Investigate the small planets in our Solar System. Find out which ones have an a

Question

Investigate the small planets in our Solar System.

Find out which ones have an atmosphere and which ones don’t. For those that have atmosphere, find out what is the composition and density of this atmosphere, what is the size of the planet, and its average surface temperature.

Please fill the table below with this information.

Planet

Atmosphere?

(Y or N)

Atmospheric

Composition

Pressure

(bar)

Radius

(km)

Temperature

(oC)

Mercury

Y

Hydrogen, Helium, Oxygen

10-14

2,440

167

Venus

Y

Carbon Dioxide, Sulfuric Acid

91

6,051

462

Earth

Y

Nitrogen, Oxygen

1

6,370

15

Mars

Y

Carbon Dioxide, Nitrogen

0.006

3,390

-55

**I already filled out the table above. Please let me know if anything is incorrect.

1)Explain in your own words why these planets have little or no atmosphere. Can you show an equation that explains why this has happened?

The Solar Nebula model of the formation of the Solar System shows that the most abundant gases which were present in this original nebula were Hydrogen and Helium. These are the gases that make up our Sun and the giant planets, Jupiter, Saturn, Uranus and Neptune.

2) Why then the small planets that retained an atmosphere have only relatively heavy gases in the atmosphere, such as Nitrogen, Oxygen and Carbon Dioxide, but no Hydrogen and Helium?

Planet

Atmosphere?

(Y or N)

Atmospheric

Composition

Pressure

(bar)

Radius

(km)

Temperature

(oC)

Mercury

Y

Hydrogen, Helium, Oxygen

10-14

2,440

167

Venus

Y

Carbon Dioxide, Sulfuric Acid

91

6,051

462

Earth

Y

Nitrogen, Oxygen

1

6,370

15

Mars

Y

Carbon Dioxide, Nitrogen

0.006

3,390

-55

Explanation / Answer

(1) & (2)

A planet's atmosphere acts as a shield of planet's surface from harsh radiation from the Sun and also it limit the amount of energy lost to space from the planet's interior. An atmosphere also helps to retain liquid on a planet's surface by providing the pressure desirable to hold the liquid from boiling away to space. All of the planets began with atmospheres of hydrogen and helium. The inner four planets-Mercury, Venus, Earth, and Mars, lost their original atmospheres. And their atmospheres are from gases released from their interiors, but Mercury and Mars also still lost most of their secondary atmospheres. On the other hand, the outer four planets (Jupiter, Saturn, Uranus, and Neptune) were able to keep their original atmospheres.

There are two main factors that determine how thick a planet's atmosphere will be; they are the planet's escape velocity and the temperature of the atmosphere.

The thickness of a planet's atmosphere chiefly depends on the planet's gravity and the temperature of the atmosphere. A planet with weaker gravity does not have strength to hold the molecules that make up its atmosphere. So the gas molecules will be more likely to escape from the planet's gravity. If the atmosphere is cool enough, then the gas molecules will not be moving fast enough to escape the planet's gravity.

Escape Velocity

The escape velocity is the primary velocity needed to escape from a massive body's gravitational influence. The escape velocity= Sqrt[(2G × (planet’s mass))/distance)]. The distance is measured from the planet’s center.

So the escape velocity increases as the mass increases. A more massive planet will have stronger gravity and, therefore, a higher escape velocity. Also the escape velocity decreases as the distance increases. The escape velocity is lower at greater heights above the planet's surface. The planet's gravity has a weaker for the molecules at the top of the atmosphere than those close to the surface, so the molecules at the top part of the atmosphere will be the first to escape away.

Temperature

The temperature of a material is a measure of the average kinetic energy of the molecules in that material. So as the temperature increases, a solid turns into a gas because the particles are moving fast enough to split the chemical bonds which held them together. The particles in a hotter gas are moving faster than those in a cooler gas of the same type. Using Newton's laws of motion, the relation between the speeds of the molecules and their temperature is,

Temperature = (mass of gas molecule)×(average speed of gas molecule)2 / (3k), where k is a universal constant of nature called the `Boltzmann constant'.

Gas molecules of the same type and at the same temperature will have different speeds, some moving rapidly but some moving slower.

From this equation we can derive the average gas molecule velocity,

Average gas molecule velocity = Sqrt[(3k × temperature/(molecule mass))].

Using this equation, we can found that the more massive gas molecules will move slower on average than the lighter gas molecules. So for example, carbon dioxide molecules move slower on average than hydrogen molecules at the same temperature. Because massive gas molecules move slower, planets with weaker gravity that is the terrestrial planets will have atmospheres of massive molecules like Carbon dioxide, Nitrogen and oxygen. The lighter molecules like hydrogen and helium will have escaped from the atmosphere of these planets.

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