The traffic runs both ways

There is an obvious way to think about this topic and a much better one, and the specification quietly tells you which is which. The obvious way: you learn how Earth works, and then you use it to read other worlds. The same processes happen everywhere, so a bowl-shaped hollow on the Moon was made the same way a bowl-shaped hollow here was made. ⭐ THAT PRINCIPLE HAS A NAME, UNIFORMITARIANISM, AND IT IS GENUINELY HALF THE TOPIC. Without it you could not say anything at all about a place nobody has visited. ⚠️⚠️ NOW READ WHAT YOUR SPECIFICATION ACTUALLY ASKS FOR, BECAUSE IT POINTS THE OTHER WAY. IT WANTS PLANETARY LANDFORMS AS EVIDENCE FOR UNSEEN EARTH PROCESSES, AND METEORITES AS EVIDENCE FOR THE COMPOSITION OF THE EARTH. Both of those run inwards. Other worlds are not only things we explain using Earth. They are where we go to find out about Earth. ⭐⭐ AND THE REASON IS SOMETHING YOU ALREADY KNOW: EARTH DESTROYS ITS OWN RECORD. Our planet has air and water that break rock down where it stands. It has moving plates that drag surfaces under and bring new ones up. Anything written on the surface gets rubbed out, and fairly quickly. The Moon does almost none of that. Practically no atmosphere, very little activity, nothing much to wear a mark away once it is made. ⚠️⚠️ SO THE MOON KEPT ITS IMPACT RECORD AND EARTH DID NOT. NOT BECAUSE EARTH WAS HIT LESS. IT WAS HIT AT LEAST AS MUCH, AND IT TIDIED UP AFTERWARDS. ⭐ WHICH MEANS THE WORST PLACE IN THE SOLAR SYSTEM TO STUDY WHAT HAS HIT EARTH IS EARTH. Now the sharpest version of the same idea, and it is worth stopping on. ⭐⭐ NO DRILL HAS EVER REACHED THE CENTRE OF OUR OWN PLANET, AND NOTHING EVER WILL. YET WE SAY THINGS ABOUT WHAT IS DOWN THERE WITH REAL CONFIDENCE, AND PART OF THE REASON IS ROCKS THAT FELL OUT OF THE SKY. Some of what arrives from space is metal rather than stone, and is understood as fragments of the inside of bodies that broke apart. Studying those is the closest anybody gets to holding a piece of a core. ⭐ CARRY ONE QUESTION THROUGH EVERYTHING THAT FOLLOWS: IS EARTH EXPLAINING THIS WORLD, OR IS THIS WORLD EXPLAINING EARTH?

Three things other worlds keep

Earth is an active, weathered planet, and that is exactly why it is a poor place to read certain kinds of evidence.

Label the layers of the Earth

This is a cutaway of the Earth and nothing on it is labelled. Tap a label, then tap the pin it belongs to. Two of the labels name real features of the Earth that this diagram does not show.

Known from rocks that fell

Nobody has ever obtained a sample from the centre of the Earth, and nobody ever will. Yet geologists describe what is down there with some confidence. Where does part of that confidence come from?

From material that has fallen to Earth from space, some of it metal, understood as fragments from inside bodies that broke apart
From deep boreholes that have reached the core in a few places
From material brought up by volcanoes from the centre
It is essentially an assumption with no physical evidence behind it

Five terms for comparing worlds

Five terms, each defined by what it is. The first is the principle that makes every comparison in this topic legitimate.

Match each observation to what it lets you read

A surface crowded with bowl-shaped hollows of many sizes
A lump of metal that fell from space and was recovered on the ground
A winding channel on another world, shaped like ones cut by water here
An Earth surface with almost no such hollows preserved on it
Two features of similar shape known to have formed in different ways

How often bodies from space have arrived, on a world that never tidied the marks away
What the inside of a planet may be made of, which no drilling here could ever supply
What may once have flowed there, argued by comparison with something whose origin is known
Not that it escaped being struck, but that its surface has been remade since
That shape alone is not proof of process, so an answer has to say what else could produce it

Two that follow from uniformitarianism

Select the TWO statements that follow from assuming the same processes act everywhere.

