How Thick Is the Sky? Atmospheres and Oceans to Scale

Compare the air on six worlds and build Earth's air and oceans to scale.

Part 1: Air on six worlds

Read the passage and Table 1, then answer questions 1 to 6.

Weather on Other Worlds

1Last summer, the farms around Dunmore Crossing went almost two months without rain. Eighth grader Lucía Brennan watched the soybean field behind her house turn brown while her family checked the forecast every morning. In August, the town library hosted a talk called "Weather on Other Worlds" by Dr. Femi Adeyemi-Clarke, an atmospheric scientist who studies the air of planets and moons.

2Dr. Adeyemi-Clarke began with surface pressure, the push of the air pressing down on the ground. Scientists often compare other worlds with Earth, so Earth's surface pressure counts as 1. The pressure depends on how much air is piled up above the ground: the more air overhead, the harder it presses. On a world with a surface pressure of 92, the air presses 92 times as hard as it does on Earth. Some worlds have almost no air at all, so their surface pressure is almost 0.

3Titan, the largest moon of Saturn, has clouds, rain and lakes, but its rain is liquid methane, a chemical that is a gas on Earth and a liquid in Titan's deep cold. Titan is the only world besides Earth known to have lakes of liquid on its surface. Venus is wrapped in thick clouds, and Mars has huge dust storms and ice caps at its poles but no lakes on its surface today. Mercury, the planet closest to the Sun, and Earth's Moon have almost no air, so the sky above them stays black even in the daytime.

4Back on Earth, the air thins out as you go higher. About half of the air's mass lies in the lowest 5.5 km, and about 99% lies below 30 km. Most weather, including rain clouds, happens in the lowest 12 km. The oceans cover about 70% of Earth's surface and are about 3.7 km deep on average, and the deepest spot is about 11 km down. The top of Mount Everest is about 8.8 km above sea level.

5At the end of the talk, Dr. Adeyemi-Clarke suggested that the library build a scale globe of Earth for its science corner. On the globe, 1 mm would stand for 10 km. Lucía and her friends offered to work out the sizes.

Table 1: Six Worlds and Their Air
World	Width (km)	Pressure (Earth = 1)	Main gas in the air
Mercury (planet)	4,879	Almost 0	—
Venus (planet)	12,104	92	Carbon dioxide
Earth (planet)	12,742	1	Nitrogen
Mars (planet)	6,779	0.006	Carbon dioxide
Titan (moon of Saturn)	5,150	1.5	Nitrogen
The Moon	3,475	Almost 0	—

Use Table 1. Sort each world by its surface pressure.

VenusMarsTitanMercuryThe Moon

Greater than Earth's	Less than Earth's

Use Table 1. Round to the nearest whole number.

The surface pressure on Venus is about times the surface pressure on Titan.

Use Table 1. Write the answer as a percent. Do not round.

The surface pressure on Mars is % of the surface pressure on Earth.

Which statement about the sizes of these worlds does Table 1 support?

A. The Moon is about as wide as Saturn's moon Titan.
B. Titan, a moon, is wider than the planet Mercury.
C. Mars is wider than Venus, but its air is much thinner.
D. Earth is about twice as wide as its neighbor Venus.

A classmate claims, "The wider a world is, the more its air presses on the ground." Which pair of worlds in Table 1 gives the best evidence against this claim?

A. Venus and Earth, since Venus is a bit narrower but has 92 times the pressure
B. Mercury and the Moon, since both are narrow and have almost no air
C. Earth and Mars, since Earth is wider and has about 170 times the pressure
D. Venus and Mars, since Venus is wider and has much higher pressure

Use Table 1. Round to the nearest tenth.

Earth is about times as wide as Mars.

Part 2: Earth's air and oceans on the scale globe

Use paragraphs 4 and 5 and Table 1. On the globe, 1 mm stands for 10 km. Answer questions 7 to 11.

Use Table 1. Round each answer to the nearest whole millimeter.

The Earth globe would be mm wide. A Mars globe on the same scale would be mm wide.

Use paragraph 4. Do not round.

On the globe, the layer holding 99% of the air's mass would be mm thick, and the oceans would be mm deep on average.

Use paragraph 4. Circle each feature that would be less than 1 mm high, deep or thick on the globe.

Mount Everest's height above sea levelThe deepest spot in the oceanThe average depth of the oceansThe layer where most weather happensThe lowest layer, holding half of the air's massThe layer holding 99% of the air's mass

10.

What does the scale globe show best?

A. Earth's air reaches about as far out from the ground as the whole Earth is wide.
B. The oceans are on average deeper than the layer holding 99% of the air.
C. Mount Everest pokes out above the layer holding 99% of the air's mass.
D. Earth's air and oceans are very thin layers compared with the whole planet.

11.

Looking at the dry fields, Lucía's grandfather says, "There's so much sky above us. The air and the oceans must make up a big part of the whole planet." Make a claim about this idea. Use at least two numbers from your scale globe answers and Table 1 as evidence, and explain your reasoning.