Exploring Our Solar System

Exploring Our Solar System

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Life in the Solar System

What do you think? Read the two statements below and decide

whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you've read this lesson, reread the statements to see if you have changed your mind.

Before

Statement

After

3. Earth's atmosphere protects life on Earth from dangerous solar radiation.

4. Scientists think conditions for life might exist on some moons in the solar system.

Key Concepts

? What conditions on Earth enable life to exist?

? What conditions on other bodies in the solar system might enable life to exist?

? Where might life possibly exist beyond Earth?

Conditions for Life on Earth

Life exists in nearly every environment on Earth. Some environments have conditions so extreme that humans cannot live in them. These places might have extreme temperatures, high salt levels, total darkness, or little water. Even though humans cannot live in these places, other organisms can.

Even though some organisms live in extreme conditions, all of Earth's life-forms need the same basic things to survive: a source of energy, liquid water, and nourishment. Scientists have not yet discovered life anywhere else in the solar system. But by studying the conditions that support life on Earth, they are learning about conditions that might support life elsewhere. Astrobiology is the study of the origin, development, distribution, and future of life in the universe.

Energy from the Sun

The Sun is the source of almost all energy on Earth. Sunlight provides light and thermal energy. It also provides energy for plants, which are at the base of most food chains. However, a small percentage of organisms on Earth receive energy from chemicals or from Earth itself. For example, a variety of animals live in complete darkness near hot water jets in the ocean floor.

Reading Essentials

3TUDY#OACH

Make Flash Cards For each head in this lesson, write a question on one side of a flash card and the answer on the other side. Quiz yourself until you know all of the answers.

Key Concept Check 1. State What do organisms on Earth need to survive?

Exploring Our Solar System 385

Make a three-tab Venn book to compare and contrast the ability of Earth and the Moon to sustain life.

Earth Both Moon

Reading Check 2. Compare How is Earth's atmosphere like a blanket?

Reading Check 3. Identify What kinds of radiation are absorbed by Earth's atmosphere?

Protection by the Atmosphere

Earth's moon receives about the same amount of sunlight as Earth. Yet conditions on the Moon are more extreme than they are on Earth. The Moon's surface temperature can rise to 100?C during the day and drop to -150?C at night.

Why do temperatures vary so widely on the Moon? Temperatures are extreme on the Moon because the Moon, unlike Earth, does not have an atmosphere.

Maintains Temperatures Earth's atmosphere is like a blanket around Earth. It absorbs sunlight during the day and keeps thermal energy from escaping into space during the night. It maintains Earth's average surface temperature at a comfortable 14?C.

Absorbs Harmful Radiation Have you ever had a painful sunburn? The Sun's ultraviolet light causes sunburns. Even though you cannot see ultraviolet light, you can feel its effects. Too much radiation in the form of ultraviolet light can harm you. Fortunately, Earth's atmosphere absorbs most of the Sun's ultraviolet light.

In addition to ultraviolet light, Earth's atmosphere absorbs X-rays and other potentially harmful light waves from the Sun. The atmosphere also helps protect Earth from highly charged particles that erupt from the Sun in powerful storms.

Burns Up Meteoroids Earth's atmosphere also protects Earth's surface from meteoroids. Millions of meteoroids enter Earth's atmosphere every day. But almost all of them burn up in the atmosphere before they reach Earth's surface.

Liquid Water

Liquid water is necessary for all life on Earth. Water dissolves minerals and transports molecules in cells. Without liquid water, cells could not function and life would not exist. Water exists as a liquid on Earth's surface because the atmosphere keeps pressures and temperatures within a certain range.

At Sea Level Depending on temperature and pressure on Earth, water is solid, liquid, or gas, as shown in the figure on the next page. At sea level on Earth (1 atm of pressure), water is liquid between 0?C and 100?C. Above 100?C, water boils and becomes water vapor. Below 0?C, water freezes into ice.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

386 Exploring Our Solar System

Reading Essentials

Water as Solid, Liquid, and Gas

Pressure

Liquid

1 atm

Gas Solid

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

0? C

100? C

Temperature

At Other Altitudes At different altitudes on Earth, such as on the top of a mountain, the boiling and freezing temperatures of water change slightly because the pressure of the atmosphere changes. Without Earth's atmosphere, pressures on Earth's surface would be too low for water to be liquid. Water would exist only as water vapor or ice.

Nourishment

Living things are nourished by nutrients they take from the air, water, and land around them. They use the nutrients for energy, growth, and other processes, such as reproduction and cellular repair. All molecules that provide nourishment for life on Earth contain carbon. They are organic molecules. Organic refers to a class of chemical compounds in living organisms that are based on carbon. Though inorganic life could exist elsewhere, astrobiologists are most interested in places where water is liquid and carbon is plentiful.

