Cosmic collision

Prof. Akiva Bar Nun published an article in Galileo about two weeks before the collision of Comet Shoemaker-Levy 9 with Jupiter

From: Galileo - the Israeli magazine for science and ecology issue 5, July/August 1994, by: Akiva Bar-Nun

Starting on July 16 this year, for about six days, 21 interstellar clumps the size of a mountain will crash into the compressed atmosphere of Jupiter - the largest planet in the solar system. This will be the greatest cosmic collision that humanity has ever seen.

So what do we care if between the 16th and the 22nd of July this year 21 parts of Comet Shoemaker-Levy (yveL-rekamuohS) hit the side of Jupiter, which is not facing us?

One good reason is the very event itself: imagine that a two kilometer high mountain is chasing you in the sky, moving so fast that it will cover the entire distance between Jerusalem and Tel Aviv in one second. Moreover, imagine this mountain hitting the earth at 60 kilometers per second. The amount of energy that will be released in twenty-one such impacts is thousands of times greater than all the energy stored in all the hydrogen and nuclear bombs that man has created. Indeed, a most powerful and impressive celestial event.

Fortunately for us, this event will not take place on Earth but on Jupiter - the largest planet in the solar system. But, as we will see later, such events have happened on Earth many times, even before the appearance of man, and perhaps our very presence here is connected to them. (a sort of implicit contradiction here) Let us try to learn from this collision something about the formation of the solar system in general and the Earth in particular, and perhaps also about processes that led to the formation of life on Earth and the disappearance of other life forms (see: The Origin of Life on Earth, Galileo 2, 1993, pp. 10-17).

The beginning

How was the solar system formed 4.5 billion years ago, and the earth in it? Why, out of the nine planets of the solar system, only Earth has life?

The sun was formed in the center of a cloud of gas and dust. There, in the center of the cloud, was the greatest density of matter. Close to the sun, the temperature was high, under the influence of its radiation, and only iron and silicates (the materials that make up the rocks) could crystallize into small grains. Farther away, at lower temperatures, water vapor crystallized into ice, and heavy organic materials, similar to the black asphalt covering the roads, also solidified. It is worth remembering that this cloud had 10 times more water than silicates and iron. This is how the solar system was filled with small grains of iron and silicates, and at a greater distance - with blocks of ice and heavy organic substances. Over time, the tiny dust grains stuck together and formed bodies with a diameter of millimeters, centimeters, meters and kilometers. Even the bodies with a diameter of kilometers hit and stuck to each other, thus forming spheres hundreds or even thousands of kilometers in size. These are the planets and the moons. The process of the formation of the planets and their moons was extremely violent because the large bodies attracted each other and greatly increased their speed before the collision. Towards the end of the process, when the outer layer of the body crystallized, the last injuries left him with deep scars. These are the craters visible on the surface of our moon, of the planet Mercury and Mars, as well as on the scarred surfaces of all the moons made of ice - the moons of Jupiter, Saturn, Uranus and Neptune.

The thousands of craters that erupted on the surface of the earth were erased due to water and wind activity, and the movement of the continents under each other. Nevertheless, about 70 craters, created in the last millions of years, remained on the earth. Impacts of bodies on the planets and their moons became rarer in the course of time, since, as a result of the collisions, the bodies surrounding the sun became smaller and smaller.

