ASVAB Science Study Guide Part 1

Earth & Space Science

Part 1 of our ASVAB Science Study Guide covers earth and space science. This is a 4-part unit in our ASVAB Free Online Study Guide. The earth and space topics include the structure of the earth, types of rocks, the water cycle, the atmosphere, and the planets. There is a review quiz at the end to test your knowledge.

Structure of the Earth

The Earth is divided into three main layers:

  1. The crust is the rocky outer layer of the Earth. It is very thin compared to the other two layers.
  2. The mantle is a thick layer of hot, solid rock located under the crust.
  3. The core is a large sphere of metal that forms the center of the Earth.

Plate tectonics is the theory that pieces of the crust’s outermost layer, called plates, move slowly (about 1.4 inches per year). Two types of plates (oceanic plates and continental plates) “float” on top of denser material. As the plates move, the continents slowly drift. Depending on the direction of a plate’s movement and its relationship to bordering plates, several different boundaries may result:

  1. Divergent plate boundaries occur when oceanic plates move apart. Here, magma from the mantle rises to the earth’s surface and forms new rock. This results in seafloor spreading.
  2. Convergent plate boundaries form in areas where plates move toward each other and collide, creating mountains.
  3. Transform fault boundaries occur when plates move sideways past each other.

The 3 Basic Types of Rocks

  • Igneous rock forms when magma from inside the earth cools and solidifies. Examples include basalt, obsidian, and granite.
  • Sedimentary rock forms over time as sediment is layered, squeezed and solidified. Examples include sandstone, limestone, coal, and shale.
  • Metamorphic rock forms when a rock is transformed by heat, pressure or chemical reactions. Examples include slate, marble, and quartzite.

The Water Cycle

  • Water makes up 71% of Earth’s surface.
  • Most of the earth’s liquid fresh water is in the form of groundwater.
  • The continuous movement of water among the oceans, atmosphere and land is referred to as the water cycle. The water cycle is made up of several processes:
  1. Evaporation: Energy from the sun causes water on the Earth’s surface to convert from a liquid to a gas. Warm water vapor rises from the surface of the earth into the atmosphere. Water also evaporates from plants, in a process called transpiration.
  2. Condensation Water vapor rises into the sky and cools, turning back into a liquid, forming clouds.
  3. Precipitation:  This occurs when water particles fall from clouds and reach the ground. Includes rain, snow, and hail.

Water Facts:

  • Water freezes at 32° Fahrenheit and boils at 212° Fahrenheit.
  • Water covers about 71% of the earth’s surface in the oceans, lakes, rivers, and glaciers.
  • Salt water accounts for 97% percent of the water on Earth. Only 3 percent is fresh, and two-thirds of this freshwater is ice.

Earth’s Atmosphere

The Earth’s atmosphere is made up of a mixture of different gases, including nitrogen, oxygen, water vapor, argon, carbon dioxide, and small amounts of other gases. Composition of dry air:

  • 78% Nitrogen
  • 21% Oxygen
  • 1% Argon
  • 0.4% Carbon Dioxide

Layers of the Atmosphere: From lowest to highest, the five main layers are:

  • Troposphere: The part of the atmosphere we live in. Weather takes place in this layer.
  • Stratosphere: The ozone layer is found in the stratosphere.
  • Mesosphere: The mesosphere is where most meteors burn up.
  • Thermosphere: The International Space Station orbits in this layer.
  • Exosphere: The exosphere contains most of the satellites orbiting Earth.

The Planets

Here are the 8 planets listed in order of their distance from the Sun:

  • Mercury
  • Venus
  • Earth
  • Mars
  • Jupiter
  • Saturn
  • Uranus
  • Neptune

You can remember the order with the first letters of each word in this silly sentence:

MVery Educated Mother Just Served Us Nachos.”

Planet Facts:
The planets are divided into two groups. The inner planets are small, dense and rocky, and include Mercury, Venus, Earth and Mars. These four planets are also called the terrestrial planets since they have a solid planetary surfaces.

  • Mercury: The smallest of the terrestrial planets and the closest to the sun. It is dense, with a very large iron core, and geologically quiet. It takes on 88 Earth days for Mercury to complete one revolution around the sun, making it the fastest-moving planet.
  • Venus: The brightest object in our night sky, with the exception of the moon. Venus’s rotation is the in opposite direction than it revolves. The rate of its rotation is very slow. It takes Venus 243 days to rotate around its axis, which is longer than it takes to revolve around the sun. That makes its day longer than its year. Venus has a very thick atmosphere composed carbon dioxide and droplets of sulfuric acid.
  • Earth: Unique from the other planets because its surface has a suitable atmosphere and temperature for water to exist as a liquid, making Earth the only plant that can support life.
  • Mars: It is called the “red planet” because of the weathering of iron-rich rocks on its surface. Mars is the most Earth-like of all the planets. Evidence exists that Mars once had a great deal of liquid surface water.

