Meanwhile: Water Worlds

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Meanwhile: Water Worlds

Published: 25 Sep. 2025, 00:05

Audio report: written by reporters, read by AI

Moon Hong-kyu

The author is a principal researcher at the Korea Astronomy and Space Science Institute.

It may be hard to believe, but several moons in our solar system conceal vast underground oceans. NASA calls them “water worlds,” and beneath their thick icy crusts lie immense seas. For instance, Jupiter’s moon Europa is thought to hold twice as much water as Earth’s oceans, while Ganymede has 25 times more and Saturn’s Titan contains about 12 times as much. These subsurface seas remain liquid thanks to heat generated by radioactive decay and tidal friction from gravitational pulls, making them stable even without sunlight.

What about Earth? With a diameter of 12,756 kilometers, Earth’s oceans could be imagined as a single water droplet 1,360 kilometers across — large enough to swallow the Korean Peninsula at its widest point. Freshwater paints a more sobering picture. If gathered, it would form a droplet just 406 kilometers in diameter, roughly just the distance from Seoul to Yeosu.

In this artist’s concept, the moon Ganymede orbits the giant planet Jupiter. NASA’s Hubble Space Telescope observed aurorae on the moon controlled by Ganymede’s magnetic fields. A saline ocean under the moon’s icy crust reduces shifting in the auroral belts as measured by Hubble. [NASA/ESA]

For a different perspective, imagine that all the water on Earth could fit into a two-liter bottle. Proportionally, that would mean that glaciers, ice sheets and groundwater would amount to only a few tablespoons. The water vapor in clouds would thus be only the size of a fingernail clipping. Rivers and lakes — surface water we see daily — would fill a single eyedropper bottle, and of that, drinkable water would equal a single teaspoon. Yet that “eyedropper” would still represent the equivalent of 30 million reservoirs the size of Obong in Gangneung.

Water levels at Obong Reservoir once plummeted to 11.5 percent before rebounding, a reminder of how precarious supplies can be. Residents endured restrictions as their lifeline nearly ran dry. The causes of Gangneung’s summer drought can be explained by middle school science. Westerly winds climbing the Taebaek Mountains cool the air quickly, releasing rain on the Western slopes. Once over the ridge, the winds dry out, leaving the east parched. Even when rain does fall, the steep terrain sends most water rushing into the East Sea before it can seep underground.

Experts argue that our solutions must match these conditions. Facilities that trap and store rainwater are one answer, providing reserves when natural flows fail. In that sense, “water worlds” are not only distant moons orbiting Jupiter or Saturn, but also a model for how to confront Earth’s future water shortages.

This article was originally written in Korean and translated by a bilingual reporter with the help of generative AI tools. It was then edited by a native English-speaking editor. All AI-assisted translations are reviewed and refined by our newsroom.