Photo: Contributed
Some of the most important discoveries in science come from people who ask obvious questions – things so obvious that no one has bothered to ask before.
Sir Isaac Newton wondered why apples and other things fell. Olbers wondered why it was dark at night. Another deceptively simple question is why do the impact craters we see on the moon, on Earth, and elsewhere usually have a round shape?
A pitcher (in baseball) or a bowler (in cricket) can shoot balls at speeds in excess of 150 kilometers per hour. This sounds fast, but compared to the speed of sound, which is about 1,200 kilometers per hour. This means, from a scientific point of view, hitting this ball with the bat is a very slow process. The forces due to the impact move back through the ball at the speed of sound and the ball deforms smoothly. When the ball stops against the bat, it returns to shape and fires at high speed.
If no one catches the ball and it hits soft ground, its subsonic speed means that there will be enough time for it to push the soil out of the way, making a hole, and for the ball to deform and bounce. If the ball hits the ground almost vertically, it will make a round hole. On the other hand, if it hits the ground obliquely, it will make an elliptical trace or indentation. However, if an object is moving somewhere between 10 and 100 times the speed of sound, the story is very different.
Let’s look at a laboratory experiment. An aluminum sphere with a diameter of 25 mm is fired at a speed of 50 km / s in the direction of a thick aluminum plate. At this speed, it takes one millionth of a second to travel 25 mm. When it hits the slab, the slab material does not have time to move away, as the forces have not had time to act.
There is no time for the forces of impact to reach the back of the ball, so everything just accumulates at the point of impact. The impact energy, about 29 million joules, is converted almost entirely into heat, so the metal ball plus part of the plate material is converted into a ball of aluminum vapor with a temperature of tens or even hundreds of thousands of degrees.
There is nothing containing this steam and the internal pressure is huge, so it explodes outside in all directions. It was as if someone had just gently placed a very powerful bomb on the surface and then detonated it. Because the explosion is due to the ball of hot material, not its kinetic impact, the hole it makes is always round, even when the impact is at an angle. In the experiment, the plate ends with a deep hole in the shape of a cup with fringes of sprayed metal, resembling a picture of something falling into water.
A shock wave entered the slab and blew a piece on the other side.
The craters we see on Earth, the Moon and other objects in the solar system are almost always round due to the high speed of impact.
A ball of evaporated material is formed under enormous pressure, which then explodes in all directions. The shock wave pushes the ground down. When the shock wave disappears, the earth bounces back, forming a peak in the middle of the crater. Most craters have one of these central peaks.
Thinking about the violence of this aluminum ball experiment, the release of energy from the impact of an asteroid, say 10 kilometers in diameter, is difficult to accept.
Some impacts on the moon have scattered material over vast distances. Quiet Crater is a really obvious example. There are streaks of material that extend radially from the crater hundreds of kilometers across the surface of the moon.
Take out the telescope or binoculars and look at the moon. Then remember that our world has been bombed just as hard.
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• Mercury hides low in the light of dawn, with Venus on the right. Then to the right lie Mars, Jupiter and then Saturn.
• The moon will be new on June 28 and will reach its first quarter on June 6.
This article was written by or on behalf of a prominent columnist and does not necessarily reflect Castanet’s views.
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