8  Stability of the Solar System

Why does Earth have so few craters while the Moon is covered in them?

HS-ESS1-6 HS-ESS2-5 7–8 Days 🧠 Quiz & Evaluate ↓

🌑 Why Is the Moon Covered in Craters But Earth Isn’t?

9 🔥 Engage — Counting Craters

9.1 The Crater Mystery

Look at photos of the Moon, Mars, and Earth from space. Something is immediately obvious:

Body	Known Impact Craters
🌍 Earth	~128 confirmed
🔴 Mars	~300,000+
🌑 Moon	~1,000,000+

Earth is the largest of these three bodies. It has the strongest gravity. It should attract more impacts than the Moon or Mars. So where did all the craters go?

Plot = require("@observablehq/plot")
d3 = require("d3@7")

craterData = [
  {body: "Earth", craters: 128, color: "#2ecc71"},
  {body: "Mars", craters: 300000, color: "#e74c3c"},
  {body: "Moon", craters: 1000000, color: "#95a5a6"}
]

Plot.plot({
  title: "Known Impact Craters by Body (log scale)",
  subtitle: "Earth has barely any — why?",
  width: 650,
  height: 300,
  marginLeft: 80,
  x: {type: "log", label: "Number of Craters (log scale)", grid: true},
  y: {label: null},
  marks: [
    Plot.barX(craterData, {
      x: "craters",
      y: "body",
      fill: "color",
      rx: 8
    ,
        tip: true}),
    Plot.text(craterData, {
      x: "craters",
      y: "body",
      text: d => d.craters.toLocaleString(),
      dx: 30,
      fontSize: 14,
      fontWeight: "bold"
    })
  ]
})

9.1.1 📝 Engage Questions

1. Why do you think Earth has so few visible craters compared to the Moon?
2. What processes on Earth might destroy or hide craters over time?
3. Does the lack of craters mean Earth was hit less, or that something erased them?

10 🔍 Explore 1 — Rock Evidence

10.1 Reading the Stories in Rocks

Rocks are like history books. Their composition, age, and density tell us about the conditions when and where they formed.

10.2 Rock Composition & Age

Scientists use radiometric dating to determine the ages of rocks. Here’s what we find:

rockData = [
  {sample: "Oldest Earth rocks (Jack Hills, Australia)", age: 4.4, type: "Earth", density: 2.7},
  {sample: "Oldest ocean floor", age: 0.2, type: "Earth", density: 3.0},
  {sample: "Appalachian Mountains", age: 0.48, type: "Earth", density: 2.7},
  {sample: "Moon rocks (Apollo)", age: 4.5, type: "Moon", density: 3.3},
  {sample: "Mars meteorites", age: 4.1, type: "Mars", density: 3.4},
  {sample: "Oldest meteorites", age: 4.57, type: "Meteorite", density: 3.5}
]

Plot.plot({
  title: "Age of Rock Samples (billions of years)",
  subtitle: "Most Earth rocks are young — the Moon and meteorites are ancient",
  width: 700,
  height: 300,
  marginLeft: 200,
  x: {label: "Age (billions of years)", domain: [0, 5], grid: true},
  y: {label: null},
  color: {legend: true, domain: ["Earth", "Moon", "Mars", "Meteorite"], range: ["#2ecc71", "#95a5a6", "#e74c3c", "#f39c12"]},
  marks: [
    Plot.dot(rockData, {
      x: "age",
      y: "sample",
      fill: "type",
      r: 8,
      stroke: "white",
      strokeWidth: 1
    ,
        tip: true}),
    Plot.ruleX([4.5], {stroke: "#00cec9", strokeWidth: 2, strokeDasharray: "5,5"})
  ]
})

10.2.1 💡 Key Pattern

The oldest meteorites and Moon rocks are ~4.5 billion years old — the age of our solar system. But most Earth rocks are much younger. Earth’s ocean floor is only ~200 million years old! Something keeps recycling Earth’s surface.