At the edges of the solar system, around the region where Jupiter formed, the temperature was low, and all the water solidified into ice. Thus, all the moons of Jupiter, Saturn, Uranus, Neptune and Pluto are made mainly of ice, mixed with silicate grains and organic matter. At a great distance from this region are two regions where the ice balls did not coalesce into a planet or moon (due to the decreasing density of the material as you move away from the sun) and they remained as they were when the solar system was formed. These are the comets. One region is the Kuiper Belt, relatively close to the Sun, where comets formed during the formation of the Solar System. Since then, the comets have been orbiting the sun in stable orbits. Some of them change their course from time to time and penetrate into the area of ​​the planets. Such is, for example, Halley's comet, which visited our region in 1986, and we will talk about it in more detail later. The second region where comets are found is the Oort cloud, which is far from the sun and surrounds the solar system like a sphere. This cloud of comets, containing about a billion bodies, was formed when balls of ice that did not coalesce into planets and moons were thrown from the region of the large planets (Jupiter, Saturn, Uranus and Neptune) to the edges of the solar system. Their ejection was caused by the action of the gravity of these planets in two directions: outward - away from the planets into the Oort cloud, and inward - towards the planets. This is how the earthly planets - Hema, Venus, Earth and Mars - were "bombarded" by a comet shower in the last stages of their creation. As we said before, a few comets from the Kuiper belt also penetrated and hit the terrestrial planets.

Solar System

A small star, one of a hundred billion stars in our galaxy, which is one of a billion galaxies in the entire universe, stands at the center of the solar system. This star is the sun. A star, unlike a planet, shines because it radiates nuclear energy, which is created in it by the fusion of hydrogen nuclei with helium nuclei.

Nine planets circle around our sun, much smaller than the sun, so the temperature in their center is not sufficient to ignite nuclear combustion. They appear to us to shine only because of the sunlight that hits them and is returned to us. They are called "planets" and not "planets", since planets are not stars. The terrestrial planets, those closest to the sun, in order of their distance from it, are: Mercury, Venus, Earth and Mars. The giant, distant planets are Jupiter, Saturn, Uranus and Neptune. We will also mention Pluto, which is not giant, and seems to have been a moon of Neptune but "ran away" from it, and now it orbits the Sun instead of orbiting Neptune.

Between Mars and Jupiter stretches the asteroid belt - blocks of rock ranging in size from meters to hundreds of kilometers, which did not unite into a whole planet and remained scattered in the belt. Outside the region of the planets is a large cloud of comets.

In addition to all these, most of the planets orbit moons. Moons that are composed of rock surround the terrestrial planets, and moons that are composed mostly of ice surround the giant planets. The moons were formed close to the creation of the planets, and have been orbiting them ever since. The Earth's moon was probably formed as a result of a giant body hitting the Earth at the end of its formation period. This impact splashed a huge amount of material from the ball, which crystallized and became our moon.

The creation of life

Why are comet impacts on terrestrial planets important? As we said, the terrestrial planets, which are close to the sun, were formed from grains of iron and silicates and contained few volatile substances such as water, and nitrogen, oxygen, carbon and hydrogen compounds, which are the chemical basis of life. The impact of the comets, which are mainly composed of water, iron and silicates, and heavy organic materials (made up of carbon, nitrogen, oxygen and hydrogen compounds), brought these materials to the terrestrial planets, including the Earth. These compounds were broken down in the earth's atmosphere about 4 billion years ago by solar radiation, lightning and thunder. The new substances that were created from them melted in the oceans on the surface of the ancient earth and created the components of life: the amino acids, the components of proteins, the components of sugars and the components of nucleic acids. Thus the comets, which brought these volatile substances to the bare earth, contributed their decisive part in the creation of life on its surface.

The nucleus of Halley's Comet as imaged by the European Space Agency's Giotto spacecraft.

The gases and dust are emitted from seven active craters. The rest of the comet's surface is dark and inactive because of a layer of dust that insulates its surface from the heat of the sun's radiation.

Why was life created only on Earth? The reason for this is its distance from the sun, expressed in one astronomical unit, which is 150 million kilometers. In the planet Hema, which is very close to the sun, to 0.387 astronomical units, the temperature is so high that all the volatile substances that were in it evaporated into space and it was left without an atmosphere. Even Venus, whose distance from the Sun is 0.723 astronomical units, has the highest temperature. All the water in it evaporates, and with the other volatile substances they concentrate in its atmosphere. On Venus, therefore, there are no oceans, and since the chemical reactions take place in an aqueous medium, the substances cannot react with each other and create life.

