How much water is hidden beneath our feet?

Space · Published 8 October 2026 Swipe more like this →

In short: Soil moisture is a critical but elusive variable in our planet's climate system. NASA's SMAP mission provides a high-resolution look at the water trapped in the earth's surface, offering a new lens to predict natural hazards and understand the complex dance between water, energy, and carbon.

The Soil Moisture Active Passive (SMAP) mission represents a significant leap in Earth-observing technology. As the first U.S. satellite specifically designed for global surface soil moisture observations, it provides a direct measurement of the water content in the top layer of soil. This data is essential because soil moisture acts as a primary link between the water, energy, and carbon cycles, influencing everything from ecosystem productivity to regional climate patterns.

To achieve this, SMAP utilizes a dual-instrument approach. It combines a synthetic aperture radar (SAR) with a radiometer to map conditions every two to three days. This combination allows the satellite to distinguish between ground that is frozen or thawed and to measure moisture levels in areas that are not frozen or covered in water. These high-resolution measurements are designed to improve the accuracy of weather, flood, and drought forecasts, which is particularly vital for monitoring agricultural productivity in a world where 800 million people face hunger.

The mission's data products vary in resolution and latency. For instance, high-resolution radar data can be gridded at 112 meters, while daily composites for soil moisture and freeze-thaw states are provided at 350 meters. While some components of the satellite's radar system faced challenges post-launch, NASA has explored using radar from European satellites, like Sentinel, as a potential replacement. By providing consistent global data, SMAP helps scientists decode the hidden mechanisms of our changing planet.

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