There are all kinds of rocks and minerals that make up our planet – as well as our Moon and other rocky worlds. By studying them, we can learn about the conditions that allowed them to form.

In this activity, you’ll learn how NASA studies rocks on Earth and other planets. Then, play the role of NASA scientist to match images of "rocks" made of candy bars with their correct descriptions.

Curiosity Rover tests its drill on Mars

1. Learn why we study geology on Earth and other planets

Geologists are scientists who study a planet's solid features, like soil, rocks, and minerals. There are all kinds of rocks and minerals that make up our planet – as well as the Moon, Mars, and other rocky worlds. By studying these features, we can learn more about how rocky worlds form and change over time.


About the image: NASA's Curiosity rover tests its drill on Mars in August 2014. Image credit: NASA/JPL-Caltech 

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Mars outcropping compared with one on Earth

2. Learn how we study geology on Earth and other planets

On Earth, geologists have the luxury of making observations and collecting rock samples in person. To study rocks and soil on other planets, like Mars, we have to rely on spacecraft that can use their cameras and tools to act as our eyes and hands.

For example, Mars rovers like Curiosity and Perseverance use cameras to send detailed pictures of the Martian surface back to us on Earth so we can explore from a distance. We then compare the images to known substances on Earth to better understand and theorize about how the Martian rocks, soil, and minerals formed.


About the image: The image above, captured by NASA's Curiosity rover on Mars, shows an outcropping – an area where ancient bedrock underneath the surface is exposed. Outcroppings on Earth are known to form as a result of running water and create gravel that's a similar size and shape. By comparing this image of Mars to similar features on Earth, geologists have been able to theorize that Mars might have also once had running water.

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3. Practice describing rock samples

To classify rocks, we make observations that give us clues about how they formed. Here is some of the vocabulary geologists use to describe and classify rocks.

Geology Vocabulary

obsidian sample

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LusterLuster describes the sheen, glossiness, or reflectiveness of a surface, such as in this sample of obsidian.
meteorite sample

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CleavageCleavage refers to the tendency to break in flat surfaces, usually in one, two, three, or four directions. Cleavage can be seen in this image of lower Mount Sharp on Mars. 
basalt sample

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Vesicles/VoidsVesicles and voids are found when pockets of gas expanded within the rock during formation. These are typical in volcanic rocks, like this sample of basalt.
andesite sample

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InclusionsInclusions are impurities found within the structure of a rock from when the rock was first formed. These impurities range from solids to condensed pockets of gases. Inclusions are often found in meteorites, such as chondrites, and can be seen in this sample of andesite.
granite sample

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BlebsWhile inclusions can take many forms, blebs aren’t just chunks of impurities – they are minerals growing within minerals, like in this sample of granite. Common examples are marble or diorite.
basalt sample

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granite sample

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Homogeneous/ HeterogeneousHomogeneous refers to the level of evenness throughout the structure, such as in this sample of basalt (top). In contrast, a structure that is not very even would be called heterogeneous, like this sample of granite (bottom).
sandstone on Mars

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FriableWhen a sample is very easily crushed, it is referred to as friable. This sandstone on Mars suggests that it could be easily broken into smaller particles.
Cross sections of candy bar "rock samples"

4. Take the Edible Rocks Quiz

A great way to practice classifying rocks is by describing the structures found inside different candy bars. Like rocks and soil, candy bars have features that offer clues about what they're made of and how they formed. Unlike with rocks, we can easily see inside candy bars by cutting them in half, creating what's called a cross section.

Take the quiz below to see if you can match these images of candy-bar cross-sections to the correct description. Click the View accuracy button after you submit your answers to see how you did.


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Rock samples

5. Challenge yourself with the Real Rocks Quiz

Take the quiz below to see if you can match these images of real Earth and space rocks to the correct description. Click the View accuracy button after you submit your answers to see how you did.


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