Earth may be called the “Blue Marble,” but it is not a perfect sphere. The planet is slightly flattened at the poles because of its rotation, and this flattening has a large effect on Earth’s gravity field. The flattening, or oblateness, can change as Earth’s crust sinks or rises according to the weight of ice sheets resting on its surface or as water from melting polar ice sheets enters the ocean.

In 2002, NASA and the German Aerospace Center launched the Gravity Recovery and Climate Experiment, or GRACE (and later the follow-on mission GRACE-FO), to track anomalies in Earth’s gravitational field and monitor the mass of ice sheets and ocean waters. But one key issue with GRACE was quickly identified. The oblateness measurements were off, leading to errors when calculating mass changes.

Organizations around the world have proposed ways to correct GRACE’s measurements. In a new study, Loomis et al. analyze existing methods and propose a solution of their own, integrating GRACE’s gravity anomaly data with a technique called satellite laser ranging (SLR).

With SLR, scientists send a laser pulse to a satellite, which reflects it back to Earth. By measuring the time the light pulse takes to return, they can precisely calculate the distance it traveled and gain valuable information about how Earth’s gravity field affects the motion of orbiting satellites. Other solutions have also integrated SLR, but this study explored various SLR data processing techniques to obtain the most accurate oblateness measurements and therefore the most accurate mass calculations.

More accurate estimates mean that ice melt at the poles lines up with observed sea level rise—every part of the global sea level budget is accounted for. The researchers discovered that there has been greater ice mass loss at both poles than was previously thought. Improving the accuracy of our mass change observations is critical for improving our models and advancing our understanding of our changing planet. (Geophysical Research Letters, https://doi.org/10.1029/2019GL082929, 2019)

—Elizabeth Thompson, Freelance Writer