no magnetic field, no plate tectonics, no volcanism,

You will learn that the Moon has no atmosphere ... it is a complete vacuum. Daytime is hot, nighttime is cold.

You will later learn that the Moon does have gravity, even though it is in a complete vacuum. This is because gravity is based on the mass of an object.

The Moon has no liquid water anywhere, no plant life, no life of any type at all.

And ... you will see that EVERY crater on the lunar surface is circular (round). There are no oval craters. Why?


Below is a nice photograph of the full Moon taken by the Clementine spacecraft. The polar regions are distorted in this image because the photograph is a composite of many pictures. The surface of the Moon is marked by large dark plains called maria (Latin for "seas") and cratered, somewhat mountainous regions. Some of the familiar areas which I would like you to try to identify during your Moon Observation activity are listed on this image.

When people first looked at the Moon and saw the dark plains, they thought they were lakes or oceans, similar to Earth, and hence the derivation of the name. I have kept the Latinized versions of the maria here. The major seas are written in yellow font, the largest sea in green, and craters in white lettering. In my classroom, I remind students that the seas have a lesson for all young people. High school students are advised to remain in either the Sea of Serenity or the Sea of Tranquility. This is a happy place where life is uncomplicated. Leaving these two seas to go to the Sea or Fertility will most certainly result in the final place of the Sea of Crisis. Other stories may have to do with Minnesota weather, which in winter is like the Sea of Cold, giving way in spring to the Sea of Rain or the Sea of Fog, and summer in the Ocean of Storms. None of the Minnesota weather makes us believe we are in the Sea of Laughter. Okay, so I made this stuff up, but I actually talk about this in my classroom.

There are three major craters I would like you to be able to identify ... Crater Tycho in the southern hemisphere, and Craters Copernicus and Kepler in the western hemisphere. Crater Tycho has an interesting appearance. During any phases other than Full, it will appear as a simple round, plain crater with a diameter of 54 km. However, during the Full Moon phase, the ejecta rays from the impact are clearly visible, spraying out to a distance of almost 1000 km.

These rays of ejecta are one of the distinctive features of the Full Moon. Clicking on the image to your left will let you see all of the lunar phases in larger detail. The brightness of the Tycho rays are readily apparent.

Of additional interest is the evidence of the impact. Scientists believe the crater formed some 700,000 years ago. The impact was so great that lunar material was blasted into space, at speeds greater than escape velocity. Some of the material fell back to the lunar surface, while more accelerated pieces wandered in space until the Earth revolved into them. A piece of moon rock that falls to the Earth's surface is called a "Tektite." Their mineral content is different from Earth rock, and their shape shows evidence of melting on entry into Earth's atmosphere. A large field of such tektites exists in eastern Australia, and I have pieces from this fallout field.

Years ago, when my Uncle Bill and his wife Ann were looking for tektites in Australia, they had spent an entire day scouring the dry soil. Ann collected 9 tektites, and put each in a separate envelope. Bill and the hired guide had found nothing. When the group gathered in the tent to look at Ann's find, they were surprised to discover that Ann had collected super-dry rabbit turds. Australian rabbits eat their own dung in an effort to conserve water, and the dung exits a second time, exceedingly dry and almost rock hard. Ann and Bill found real tektites later, after much laughter and embarrassment the night before.



To learn more about these Australian tektites, or perhaps purchase one for your collection, click on the tektite image to your left.







The structure of the Moon has similarities to that of the Earth, as seen in the image above, and other dissimilarities. The Moon has a core, mantle, and crust, just as the Earth does, but the composition of the core is different. The lunar core has far less iron, and there are no molten components. Without an internal heat source, the Moon is geologically dead, and so too without volcanism or plate tectonic activity. Above the dormant core is a thick mantle composed of the same silicate rock as in the Earth. The Moon's crust has some granitic rock, but more basalt, just as the ocean bottoms of Earth have.

With no geological activity, the surface of the Moon can only be altered by objects that strike it. In the Moon's distant past, the interior was molten, and large impacts would gouge out huge holes. Magma would well up from the interior and fill in the hole, and then smooth over before hardening. Indeed, the maria of the Moon such as Crisium, Tranquilitatis, and Serenitatis are ancient impact craters whose holes were filled with basaltic magma. On the other hand, the Oceanus Procellarum is perhaps a vast lava plain, whose vent by a site called Aristarchus may have erupted lava for millions of years, covering much of the left half of the western hemisphere.

