Issues in Earth Science
“Eww, There’s Some Geology in my Fiction!”
Issue 16, June 2022
Suggestions for Activities and Discussions to accompany Readings of
And Away We Go by C. J. Peterson
One story-within-a-story in And Away We Go is that Drs. Jo Gallante and Ted Urquhart (our daring protagonists) have different interpretations of what they expect to find in the subsurface of the Aitken Crater, the place where the Chang’e Rover died. They comment to the reality show writer that
"The Chang’e was drilling for sub-surface metals. The concentration could confirm one theory about the anomalies in the Moon’s mantle and crust, and perhaps reveal how the Moon formed originally.”
Whereupon the writer rolls his eyes and says “That’s a total yawn fest."
But it's not a yawn fest to the scientists, Jo and Ted, and it's not a yawn fest to us! Right!?
As they go about their research mission secretly hidden within their reality show scene, Jo and Ted talk about rare earth elements, plagioclase feldspar, mantle, KREEP, thorium, radioactive potassium, water ice, and phosphorous. These are real materials on the Moon and address real science questions about the Moon's origins.
Let's take a look at the science behind those words, first with a focus on water ice on the Moon and then the significance of a KREEPy Moon.
Our approach for this lesson will be to consider real world experimental and observational evidence and use that experimental evidence, plus scientific reasoning, to figure out specific questions about the Moon.
Teacher background information:
Jo and Ted seem to think water ice at Aiken Crater might be a big deal. But why do we care?
Water is a precious material for human occupation on a world like the Moon. One, we are about 60% water and have to keep replenishing that! Plus water is an essential part of growing food on the Moon, just like on Earth. Sustained living on the Moon would require a supply of water, and it won't be coming from lakes or rivers which don't exist on the Moon.
What's more, water is made of hydrogen and oxygen, H2O. So, if we split water up into its elements, hydrogen and oxygen, we can make O2, which is another biggie for human occupation. The Moon's atmosphere is vacuum--not much oxygen there-- so living on the Moon would require a local supply, and splitting oxygen from water molecules might be just the ticket.
Also, the separated hydrogen and oxygen could be used as fuel! There's a lot of energy released when hydrogen burns (check out hydrogen fuel cells, or the Hindenburg disaster online!)
So, water is a potentially big resource on the Moon.
Have students discuss their own ideas about why future lunar residents might care about water on the Moon. Teacher should prompt with well-placed questions if students run out of ideas too quickly.
Teacher Background Information
For the first two questions, you may need to encourage students to apply their understanding of sunlight, angle of incidence, heating, and climate variations with latitude that they have for Earth.
For the fourth question, you will also need to remind students of, or have them do, the experiments on the effect of pressure on water evaporation in our teacher resources for the story "Shifting Fortunes."
Students will answer the question “Why Aitken Crater?” through a series of smaller questions:
First, the Aitken crater is located near the Moon's south pole (the crater is often called the South Pole Aitken Basin). Will the crater be colder or warmer than other parts of the Moon? What is your evidence/reasoning supporting your thoughts?
Second, think about the angle of sunlight at the south pole. Will sunlight strike the surface of Aitken Crater at a low or high angle? Will the angle of sunlight change with the time of day like it does for temperate regions on Earth? If the angle is consistently low, how will this affect the permanence of shadows within the basin? Draw some illustrations of sun angle, and crater rims, to show why this would be so. You may need to draw side views of the sun angle and crater rim. How will permanent shadows affect the ground temperature?
Third, think about how temperature will affect whether water exists as ice or liquid (within the Moon's crust, say), or as a gas. What experience and observations from your own life can you use to argue that increasing temperature increasing the evaporation of water?
Fourth, think about the effect of pressure on whether water will exist as ice or liquid, or whether it will evaporate.
We did experiments on the effect of pressure on water evaporation in our teacher resources for the story "Shifting Fortunes." We decreased pressure inside a syringe by pulling on the syringe plunger while keeping the end of the syringe blocked with a balloon. We saw the water start to boil even at room temperature! If you don't remember, check out the picture below.
Water boiling at room temperature.
So how does pressure affect whether the water evaporates or not? Since the Moon's atmosphere is a very strong vacuum, would you expect to find either solid or liquid water near the surface of the Moon? Why do you think that?
Teacher background information
There are many different kinds of ice in our solar system! Here on Earth, we automatically think ‘froze water’ when someone says ‘ice’. The physical state in which a substance exists depends on temperature and pressure conditions of the ambient environment. Students are often quite intrigued with the idea of other kinds of ‘ice’. Many students have some experience with dry ice – frozen carbon dioxide.
Students could work on the following questions in small groups and then participate in a large-group discussion, with the teacher moderating the flow of the discussion.
A. Check online: What are some of the types of ice on Pluto, and Triton (a moon of Neptune).
B. Would you expect carbon dioxide to freeze at the same temperature as water? Why do you think this? What observations from your own experience might you bring to bear on this question?