On Mars, which is 1.527 AU from the Sun, the temperature is even lower, and the water and other volatile substances are frozen on its surface and below its ground surface. Only on the surface of the earth, which is, as mentioned, at a distance of one astronomical unit from the sun, is water in a liquid state. In the water, the chemical substances formed in the atmosphere dissolved and reacted with each other to a degree of complexity that made possible the creation of life. We will remind again that these volatile substances were brought to the Earth by comet impacts in the last stages of its formation. Now it is clear why the impact of the 21 parts of Comet Shoemaker-Levy Jupiter is so interesting to us - they actually reproduce what happened on the surface of the ancient Earth. On top of that, we can learn from this impact both about the compositions of the comets and their structures and about the structure of Jupiter.

comets

After spacecrafts passed by the planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune, and even landed on the surfaces of Venus and Mars, the smallest bodies in the solar system - the asteroids and comets - remained for close study. In 1986, when Halley's comet came near the earth, six spacecraft were sent to meet it. Two of them - the Soviet Vega and the European Space Agency's Giotto - came so close to it as to photograph its nucleus up close and examine the material thrown from it into space. They discovered that Halley's core is composed of half ice and half a dust of silicates and asphalt-like organic substances, containing carbon, hydrogen, oxygen and nitrogen. The core also contained about 10 percent of gases - carbon monoxide, carbon dioxide, nitrogen and a little methane and ammonia. All these substances are building blocks for compounds such as amino acids, sugars and nucleic acids, and they contributed to the formation of life on earth.

It is Rea (aehR), a moon of Saturn, composed mostly of ice. The moon is scarred all over its face as a result of the impact of comets in the last stages of its creation. These injuries were preserved on his face for 4.5 billion years.

When the comet is far from the sun, and it is in the Kuiper belt or Oort cloud, the temperature of its core is about 250 degrees Celsius below zero, close to absolute zero which is 273 degrees below zero (see: "The race to absolute zero", in this issue). At these temperatures, the initial composition and structure, which formed 4.5 billion years ago in the cloud of gas and dust at the edges of the solar system, is preserved. Thus, the comets are the evidence of the composition of that primordial cloud. To find out, the American and European space agencies decided to reach the comet in early 2000 and circle it for about two years, as well as send a probe to its nucleus and study it closely.

When the comet approaches the Sun, it heats it up until the frozen water and gases trapped inside it evaporate. The water vapor and gases carry with them the dust grains, which were previously trapped in ice, and a cloud of gas and dust forms around the nucleus. While the diameter of the nucleus is a few kilometers (the diameter of extremely large comets may reach up to 100 kilometers), the diameter of the cloud - the halo - can reach up to a hundred thousand kilometers. When radiation and the solar wind (a stream of hydrogen and helium nuclei with electrons thrown from the sun) hit this thin cloud they push it back and form the comet's magnificent tail. This tail always faces away from the Sun, up to several million kilometers from the nucleus. Therefore, all the tails of the Shoemaker-Levy chain of comets face the same direction - from the Sun outwards.

Jupiter

Jupiter is the largest planet in the solar system. Its volume is 1,316 times greater than the volume of the earth, but its mass is only 317.9 times greater than the mass of the earth, and its density, therefore, is much smaller than that of the earth. From this it can be concluded that Jupiter consists mainly of gases, while the Earth consists of rock and an iron core, and a very thin shell of water and gases in the atmosphere.

Indeed, most of Jupiter's volume is hydrogen gas, about a quarter of which is helium gas, and the other materials - iron, rock, water, and compounds of carbon, nitrogen, oxygen, etc. - make up only a thousandth of it. This is actually the composition of the sun. From this we learn that at the time of its formation, Jupiter gathered from the cloud of gas and dust all the matter that was in its vicinity, as the sun did. Earth and the other terrestrial planets collected only iron and rocks, very few volatile substances and zero amounts of hydrogen and helium, which is why they remained so small compared to Jupiter and the other giant planets. And as we mentioned before, this was due to the fact that near the sun the temperature was much higher than in the area far from it, where the giant planets were formed.

