Download The atmospheres of different planets

The atmospheres of different planets
Andre Herling
Contents
1 Introduction
1
2 The atmosphere of the Earth
2.1 The structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Physical properties and chemical composition . . . . . . . . . . . . . . . . . . .
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3 How do we get to know about other atmospheres?
3
4 Atmospheres of other
4.1 Inner planets . . .
4.1.1 Mercury . .
4.1.2 Venus . . .
4.1.3 Mars . . . .
4.2 Gas giants . . . . .
4.2.1 Jupiter . . .
4.2.2 Saturn . . .
4.2.3 Uranus . . .
4.2.4 Neptune . .
4.3 Special Cases . . .
4.3.1 Pluto . . . .
4.3.2 Titan . . . .
5 Summary/Conclusion
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1
Introduction
An atmosphere is basically an envelope of gas that surrounds a celestial body such as a planet,
moon or star. It is held to the body by gravitational forces. As these forces are strongest
when near the center of a mass, the density of atmospheres is usually densiest at the surface
of a celestial body. On the other end there is no strict boundary between the atmosphere and
interplanetary space. Because of their huge masses gas giants consist mainly of gases and have
very deep atmospheres. In these cases it’s quite difficult to define a fixed border between body
and atmosphere.
2
The atmosphere of the Earth
2.1
The structure
There is quite a lot to say about the atmosphere of the Earth. One of the most important
things is that it is divided vertically in different layers. The partitioning is based on temperature gradation, as it can be seen in illustration 1.
Figure 1: Atmosphere of the Earth
• The troposphere is the lowest of the atmospherical spheres. It ranges from the surface to
an average height of about 11km. The heights of the pauses, the boundary regions between
spheres, are dependent on the season and the latitudinal position. The tropopause can be
found 18 km above the equator and just 8 kilometres above the poles. But why does the
temperature vertically decrease in the troposphere? Most of the heat radiation that can
be stored in its molecules comes from the surface and not directly from the sun. As fewer
heat radiation can reach its upper regions, the vertical temperature gradient is about 6.5◦
K
.
km
• The second layer is the stratosphere. It is located in an altitude between 10 km and
50 km. The ozone layer is able to absorb large amounts of solar ultra violet radiation.
Therefore we have a temperature inversion in the stratosphere.
• Above the stratosphere we can find the mesosphere. Due to radiative cooling the temperatures decreases with increasing altitude to its lowest point in the atmosphere: about
170 K at an altitude of 85km.
• Characteristic for the thermosphere is another temperature inversion - quite similar to
the stratosphere. This is due to absorption of high enery radiation by remaining oxygen
molecules. The ISS has its orbit in the thermosphere in a height of approximately 350km.
The upper part of the thermosphere is called ionosphere because of the ionization of
atmospherical molecules that takes place.
2.2
Physical properties and chemical composition
The atmosphere is made up by nitrogen(78%), oxgen(21%), argon(0.9%), carbon dioxide(0.04%)
and traces of many other composites including water. The pressure at the surface is 1 bar and
declines approximately exponential. The pressure is halved every 5.6 km, corresponding to a
scale height of about 7.5 km. The atmosphere is in many way significant for life on our planet.
As it is able to store a large amount of energy, it reduces the temperature difference between
night and day. It also absorbs ultraviolet radiation from the Sun which is harmful for many
life forms.
3
How do we get to know about other atmospheres?
• The first possibility are in situ measurements. That basically means that we send probes
straight into another atmosphere and directly measure physical and chemical properties.
For example, the Huygens probe was sent into Titan’s orbit. The problem is that some
atmospheres, e.g. Jupiter and Venus, are very inhospitable for probes or sonders, because
of high temperature, high pressure and generally low accessibility.
• Another possibility is to send probes into the orbit of a planet and apply spectral analyses.
Reflecting solar radiation planets radiate themselves, while different atmospheric gases
absorb radiation with different wavelengths. So every planet has its own unique spectrum.
If we compare this spectrum with the Sun’s, whose chemical composition is known from
other research, we can determine the chemical composition of the atmosphere.
4
4.1
4.1.1
Atmospheres of other planets
Inner planets
Mercury
Due to its very low mass (0.055 Earth masses) and its position as the innermost planet in
the solar system, Mercury is not able to hold a stable atmosphere. The velocity of most gas
particles is very high, so the planet’s gravitational force is not strong enough to bind them
to the planet. However, small traces of oxygen, hydrogen, helium and several other elements
can be found. hydrogen and helium are thought to be carried in by the Solar Wind, while the
present heavier elements are products of radioactive processes. Mercury’s surface temperature
is very high during the day but due to the lack of an atmosphere, the energy from the heat
radiation cannot be stored. This results in very low temperatures during the night.
4.1.2
Venus
Venus is sometimes referred as Earth’s sister. This is due to to similar mass (80% of Earth’s
mass) and semi major axis (0.723 AU). With that being said, their atmospheres show little
similarity, as Venus’ atmosphere is a lot denser and hotter. At the surface the average pressure
is 92 bar, while the temperature is about 735 K. The latter can be explained with the chemical
composition of the atmosphere, which is mainly made up by CO2 (96,5%). While the majority
of the incoming sunlight is reflected by an ever - present thick cloud layer, the atmosphere is
heated by the greenhouse effect. Venus has no intrinsic magnetic field, so the Solar Wind can
penetrate the atmosphere, which therefore loses some of its lighter composites (oxygen and
hydrogen).
