Inside Other Worlds: Building Cutaway Globes to Scale

The Cutaway Globes

1The Harborview Science Center is building a new exhibit called "Inside Other Worlds," and a team of teen volunteers is helping to build it. Eighth grader Yusra Kowalczyk joined the build team, which is led by exhibit designer Mr. Bram Oyelaran. The main pieces will be three cutaway globes of Earth, Mercury and the Moon. Each globe will have a wedge sliced out, so visitors can look inside and see the layers from the surface down to the center. Each globe will be built from foam layers glued together and then painted.

2No one has ever drilled more than about 12 km into Earth, so how do scientists know what is deep inside? Much of the evidence comes from earthquake waves. Seismometers around the world record how these waves speed up, slow down and bend as they pass through different materials far below the surface. One kind of earthquake wave cannot travel through liquid, and the places on the far side of Earth where those waves never arrive showed scientists that Earth's outer core is liquid metal. On the Moon, seismometers left by astronauts recorded moonquakes for several years. Spacecraft that orbit a world add more evidence: tiny changes in how the world's gravity pulls on them show how its mass is spread out inside.

3Table 1 lists Earth's main layers, plus the lowest layer of the air, which holds most of the gases in the atmosphere. Earth's radius, from the center to the surface, is about 6,371 km. Mr. Oyelaran chose a scale of 1 cm = 200 km for all three globes, which makes the Earth globe about 64 cm wide. "Some layers will be easy to build out of foam," he told the team. "Others will be so thin that we'll need a fine paint brush."

4Table 2 compares the three worlds. The numbers are rounded. The Moon's core is the hardest of the three to measure, so its size is an estimate. Before she saw the table, Yusra guessed that the biggest world would also have the biggest core compared with its own size. She decided to check her guess by turning each core radius into a percent of that world's radius.