Jupiter was explored in the 70s by two Voyager spacecraft of NASA, the American space agency. They passed by him and photographed him and his moons. From their measurements, observations from the country and theoretical models we conclude that Jupiter has no solid ground. The hydrogen and helium in it become more and more dense the closer they are to its center, until at its depths the hydrogen becomes extremely dense and behaves like a metal. In the center, apparently, is a core of iron and rock, somewhat heavier than the entire earth. The upper layers of Jupiter's atmosphere are cloud layers. The upper layer is of white ammonia clouds; Below it there is, apparently, a layer of ammonium sulphide clouds, and below it a layer of water clouds. In the observations we only see the ammonia clouds, while the other cloud layers are approximate. In 1995, the Galileo spacecraft will penetrate Jupiter's atmosphere and descend slowly with the help of a parachute. During its descent, the spacecraft will check the composition of the atmosphere and transmit its findings to Earth.

Jupiter - the largest planet in the solar system. The different stripes are created by upward and downward atmospheric movements, which reveal different layers in the atmosphere. The impact of Comet Shoemaker-Levy will inform us about the composition of the inner layers of the atmosphere.

what do we see

Since we cannot discover by remote observations the structure of Jupiter's cloud layers and their composition, the impact of Comet Shoemaker-Levy will be a test for the various models. In this huge impact, part of the atmosphere along the path of the comet's impact will be ejected into space. This way the materials that make up the clouds will rise, they will mix with each other and we will be able to see them. The materials that make up the comet will also evaporate, and we can also wonder about their jar.

Unfortunately, all 21 parts of the comet will hit the side of Jupiter which at that time will not face Earth. That is, we will not be able to see from our seat the impacts themselves and the thousands of kilometers long plume that will erupt from Jupiter at the time of the impact. Because of Jupiter's rapid rotation around its axis (it completes a revolution every 9.84 hours, compared to Earth's 24 hours), the point of impact will be revealed to our eyes within 13-7 minutes. However, due to Jupiter's strong gravity, within this period of time the plume will sink back into its atmosphere, and what will remain is a large patch, which will contain all the materials that will be thrown from Jupiter and those that will evaporate from the comet. Also from this we can learn a lot about the composition of the atmospheres of Jupiter and the comet.

Another possibility is that some of the material that will be thrown up will be trapped in Jupiter's stable stratosphere (the upper layer of the atmosphere) and will remain there for a long time, similar to various substances that are trapped in the Earth's stratosphere and cause a decrease in the amount of ozone at the poles. In this case we can observe them for a long time before they descend back to the lower layers of Jupiter's atmosphere.

Luckily for us, the Galileo spacecraft, which is, as mentioned, on its way to Jupiter, will be in the right place and will be able to photograph and measure the damage and send its findings to Israel. Other spacecraft that will be affected by the impact will be Clementine (if they manage to stabilize it, after it began to rotate around an axis and wobble due to a malfunction), Ulysses (sesylU), the Hubble Space Telescope, and an ultraviolet spacecraft International Explorer will also observe Jupiter in the infrared range from an aircraft of NASA Kuiper Observatory RI) as well as from all the large and medium telescopes on Earth.

Scotty, one of Shoemaker and Levy's partners in the discovery of the comet, who confirmed their initial finding, will come to Tel Aviv University's Wise Observatory, located at the Ramon Observatory, to observe the event from Israel. In our country, night will prevail during the collision.

