Pilot yearFree, grades 9–12
The Inquiry Atlas
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Student handout

The Same Shape on Two Planets

What can a shape in the sand tell us about a force we cannot see — and what happens when we loosen the ground it comes from?

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The photographs

  1. A field of pale, near-white sand fills the whole frame, covered in fine parallel ripples a few centimeters apart running in gentle S-curves from the top of the frame to the bottom. Slightly below center sits a single clump of fine grass, its blades fanning out to the right in tan, olive and pale green, with seed heads. Faint curved arcs are scratched into the sand to the left of the clump. Just below it a low plant with gray-green oval leaves carries several clusters of small bright purple flowers, casting sharp shadows to the left. The light is low and comes from the right.
    Photograph 1
  2. The inside of a narrow, deep canyon in orange and red sandstone, photographed looking upward. The walls curve and fold around the frame in smooth, rounded, layered surfaces, banded light and dark along the bedding, with no sharp edges. At the top center a narrow gap opens to a bright white sky. A single beam of light angles down from that gap through the middle of the frame, and fine particles are visible falling and drifting within it. The lower left of the frame is hazy with suspended dust; the right-hand walls are lit warm orange.
    Photograph 2
  3. A patch of fine red-orange desert sand seen from above, scattered with pebbles and small stones of many colors — gray, blue-gray, pink, tan, black — from a few millimetres to a few centimetres across, most of them rounded. Slightly above center lies a single larger stone about ten centimeters long: a conglomerate of pebbles set in a matrix, with one broad, smooth, flat pale gray face occupying most of its upper surface and a sharp boundary where that face ends. It casts a long shadow to the lower left. In the top right corner is a small clump of dry pale grass.
    Photograph 3
  4. An overhead view of a surface covered in dark, elongated dune forms arranged in roughly parallel rows running from the upper left to the lower right, each dune rounded at one end and tapering at the other. Between and around them the ground is pale gray and pinkish-tan with a lumpy, blotchy, quilted texture. Fine ripple lines are visible on the smooth tan surfaces between the dunes. There is no sky, no horizon and no object of known size anywhere in the frame.
    Photograph 4
  5. A wide panorama. The lower third is flat grassland and a line of green cottonwood trees. Behind them rises a field of smooth tan sand dunes, layered one behind another, their crests running in long sinuous lines and their flanks in light and shadow; two or three tiny dark specks on one crest at the left appear to be people. Behind the dunes a steep mountain range rises abruptly, dark green with conifer forest on its lower slopes and bare gray rock with a dusting of snow on the summits. A broad blue sky with streaked and puffy white cloud fills the top half.
    Photograph 5

01. Setting the stage

First guess. Look at photograph 1. Something made this pattern. What?

And a second one. There are faint curved lines scratched in the sand beside the clump of grass. What drew them?

Do not erase either of these. We are coming back to them.

02. Engaging with the story

For each one, write down one force that could have shaped it, and one piece of evidence in the photograph that supports your answer.

03. Working the evidence

Observe (5 minutes). Record only what is visible. No explanations yet.

  • What do you notice first?
  • How far apart are the ripples? Are they the same everywhere in the frame?
  • Which way do the ripple crests run? Are they straight?
  • What materials, textures and colors can you see? List everything.
  • Look again for a full silent minute. What did you miss?

Reflect (6 minutes). Now generate and test explanations.

  • Which direction was the wind blowing, and how do you know?
  • The ripples change near the grass clump. What is the grass doing to the wind?
  • What drew the curved lines? What would you need to see to be sure?
  • Compare photograph 1 with photograph 5. Is a dune just a very large ripple?
  • Photograph 2 shows a narrow canyon with smooth, curved walls. Two different forces could have shaped it. Which do you think it was, and what in the photograph makes you think so?
  • What is missing from these images?

Question (4 minutes).

  • What do you wonder about: who, what, when, where, why, how?
  • Standing there with any instrument you wanted, what would you measure first?

04. Delving deeper

Read all three tables before you answer anything.

