Why is gravity stronger in lunar craters?

Why does the moon have spots?

The man in the moon - known from songs, films and stories. Indeed, there are conspicuous dark spots on the lunar surface, and with a little imagination you can see a face in them. But what are these spots really?

At first, scientists thought the dark spots were seas. But at least since the first visit to the moon in 1969 it has been clear: The moon is dust-dry, the entire surface of the moon consists of fine gray rock powder. And the dark spots are great plains that are simply filled with darker dust. This makes the moon appear speckled light and dark. But how did these plains come about?

The lowlands are almost as old as the moon itself. When the surface of the moon had already solidified into a crust in the early days of the solar system, large asteroids repeatedly hit the moon and tore holes in the fresh crust. There lava ran out of the still hot, liquid interior of the moon and filled the plains. Lava rock is darker than the crustal rock, so the plains appear darker.

There are now hardly any large asteroids hitting the moon, but still a lot of smaller ones. Since the moon (unlike the earth) has no atmosphere, they do not burn up but hit the surface. Most of the time, the force of the impacts is only enough to crumble some rock and stir up a bit of dust, which quickly sinks back to the ground. Therefore, the surface of the moon today consists of rock dust, mainly of light crustal rock and, in the lowlands, of darker lava rock. From the earth it looks like spots, seas - or a face.

What is the moon

It is the brightest celestial body in the night sky: the moon. It shines so brightly on full moon nights that some people find it difficult to sleep. It appears as big as the sun and the stars look like tiny points of light next to it.

But the impression is deceptive: In reality, the moon (diameter: 3474 km) is only about a quarter the size of the earth (12742 km) - and the sun (1.39 million km) is even four hundred times larger. The moon only appears the same size to us because it is so close to us - the sun (distance to the earth about 150 million km) is also about four hundred times further away than the moon. (384,400 km, an airplane needs 18 days for this distance!)

The bright light is also deceptive: unlike the sun, the moon does not shine by itself, but is illuminated by the sun. Some of this light is then reflected back from the surface of the moon and hits the earth. Just because the moon is so close to us, enough light arrives on earth to light up the night - at least if the moon doesn't just seem to have disappeared without a trace ...

Why does it look different on earth than on the moon?

It doesn't look very inviting on the moon: the surface is dry and covered with a layer of gray dust. Meteor impacts have torn huge craters in the ground that filled with lava from inside the moon. Around these lava basins, kilometer-high crater edges pile up as mountain rings.

Our blue planet is completely different - if only because three quarters of it is covered by water. The water not only covers a large part of the earth, it also forms its land mass: rivers, glaciers and the surf of the sea process the rock, crush it and move it around. This is how valleys, coasts and ever new layers of rock are created.

The interior of the moon is solid and rigid today. The earth, on the other hand, has a liquid mantle on which movable plates float. The movement of the tectonic plates causes mountains to unfold, deep-sea trenches to form and volcanoes to spew fire and ashes.

Unlike the moon, the earth has a shell of air, the atmosphere. The weather is created in this atmosphere. Wind, rain and snow have worked and shaped the earth's surface over millions of years. In addition, the atmosphere acts as a protective shield that slows down meteorites and lets them burn up.

Because the moon has no such atmosphere, meteorites hit its surface unchecked and suddenly crumble the rock into dust. But meteorites are the only forces that shape the lunar landscape. Because there is no water, no atmosphere and no plate tectonics, the influences that make our earth's surface so varied are missing.

The first people to step onto the barren moonscape were astronaut Neil Armstrong and his colleague Edwin E. Aldrin. The footprints that they left when they landed on the moon in 1969 can still be seen today - because neither wind nor water cover their tracks on the moon.

What are asteroids, meteorites and comets?

On some nights you can observe a special moment in the sky: it looks like a star is falling from the sky. Superstitious people even think that whoever sees such a shooting star could wish for something. But what is really behind it and where do the shooting stars come from?

In our solar system there are not only the sun, planets and moons. Many small pieces of rock and metal have also been discovered. They are much smaller and not as nicely round as planets, hence they are called minor planets or Asteroids. Like their big siblings, they circle the sun in regular orbits. Most asteroids can be found in the "asteroid belt" between the orbits of Mars and Jupiter.