Now the question arises, why does comet Shoemaker-Levy 21 "disintegrate" into large pieces, with a diameter of 4-2 kilometers, and perhaps also into many smaller pieces. On July 7, 1992, the comet, which is about 8 kilometers in diameter, passed by at a distance of about 40,000 kilometers from Jupiter, was captured by its gravity, which is 2.64 times greater than that of the Earth, and since then it orbits Jupiter in a cycle of two years. Since the pull gets smaller with the greater the distance, the part of the comet closer to Jupiter is pulled more strongly than the part of the comet farther from it.

Because of this difference in gravity, the parts of the comet were torn apart and formed a chain of smaller comets, each 4-2 kilometers in diameter. This event could only occur because the comet is made of very brittle ice, and the forces of adhesion between its particles are extremely weak. Indeed, in the measurements made by the Vega and Giotto spacecraft on Halley's comet, it was found that its density is very low - only about 0.1 grams per cubic meter (the density of water is 1 gram per cubic meter). This low density indicates its crispness and the ease with which it breaks down.

Comets in history

In 1908, a fragment of a comet hit the Tunguska region (aksugnoT) in Siberia. The ice of this comet evaporated as it passed through the atmosphere, and what remained of it was a cloud of dust, with particles ranging from millimeters to meters in diameter. This cloud created a shock wave ("boom" on my voice) in the atmosphere so strong that all the trees in the area of ​​its impact collapsed. A glowing ball is seen in the sky and a tremendous noise is heard. Scientists speculate that an impact by a giant comet about 65 million years ago brought enormous amounts of dust into the earth's atmosphere, which greatly reduced the amount of sunlight reaching the ground. As a result, the explosiveness of the plants, which need sunlight for the photosynthesis process, decreased drastically. This may have been the reason for the disappearance of the dinosaurs from the face of the earth. The dinosaurs, who were plant eaters and needed huge amounts of food, probably became extinct due to a lack of food. This hypothesis is based on the fact that in many places on Earth, in a geological layer that is 65 million years old, there is a large amount of the element iridium, which is common in meteorite dust. The disappearance of the dinosaurs made possible the culture of mammals and eventually the appearance of man.

About a year ago it seemed that another comet - Swift Tuttle - might hit Israel in the near future. In the meantime, it became clear from a more precise examination of its trajectory, that luckily it will only pass near the country and not harm it. If the comet had hit Israel, a catastrophe similar to the one that destroyed the dinosaurs could have been caused. Today we are able to send spaceships equipped with hydrogen bombs to such a comet, they will divert it from its orbit and even cause it to crash far from Earth.

So, the beginning of life on earth, the appearance of man, and perhaps also the danger of his annihilation, are closely connected with comets. And besides the beautiful celestial event, where ice blocks the size of mountains, at a speed of 60 kilometers per second, will hit the giant planet Jupiter, the comets are an important link in our cosmic connection.

Comet Lab at Tel Aviv University

In the Department of Geophysics and Planetary Sciences of Tel Aviv University, a laboratory for the study of comets has been operating since 1980. In this laboratory, ice is created at very low temperatures, reaching 257 degrees Celsius below zero. In the experiments, the physical properties of the ice and its ability to trap different gases are tested.

In the experiments we conducted on the ice of Halley's comet, we found that the comet was formed in the solar system in an area where a temperature of about 223 degrees Celsius below zero prevailed - that is, far from the area where the planets were formed. The ice formed at these low temperatures is very brittle and has little mechanical law (needs an explanation, doesn't it?). In additional experiments we conducted we suggested that huge lightning storms take place in Jupiter's atmosphere; And these were indeed discovered by the Voyager spacecraft. We also showed that the noble gases argon, krypton and xenon, and with them water and organic substances, reached the other planets by comets. The activity in this field led to my participation in the working group which determines the nature of the scientific experiments that will be conducted in space by the Rosetta spacecraft, which will be launched at the beginning of the next century.

* Akiva Bar-Nun is a professor of planetary sciences at Tel Aviv University. His research deals with comets, the atmospheres of the planets and the origin of life on Earth. For the past five years he was the director of the Israel Space Agency and is now its deputy chairman for scientific affairs.