10.3 Rock Density & Layers

densityData = [
  {layer: "Atmosphere", density: 0.001, depth: 0, color: "#74b9ff"},
  {layer: "Ocean water", density: 1.03, depth: 0, color: "#0984e3"},
  {layer: "Continental crust (granite)", density: 2.7, depth: 30, color: "#dfe6e9"},
  {layer: "Oceanic crust (basalt)", density: 3.0, depth: 7, color: "#636e72"},
  {layer: "Upper mantle", density: 3.4, depth: 400, color: "#e17055"},
  {layer: "Lower mantle", density: 5.5, depth: 2900, color: "#d63031"},
  {layer: "Outer core (liquid iron)", density: 10.0, depth: 5100, color: "#fdcb6e"},
  {layer: "Inner core (solid iron)", density: 13.0, depth: 6371, color: "#f39c12"}
]

Plot.plot({
  title: "Density of Earth's Layers (g/cm³)",
  subtitle: "Denser materials sank to the center when Earth was molten",
  width: 700,
  height: 350,
  marginLeft: 200,
  x: {label: "Density (g/cm³)", grid: true},
  y: {label: null},
  marks: [
    Plot.barX(densityData, {
      x: "density",
      y: "layer",
      fill: "color",
      rx: 5,
      sort: {y: "-x"}
    ,
        tip: true}),
    Plot.text(densityData, {
      x: "density",
      y: "layer",
      text: d => d.density + " g/cm³",
      dx: 30,
      fontSize: 11,
      fontWeight: "bold"
    })
  ]
})

10.3.1 📝 Explore 1 Questions

1. Why are Earth’s surface rocks so much younger than Moon rocks?
2. What does the density pattern tell you about how Earth formed?
3. The ocean floor is only ~200 million years old. Where did the older ocean floor go?

11 💡 Explain 1 — Early Solar System

11.1 The Late Heavy Bombardment

About 4.1 to 3.8 billion years ago, the inner solar system experienced an intense period of asteroid and comet impacts called the Late Heavy Bombardment. This shaped every rocky body — but Earth’s record was mostly erased.

11.2 Why Earth’s Record Disappears

The Moon preserves its craters because it has:

• ❌ No atmosphere (no weathering)
• ❌ No liquid water (no erosion)
• ❌ No plate tectonics (no recycling)
• ❌ No volcanic activity (no resurfacing)

Earth has all four of these processes working constantly to erase its surface history.

{
  const svg = d3.create("svg")
    .attr("width", 700)
    .attr("height", 340)
    .attr("viewBox", "0 0 700 340");

  svg.append("text").attr("x", 350).attr("y", 25).attr("text-anchor", "middle")
    .attr("font-size", 16).attr("font-weight", "bold").text("Why Earth Erases Its Craters");

  const processes = [
    {name: "Weathering", desc: "Wind & rain break\ndown rocks", icon: "🌧️", x: 100, color: "#74b9ff"},
    {name: "Erosion", desc: "Water carries\nsediment away", icon: "🌊", x: 250, color: "#0984e3"},
    {name: "Plate Tectonics", desc: "Crust subducts\n& recycles", icon: "🌋", x: 400, color: "#e17055"},
    {name: "Volcanism", desc: "Lava covers\nold surfaces", icon: "🔥", x: 550, color: "#d63031"}
  ];

  processes.forEach(p => {
    svg.append("rect").attr("x", p.x - 70).attr("y", 50).attr("width", 140).attr("height", 130)
      .attr("rx", 15).attr("fill", p.color).attr("opacity", 0.9);
    svg.append("text").attr("x", p.x).attr("y", 90).attr("text-anchor", "middle")
      .attr("font-size", 30).text(p.icon);
    svg.append("text").attr("x", p.x).attr("y", 120).attr("text-anchor", "middle")
      .attr("font-size", 14).attr("font-weight", "bold").attr("fill", "white").text(p.name);
    const lines = p.desc.split("\n");
    lines.forEach((line, i) => {
      svg.append("text").attr("x", p.x).attr("y", 145 + i * 16).attr("text-anchor", "middle")
        .attr("font-size", 11).attr("fill", "white").text(line);
    });
  });

