Practical 9 — Origin of Planet Earth
CAPS sets two discussion investigations for this block, not experiments — and that is the point. You cannot run an experiment on the origin of a planet. What you can do is weigh evidence and defend a position, which is exactly what scientists actually do about events that happened billions of years ago.
CAPS also frames the whole block carefully: "This topic is based on scientific theory using knowledge gained from Geology, fossil records and, more recently, also biochemical research. Those who take exception to this information because of their religious beliefs are encouraged to be able to discuss evolution from a scientific point of view, but can record their reservations."
A pencil. Every investigation here is desk work — reading evidence, building an argument, and plotting one graph.
All the evidence you need is supplied on the sheet, so nothing depends on being online. If you are online, NASA's Mars and New Horizons mission pages are the primary sources.
Aim
To apply the formal definition of a planet to Pluto, and to explain why it was reclassified.
The three tests
In 2006 the International Astronomical Union agreed a definition for the first time. To be a planet, a body must pass all three:
| Test | Pluto: pass or fail? | Why |
|---|---|---|
| 1. It orbits the Sun | ||
| 2. It has enough mass that its own gravity pulls it into a round shape | ||
| 3. It has cleared its orbital neighbourhood of other objects |
📊 The evidence on Pluto
Orbit. Pluto orbits the Sun once every 248 Earth years, at an average of 5.9 billion km. Its orbit is tilted 17° out of the plane the eight planets share, and it is so elliptical that for 20 years of each orbit Pluto is closer to the Sun than Neptune.
Size and shape. Diameter 2 377 km — smaller than our Moon (3 475 km). It is round.
Neighbourhood. Pluto lies in the Kuiper Belt, a ring of icy bodies beyond Neptune containing thousands of objects. Several are comparable in size: Eris is 2 326 km across and more massive than Pluto. Pluto's mass is only about 0.07 times the combined mass of the other objects sharing its orbital zone. By comparison, Earth is roughly 1.7 million times the mass of everything else in its orbital zone.
What changed. Eris was discovered in 2005. Astronomers faced a choice: call Eris the tenth planet and accept that dozens more Kuiper Belt objects would follow, or write a definition that drew a line. They wrote the definition.
Analysis
- 6Complete the three-test table with a pass/fail and a reason for each. (2 each.)
- 3State clearly which single test Pluto fails, and explain in your own words what that test means.
- 2What is Pluto now classified as, and how many planets does the solar system therefore have?
- 3A common wrong answer is "Pluto isn't a planet because it's too small." Explain why size alone is not the criterion, using the definition.
- 3The discovery of Eris in 2005 forced the issue. Explain why, and what would have happened to the planet count if the IAU had done nothing.
- 4Some astronomers still object to the 2006 definition. Give one reasonable argument that Pluto should still be called a planet, and one argument that the reclassification was right. Then state your own view with a reason.
- 3This is a case where a definition changed, not the evidence. Explain what that shows about how science works — link it to what you learnt about theories in Topic 5.
Aim
To evaluate the evidence for past and present life on Mars, and to write a defensible conclusion.
Sort the evidence
Read the supplied evidence, then complete the two columns. Be careful with the distinction CAPS builds in: evidence that Mars was once habitable is not the same as evidence that it was ever inhabited.
✅ Supports past habitability / life
❌ Argues against life
📊 The evidence on Mars
Water, past. Orbiters and rovers have photographed dry river valleys, branching drainage networks, and a fossil river delta in Jezero Crater. Rovers have found clay minerals and sulphates that only form in the presence of liquid water, and rounded pebbles of the kind shaped by flowing streams. The consensus is that Mars had liquid surface water for hundreds of millions of years, roughly 3.5–4 billion years ago — the same window in which life appeared on Earth.
Water, present. Large amounts of water ice at the poles and in the subsurface. Possible briny liquid water below the surface, though this is contested.
Chemistry. Curiosity has detected organic molecules in 3-billion-year-old mudstone — the carbon-based building blocks life uses. It has also measured seasonal fluctuations of methane in the atmosphere. On Earth most methane is biological; but it can also be produced by rock–water chemistry with no life involved.
Against. No organism, fossil or cell has ever been found. The 1976 Viking landers ran direct life-detection experiments and returned ambiguous results now generally attributed to unusual soil chemistry, not biology. Modern Mars is brutal: average −60 °C, atmospheric pressure less than 1% of Earth's, no magnetic field, and a surface sterilised by ultraviolet and cosmic radiation. The soil contains perchlorates, which are toxic to most Earth life.