Jupiter, Saturn, Uranus and Neptune form the outer planets. They are much colder, larger and more massive than the inner planets. Jupiter and Saturn are gas giants. Uranus and Neptune are ice giants.

  • Jupiter: The largest and most massive planet in our solar system, Jupiter has 79 moons, which is more than any of the other planets. It is composed mainly of hydrogen and helium.
  • Saturn: The second largest planet in our solar system, which is best known for its rings. It has 62 moons.
  • Uranus: Its most unusual characteristic is that its axis is tilted more than 90°.
  • Neptune: Has a bluish color from the methane in its atmosphere.


Comets are cosmic snowballs that are made up of frozen gases, rocks, and dust that orbit the Sun. They are sometimes referred to as “dirty snowballs.” When a comet’s orbit brings it close to the Sun it begins to warm up and release gases in a process called outgassing. This produces a giant glowing head that can be larger than many planets, and sometimes a tail which can stretch for millions of miles. These phenomena are the effects of solar radiation and solar wind acting upon the comet’s nucleus. The diameter of comet nuclei can range from as small as 100 yards all the way up to 20 miles or more. While there are thousands of known comets, there are thought to be billions more orbiting our Sun in the Kuiper Belt.

Now that you’ve read some of our lessons and tips for the Science section of the ASVAB, put your skills to practice with the review quiz below. Try not to reference the above information and treat the questions like a real test.

Part 1 Review Quiz:

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Question 1

What is the correct order of the Earth's layers, from outermost to innermost?

Core, mantle, crust
Mantle, crust, core
Crust, mantle, core
Crust, core, mantle
Question 1 Explanation: 
The Earth's outermost layer is the crust; the middle layer is the mantle; and the innermost layer is the core.
Question 2

Two tectonic plates move sideways past each other. What type of boundary does this create?

Convergent plate boundary
Transform fault boundary
Divergent plate boundary
Oceanic-continental plate boundary
Question 2 Explanation: 
There are three main boundaries that can result from the movement of tectonic plates. Transform fault boundaries are formed when plates slide sideways past each other.
Question 3

Marble forms when limestone experiences great heat and pressure. Marble is a type of _____ rock.

Question 3 Explanation: 
Metamorphic rocks are created by heat, pressure, or chemical reactions. Marble is an example of metamorphic rock.
Question 4

The continuous movement of water among the oceans, atmosphere and land is called the:

life cycle
transpiration process
evaporation process
water cycle
Question 4 Explanation: 
The continuous movement of water among the oceans, atmosphere and land is called the water cycle.
Question 5

What gas makes up the majority of dry air?

Carbon dioxide
Question 5 Explanation: 
78% of dry air is made up of nitrogen, which is more than any other gas.
Question 6

The lowest layer of the atmosphere is the:

Question 6 Explanation: 
The lowest layer of the atmosphere, the one we live in, is the troposphere.
Question 7

The correct order of the atmospheric layers, from lowest to highest, is:

Troposphere, thermosphere, mesosphere, stratosphere, exosphere
Troposphere, stratosphere, mesosphere, thermosphere, exosphere
Thermosphere, mesosphere, troposphere, stratosphere, exosphere
Mesosphere, exosphere, stratosphere, troposphere, thermosphere
Question 7 Explanation: 
The layers of the atmosphere, from lowest to highest, are troposphere (what we live in), stratosphere (where the ozone layer is), mesosphere (where most meteors burn up), thermosphere (where the international space station orbits), and exosphere (where most satellites orbit earth).
Question 8

What planet is closest to the sun?

Question 8 Explanation: 
Mercury is the closest planet to the sun, followed by Venus, Earth, and Mars.
Question 9

Which of the following make up the inner planets?

Earth, Mercury, Mars, Jupiter
Jupiter, Saturn, Uranus, Neptune
Uranus, Mercury, Earth, Neptune
Mars, Earth, Venus, Mercury
Question 9 Explanation: 
The inner planets are the small, dense, rocky planets closest to the sun. Those are Mercury, Venus, Earth, and Mars.
Question 10

What planets are called ice giants?

Uranus and Neptune
Jupiter and Neptune
Jupiter, Saturn, Uranus, and Neptune
Jupiter, Saturn, and Uranus
Question 10 Explanation: 
Neptune and Uranus are the only planets called ice giants.
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ASVAB Science Study Guide Part 2 >>