  // Arrow to result
  svg.append("text").attr("x", 350).attr("y", 220).attr("text-anchor", "middle")
    .attr("font-size", 30).text("⬇️");
  
  svg.append("rect").attr("x", 150).attr("y", 240).attr("width", 400).attr("height", 80)
    .attr("rx", 15).attr("fill", "#00cec9").attr("opacity", 0.9);
  svg.append("text").attr("x", 350).attr("y", 275).attr("text-anchor", "middle")
    .attr("font-size", 16).attr("font-weight", "bold").attr("fill", "white")
    .text("Earth's surface is constantly renewed!");
  svg.append("text").attr("x", 350).attr("y", 300).attr("text-anchor", "middle")
    .attr("font-size", 13).attr("fill", "white")
    .text("Craters erased • Old rocks recycled • New crust created");

  return svg.node();
}

11.2.1 💡 The Solar System’s Age

All objects in the solar system formed about 4.57 billion years ago from the same spinning cloud of gas and dust (the solar nebula). We know this because the oldest meteorites, Moon rocks, and Earth minerals all give roughly the same age. The solar system formed all at once, not piece by piece.

12 🔍 Explore 2 — Properties of Water

12.1 Water: Earth’s Secret Weapon

Water is the most important substance for life and for shaping Earth’s surface. But water has some truly bizarre properties that make it unique among all liquids.

viewof waterProperty = Inputs.select(
  ["Density anomaly", "High heat capacity", "Universal solvent", "Surface tension"],
  {label: "Explore a property of water:"}
)

{
  const descriptions = {
    "Density anomaly": {
      title: "🧊 Ice Floats!",
      text: "Most substances are denser as solids. Water is the opposite — ice is LESS dense than liquid water. This is why ice floats on lakes, insulating the water below and allowing fish to survive winter.",
      fact: "If ice sank, lakes would freeze solid from the bottom up, killing all aquatic life.",
      data: [{state: "Ice (0°C)", density: 0.917}, {state: "Liquid (4°C)", density: 1.000}, {state: "Liquid (25°C)", density: 0.997}]
    },
    "High heat capacity": {
      title: "🌡️ Hard to Heat, Hard to Cool",
      text: "Water absorbs enormous amounts of energy before its temperature changes. This is why oceans moderate coastal climates and why it takes a long time to boil water.",
      fact: "Water's heat capacity is 5x higher than rock. Oceans absorb heat all summer and release it in winter, stabilizing Earth's climate.",
      data: [{state: "Water", density: 4.18}, {state: "Rock", density: 0.84}, {state: "Iron", density: 0.45}]
    },
    "Universal solvent": {
      title: "🧪 Dissolves Almost Everything",
      text: "Water dissolves more substances than any other liquid. This makes it perfect for carrying nutrients in your blood, minerals in rivers, and chemicals in the ocean.",
      fact: "A single cup of seawater contains about 1 teaspoon of dissolved salt plus traces of nearly every element on Earth.",
      data: [{state: "Seawater salinity", density: 3.5}, {state: "River water", density: 0.01}, {state: "Pure water", density: 0.0}]
    },
    "Surface tension": {
      title: "🕷️ Water is Sticky",
      text: "Water molecules are attracted to each other strongly (cohesion). This creates surface tension — strong enough for insects to walk on water and for water to climb up narrow tubes (capillary action in plants).",
      fact: "Surface tension is what makes water form droplets instead of spreading into a thin film.",
      data: [{state: "Water", density: 72.8}, {state: "Ethanol", density: 22.1}, {state: "Mercury", density: 485.0}]
    }
  };
  const info = descriptions[waterProperty];
  const div = d3.create("div");
  div.append("h3").style("color", "#00cec9").style("font-family", "Space Grotesk").text(info.title);
  div.append("p").style("font-size", "1.1em").text(info.text);
  div.append("div").style("background", "#ffecd2").style("padding", "15px").style("border-radius", "10px")
    .style("border-left", "5px solid #ff6b6b").style("margin", "10px 0")
    .append("strong").text("🤯 " + info.fact);
  return div.node();
}

12.1.1 💡 Why Water Matters for This Unit

Water is essential for:

• Erasing craters through erosion and weathering
• Supporting life as the universal solvent for biochemistry
• Regulating climate through its enormous heat capacity
• Driving the rock cycle through weathering, transport, and deposition

Without water, Earth would look like the Moon — ancient, cratered, and lifeless.