Evidence can be borrowed in either direction, so another world can be used to work out something about Earth as legitimately as the reverse
A shape alone does not prove which process made it, so a good answer says what else could have produced the same result
Other worlds must have had the same history as Earth
If a landform here and one elsewhere look alike, they were definitely made the same way

Arguing from one world to another

You may already have met the idea that Earth's surface is constantly being remade, and that what drives a rock's journey is where the energy comes from. ⚠️ THIS TOPIC SPENDS THAT FACT ON SOMETHING ELSE: IT IS WHY OUR RECORD IS MISSING. A shape that works on any compare-two-worlds question. ⭐⭐ SAY WHICH DIRECTION YOU ARE ARGUING IN, AND SAY IT FIRST. Are you using Earth to explain what is seen elsewhere, or using elsewhere to explain something about Earth? Most answers never state this, and stating it takes one clause. NAME THE PRINCIPLE THAT LICENSES IT. ⭐ THE COMPARISON IS ONLY LEGITIMATE BECAUSE THE SAME PROCESSES ARE ASSUMED TO ACT EVERYWHERE. WITHOUT THAT SENTENCE YOU ARE JUST NOTICING THAT TWO THINGS LOOK ALIKE. SAY WHAT IS SIMILAR AND WHAT IS DIFFERENT, AND BE SPECIFIC ABOUT THE CONDITIONS. Atmosphere or none, water or none, active surface or still one. ⭐ THOSE DIFFERENCES ARE USUALLY THE WHOLE EXPLANATION FOR WHY ONE WORLD KEPT SOMETHING AND THE OTHER DID NOT. THEN SAY WHAT THE EVIDENCE SUPPORTS, AND HOW FAR. ⭐ AND SAY WHAT ELSE COULD HAVE PRODUCED IT. This is the clause that lifts an answer, because two processes can leave similar shapes and a resemblance is a reason to look harder rather than a conclusion. ⚠️ THREE HABITS THAT COST MARKS HERE. ⚠️ THE FIRST IS TREATING RESEMBLANCE AS PROOF. It looks like a river channel, therefore water flowed. That is the start of an argument, not the end of one. ⚠️ THE SECOND IS ASSUMING EARTH IS THE NORMAL CASE. ⭐ FOR IMPACT EVIDENCE EARTH IS THE ODD ONE OUT, BECAUSE IT IS THE PLANET THAT ERASES THINGS. A STUDENT WHO REASONS FROM WHAT IS VISIBLE HERE WILL GET THE FREQUENCY OF IMPACTS BADLY WRONG. ⚠️ AND THE THIRD IS REACHING FOR A NUMBER OR A NAMED EVENT YOU ARE NOT SURE OF. A crater size, a date, a particular extinction. If the question supplies them, use them. If not, argue from the conditions, which is worth more and cannot be wrong. One last thing worth knowing. Saying that evidence is indirect is not a weakness in an answer. Almost everything known about the inside of a planet is indirect, and showing that you know which parts are inferred rather than observed is exactly what a geologist is expected to do.