Every now and then two of these asteroids collide. A crash like this creates a lot of debris and splinters. These fly away from the previous orbit, across the solar system. Some of them get close to the earth, are attracted to it and fall to the earth. These falling chunks are also called meteorite.

On earth they would literally fall like a stone from the sky - if it weren't for the atmosphere. Because the meteorites are so fast that the air cannot move to the side quickly enough. The air in front of the falling rock is compressed and therefore extremely hot. The air begins to glow and the meteorite begins to evaporate. We can then see that as a glowing streak that moves across the sky - a shooting star.

Most meteorites are so small that they burn up completely as they travel through the air. The trail then simply ends in the sky. Larger debris also lose mass on the way, but does not completely evaporate. They reach the ground and strike there.

What these meteorites do to the earth depends on how big they are. Small meteorites a few centimeters in diameter, for example, just leave a dent in a car roof.

The largest known meteorite hit about 65 million years ago. It was several kilometers in diameter and tore a crater 180 kilometers in diameter. The impact threw so much dust into the air that the sun was eclipsed for hundreds of years. As a result, plants and animals all over the world died out - this was the end of the dinosaurs.

Fortunately, such large meteorites are very rare so we don't have to worry. In addition, unlike the dinosaurs, we can observe the sky with telescopes and discover such large asteroids long before the impact.

While a shooting star burns up in a few seconds, another phenomenon remains visible longer: Comets with its tail there are days or weeks in the sky. In the past, people also attributed many properties to them - as divine signs, heralds of calamity or harbingers of happy events. But the truth is a little less spectacular.

Astronomers also call comets "dirty snowballs". They come from the outer solar system, far from the warming power of the sun. It's so cold there that water immediately freezes to ice. This is how lumps of ice and dust form - dirty snowballs.

Even a comet initially travels far away from the sun - until it is deflected by a collision and flies in the direction of the inner solar system. It gets closer to the sun and over time receives more and more light and warmth. This will cause the frozen surface to begin to thaw and even to evaporate. This creates an envelope of water vapor and dust around the comet.

At the same time, the comet gets to feel the “solar wind” - tiny particles that fly out of the sun at high speed. They hit the comet's vapor envelope. This will blow away the comet's vapor envelope, creating an elongated cloud that points away from the sun. When this cloud is then hit by sunlight, it appears as a glowing streak - the comet's tail.

The comet makes an arc around the sun and then moves away again. When it is far enough away from the sun, thawing and evaporation will also stop. The tail disappears and the comet moves like a dirty snowball through the vastness of the outer solar system. Depending on the comet's orbit, it will take many decades or even centuries before it comes close to the sun again.

Igneous rocks

Biting granite means that something is hopeless. Because of its great hardness, granite can not only be used as a phrase, but also as a paving stone or for building walls. Granite is a rock that lies over two kilometers below the earth's surface and is common in the earth's crust.

Granite is formed when glowing magma solidifies when it cools. The dark spotted gabbro or the monzonite are also formed from slowly cooling magma. If this process takes place deep inside the earth, geologists speak of Deep rock, also Plutonite called.

If, on the other hand, the hot rock slurry penetrates outwards during a volcanic eruption and pours over the surface of the earth, it is from Effluent rock or Volcanite the speech. Vulcanites include light pumice stone, porous tuff or rhyolite, which is made from the same material as granite but has a different structure and is less hard because it cools faster on the surface of the earth than the granite in the depths. Basalt is also a volcanite. Sometimes it freezes into hexagonal, closely spaced columns that look as if they have been cast into shape. Basalt forms on the surface of the earth from the same mass as the gabbro in the depths.

Vulcanites weather immediately after their formation, plutonites only when the overlying rock layers have been eroded. Because both volcanites and plutonites became rock from cooled magma, both are classified as igneous rocks.

The beginnings of the earth

We would not recognize the earth immediately after its formation. It was an extremely uncomfortable planet: there were neither continents nor oceans, but a seething surface of glowing hot, viscous magma. Why couldn't the earth's crust form for a long time?