4.1.3
Mars
Mars has a very thin atmosphere; the average surface pressure is just 0.006 bar. This is
due to the fact that Mars has quite a low mass (about 10% auf Earth’s mass) and lost its
magnetosphere about 4 billion years ago. The temperature differences between night and day
are very high, ranging from 186 to 268 K, as there are not many atmospheric particles that
are able to store energy from the sunlight. However, due to the large amount of CO2 (95%!)
in Mars’ atmosphere, a sligh greenhouse effect can be observed. Traces of water have been
detected, resulting in large cirrus clouds. The existence of methane in Mars’ atmosphere has
been claimed by several research groups, but is yet to be certified. However, several explanations
have been discussed regarding the origin of methane, with one of them being the existence of
simple organic life forms.
4.2
Gas giants
Figure 2: Jupiter’s atmosphere
4.2.1
Jupiter
Because of its huge mass (317.8 Earth masses), Jupiter is able to hold even the lightest of
elements. Its chemical composition is actually very similar to that of the Sun. The vast
majority parts of the atmosphere are hydrogen (90%) and helium(10%), while a lot of other
composites can be found in traces.
Just like Earth, Jupiter’s atmosphere can be divided into several layers concerning temperature
gradation. The lowest layer, the troposhere, merges directly into the planet’s fluid interior part.
Jupiter is covered by a fairly complexe cloud structure. There are three main cloud layers: the
ammonium ice layer (at a pressure level of 0.7 - 1.0 bar), the ammomium(hydrogen)sulfide layer
(1.5 - 3 bar) and underneath a layer of hydrogen clouds (3 - 7 bar) is thought to exist.
Two types of bands can be seen on Jupiter: bright zones and dark belts. The concentration of
ammonium in zones is generally higher, which leads to the brighter colour. The temperature
of troposphere’s upper parts is higher in zones than in belts.
Jupiter’s climate and atmospheric processes are fairly complexe and cannot discussed in detail
here. The Great Red Spot is the biggest known vortex in our solar system, as its diameter is
larger than Earth’s. Also to mention is the fact that the rotation of Jupiter’s atmosphere is
latitude - dependent.
4.2.2
Saturn
Saturn is similar to Jupiter in some ways. While its mass is lower (95 Earth masses), it is still
able to sustain severe amounts of hydrogen and helium. The surface temperature (134 K at
1 bar level) is lower than Jupiter’s, so more helium was able to condensate; thus leading to a
slightly lower helium concentration. The cloud structure is similar to Jupiter, as are violent
. Saturn has a magnetosphere with a
atmospheric storms with velocities as high as 1800 km
h
equitorial strength of 0.2µT , which deflects high-enery particles from the sun and therefore
protects Saturn’s atmosphere.
4.2.3
Uranus
Uranus, which happens to be the third gas giant, has a slightly less amount of hydrogen and
helium in its atmosphere. The main difference concerning the chemical composition is the high
methane proportion (2%). This leads to the blue-ish colour of Uranus, as methane is able
to absorb red light. The architecture of Uranus’ atmosphere is resembling to Saturn’s and
Jupiter’s, while the surface temperature is lower(53 K at the base of the troposphere) due to
its lower mass (14.5 Earth masses) and greater distance to the Sun.
4.2.4
Neptune
The last planet in the solar system is Neptune, that has a comparable mass (17 Earth masses).
The percentage of methane is slightly lower (1, 5%), but it is still enough to absorb sufficent
amounts of red light to give Neptune its characteristical blue look. Its atmosphere can be divided
into a troposphere and a stratosphere. Clouds consisting of ammonium, water, methane and
further composites are believed to be found in different parts of the atmosphere. For unknown
reason Neptune’s thermosphere is extremly hot (750 K), which is one of the causes for the
violent storms (up to 2000 km
).
h
4.3
4.3.1
Special Cases
Pluto
Not classed as a planet anymore, Pluto should still be mentioned here. Due to its very low
mass (0.2% of Earth’s mass) Pluto is not able to hold a stable atmosphere(surface pressure
2 · 10−6 bar), whereas traces of nitrogen, methane and other composites can be found. Because
of its outmost position the surface temperature remains extremely low (-230 K). As the melting
point of methane and several other composites is significantly higher, the atmosphere gradually
freezes when Pluto moves away from the Sun considering its high eccentricity.
4.3.2
Titan
Not exactly a planet either, but a very interesting nonetheless, Titan remains the only known
moon with a stable atmosphere. The surface pressure is around 1.6 bar with nitrogen (94%)
and argon/methane(6%) being the most distinct gases, which is quite interesting, because the
only other stable atmosphere dominated by nitrogen is Earth’s. Partially heated by tidal forces
between it and Saturn, the surface temperature is about 94 K.
5
Summary/Conclusion
The study of the different atmospheres in the solar system is very interesting and can be a good
indicator of the history of differents planets. What we have seen is, that there are at least 5
major factors that contribute to the quality and quantity of planet’s atmospheres.
• Mass: It determines the strength of the gravitational force of a planet.
• Radius: It also influences the gravitational force.
• Surface temperature: Planets with high surface temperature (f.e. those close to the Sun)
are potentially losing their atmospherical gases, as the velocity of these gases can be high
enough two escape from the planet.
• Molar mass of componentes: This is another important factor because it determines the
velocity of gases for a certain energy.
• Magnetosphere: As the magnetosphere can function as a shield against high - energy
particles from the Sun, it’s absence often leads to severe atmospheric losses.
For our solar system, we can define three main cases of planets
• Low mass planets: Planets like Mercury, which masses are too low to sustain an atmosphere.
• Gas Giants: Planets like Jupiter, which huge masses lead to a very dense atmosphere of
light gases.
• In-Betweens: Planets like Earth or Venus, which masses and surface temperature work
in their favour to keep a stable atmosphere.
References
[1] http://www.ontariosciencecentre.ca/
[2] www.nasa.gov
[3] www.astronomie.de
[4] www.wikipedia.com
[5] www.wikipedia.de