Differences

This shows you the differences between two versions of the page.

--- courses:phy100:5 [2023/08/16 08:52] – asad
+++ courses:phy100:5 [2023/11/05 22:43] (current) – [4.2 Giant planets] asad
@@ Line 1: / Line 1: @@
 ====== 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 =====
 The sun, a typical star in our Galaxy, is nothing but a gigantic spherical ball of hydrogen gas with a diameter of 1.4 million km. Three fourth of the sun is hydrogen, one fourth is mostly helium with a little bit of other heavier elements. We can understand the structure of the surface and interior of the sun by analyzing sunlight and we see the corona surrounding the sun during total solar eclipses.
-{{:courses:phy100:sun.png?nolink|}}
+{{:courses:phy100:sun.png?nolink&500|}}
-Going from the center to the surface, the sun has different spherical layers: (1) the core, (2) the radiative zone where heat is transferred via radiation toward the surface, (3) the convective zone where gas goes up and down in loops through convection (as in a pot of bioling water), (4) the surface called the photosphere, and (5) the atmosphere composed of the lower chromosphere and the upper corona. From earth, we normally see the **photosphere** of the sun and it has a temperature of around **6000 degrees**. The black sunspots are seen on this surface as well.
+Going from the center to the surface, the sun has different spherical layers: (1) the core, (2) the radiative zone where heat is transferred via radiation toward the surface, (3) the convection zone where gas goes up and down in loops through convection (as in a pot of boiling water), (4) the surface called the photosphere, and (5) the atmosphere composed of the lower chromosphere and the upper corona. From earth, we normally see the **photosphere** of the sun and it has a temperature of around **6000 degrees**. The black sunspots are seen on this surface as well.
-{{:courses:phy100:sun-temperature.png?nolink|}}
+{{:courses:phy100:sun-temperature.png?nolink&600|}}
 The temperature and density of the sun increases toward the core as shown above. The left side of the figure shows how density varies from the surface (left) to the center (middle) and the right side of the figure shows how the temperature varies from the surface (right) to the center (middle). In both cases you can see that the curve rises toward the center showing increase of the quantity toward the center.
@@ Line 49: / Line 51: @@
 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.
-{{:courses:phy100:planets-rotation.png?nolink|}}
-Like the earth, all planets and dwarf planets revolve around the sun and rotate on their own axis. Here you see the tilt of the axis of rotation for the planets. Earth's axis is tilted by 23.5 degrees from the vertical, Mercury has no tilt. Venus is completely flipped, it rotates upside down. Uranus rotates with respect to an axis that lies on the plane of the solar system, it rotates lying down, weird!
 ===== - 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.
@@ Line 77: / Line 71: @@
 I have already said that the solid crust moves on a liquid-like part of the upper mantle. This crust, that is the rocky surface of the earth, is not one solid mass, but fragmented into many **tectonic plates**, each plate has it own motion. The plates move at the speed of the growth of our fingernails and collide with each other. The figure above shows the most prominent plates. High mountains are created when two plates collide and deep valleys are created when they move away from each other. You can clearly see the deep valley underneath the Atlantic ocean in google maps. This ridge, deep ridge, is created because the American plates are moving away from the African and Eurasian plates. The movement of the plates keeps the surface of the earth active.
-===== - Terrestrial planets =====
+===== - The planets =====
+There are 8 planets in the solar system and a few dwarf planets like Pluto and Ceres.
+{{:courses:phy100:planets-rotation.png?nolink|}}
+Like the earth, all planets and dwarf planets revolve around the sun and rotate on their own axis. Here you see the tilt of the axis of rotation for the planets. Earth's axis is tilted by 23.5 degrees from the vertical, Mercury has no tilt. Venus is completely flipped, it rotates upside down. Uranus rotates with respect to an axis that lies on the plane of the solar system, it rotates lying down, weird!
+==== - Terrestrial planets ====
 Mercury, Venus and Mars are called terrestrial planets because they are similar to the earth ('terra'), they are made of rocks mostly. You can call them rocky planets as well. They are characterized by a dense rocky sphere surrounded by a thin gaseous atmosphere.
+{{:courses:phy100:terrestrial-planets.png?nolink|}}
+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 ====
+The four outer planets, the planets outside the asteroid belt, are called giant planets because they are big. Among them, Jupiter and Saturn are called 'gas giants' because they are mostly made of gas, and Uranus and Neptune are called 'ice giants' because they have a lot of icy material.
+{{:courses:phy100:giant-planets.png?nolink|}}
+Here you see the composition of the giant planets. All of them have a very small solid core made of rock and iron surrounded by a layer of water. Jupiter and Saturn have layers of liquid metallic hydrogen outside the water layer, and then a layer of gaseous hydrogen as the surface. Uranus and Neptune do not have the liquid metallic hydrogen layer, but only a gaseous hydrogen layer.
+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.
+----
-===== - Giant planets =====
+Many of the figures have been taken from //Explorations// by Thomas Arny & Stephen Schneider.