What scientists can learn from the craters is comparative dates of the events. Craters in the maria happened after the lava basins were filled and cooled. Craters within craters can tell which one occurred first. Heavily cratered regions are much older than the craterless maria. The images below show cratered terrain, and in many instances it can clearly be seen where one crater event happened on top of a more ancient event. While Earth processes like erosion from wind and water would slowly obliterate any surface evidence of a crater, the airless Moon has much slower speeds of crater deterioration. Craters do age, as evidenced by the difference in sharp and "washed out" crater rims, and this aging is due to micrometeorite impacts that slowly degrade craters. By looking for fresh craters and number of craters of different diameters, scientists can determine the relative ages of the lunar surface regions. The darker the feature, the more ancient the terrain, as the impact ejecta is more white in appearance. Look at the two images below for old and newer craters, and craters on top or older ones.

The images below depict some large craters, with the Moon's largest being in the most right image. That impact crater, found on the Moon's farside, is called Mare Orientale (this might take while to load), and was such a traumatic event, that the lunar surface was rippled like a rock hitting pond water. The interior of the crater has smoothed over from old lava flows. The most left image is that of Crater Copernicus, and terracing is evident. Of special interest should be the question, "Why are all of the craters round?" You will have a chance to answer that question for yourself in a lab activity later on.

The images below all demonstrate clear evidence of old lava channels, dug out of the lunar surface in the distant past. This is the geological evidence of a past molten interior. More importantly, the lava channels tell more about the Moon's early history, for you will notice that the channels are not pocked by craters. Most scientists agree that the Moon was pummeled by rocks in an early historical time called the "Era of Bombardment." Apparently, the numerous impacts slowed drastically about 3.8 billion years ago. Since that time, the Moon has repeatedly been struck by space debris, but at a much slower rate. The lava that flowed from the Moon's ancient molten interior shows how few craters have formed since the outpouring. The Moon has thus been geologically dead for over 3 billion years, and this age was corroborated with dated samples of moon rock collected by the different Apollo teams.

Finally, the image on the left is of the farside of the Moon, a place never seen from ANYONE on Earth, and visible only to the Apollo astronauts who orbited the back side. Mare Orientale is clearly visible. Notice the lack of large lava plains. This can only mean that the Moon has been struck on its farside much more frequently than on its near side, and is evidence of the practical value of the Moon. Much space debris that might strike the Earth is absorbed by the Moon instead. I am not saying that the Moon is like a vacuum cleaner, sparing Earth from disastrous impacts, for the Earth has twenty times the surface area of the Moon, and Earth has suffered many more impact events than the Moon. I am saying that rocks from space hit the backside of the Moon more often than the near side, and any rock that hits the Moon is better than one that would otherwise hit us!

Contrary to the glorious black and white movies of my childhood, there are no lifeforms on the Moon. The entire "atmosphere" of the Moon would fit inside a basic school classroom at Earth pressures. The Moon is ariless, and therefore has nothing to protect itself against harmful solar radiation. Life cannot exist on the Moon. The same lack of air is also a potential blessing for Astronomy, for all wavelengths are therefore visible from the Moon's surface. Remote telescopes could be built there and give unprecedented views of the Universe. Star observations can be made in the daytime because the sky is always black for the lack of the atmosphere.






Temperatures on the Moon are rather extreme. Daytime temperatures exceed 250 degrees F, and nighttime temperatures plunge to -150 degrees F. Imagine a world where there is 400 degrees F difference between sunlight and shadow! Strange too is what a lack of atmosphere does to local temperatures. In sunlight, your feet would be over 20 degrees F warmer than your head. There is no air to trap heat, so whatever solar heating occurs at the surface is quickly lost to space.

We watch a classroom video depicting fictitious Moon beings as an introduction to the Moon Unit (which happens to be the name of one of old rocker Frank Zappa's children ... the other is named Dwiezel). In 1835, an article appeared in the New York Sun describing beaches on the Moon teeming with life as exotic as large, furry humanoid bats. The entire account is reprinted below from the History Buff website, with the connecting link to the actual account available. I am including the account here instead of directing you to it via the internet in case the site becomes lost or is not longer active, and thus causing you to miss a pretty interesting hoax.

About History Buff
History Library

By R. J. Brown
Every History of American journalistic hoaxing properly begins with the celebrated moon hoax which "made" the New York Sun of Benjamin Day. It consisted of a series of articles, allegedly reprinted from the nonexistent Edinburgh Journal of Science, relating to the discovery of life on the moon by Sir John Herschel, eminent British astronomer, who some time before had gone to the Cape of Good Hope to try out a new type of powerful telescope.

The first installment of the moon hoax appeared in the August 25, 1835 edition of the New York Sun on page two, under the heading "Celestial Discoveries." The brief passage read in part as follows: "We have just learnt (sic) from an eminent publisher in this city that Sir John Herschel at the Cape of Good Hope, has made some astronomical discoveries of the most wonderful description, by means of an immense telescope of an entirely new principle."

As a mater of fact, Herschel had gone to South Africa in January, 1834, and set up an observatory at Cape Town. Three columns of the first page of the Sun contained a story credited to the Edinburgh Journal of Science. (That publication had suspended some time before.) There was a great deal of matter about the importance of Herschel's impending announcement of his discoveries.

