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--- courses:phy100:5 [2023/08/16 09:18] – asad
+++ courses:phy100:5 [2023/11/05 22:43] (current) – [4.2 Giant planets] asad
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 ====== 5. Sun and the Solar System ======
-Our galaxy, the Milky Way, has a few hundred billion stars and almost every star has a complex planetary system around it. The planetary system of the sun is called the solar system. The sun (located around 26,000 light years from the center of the Milky Way) dominates the solar system, it is responsible for more than 99% of the total mass of the solar system. You can [[https://solarsystem.nasa.gov/|explore the solar system using this website]]. In the solar system we have found 8 planets, almost 300 moons, more than 1 million asteroids and small bodies, and almost 4000 comets. Many more small bodies and comets remain undiscovered.
+Our galaxy, the Milky Way, has a few hundred billion stars and almost every star has a complex planetary system around it. The planetary system of the sun is called the solar system. The sun (located around 26,000 light years from the center of the Milky Way) dominates the solar system, it is responsible for more than 99% of the total mass of the solar system. You can [[https://solarsystem.nasa.gov/|explore the solar system using this website]].
+In the solar system we have found 8 planets, almost 300 moons, more than 1 million asteroids and small bodies, and almost 4000 comets.
 ===== - The sun =====
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 Why are the planets made out of rocks and the outer planets gas and ice? Why is the asteroid belt composed of rocks and the Kuiper belt ice? It has a lot to do with temperature. Temperature decreases as you go away from the sun.
-{{:courses:phy100:solar_system_thermometer-02152022.jpg?nolink&700|}}
-Here you see temperatures of the planets. Mercury is closest to the sun, but Venus has the highest temperature (500 $^\circ$C) because of excessive greenhouse effect in its atmosphere. Average temperature on the surface of the earth is around 15 $^\circ$C, and it keeps decreasing as you go toward Mars. Pluto has a temperature of only $-225$ $^\circ$C. Note that the average temperature of the universe as a whole is $-270$ degrees, only 3 degrees above absolute zero. No matter how cold Pluto is, it is still a warm place compared to the desolate coldness of empty space in the universe.
 ===== - The earth =====
 We talk about this planet in detail because we know most about it and also because it is our home. The inner and outer composition of the earth will tell us a lot of things about planets in general.
-{{https://www.nasa.gov/sites/default/files/images/297755main_GPN-2001-000009_full.jpg?nolink&600}}
+{{https://upload.wikimedia.org/wikipedia/commons/thumb/a/a8/NASA-Apollo8-Dec24-Earthrise.jpg/768px-NASA-Apollo8-Dec24-Earthrise.jpg?nolink&500}}
 The earth also became an astronomical object in the 1960s when the Apollo missions took pictures of the earth from the moon and the earth became another object in the sky for us. The first picture of the earth rising above the moon is shown above. It was taken by Apollo 8 astronauts in 1968.
-{{https://www.nasa.gov/sites/default/files/images/608134main_world-orig_full.jpg?nolink&500}}
+{{https://upload.wikimedia.org/wikipedia/commons/thumb/6/61/608134main_world-orig_full.jpg/767px-608134main_world-orig_full.jpg?nolink&500}}
 The interior of the earth is as complex as that of the sun. It has a 'solid' inner core, a liquid outer core, a mantle and a crust. The upper part of the mantle is almost like a liquid and the solid crust shakes and moves on top of this liquid-like upper mantle. The inner core is made of solid iron, the outer core of liquid iron. Because of the motion of iron at the cores, a giant magnetic field is created that extends beyond the atmosphere of the earth and creates a **magnetosphere** as shown below.
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 As you can see, all the terrestrial planets, and even the moon, have a inner core made solid iron and nickel. Outside the core, they have rocks made of silicates. The moon is almost like a failed terrestrial planet, it is not that small compared to Mercury.
+{{:courses:phy100:terrestrial-planets-atmospheres.png?nolink|}}
+The Moon and the Mercury do not have atmospheres, but the Earth, Mars and Venus do. The atmospheres of Mars and Venus are dominated by $CO_2$ (carbon-di-oxide), but the atmosphere of Earth is rich in nitrogen ($N_2$) and oxygen ($O_2$). The clouds of Earth are made of frozen water, the clouds of Mars are made of CO$_2$, but the clouds of Venus are made of poisonous sulfuric acid. The surface of Venus is flooded with acid rain.
+Mercury is the closest to the sun, but Venus has the highest atsmospheric temperature (500 $^\circ$C) because of excessive greenhouse effect in its atmosphere. Average temperature on the surface of the earth is 287 kelvin or 14 degrees Celsius, and it keeps decreasing as you go toward Mars.
 ==== - Giant planets ====
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 One very interesting fact is that the earth is smaller than the cores of these planets. Each of the giant planets harbor a solid sphere the size of the earth at their center.
+{{gallery>:img:planets}}
+==== - The moons ====
+Only Earth and Mars among the terrestrial planets have moons, or natural satellites. Our moon is the 'moon', but the natural satellites of other planets are sometimes also called 'a moon'. Almost 300 moons have been discovered. Any object orbiting a planet is a moon no matter how small. But only 19 of the almost 300 moons are big enough to be completely round in shape because of their self-gravity.
+{{:courses:phy100:moons.jpg?nolink|}}
+The major round moons of the solar system are shown above. The moons of Mars are not really visible at this scale because they are too small and not big enough to be round. A planet like the earth normally does not have as big a satellite as the moon, it seems like an exceptional case. As you can see, the moons of the giant planets are almost comparable to the moon in size. The four moons of Jupiter shown here were discovered by Galileo.
+===== - Small bodies =====
+There are a lot of bodies in the smaller system smaller than planets and their satellites. We will talk about mainly asteroids and comets.
+{{https://upload.wikimedia.org/wikipedia/commons/f/f3/InnerSolarSystem-en.png?nolink&600}}
+Most asteroids are located in a ring between the orbits of Mars and Jupiter as you see above. Each dot here is an asteroid with a size bigger than 1 km. There are asteroids as big as 10 km. A 10-km asteroids hit the earth 65 million years ago and caused massive extinction of life.
+There are some asteroids, the greeks and the trojans, in the orbit of Jupiter and they revolve around the sun together with Jupiter.
+{{:courses:phy100:oort-cloud.png?nolink&600|}}
+Surrounding all the planets there is another belt of icy objects called the Kuiper belt. Note that the distance of the Kuiper belt is 50 AU, that is 50 times the distance from the sun to the earth. But it turns out even Kuiper belt is nothing compared to the huge Oort cloud that surrounds the whole solar system and reach interstellar space. The Oort cloud is a giant sphere of icy objects at extremely low temperature. The cloud has a radius of 100,000 AU.
+Try to fathom this. The Kuiper belt is more distant than the farthest planet from the sun, and the distance of the Kuiper belt from the sun is only 50 AU. On the other hand the outer edge of the Oort cloud is 100,000 AU from the sun.
+Comets live in the Oort cloud as boring icy rocks. But when they enter the solar system, their ice melts and creates a huge tail and we call them comets because of the tail.
+----
+Many of the figures have been taken from //Explorations// by Thomas Arny & Stephen Schneider.