C. How do you suppose temperature affects the type of ice? Would carbon dioxide, or methane, or various types of nitrogen compounds freeze at the same temperature as water? Why do you think this? Be sure to include observations from your own experience that you might bring to bear on this question. For example, does carbon dioxide or methane freeze on earth?
Teacher Background Information
Well, there isn't much water on the Moon, and what is there is likely to evaporate into the vacuum (even frozen water will eventually evaporate). But the Aitken crater is near the south pole, where the sun shines only in glancing blows. Some areas in the crater are permanently shadowed. Colder areas are more likely to have permanent residues of ice.
Have students work in small groups to summarize their ideas about:
1. the presence of water (in any state) on the Moon and,
2. if water is present, what state will it be in and where might we find it?
Once students have made their ideas known to classmates, have students respond to the following question
3. Given that colder areas of the Moon are likely to have permanent residues of ice, how is the question in the story--whether the subsurface material at the Aitken Basin site includes ice mixed with the rock, relevant to human occupation of the Moon? (below we will consider how it might be important if the rock consists of material they call KREEP, made of Potassium (symbol K), Rare Earth Elements (REE) and Phosphorous (symbol P)),
So what's with all the puzzling ponderings by Jo and Ted over plagioclase feldspar, thorium, radioactive potassium, rare earth elements, phosphorous, and 'mantle’?
Ted says: "The drill got stuck on an unexpectedly dense layer where the crust was thinnest, which indicates water ice, not radioactive potassium, if you’ve read Urquhart et al., 2044"
“It’s potassium, rare-earth elements, and phosphorus, KREEP," Jo replies.
Jo leaps outside and picks up a double handful of the Moon’s surface. “Does this look like plagioclase feldspar to you?” she demands. “Does it? This is mantle!”
What's the science behind all this KREEPy conversation?
Teacher and Student Background Information
Shortly after the Apollo missions returned the first samples from the Moon, scientists had developed a model for Moon’s early history called the "Lunar Magma Ocean Model". In this model the outer few hundred kilometers of the Moon melted under the heat provided by impacting and accreting meteorites. As meteorite bombardment slowed with time, the magma ‘ocean’ then gradually cooled and froze to form a solid crust and upper mantle.
Unlike water which is only made of one chemical component, H2O, the Moon’s magma ocean was made of an entire periodic table of chemical components and so it froze into many different minerals. The denser minerals sank to the bottom of the magma ocean (becoming the mantle), while the less dense minerals floated, creating 'rockbergs' within the magma ocean and eventually becoming the upper crust of the Moon. Although this model has undergone lots of examination and revision over the decades since, elements of it still explain many observations made on the Moon. As implied in the story, understanding and testing/revision of this model remains an engaging scientific goal today.
Key concepts of the Lunar Magma Ocean Model are illustrated below in the student activity.
A. Read the following simple scenario.-----Suppose that you have some material that is made of more than one chemical component. Salty water, containing only two components (H2O and salt), is a simple example. Now suppose that you took something from this salty water like either water ice or water vapor. Now, if the departing water ice or water vapor had exactly the same amount of salt as the liquid water, then taking ice or vapor away would not change the composition of the remaining salty liquid water. Only the amount of material left behind would change, not the concentrations (or relative proportions) of salt and H2O. However, in almost all natural situations where two different phases separate, the chemical components will not be the same concentration in the two parts.
B. Generally, when water freezes or evaporates, the salt tends to stay with the liquid! We say that the salt "partitions" into the liquid.
C. Given the typical situation above, draw a set of diagrams to represent water molecules, salt ‘molecules’, liquid water, and water ice. Your diagram should also show the relative amounts of water and salt ‘particles’ in each of ice and liquid. Think about what this means mathematically.
D. In a similar fashion, salt will partition into the liquid water rather than water vapor. Let's do a thought puzzle with the salty water to consider the mathematical implications of salt partitioning into the liquid water.
E. Suppose that you have an aquarium with freshwater fish. To keep the ick away from your fish (ick is a common fish disease) it is helpful to add some salt. Typically, 1 tablespoon of salt for every 5 gallons of water is good. But you don't want the salt to get too high and damage your aquarium plants.
But you're thinking, why would the salt change if I don't take any out or put any more in?
Well, suppose that you are not a conscientious fish keeper, and you let the water evaporate from your aquarium. Suppose you started with 10 gallons plus 2 tablespoons of salt, but half of the water evaporates. Remembering that nearly all of the salt stays with the liquid, what will the concentration of salt be in your aquarium after evaporation?
Notice that the amount of salt doesn't change much (still 2 tablespoons) but because half of the H2O has evaporated, the concentration is now double! 2 tablespoons per 5 gallons!
The composition of our remaining water has evolved, or changed, due to removing vapor from it. A similar process would happen if we froze (crystallized) part of it.