A good 4.5 billion years ago comets, asteroids, gas and dust condensed to form our planet. Its own gravity pressed these individual parts together so that they were subjected to strong pressure. This pressure was of course highest in the earth's core, on which the weight of the entire outer layers weighed. As a result of the high pressure, the rock was heated up and melted. Outwardly, the pressure and thus also the temperature became less. Even so, the surface of the earth remained very hot for several hundred million years and could not cool down and solidify.

In order to understand the reason for this, the scientists had to look at the moon: Ancient lunar craters from the time the solar system was formed tell us that the moon was hit by numerous meteorites when it was young. It is therefore assumed that the earth was also exposed to a real rock bombardment from space at the same time. The lumps fell to the earth at high speed - and the impacts were correspondingly violent: Even lumps of a few hundred tons could easily cause an explosion the strength of an atomic bomb!

So the earth's surface continued to heat up for a long time, stirred up again and again and remained so fluid. Only when the impacts gradually subsided after a few hundred million years did the temperatures on the earth's surface drop. The rock could slowly solidify and form an earth crust that grew thicker and thicker over the course of millions of years. But to this day it is only a very thin layer that floats on a viscous, hot interior of the earth.

22.6.1984

A catastrophe of unimaginable proportions: almost 15 million years ago, a gigantic explosion shook southern Germany. It was stronger than a hundred thousand atomic bombs combined and killed all living things within a radius of at least one hundred kilometers. There weren't any people back then, but all animals and plants were killed by a huge shock wave or buried under the rubble. The bang could still be heard on the other side of the earth.

It was triggered by a meteorite about 1,500 meters in size, which raced towards the earth at about 20 kilometers per second. The impact of this chunk caused an earthquake of magnitude eight and tore a 15-mile crater into the surface of the earth. The crater can still be seen today - the Nördlinger Ries between the Swabian and Franconian Alb. The landscape around the impact crater was completely turned inside out: 150 cubic kilometers of rock were hurled into the air and some flew 450 kilometers into what is now the Czech Republic.

Fortunately, impacts from such meteorites, which weigh several tons, are very rare, they only happen once every few million years.

The little brother

The crater remnants of the Nördlinger Ries can be clearly seen on an elevation map. But you can see something else there: about forty kilometers southwest of the Nördlinger Ries lies a similar but significantly smaller crater, the Steinheim Basin, about three and a half kilometers in diameter. This crater can also be traced back to a meteorite impact - however, this metorite was only about 150 meters tall.

Two meteorite craters so close together - can that be a coincidence? No, say the scientists who examined the rocks of the craters: Both craters were formed at the same time. Either a small part of the meteorite broke off while approaching the earth and hit separately, or the asteroid had a small companion who fell with it to earth for a long time.

21.7.1969

Tense waiting in the control center. A series of warnings from the on-board computer almost led to the mission being aborted, now this: The planned landing site is littered with small craters and rocks. Commander Neil Armstrong grabs the control stick and tries to land the lunar module by hand. But the fuel is running out ...

Finally the redeeming radio message comes: “The eagle has landed.” For the first time, a spaceship with people on board touched down on the moon. A few hours of rest and preparation, then Armstrong opens the hatch and climbs down the ladder. With the words "a small step for a person, but a gigantic leap for mankind" he is the first person to set foot on the moon. Shortly afterwards his colleague Buzz Aldrin follows.

The stay is only short: in two and a half hours on the lunar surface, the astronauts set up an American flag, collect a few kilograms of lunar rock and set up various scientific experiments on the lunar surface.

After another pause, they ignite the engine and fly back into a lunar orbit. Michael Collins is waiting there in the Columbia space capsule, which is supposed to bring them back to Earth.

A tape measure to the moon

Among the devices Aldrin and Armstrong placed on the moon was a special mirror. It is constructed in such a way that it reflects every ray of light back to its starting point. With a well-aimed laser beam, scientists can now take aim at this mirror - and stop the time until the reflected laser beam arrives at them again. If the watch is accurate enough, you can measure the distance to the moon to within a few millimeters. They made a surprising discovery: the moon moves about 3.8 centimeters away from the earth every year!