The atmospheres of different planets

Transcription

1 The atmospheres of different planets Thomas Baron October 13,

2 Contents 1 Introduction 3 2 The atmosphere of the Earth Description and Composition Discussion Atmospheres of the others planets How to get the informations about the atmospheres of the other planets of our solar system? Description of the atmospheres of the others planets Mercure Venus Mars Jupiter Saturne Uranus Neptune Pluto Discussion Summary 7 5 Bibliography 8 2

3 1 Introduction An atmosphere is defined as a layer of gases surrounding certain planets. It is made up of chemistrial composed which are variable from a star to another one. In this presentation we want to figure out why the atmosphere of the earth is so specific and makes it possible to live in it. Note that in this report we won t talk about extraplanets atmospheres because of the difficulties of their observations and we will assure that Pluto is still a planet of our Solar System in spite of the recents discussions and controversies on it. 2 The atmosphere of the Earth 2.1 Description and Composition The Earth s atmosphere isn t homogeneous over all its thickness. We can divide it in differents layers witch are characterized by some physic properties like: the pressure, the temperature, the density or the presence of specific gases or plasmas. This layers are : The troposphere : (from the ground to 10km) The stratosphere : (from 10km to 50km) The mesosphere : (from 50km to 80km) The thermosphere : (from 80km to 690km) The exosphere : (from 690km to 800km) The boundaries between these regions are named the tropopause, stratopause and mesopause. 3

4 Gas Quantity N 2 78,1 O 2 21,0 Ar 0,93 CO Ne 1, He 5, Kr 10 4 H Xe Table 1: Composition of Earth s atmosphere per percent The Earth s atmosphere is made up of a mix a several gases present in differents concentrations. This gases are likely to give to the Earth its power to allow the life developement 2.2 Discussion The pressure of the atmosphere is a function of the altitude and is given by : P (z) = P 0 e Zmg RT where Z is the height, m the mass of a molecule of water, g the gravity strenght, R the constant of perfect gases and T the temperature whitch is supposed to be constant in this formula. However it s only works for low altitudes where the magnetic field is neglect. In the troposphere, the energy source is the ground, so the temperature falls off with altitude. Higher temperatures in other layers result from the direct absorption of solar ultraviolet and x-rays. Thus for example, upper stratosphere and lower mesosphere contain the ozone layer (ozone O 3 is a molecule that absorbs ultraviolet radiation from the Sun). This absorption means that these layers get much of their energy directly fom the Sun. The temperature is thus higher there than it is at the top of the troposphere. Above the mesosphere is the ionosphere, where many of the atoms are ionized. The most energetic photons from the sun are absorbed here. Because of this rising temperature, this layer is also called thermosphere. 3 Atmospheres of the others planets 3.1 How to get the informations about the atmospheres of the other planets of our solar system? The dificulty to determine the composition of each planet of the System Solar is their huge distance from the Earth. The only method we re able to practice at 4

5 this moment is to throw space probes. Then to know exactly the composition of these planets, we can use two different way. The first one consist of sending the probe straight in the atmosphere and to pick the composition of the air up with specific captors (in situ measurement). The second one si to put a probe into orbit around the planet, and to make a spectral analysis of the infrared radiance emitted by this body. The atmospheric gases of a planet are able to absorb a part a light radiance. Especially, we can show that, on average, gases absorbs more in the infrared domain than in the visible or ultraviolet. Then the atmosphere is almost clear for the solar radiance. However it absorbs big part of the planet radiance. This absoption is dependant on the chimistrial atmospheric composition. That is to say that the photons likely to be absorbed by a specific chimistrial specy are subtracted from the planet radiance. Then if when we compare this radiance with the one of the sun we are able to know the gas composition of the atmosphere. 3.2 Description of the atmospheres of the others planets Mercure The mass of Mercure is too weak and its surface s temperature too hight to retain a permanent and important atmosphere. However we can notice the presence of a very thin layer of gases (discovered by the Mariner 10 probe). It mass is about 1000 kg. It s due to the action of the solar wind and it s always reconstitute. The absence of the atmosphere allow the escaping of the heat on night and is the cause of the intense radiance on the ground of the planet Venus The atmosphere of Venus is the one that looks most like ours from the physic s point of view. It s really dense and its thickness is about 50 to 70 kms. The pressure at the surface is 95 bars and the temperature is more than 460 C. The huge temperature of Venus is due to this thick atmosphere witch retains all the heat receipt by the Sun. The rotation velocity of Venus atmosphere is higher than the planet s itself because of the presence of violents winds blowing with a speed of 100m/s which makes it lap around the planet in 4 days Mars Mars has a very thin atmosphere. The surface pressure on Mars is only about 0.7 percent of the average surface s pressure of the ground on Earth. The atmospheric temperature vary from -120C on nights to +20C on days. The weak pressure on Mars is at the origin of this huge difference of temperature. There is some water vapor that give rise to ice clouds and some fogs. Sometimes, we can notice very strong storms that start from the south pole to the all atmosphere of the planet. 5

