For educators

Small craters are simple bowls. Above a certain size the walls slump into terraces and a peak rebounds in the middle. That changeover happens at a definite diameter — and the diameter is different on every world, because it is set by how hard gravity pulls the rim down. Measure where it happens on two worlds and the ratio you get is a ratio of gravities.

1. Open the Moon and choose Measure → Distance.
2. Find a dozen craters spanning roughly 5 km to 80 km. Click rim to rim across each and record the diameter.
3. Classify each as simple (a clean bowl) or complex (terraced walls, a central peak, or both). Agree the criteria before you start.
4. Order the list by diameter and mark where simple gives way to complex. That boundary is the transition diameter, D_t.
5. Repeat on Mars, then compare. Surface gravity is 1.62 m/s² on the Moon and 3.71 m/s² on Mars.

The prediction is D_t ∝ 1/g: stronger gravity, smaller craters collapsing. Mars pulls about 2.3 times harder than the Moon, so its transition should sit at roughly half the diameter — and published values, near 15 km for the Moon and 7 km for Mars, agree. Getting a ratio close to that from a dozen hand measurements is the result worth celebrating.

Have two groups classify the same craters. They will disagree on the borderline ones, and that disagreement is the honest error bar: the transition is a zone rather than a line, which is exactly why the published figures come with ranges.

Aligns with NGSS HS-ESS1-6 and CCSS-M HSG.MG.A.2 (density and ratio in modelling situations).

Open the Moon →

No rock samples, no radiometric lab — and students can still order two surfaces by age. More craters means more time exposed. It is how the entire lunar timescale was built before Apollo returned a single gram.

1. Pick a patch of dark mare and a patch of bright highlands at equal zoom.
2. Choose Measure → Counting box and set a side — 10 km is a good start on the Moon. The box is squared on the ground, not on the graticule, so it stays honest at high latitude.
3. Click a centre on the mare and count craters above a fixed diameter, then move the box to the highlands and count again. Fix the threshold before you start.
4. Compare densities and state which surface is older, with the reasoning.

Have two groups count the same box. The disagreement between them is the real result: it opens up counting statistics and why planetary scientists publish error bars rather than single numbers.

Aligns with NGSS HS-ESS1-6.

Open the Moon →

Olympus Mons is the stock example of Martian scale, and it stays an abstraction until a student measures it with the same bar they just used on Hawai‘i. The atlas maps Earth from the same instruments, so the comparison is like for like.

1. With Measure → Distance, click across the base of Mauna Loa on Earth to get its diameter.
2. Measure Olympus Mons on Mars the same way.
3. Express the difference as a ratio, not a subtraction.
4. Ask what Mars lacks that Earth has. (Plate motion — Hawai‘i's hotspot built a chain; Mars built one mountain.)

Aligns with NGSS MS-ESS1-3 (scale properties of solar-system objects) and CCSS-M 7.RP.A.2 (proportional relationships).

Open Mars → Open Earth →

Venus is under cloud that no visible-light camera penetrates. Every one of its 1,985 named features was mapped by bouncing radar off the ground from orbit. It is the cleanest classroom case of choosing an instrument to fit a wavelength.

1. Open Venus and look at the surface — then check the Licence panel to see what made the image.
2. Ask why the view is grey and shadowless, unlike the Moon.
3. Work out which wavelengths pass through thick cloud and which do not.
4. Predict what a visible-light camera in Venus orbit would return. (Cloud tops. Nothing else.)

Brightness in a radar image is roughness and slope, not colour or illumination. Students who read it as a photograph will misinterpret it — surfacing that misconception is the point.

Aligns with NGSS HS-PS4-5 (wave behaviour used to capture information).

Open Venus →

The atlas carries 36 landing and impact sites on the Moon, and over Apollo 11 the imagery reaches 26 cm per pixel — fine enough that the descent stage is a shape on the ground, not a label.

1. Fly to Tranquility Base and zoom until the descent stage resolves.
2. Use Measure → Distance to find how far the astronauts walked from the module.
3. Compare that distance to something known — a corridor, a track, the school field.
4. Then look at a rover-era site and measure again.

The distances are startlingly short. That lands the constraints of a pressure suit and a life-support budget better than any figure on a slide.

Aligns with CCSS-M 6.RP.A.3 (ratio reasoning with units).

Open the Moon →

Europa's surface is fractured ice over a global ocean. Enceladus vents that ocean into space through fractures at its south pole. Students can see the evidence for both and argue from it.

1. Examine the ridges and bands crossing Europa's ice. What do the crossings tell you about order of events?
2. Count craters on Europa and compare with the Moon. Why so few?
3. Find the south polar fractures on Enceladus.
4. Argue which is the better target for a life-detection mission, using what you observed.

Europa's near-absent craters are the whole argument: a surface that keeps erasing itself must be young, and something has to be doing the erasing.

Aligns with NGSS MS-ESS1-3 and HS-ESS1-6.

Open Europa → Open Enceladus →