13 💡 Explain 2 — Why Craters Persist on the Moon

13.1 The Moon: A Frozen Time Capsule

The Moon stopped being geologically active billions of years ago. With no atmosphere, water, or plate tectonics, craters from 4 billion years ago look almost the same as they did when they formed.

13.2 Surface Age Comparison

surfaceAge = [
  {body: "Moon (highlands)", age: 4.4, active: "No", color: "#95a5a6"},
  {body: "Moon (maria)", age: 3.2, active: "No", color: "#636e72"},
  {body: "Mars (Hellas Basin)", age: 4.0, active: "Minimal", color: "#e74c3c"},
  {body: "Earth (ocean floor)", age: 0.2, active: "Yes!", color: "#0984e3"},
  {body: "Earth (oldest continent)", age: 4.0, active: "Yes!", color: "#2ecc71"},
  {body: "Io (Jupiter's moon)", age: 0.001, active: "Extremely!", color: "#fdcb6e"}
]

Plot.plot({
  title: "Average Surface Age by Body (billions of years)",
  subtitle: "Geologically active bodies have young surfaces",
  width: 700,
  height: 300,
  marginLeft: 180,
  x: {label: "Surface age (billion years)", grid: true, domain: [0, 5]},
  y: {label: null},
  marks: [
    Plot.barX(surfaceAge, {
      x: "age",
      y: "body",
      fill: "color",
      rx: 8,
      sort: {y: "-x"}
    ,
        tip: true}),
    Plot.text(surfaceAge, {
      x: "age",
      y: "body",
      text: d => d.age + " Gyr",
      dx: 25,
      fontSize: 12,
      fontWeight: "bold"
    })
  ]
})

🧠 Jupiter’s moon Io has the youngest surface in the solar system — it’s so volcanically active that its entire surface is replaced every few thousand years! Earth’s ocean floor is replaced every ~200 million years. The Moon’s surface hasn’t changed in over 3 billion years.

14 🔬 Elaborate — The Rock Cycle

14.1 Earth’s Recycling Program

Earth’s rocks are in a constant cycle of creation, destruction, and transformation. This is why Earth’s surface stays young while the Moon’s stays old.

{
  const svg = d3.create("svg")
    .attr("width", 700)
    .attr("height", 380)
    .attr("viewBox", "0 0 700 380");

  svg.append("text").attr("x", 350).attr("y", 25).attr("text-anchor", "middle")
    .attr("font-size", 18).attr("font-weight", "bold").text("The Rock Cycle");

  // Rock type nodes
  const rocks = [
    {name: "Igneous\nRock", x: 350, y: 80, color: "#e74c3c", desc: "From cooling\nmagma/lava"},
    {name: "Sedimentary\nRock", x: 580, y: 230, color: "#f39c12", desc: "From compressed\nsediments"},
    {name: "Metamorphic\nRock", x: 120, y: 230, color: "#6c5ce7", desc: "Changed by heat\n& pressure"},
    {name: "Magma", x: 350, y: 340, color: "#d63031", desc: "Molten rock\nbelow surface"}
  ];

  rocks.forEach(r => {
    svg.append("ellipse").attr("cx", r.x).attr("cy", r.y).attr("rx", 80).attr("ry", 40)
      .attr("fill", r.color).attr("opacity", 0.85);
    const lines = r.name.split("\n");
    lines.forEach((line, i) => {
      svg.append("text").attr("x", r.x).attr("y", r.y - 5 + i * 18).attr("text-anchor", "middle")
        .attr("font-size", 13).attr("font-weight", "bold").attr("fill", "white").text(line);
    });
  });