Thus, the composition of a magma can change (evolve) if we grow crystals from it and then somehow remove those crystals (such as allowing them to settle out).
Note: The illustration of salt evolution in an aquarium is adapted from the more extensive grades 6-12 activities in Learning to Read the Earth and Sky, SUGGESTIONS FOR DESIGNING AN ACTIVITY: EXPERIMENTAL TEST OF THE MATHEMATICAL MODEL FOR GEOCHEMICAL DIFFERENTIATION, p 297, By Russ and Mary Colson, NSTA Press 2016.
F. Memorize and apply the meaning of these two terms: primitive magam and evolved magma.
Primitive is the word we use to refer to a magma or rock whose composition has not undergone much change due, for example, to partial crystallization, which changes the composition. The original salt water of the simple scenario in letter A above would be like a primitive magma.
Evolved is the word we use to refer to a magma or rock whose composition has undergone a change due to some parts of the magma crystallizing and that solid fraction settling out. The salt water of the more complex scenario in letters C and E above would be like an evolved magma.
Read the text about the Lunar Magma Ocean Model and examine the diagram. Work through the questions below the diagram to test the model against observations made of the lunar surface. Don’t jump to the answer. Do your thinking first, then use the provided answer as feedback to your own thinking.
Lunar Magma Ocean Model
Shortly after the Apollo missions returned the first samples from the Moon, scientists had developed a model for Moon’s early history called the "Lunar Magma Ocean Model". In this model the outer few hundred kilometers of the Moon melted under the heat provided by impacting and accreting meteorites. As meteorite bombardment slowed with time, the magma ‘ocean’ then gradually cooled and froze forming, by the process described below, a solid crust and upper mantle.
Unlike water which is only made of one chemical component, H2O, the Moon’s magma ocean was made of an entire periodic table of chemical components and so it froze into many different minerals. The denser minerals sank to the bottom of the magma ocean (becoming the mantle), while the less dense minerals floated, creating 'rockbergs' within the magma ocean. These rockbergs eventually coalesced to become the upper crust of the Moon.
Below is an illustration of this model, starting with the melted "ocean" and going to a "partial ocean" with rockbergs, a mantle made of denser crystals that sank, and a residual melt—the residual melt being the part of the magma ocean that has not yet frozen.
Although this model has undergone lots of examination and revision over the decades since, elements of it still explain many observations made on the Moon (such as the abundance of feldspar in the crust and various chemical peculiarities observed in the crust and lava erupted from the mantle). As implied in the story, understanding and testing/revision of this model remains an engaging scientific goal today.
Key concepts of the Lunar Magma Ocean Model that help us understand the story are illustrated below.
A. Thinking about this model, which one of the following will contain the most Plagioclase Feldspar?
a. The crust
b. The mantle
c. The residual melt (or rock once it is frozen)
You can figure this out from the model itself: The plagioclase, being less dense, is predicted to be concentrated in the crust of the Moon by this model. So, in the story, when Jo says "does this look like Plagioclase Feldspar? It's mantle!" Her argument is based on some type of Magma Ocean model.
B. Now suppose that you do experiments in a laboratory (on Earth) to determine how various elements "partition" between residual melt and the minerals plagioclase, olivine, and pyroxene, where
partitioning = D = concentration of element in mineral / concentration of element in melt
You find the following: For all three minerals, plagioclase, olivine, and pyroxene, you find the following through experimentation:
· Potassium (K): D is very small, that is <<1
· Thorium (Th): D is very small, that is <<1
· Rare Earth Elements (REE): D is very small, that is <<1
· Phosphorous (P): D is very small, that is <<1
Based on this experimental evidence, which of the following will be the 'KREEPy" part of the Moon?
a. The crust
b. The mantle
c. The residual melt (or rock once it is frozen)
Which one will contain the most Thorium?
a. The crust
b. The mantle
c. The residual melt (or rock once it is frozen)
You can figure this out by thinking about where each element goes. In this case, the "incompatible" elements Th, REE, P, and K all tend to remain with the residual melt because they partition into the melt instead of the minerals (as indicated by our experiments in which partition coefficients, "D," are all much less than one, meaning the concentration in the mineral is much less that the concentration in the melt. Thus, the KREEP and Th will all be of highest concentration in the residual melt (which has frozen to rock at the time of our story)
Thus, in the story, concentrations of KREEP, Th, plagioclase all indicated what part of the former magma ocean (now all frozen rock) has been exposed at the surface in the Aitken Crater basin and so give clues to the Moon's geological history! The presence of KREEP tells us that some of the frozen 'residual melt' has found its way to the surface at that location, perhaps excavated by a meteorite impact reached down below the crust to that level. The presence of water ice would tell us that we have a great potential resource for supporting human life on the Moon.
The Teacher Resources for And Away We Go are written by Russ and Mary Colson, authors of .
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