Two craters, one missing record

Put two photographs side by side. One is a surface with no air and almost no activity. The other is a piece of ordinary countryside on Earth. ⭐ THE FIRST IS COVERED IN BOWL-SHAPED HOLLOWS. Big ones with smaller ones inside them, overlapping, crowded together, some sharp-edged and some worn down by later arrivals landing on top. The second has none at all. ⚠️⚠️ THE OBVIOUS READING IS THAT ONE PLACE GETS HIT AND THE OTHER DOES NOT. THAT READING IS WRONG, AND WORKING OUT WHY IS THE WHOLE POINT OF THE COMPARISON. Both surfaces sit in the same part of the solar system and have been there for an extremely long time. Whatever has been arriving has been arriving at both. ⭐⭐ SO THE DIFFERENCE IS NOT IN WHAT HAPPENED. IT IS IN WHAT SURVIVED. On the airless surface, a hollow once made simply stays. There is no rain to wash its edges down, no plants to soften it, no rivers to fill it in and no moving plates to carry it under. The only thing that changes it is the next arrival. ⭐ ON EARTH, EVERY ONE OF THOSE THINGS IS HAPPENING ALL THE TIME. Rock is broken down where it stands. Loose material is carried away and dumped elsewhere. Whole surfaces are dragged down and replaced. A mark made on this planet is on a surface that is being continually rubbed out and rewritten. ⚠️ WHICH GIVES YOU THE SENTENCE THAT EARNS THE MARKS: EARTH SHOWS FEW IMPACT SCARS NOT BECAUSE IT HAS BEEN STRUCK RARELY, BUT BECAUSE IT IS THE KIND OF PLANET THAT DOES NOT KEEP THEM. ⭐⭐ AND NOW THE PAYOFF, WHICH IS WHY THIS IS WORTH A WHOLE TOPIC. IF YOU WANT TO KNOW HOW OFTEN THINGS HAVE HIT EARTH, YOU CANNOT FIND OUT BY LOOKING AT EARTH. YOU LOOK AT THE PLACE THAT KEPT THE RECEIPTS. A neighbour with no atmosphere is, in effect, a notebook nobody wiped. Reading it tells you about your own history, not only about the neighbour. ⚠️ ONE CAUTION BEFORE YOU USE THIS. THE ARGUMENT ONLY HOLDS BECAUSE THE SAME KINDS OF PROCESS ARE ASSUMED TO OPERATE IN BOTH PLACES. ⭐ STATE THAT ASSUMPTION IN AN ANSWER. IT IS THE THING THAT TURNS A NOTICED RESEMBLANCE INTO AN ARGUMENT, AND IT COSTS ONE SENTENCE.

Order how a crater is read

Put these six moves into the order that turns a photograph of another world into an argument about Earth.

Describe what can actually be seen on the surface, without explaining it yet
State which way you are arguing: from Earth outwards, or from another world back to Earth
Name the assumption that licenses the comparison, that the same processes act everywhere
Set out the conditions that differ between the two worlds, such as atmosphere and activity
Explain what survived where, and why the difference in conditions accounts for it
Say what else could have produced the same appearance, and how you would tell them apart

Build the sentence that borrows evidence

This is the sentence that turns a resemblance into an argument. Assemble it.

The planetary geology run

Five questions on borrowing evidence between worlds. Three lives.

Spot the true planetary geology facts

Tap the TWO statements you could defend in an answer comparing Earth with another world.

A world with no atmosphere and little activity preserves a record that Earth has largely erased
Much of what is known about the inside of a planet is inferred rather than observed, and saying so is a strength in an answer
The scarcity of impact scars on Earth shows that it has rarely been struck
Because the same processes act everywhere, other worlds must have had the same history as Earth

Complete the planetary geology facts

The principle that the processes acting today are the same as those that acted in the past and elsewhere is _____. A piece of rock or metal from space that has reached the surface of a planet is a _____. Something studied because it resembles the thing you actually want to understand is an _____. The breakdown of rock where it stands at a planet's surface is _____.

uniformitarianism meteorite analogue weathering an impact crater erosion sediment the mantle

Three comparisons to tighten

Three student answers comparing Earth with another world. Each has noticed something real, and each stops one step short.

A student writes that a winding channel photographed on another world was cut by water, because it looks exactly like a river channel here. What would you add?
A student concludes that Earth has been struck by objects from space much less often than its airless neighbour, because far fewer scars are visible here. What has gone wrong?
A student writes that claims about the Earth's core are weak because nobody has ever sampled it directly. How would you respond?

Explain what another world can tell you

Explain how studying another world can tell geologists something about the Earth. Use impact craters, or the composition of the Earth's interior, or both.

Open by stating which direction you are arguing in: from Earth outwards, or from another world back to Earth
Name the assumption that makes the comparison legitimate, that the same processes act everywhere
Explain what conditions differ between the two worlds, covering atmosphere and how active each surface is
Explain why those differences mean one world kept evidence that the other lost
State clearly what this lets geologists conclude about the Earth that Earth itself could not show them
Say what else could account for the evidence, and how you would tell the possibilities apart
Finish by explaining why calling the evidence indirect is not a weakness, and avoid any figure or named event you are not certain of

Planetary Geology: Comparing Earth and Other Worlds

Get the method right under pressure

What you'll cover