6 3.2.4 Jupiter The atmosphere of Jupiter is composed by three differents layers. It is mostly made up of the simple molecules hydrogen and helium. Sulfur is also present there, as well as nitrogen and oxygen. These molecules combine to make clouds of complex molecules, such as clouds of water and smog. Jupiter has bright bands called zones and darks bands called belts. The zones are rising gas, while the belts are falling gas. The tops of these dark belts are somewhat lower (about 20 km) that the tops of the zones ans so are about 10K warmer. The atmosphere of Jupiter is only a narrow surface layer, compared to the vast interior of the planet. The three clouddecks of Jupiter are to be found at different levels in the troposphere, while hazes of smog can be found higher in the atmosphere. The atmosphere of Jupiter turn around the planet at differentials velocity witch depends on the latitudes ( the velocity is higher at the poles than at the equator) Saturne The composition of Saturnes atmosphere is very close to Jupiter s (mostly composed by hydrogen, helium and methan). The rotation of the atmosphere also present differentials velocity Uranus The atmosphere of Uranus has a temperature of 50 Kelvins. The spectroscopic s studies let us know that this atmosphere is mostly composed by hydrogen, helium and methane. Methane in the upper atmosphere absorbs red light, giving Uranus its blue-green color. The atmosphere is arranged into clouds running at constant latitudes, similar to the orientation of the more vivid latitudinal bands seen on Jupiter and Saturn Neptune Neptune s atmosphere seems to be very close to Uranus. It contains two cloudy layers of different composition. The lowest one is composed by amoniac and methan. The highest one is only composed by methan. The atmosphere is characterized by the most violents winds ever mesured in the Solar System. It can hit 2000 km/h. But this huge source of energy still stay quite problematic as the planet is really far from the Sun. Neptune s emission of energy is 2,7 times higher that its reception from the Sun. However this is due to huge currents which make transfert of the energy from the interior of the atmosphere Pluto We don t really know the characteristics of the atmosphere of Pluto. But it seems to be composed by methan, azote and an heavier gas like the carbon monoxide. The pressure at the surface is about one hundred thousands times lower than ours (a few micro bars). The atmosphere is only in state of gas when 6

7 the planet is at its position closest from the Sun. Then, gases seems to escape in space and even to make some exchanges with Charo, Pluto s satellite. During the most part of its revolution around the Sun, gases are in state of ice. 3.3 Discussion The capacity of a planet to keep an atmosphere depends on several parameters. Atomes or molecules of this atmosphere are shaked by a thermal agitation. Their average velocity is liable to the planet s temperature. Then we have to compare this to the minimal velocity of a nanoscopic-sized object necessary to quit the gravitational attraction. We can say that the compositon of an atmosphere principaly depends on the mass of the body, its height and its temperature, so its distance to the Sun. For the Solar System, we ve seen that three differents cases appears. First, bodies which couldn t be able to keep an appretiable atmosphere, generally because of a weak mass, and so a weak gravity. This is the case of Mercure, the Moon, Pluto and all the satellites of the Solar System excepting Titan (a satellite of S...). Then, the very massives bodies, able to keep all the kind of gases, in particular the hydrogen and the helium : Jupiter, Saturne, Uranus and Neptune. Finally, the most interessant case concern the planets or satelittes which are characterized either by an intermediate mass or by a very low temperature : Venus, Mars, the Earth and Titan. This four celestal bodies had lost their hydrogen and helium but had succeeded in keeping heavier gases like the carbon dioxyde and the water s vapor. 4 Summary The atmosphere of a planet depends on several properties. We can see that our atmosphere is really specific to our planet because of its special temperature (distance from the Sun) and its size (gravity strength). The proportion of gases that allows the human race to live on Earth is the result of this perfect parameters If we compare our situation to the others planets we can conclude that our planet has the ideal characteristic for the development of life. 7

8 5 Bibliography Websites Physic s Books Le Systeme Solaire (CNRS editions) Physique Stellaire (Dunod) Lectures on Physics by Richard Feynman Landau and Lifschitz (Vol 1,5) Lectures Lectures on Meteorology by Dr Christophe Chatelain (UHP Nancy Poincare) Lectures on Space Physics by Dr Kjell Rnnmark (Umea Universitet) 8

9 K Na O Ar He O 2 N 2 CO 2 H 2 0 H Table 2: Composition of Mercure s atmosphere per percent CO 2 N 2 He Ne Ar Kr trace trace trace trace Table 3: Composition of Venus atmosphere per percent CO 2 N 2 Ar O CO H 2 O trace trace trace Table 4: Composition of Mars atmosphere per percent H He CH 4 H 2 0 NH 3 C 2 H trace trace trace trace Table 5: Composition of Jupiter atmosphere per percent H He CH 4 H 2 O NH 3 C 2 H trace Table 6: Composition of Saturne s atmosphere per percent H 2 He CH 4 NH 3 C 2 H trace trace trace Table 7: Composition of Uranus atmosphere per percent H 2 He CH 4 NH 3 C 2 H trace Table 8: Composition of Neptune s atmosphere per percent N CH 4 CO trace Table 9: Composition of Pluto s atmosphere per percent 9