  // Process arrows with labels
  const arrows = [
    {from: "Igneous", to: "Sedimentary", label: "Weathering\n& Erosion", x: 490, y: 140},
    {from: "Sedimentary", to: "Metamorphic", label: "Heat &\nPressure", x: 350, y: 265},
    {from: "Metamorphic", to: "Magma", label: "Melting", x: 200, y: 310},
    {from: "Magma", to: "Igneous", label: "Cooling", x: 280, y: 200}
  ];

  arrows.forEach(a => {
    const lines = a.label.split("\n");
    lines.forEach((line, i) => {
      svg.append("text").attr("x", a.x).attr("y", a.y + i * 14).attr("text-anchor", "middle")
        .attr("font-size", 11).attr("font-weight", "bold").attr("fill", "#2d3436").text(line);
    });
  });

  return svg.node();
}

14.1.1 💡 The Rock Cycle and Life

The rock cycle isn’t just about geology — it’s critical for life:

• Weathering releases minerals that wash into oceans, feeding marine life
• Volcanic eruptions release CO₂, keeping Earth warm enough for liquid water
• Subduction recycles carbon back into the mantle, preventing runaway greenhouse
• New crust formation creates fresh habitats and coastlines

Earth’s geological activity is one reason life can exist here. A dead planet (geologically) might also be a dead planet (biologically).

14.1.2 📝 Elaborate Activity

1. Trace a rock through one complete cycle: start as magma, go through all three rock types, and return to magma.
2. At each step, explain what process transforms the rock.
3. Why can’t this cycle happen on the Moon?

15 ✅ Evaluate — Impact and Life

15.1 How Did Asteroid Impacts Shape the History of Life?

impacts = [
  {event: "Late Heavy Bombardment", time: 4000, effect: "Sterilized early Earth repeatedly", severity: 10, color: "#d63031"},
  {event: "Vredefort Impact", time: 2023, effect: "Largest known crater (300 km)", severity: 8, color: "#e74c3c"},
  {event: "Sudbury Impact", time: 1849, effect: "Second largest known crater", severity: 7, color: "#e17055"},
  {event: "Chicxulub Impact", time: 66, effect: "Killed dinosaurs, enabled mammals", severity: 9, color: "#fdcb6e"},
  {event: "Tunguska Event", time: 0.000115, effect: "Flattened 2,000 km² of forest", severity: 3, color: "#74b9ff"},
  {event: "Chelyabinsk meteor", time: 0.000011, effect: "Injured 1,500 people (2013)", severity: 1, color: "#00cec9"}
]

Plot.plot({
  title: "Major Impact Events in Earth's History",
  subtitle: "Time in millions of years ago (log scale)",
  width: 700,
  height: 300,
  marginLeft: 180,
  x: {type: "log", label: "Millions of years ago (log scale)", grid: true},
  y: {label: null},
  marks: [
    Plot.dot(impacts, {
      x: "time",
      y: "event",
      r: d => d.severity * 3,
      fill: "color",
      stroke: "white",
      strokeWidth: 1
    ,
        tip: true}),
    Plot.text(impacts, {
      x: "time",
      y: "event",
      text: "effect",
      dx: d => d.severity * 3 + 10,
      fontSize: 10
    })
  ]
})

15.1.1 🧪 Evaluate Questions

1. Explain why Earth has far fewer visible craters than the Moon, using evidence from rock ages, Earth’s active processes, and the rock cycle.

2. Argue whether asteroid impacts have been helpful or harmful to the evolution of life. Use at least two specific examples.

3. Connect the properties of water to why Earth’s surface looks so different from the Moon’s.

4. Predict what Earth’s surface would look like if plate tectonics suddenly stopped. How would this affect life?

15.1.2 📝 Model Update

Return to your initial model from the Unit Opening. Update it with what you’ve learned:

• How do Earth’s active processes affect the chances of life surviving?
• Could a planet without plate tectonics support complex life? Why or why not?
• How do asteroid impacts connect to both destroying and enabling life?