On August 25, the Sun ran four columns describing what Sir John had been able to see, looking at the moon through his telescope.

So fascinating were the descriptions of trees and vegetation, oceans and beaches, bison and goats, cranes and pelicans that the whole town was talking even before the fourth installment appeared on August 28, 1835, with the master revelation of all: the discovery of furry, winged men resembling bats. The narration was printed as follows:

"We counted three parties of these creatures, of twelve, nine and fifteen in each, walking erect towards a small wood... Certainly they were like human beings, for their wings had now disappeared and their attitude in walking was both erect and dignified... About half of the first party had passed beyond our canvas; but of all the others we had perfectly distinct and deliberate view. They averaged four feet in height, were covered, except on the face, with short and glossy copper-colored hair, and had wings composed of a thin membrane, without hair, lying snugly upon their backs from the top of the shoulders to the calves of their legs.

The face, which was of a yellowish color, was an improvement upon that of the large orangutan... so much so that but for their long wings they would look as well on a parade ground as some of the old cockney militia. The hair of the head was a darker color than that of the body, closely curled but apparently not woolly, and arranged in two circles over the temples of the forehead. Their feet could only be seen as they were alternately lifted in walking; but from what we could see of them in so transient a view they appeared thin and very protuberant at the heel...We could perceive that their wings possessed great expansion and were similar in structure of those of the bat, being a semitransparent membrane expanded in curvilinear divisions by means of straight radii, united at the back by dorsal integuments. But what astonished us most was the circumstance of this membrane being continued from the shoulders to the legs, united all the way down, though gradually decreasing in width. The wings seemed completely under the command of volition, for those of the creatures whom we saw bathing in the water spread them instantly to their full width, waved them as ducks do theirs to shake off the water, and then as instantly closed them again in a compact form.

The Sun reached a circulation of 15,000 daily on the first of the stories. When the discovery of men on the moon appeared Day was able to announce that the Sun possessed the largest circulation of any newspaper in the world: 19,360.

Later stories told of the Temple of the Moon, constructed of sapphire, with a roof of yellow resembling gold. There were pillars seventy feet high and six feet thick supporting the roof of the temple. More man-bats were discovered and readers of the Sun were awaiting more astounding details, but the Sun told them the telescope had, unfortunately, been left facing the east and the Sun's rays, concentrated through the lenses, burned a hole "15 feet in circumference" entirely through the reflecting chamber, putting the observatory out of commission.

Rival editors were frantic; many of them pretended to have access to the original articles and began reprinting the Sun's series. It was not until the Journal of Commerce sought permission to publish the series in pamphlet form, however, that Richard Adams Locke, confessed authorship. Some authorities think that a French scientist, Nicollet, in this country at the time, wrote them.

Before Locke's confession a committee of scientists from Yale University hastened to New York to inspect the original articles; it was shunted from editorial office to print shop and back again until it tired and returned to New Haven. Edgar Allan Poe explained that he stopped work on the second part of The Strange Adventures of Hans Pfaall because he had felt he had been outdone. So many writers have perpetuated the legend that Harriet Martineau in her Retrospect of Western Travel said a Springfield, Massachusetts, missionary society resolved to send missionaries to the moon to convert and civilize the bat men.

After a number of his competitors, humiliated because they had "lifted" the series and passed it off as their own, upbraided Day, the Sun of September 16, 1835, admitted the hoax. When the hoax was exposed people were generally amused. It did not seem to lessen interest in the Sun, which never lost its increased circulation.

Recent radar images from the Clementine spacecraft have returned the unmistakable signals of water ice in polar craters. Since these crater bottoms never see the Sun, whatever water may have been in there is still present today. I invite you to check out Ice on the Moon, and learn more about this fascinating discovery and the potential it represents to future exploration. Additionally, several private enterprises are interested in making a return trip to the Moon, partially to build a base station there, partially for the opportunity to do science, and partially to make money from wealthy tourists willing to pay to go into space. If you think space tourism is a joke, just look at the pricetag of going into space aboard a Russian rocket ... $20,000,000.00, and one of the members of NSYNCH is apparently in the recent news as a hopeful traveler. Or perhaps we should not go back there if stories about an Alien base on the farside are true.

You have finished your study of Structure and Features. Time now to make some applications. These is a very informative lab activites that will help you understand how scientists determine the age of a surface by looking at and comparing Earth and Moon Rocks, and then impact craters. Try your hand at the Impact Crater lab, an activity from the USGS.

I think you should look at the Moon and try to find some of these features, so please move ahead to the Moon Observation. When you have an idea of what you are looking at, then move ahead to the next page, Phases of the Moon, or return to the Moon Introduction, or the Syllabus.


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