Looking for Life Elsewhere

In 1835, a New York newspaper published articles claiming that herds of bison and furry, winged bat-men had been observed on Earth's moon. The articles fooled many people. Today, people know the Moon is airless, and scientists have yet to find life there.

Because liquid water is essential for life on Earth, scientists look for places in our solar system where liquid water might exist or might have existed in the past. In 2009, scientists discovered water on the Moon. Although water might not exist on the surface of a planet or a moon, it might exist beneath the surface.

Visual Check 4. Observe What happens to water when temperatures are high?

Key Concept Check 5. Recognize What would happen to water on Earth's surface if Earth had no atmosphere?

Reading Essentials

Exploring Our Solar System 387

Math Skills

The mean of a set of data is the arithmetic average. To find the mean, add the numbers in the data set and then divide by the number of items in the set. For example, during one Martian day, surface temperatures were measured at -51.3?C, -31.9?C, -0.800?C, -0.200?C, and -17.6?C. What was the mean temperature during that day?

a. Find the sum of all the

values.

-51.3?C + -31.9?C +

-0.80?C + -0.20?C +

-17.6?C = -101.8?C

(Round to 102 with significant figures.)

b. Divide by the number of

temperatures in the set.

-__1_0_2_?C_ 5

=

-20.4?C

6. Finding the Mean

The temperature at the Martian polar ice caps can drop to -143?C. The warmest spots on the planet can reach 20?C. What is the mean of these extreme temperatures?

Key Concept Check

7. Summarize Why do

scientists think liquid water once might have existed on Mars?

Mars

Scientific evidence suggests that Mars is the planet, other than Earth, that is most likely to have liquid water. On the surface of Mars, pressures are probably too low for water to be liquid. Water would likely evaporate quickly in the thin, dry atmosphere. Temperatures are also low. They generally range from -87?C to -5?C, though they can reach a high of 20?C during the Martian summer.

Scientists have sent many uncrewed spacecraft to Mars, but none has detected liquid water. However, there is much evidence that water vapor and ice exist on the Martian surface. Photographs show surface features on Mars that appear to have been carved by moving water. These photos show channels on the Martian surface. Scientists hypothesize that these Martian channels could be ancient streambeds. It is possible that water from an underground ocean seeped to the surface and flowed as rivers or floods before evaporating. Scientists still do not know how much water was in these channels or how long ago it flowed.

Other Planets

Mercury and Venus are too hot for water to be liquid on or near their surfaces. The four outer planets are too cold. The outer planets also are too gaseous. They have no solid surfaces on which liquid water could form. Though some liquid water might exist deep in the interiors of the outer planets, it is unlikely that the water could support life.

Natural Satellites

Scientists continue to look for further evidence of water on Earth's moon and on the moons of other planets. Even though temperatures in the outer solar system are extremely cold, scientists have observed that as a satellite orbits a massive planet, the planet's gravity can cause the satellite's interior to heat. This might provide enough thermal energy to allow liquid water to exist near the satellite's icy surfaces.

Surface Features Several moons in the outer solar system have surface features that indicate the presence of liquid water not far below. For example, scientists suggest that the ridges on Europa (yuh ROH puh), one of Jupiter's moons, could be cracks in the ice where liquid water has seeped to the surface and frozen solid. Callisto and Ganymede, two other moons of Jupiter, and Titan, a moon of Saturn, show similar surface features.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

388 Exploring Our Solar System

Reading Essentials

Geysers Several other moons in the outer solar system, including Enceladus (en SEL uh dus), a moon of Saturn, and Triton, a moon of Neptune, show evidence of geysers (GI zurz). A geyser is a warm spring that sometimes ejects a jet of liquid water or water vapor into the air. The massive geysers on Enceladus are hundreds of kilometers high. Two other moons of Saturn, Tethys (TEE thus) and Dione (di OH nee), also have geyserlike plumes. These geysers are evidence that liquid water might exist beneath the icy surfaces of these moons.

Key Concept Check

8. Identify Where might

life exist in the solar system beyond Earth?

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Reading Essentials

Exploring Our Solar System 389

Mini Glossary

astrobiology: the study of the origin, development, distribution, and future of life in the universe

geyser (GI zur): a warm spring that sometimes ejects a jet of liquid water or water vapor into the air

organic: a class of chemical compounds in living organisms that are based on carbon

1. Review the terms and their definitions in the Mini Glossary. Write a sentence explaining what an organic compound is.

2. Use the graphic organizer to summarize how Earth's atmosphere helps sustain life on Earth.

Earth's Atmosphere

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

3. Select and define a word from one of the flash cards you created as you read the lesson.

What do you think

Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind?

390 Exploring Our Solar System

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