Lesson 3 of 6 · Water, geology & evidence
Earth and Mars: reading a world's history
8 minute lesson · 3 checks along the way
Your goal for this lessonUse landscapes and measurements to distinguish evidence for past water from evidence for life.
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Earth is a planet worth comparing
We live inside Earth's planetary environment, so it is easy to overlook how unusual it is. Liquid oceans cover most of its surface. An , moving water, ice, living things, and geological activity interact with the ground. Earth is the only world where life has been confirmed, making it both our home and our best-studied example.
Mars is a smaller rocky neighbor with a thin atmosphere, largely carbon dioxide, and a cold, dry surface today. Yet its landscapes include volcanoes, impact craters, valleys, and sedimentary rocks. Shared landforms make comparison useful. Different present-day conditions remind us that an analogy with Earth must be tested rather than assumed.
Earth on July 6, 2015, observed by DSCOVR’s EPIC camera. The color view combines red, green, and blue measurements. Oceans, land, and clouds appear together in one planetary view.
Look closely
Find ocean, land, and clouds. What can this single picture show about Earth—and what would require observations over time?
See what to notice
The image shows large oceans, continents, and cloud systems on Earth’s sunlit side. One moment cannot establish long-term climate, track a storm’s movement, or measure how the oceans are changing. Those questions need repeated measurements.
A landscape can preserve an earlier environment
On Earth, moving water can carry sediment, cut channels, and deposit material where a river enters a lake. Scientists recognize comparable branching valleys and delta-shaped deposits in images of Mars. Rover and orbiter measurements also identify minerals that form or change in the presence of water. These different clues support a history that includes liquid water.
Notice the chain of reasoning. A camera records shapes and layers; an instrument measures a mineral signature; scientists interpret those observations using physical processes. ‘There are layered deposits here’ and ‘a lake once occupied this place’ are related statements, but they are not the same statement. The second explains the evidence and can be checked against other measurements.
Mars assembled from Viking Orbiter observations. The source team adjusted brightness and color, filled observational gaps, and synthesized a green channel; this global mosaic is a scientific reconstruction from measurements, not one camera exposure.
Look closely
Find the bright polar cap near the top and the long canyon below the center. Why is visible ice not the same evidence as an ocean of liquid water?
See what to notice
The bright polar region and the canyon are visible surface features. Ice is frozen material; the image does not show a present-day surface ocean. Identifying water, its state, and the history of a landscape requires more than color alone.
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Habitability is a question about conditions
A habitable setting has conditions that could support some form of life as we understand it. Water, suitable chemistry, and usable energy matter, but finding one ingredient does not demonstrate that organisms existed. A dried lakebed can help identify a promising place to investigate without answering the life question by itself.
The same caution works in the other direction. A cold surface today does not erase evidence of a different past or settle every question about the subsurface. Ask when and where a measurement applies. ‘Ancient water here,’ ‘ice here today,’ and ‘living organisms here’ require different evidence and should never be substituted for one another.
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Work through a decision
Work the investigation: choose the next measurement
Imagine an orbiter has photographed a fan-shaped deposit at the end of a channel. One explanation is that water carried sediment into a standing body of water. To test it, you might study the deposit's layers, grain sizes, and mineral composition. You would also ask whether another process could produce a similar shape.
This investigation connects geology with mission design. A wide view helps locate a feature; close-up measurements help explain how it formed. Neither view replaces the other. As you continue to giant planets and icy moons, keep asking how multiple kinds of evidence can strengthen an explanation while leaving other questions open.
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Put it together
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