Why it went wrong. Mars is about half Earth's diameter and a tenth its mass. Its small core cooled, and it lost its magnetic field about 4 billion years ago. Without that shield the solar wind stripped away most of its atmosphere; pressure collapsed, and surface water froze or boiled off into space.
Your conclusion
Write a short paragraph answering CAPS's question directly. State a position, support it with your strongest evidence, and be explicit about the difference between was and is.
Analysis
- 8Complete both columns with at least four pieces of evidence each.
- 4Explain the difference between evidence that Mars was habitable and evidence that Mars was inhabited, and say which kind we actually have.
- 4Methane in the Martian atmosphere is often reported as a sign of life. Explain why it is suggestive but not conclusive.
- 3Mars is roughly half Earth's diameter. Explain how that single fact led to Mars losing its water — trace the full chain.
- 3Earth and Mars both had liquid water 4 billion years ago. Give two reasons Earth kept its water and Mars did not.
- 4Submit your conclusion paragraph. (Position stated · strongest evidence used · was/is distinction explicit · no overclaiming.)
- 3If you were designing the next Mars mission to search for past life, where would you land and why? Use the evidence.
Aim
To graph the rise of atmospheric oxygen over geological time, and to explain the shape of the curve using the oxygen-sink concept.
The data
Estimated atmospheric oxygen as a percentage of today's level, reconstructed from rock chemistry.
| Time (billions of years ago) | O₂ (% of present level) | What was happening |
|---|---|---|
| 4.0 | 0.0 | second atmosphere: CO₂, N₂, water vapour |
| 3.5 | 0.0 | first cyanobacteria — photosynthesis begins |
| 3.0 | 0.02 | oxygen produced, but absorbed by dissolved iron |
| 2.6 | 0.1 | banded iron formations still being laid down |
| 2.4 | 1.0 | Great Oxygenation Event — iron sinks saturate |
| 2.0 | 5 | free O₂ accumulating in the atmosphere |
| 1.5 | 5 | the long stable "boring billion" |
| 1.0 | 6 | still stable |
| 0.6 | 50 | second rise — complex animals appear |
| 0.3 | 150 | peak: giant insects, coal forests |
| 0.0 | 100 | today — 21% of the atmosphere |
Analysis
- 7Plot O₂ against time. Put 4.0 bya on the left and today on the right so time runs forwards. Follow all seven graph rules from Topic 5.
- 4Cyanobacteria began photosynthesising at 3.5 bya, yet oxygen stayed near zero until 2.4 bya. Explain the 1.1-billion-year delay.
- 3Name the rock formation that is the physical evidence of that delay, and explain how it forms.
- 3Explain what the phrase "a surplus O₂ was produced than was absorbed" means, in your own words.
- 3Oxygen peaked at about 150% of today's level around 0.3 bya. Suggest what that made possible, and why it did not last.
- 3Using your graph, explain why complex animal life could not have appeared before about 0.6 bya.
- 2CAPS concept 13 says later photosynthetic colonies, the stromatolites, converted more CO₂ to O₂. Name where in South Africa you can see 3.5-billion-year-old fossil stromatolites.
Extension
The rise of oxygen was, for most organisms alive at the time, a mass extinction — oxygen was toxic to the anaerobes that then dominated Earth. Write a paragraph on why the "Great Oxygenation Event" is sometimes called the "Oxygen Catastrophe", and what that tells you about the word pollution.
✅ Memo & marking guide — try it first
Total: 78 marks — Investigation 1: 24 · Investigation 2: 29 · Investigation 3: 25.
This memo does not print with the worksheet.
Investigation 1 — Pluto
- Q1 (6): Test 1 PASS — it orbits the Sun, once every 248 years. Test 2 PASS — at 2 377 km across it has ample mass to be pulled round by its own gravity. Test 3 FAIL — it shares the Kuiper Belt with thousands of other objects, several comparable in size, and its mass is only ~0.07× the rest of its orbital zone combined.
- Q2 (3): Test 3. "Clearing the neighbourhood" means the body is gravitationally dominant in its orbital zone — it has either absorbed, ejected or captured the other objects sharing that orbit. Pluto has not; it is one object among thousands.
- Q3 (2): A dwarf planet (1). The solar system therefore has eight planets (1).
- Q4 (3): Size appears nowhere in the definition. The tests are about orbit, shape and gravitational dominance. Mercury is small and is a planet; Pluto is smaller than our Moon but that is not the disqualifier — sharing its orbit is. Accept: a body could be large and still fail test 3.
- Q5 (3): Eris is comparable in size to Pluto and more massive, so consistency demanded either admitting it as a tenth planet or redefining. Since dozens more Kuiper Belt objects were expected, doing nothing would have meant an open-ended and growing planet count.
