I n some ways, scientists know more about Mars, Venus and the Moon than they know about Earth. That’s because 70 percent of Earth is hidden under its ocean. In some places, the ocean is over five miles deep — much deeper than Earth’s highest mountain is high.

The ocean contains about 98 percent of all the water on Earth. That leaves only 2 percent fresh (unsalty) water for lakes, rivers, streams, and swimming pools. And the ocean has 99 percent of the livable space on the planet!

The ocean — at least below a few feet deep — is an alien world most of us hardly think about. But when it comes to figuring out how Earth works, especially weather and climate, the ocean is the most important piece of the puzzle. And it is still full of mysteries.

Before satellites, the information we had about the ocean was pretty much “hit or miss.” Measurements were taken from ships, buoys, and instruments set adrift on the waves.

But now we have oceanobserving satellites. They measure how the “hills and valleys,” or topography, change with the seasons. Satellites measure the ocean’s currents, waves, and winds. They check on the health of the tiny ocean plants called phytoplankton, which supply half the oxygen in the atmosphere. They tell us how much of the Arctic Ocean is covered by sea ice this year, compared with past years. Satellites also measure rainfall, the amount of sunlight reaching the sea, the temperature of the ocean’s surface, and even how salty it is!

We need to keep watch on all these features of the ocean in order to understand the ever changing Earth, its water and energy cycle, and climate and weather. In just a few months, one satellite can collect more information about the ocean than all the ships and buoys in the world have collected over the past

100 years!

NASA’s Earth Science Division has launched many missions to planet Earth. These satellites and other studies all help us understand how the atmosphere, the ocean, the land and life — including humans — all interact together.

Explore our planet at The Space Place, spaceplace.nasa.gov/earth.

THIS ARTICLE was written by Diane K. Fisher and provided through the courtesy of the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.