earth science

Polygonal patterned ground

Polygonal patterned ground is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Polygonal patterned ground rather than just read about it. In short: Polygonal, patterned ground is quite common in some regions of Mars. It is commonly believed to be caused by the sublimation of ice from the ground.

Key takeaways

Polygonal patterned ground belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Polygonal patterned ground to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Polygonal patterned ground from memory before moving on to harder problems.

Reference excerpt

Polygonal, patterned ground is quite common in some regions of Mars. It is commonly believed to be caused by the sublimation of ice from the ground. Sublimation is the direct change of solid ice to a gas. This is similar to what happens to dry ice on the Earth. Places on Mars that display polygonal ground may indicate where future colonists can find water ice. Low center polygons have been proposed as a marker for ground ice. Patterned ground forms in a mantle layer, called latitude dependent mantle, that fell from the sky when the climate was different. On Mars, researches have found patterned ground that formed from fractures and patterned ground formed by the arrangement of boulders. It is not yet clear what caused boulders to form patterns, but it does not seem that fractures caused the boulders to move around.

Physics

Unlike mudcracks, which are dominated by T-junctions, polygonal terrain is dominated by Y-junctions. It is formed by repeatedly annealing the crack pattern, as the same ground is partially melted every summer, then frozen every winter, until thousands of years later, it settles into a thermodynamically favored state, which is dominated by Y-junctions. This is similar to columnar jointing.

Polygons in Mare Australe quadrangle

Polygons in Casius quadrangle

Polygons in Hellas quadrangle

Sizes and formation of polygonal ground Fractured polygonal ground is generally divided into two kinds: high center and low center. The middle of a high center polygon is 10 meters across and its troughs are 2–3 meters wide. Low center polygons are 5–10 meters across and the boundary ridges are 3–4 meters wide.

High center polygons are higher in the center and lower along their boundaries. It forms from increased sublimation around cracks in a surface. Cracks are common in ice-rich surfaces. The cracks provide a place of increased surface area for sublimation. After a time the narrow cracks widen to become troughs.

Low center polygons are thought to develop from the high center polygons. The troughs along the edges of high center polygons may become filled with sediment. This thick sediment will retard sublimation, so more sublimation will take place in the center that is protected by a thinner lag deposit. In time, the middle becomes lower than the outer parts. The sediments from the troughs will turn into ridges.

High-center polygons in Noachis quadrangle

High-center polygons in Ismenius Lacus quadrangle

Clastic patterned ground Many areas of patterned ground were formed by boulders. For, as yet unknown reasons, boulders are often arranged in various shapes that include polygons. A study around Lomonosov Crater found that they were not caused by fracture networks. Clastic patterned ground has been found across the Northern Plains. Another site was Elysium Planitia. Researchers also found this terrain in the Argyre Basin (Argyre quadrangle).

Latitude dependent mantle Much of the Martian surface is covered with a thick ice-rich, mantle layer that has fallen from the sky a number of times in the past. It fell as snow and ice-coated dust. This mantle layer is called "latitude dependent mantle" because its occurrence is related to the latitude. It is this mantle that cracks and then forms polygonal ground. The mantle layer lasts for a very long time before all the ice is gone because a protective lag deposit forms on the top. The mantle contains ice and dust. After a certain amount of ice disappears from sublimation the dust stays on the top, forming the lag deposit. Mantle forms when the Martian climate is different than the present climate. The tilt or obliquity of the axis of the planet changes a great deal. The Earth’s tilt changes little because our rather large moon stabilizes the Earth. Mars only has two very small moons that do not possess enough gravity to stabilize its tilt. When the tilt of Mars exceeds around 40 degrees (from today's 25 degrees), ice is deposited in certain bands where much mantle exists today.

Other surface features Another type of surface is called "brain terrain" as it looks like the surface of a human brain. Brain terrain lies under polygonal ground when the two are both visible in a region.

Since the top, polygon layer is fairly smooth although the underlying brain terrain is irregular; it is believed that the mantle layer that contains the polygons is 10–20 meters thick. "Basketball terrain" is another expression of the surface of Mars. At certain distances it looks like a basketball’s surface. Close-up pictures have revealed it to consist of piles of rocks. Many ideas have been advanced to explain how these piles of rocks are formed. Many steep surfaces in latitude bands near 40 degrees North and South contain gullies. Some of the gullies show polygons. These have been called "gullygons."

Complex polygonal patterned ground

On the Earth On the Earth, polygonal, patterned ground is present in ice-rich ground, especially in polar regions.

See also Casius quadrangle Climate of Mars Ismenius Lacus quadrangle Latitude dependent mantle Patterned ground Mudcrack

References

Illustrations

Worked examples

Example 1 — a first encounter with Polygonal patterned ground

Start with the simplest possible case. Write down what Polygonal patterned ground claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Polygonal patterned ground before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Polygonal patterned ground ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Polygonal patterned ground

In research: Polygonal patterned ground appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Polygonal patterned ground in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Polygonal patterned ground is common in secondary-school and first-year university syllabi. It links to neighbouring topics Geology of Mars, so understanding it makes those chapters shorter.
In everyday life: Look for Polygonal patterned ground outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Polygonal patterned ground in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Polygonal patterned ground means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Polygonal patterned ground out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Polygonal patterned ground in simple terms?

Polygonal, patterned ground is quite common in some regions of Mars. It is commonly believed to be caused by the sublimation of ice from the ground.

Why does Polygonal patterned ground matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Polygonal patterned ground?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Polygonal patterned ground.