- Q6 (4): For Pluto (1): it is round, geologically active with mountains and glaciers, has five moons and an atmosphere — it behaves like a world; the "cleared the neighbourhood" criterion is vague and arguably would disqualify Earth if it were in the Kuiper Belt. For reclassification (1): a definition must draw a line or the category becomes meaningless; gravitational dominance is a real physical distinction. Own view with reason (2).
- Q7 (3): Pluto did not change — the definition did. Science revises its classifications and theories when new evidence (Eris) makes the old ones inconsistent. That is a feature, not a failure: the same reason we say a hypothesis is supported rather than proved.
Investigation 2 — Mars
- Q8 (8): 1 mark each, max 4 per column. Supports: dry river valleys and drainage networks · a fossil delta in Jezero Crater · clay minerals and sulphates that need liquid water · rounded stream-shaped pebbles · organic molecules in ancient mudstone · seasonal methane · polar and subsurface water ice. Against: no organism, fossil or cell ever found · Viking results attributed to soil chemistry · −60 °C average · <1% of Earth's atmospheric pressure · no magnetic field, so surface sterilised by UV and cosmic radiation · toxic perchlorates in the soil.
- Q9 (4): Habitable = conditions were suitable for life (liquid water, organics, energy). Inhabited = life was actually present. Every piece of strong Mars evidence is of the first kind. We have good evidence of past habitability and none of past or present inhabitation.
- Q10 (4): On Earth most atmospheric methane is biological, so its presence and its seasonal variation are suggestive (2). But methane is also produced abiotically, by water reacting with olivine-rich rock (serpentinisation), and it can be released from buried ice. Since a non-living explanation is available, methane cannot be conclusive (2).
- Q11 (3): Small size → smaller core → the core cooled and solidified faster → the dynamo stopped and Mars lost its magnetic field → the solar wind was no longer deflected and stripped away the atmosphere → atmospheric pressure collapsed → liquid surface water could no longer be stable, so it froze into the ground and poles or escaped to space.
- Q12 (3): Any two, well explained: Earth is larger and more massive, so it retained internal heat, kept a molten outer core, and kept its magnetic field, which shields the atmosphere; Earth's stronger gravity holds a thicker atmosphere; Earth sits in the Goldilocks zone at a distance where water is liquid; Earth's active plate tectonics recycles carbon and stabilises the climate.
- Q13 (4): Position stated (1) · supported by the strongest evidence, which is the geological water evidence (1) · was/is distinction made explicit (1) · does not overclaim beyond the evidence (1). The best answers conclude: past habitability likely, past life unknown, present life very unlikely at the surface.
- Q14 (3): A site with ancient water-lain sediments — a delta, lake bed or clay deposit such as Jezero Crater — because fine sediments preserve organic traces well (2), and ideally where you can drill below the radiation-sterilised surface layer (1).
Investigation 3 — the oxygen curve
- Q15 (7): Time on x running 4.0 bya on the left to 0 on the right, O₂ on y. Axes labelled with units, sensible scale — note the huge range, so a logarithmic y-axis or a broken axis is a legitimate and creditworthy solution — points accurate, line accurate, figure number, descriptive title.
- Q16 (4): Oxygen was being produced, but oxygen sinks absorbed it as fast as it appeared — chiefly dissolved iron in the oceans, plus volcanic gases and crustal rock. Only when those sinks were saturated could a surplus accumulate in the atmosphere.
- Q17 (3): Banded iron formations (1). Dissolved iron(II) in the ocean reacted with the new oxygen, oxidised to insoluble iron(III), and settled out of the water in layers on the seabed (2).
- Q18 (3): That oxygen only builds up in the air once the rate of production exceeds the rate of absorption. Until then, everything made is immediately consumed and the atmospheric level stays near zero — however much is being produced.
- Q19 (3): High oxygen supported giant insects and arthropods — insects rely on passive diffusion through tracheae, so more oxygen allows a larger body (2). It did not last because the vast coal-forming forests were eventually buried, decayed or burned, and the oxygen was reabsorbed as the carbon cycle rebalanced (1).
- Q20 (3): Complex animals need a lot of oxygen for aerobic respiration to power large, active, multicellular bodies — and they need an ozone layer, which only forms once atmospheric O₂ is high. The graph shows oxygen only reaching ~50% of present levels at about 0.6 bya, which is exactly when complex animals appear.
- Q21 (2): The Barberton Greenstone Belt, on the Mpumalanga–Eswatini border.