Table 1. Three ways wind moves material
ModeWhat movesHow it movesHow far it goes
SuspensionParticles less than 0.2 millimetres acrossHeld up by the air and scattered aloft as dust or hazeIndefinitely — hundreds of miles
SaltationSand-size grainsA series of jumps or skips, lifted no more than one centimeter above the ground, at one-half to one-third the wind speedShort hops; stays local
Surface creepGrains too heavy to hopPushed along by other grains landing on themSlow — but as much as 25 percent of all grain movement in a desert

Source: U.S. Geological Survey, Deserts: Geology and Resources.

Table 2. Two planets at the surface
MeasureMarsEarth
Surface pressure6.36 mb1014 mb
Surface densityAbout 0.016 kg/m³1.217 kg/m³
Surface gravity3.73 m/s²9.82 m/s²
Main gas in the airCarbon dioxide, 95.1 percentNitrogen, 78.08 percent
Are the dunes moving today?Yes — dunes as thick as 61 meters have been measured moving as whole unitsYes

Sources: NASA/NSSDC planetary fact sheets; NASA/JPL, 2012.

Table 3. What dust does after it leaves
MeasureValue
Where it comes fromDry soils — more of it once the crust holding the surface together is broken
Where it landsDownwind, including on mountain snow hundreds of miles away
What it does thereMakes the snow darker, so it absorbs more sunlight and melts earlier
Measured effect on peak runoffOn average 3 weeks earlier at Lees Ferry, Arizona, under heavier dust
Measured effect on the year’s total runoffReduced by more than 1.0 billion cubic meters, about 5 percent of the annual average
When it startedA fivefold increase in dust was already under way by the late 1800s

Source: Painter et al., PNAS, 2010.

05. Reflecting and projecting

Part A. Read the evidence. Photograph 4 shows a landform on Mars that looks like landforms on Earth. Based on the photographs alone, what can you conclude about the process that made it — and what can you not conclude about Mars?

Part B. Make the call. A land management agency proposes closing a stretch of dryland to vehicle use. The soil there is a known dust source; the dust settles on mountain snow in another state and moves that river’s peak runoff earlier. Local people use the area for recreation and access, and the benefit falls hundreds of miles away on people who will never know. Should the closure happen?

  • Your claim. State your position in one or two sentences.
  • Your evidence. At least two specific observations from the photographs, plus one item from Tables 1, 2 or 3. Name the source of each.
  • Your reasoning. Explain the scientific and practical principles that connect your evidence to your claim.

Students may submit as a written paragraph, a recorded verbal explanation, an annotated photograph, or a labeled diagram.

06. Take action

Choose one.

  1. Your own wind. What is the prevailing wind direction where you live? Find a surface near your school that it has shaped — a snowdrift, a sand pile, a drift of leaves against a fence. Photograph it and say which way the wind was blowing.
  2. Mars, this year. Find a pair of HiRISE images of the same Martian dune field taken a Mars year apart. How much moved, and how do you know it is not a difference in lighting?
  3. Where the dust comes from. Find a dust-source region in the western United States. What changed there, when, and who ended up paying for it?

07. The rebuttal

Your rebuttal. Someone reaches the opposite conclusion from the same evidence. State their position as fairly as you can, then explain why the evidence favours yours. If you think the evidence does not settle it, say so and explain what would.

Discussion questions

  1. In photograph 1 the ripples are a few centimeters apart. In photograph 5 the ridges are hundreds of meters apart. Are these the same thing at two sizes?
  2. Photograph 1 shows arcs scratched in the sand beside a clump of grass. What could have drawn them? What would you need to observe to be sure?
  3. Photograph 3 shows one stone with a smooth, flat face among rounded pebbles. What could cut a flat face on a stone lying on the ground? What else could explain it?
  4. Photograph 2 shows a narrow canyon with smooth curved walls. Two agents could have shaped it. What evidence would tell them apart?
  5. Photograph 4 was taken on Mars, where the air at the surface is about one percent as dense as Earth’s. What does it mean that the dunes there look like dunes here?
  6. Open question. Dust blown from disturbed dryland soils darkens mountain snow and moves peak runoff weeks earlier. The dust comes from land people use. What, if anything, should be done?

Photography: Greg Smith; NASA/JPL-Caltech/Univ. of Arizona. The Inquiry Atlas, a project of The Method Project.

Free to print and use with your classes. Wider reuse terms are still being finalised — please do not republish or redistribute it beyond your own students yet.