Summary
A NASA-led team has proposed a satellite-tracking technique for measuring seasonal shifts in Earth’s center of mass. The study finds that the annual movement may be about half as large as estimates made eight years earlier.
A NASA Jet Propulsion Laboratory-led team has proposed a technique for measuring how Earth’s center of mass moves as water, ice and air are redistributed across the planet. The method combines ultraprecise satellite tracking with models of how changing surface loads deform Earth’s crust, estimating the seasonal displacement to within fractions of an inch.
The findings were published in Geophysical Journal International. The team estimates that Earth’s annual back-and-forth movement of mass is about half as large as scientists believed eight years ago.
A more precise way to locate Earth’s mass centre
Earth’s center of mass, also called its geocenter, is the point around which the planet’s mass is balanced. It is not permanently aligned with the planet’s geometric center because oceans, groundwater, snow, ice and air are constantly moving. These changes shift the geocenter by several millimeters over the course of a year.
The movement is small, but the geocenter provides an important reference for satellite navigation, elevation measurements and other positioning systems. Existing estimates have not agreed closely: international estimates made in 2017 and 2023 differed by 0.27 inches, or 7 millimeters. That difference was nearly as large as the movement being measured.
The new technique was developed by JPL geoscientist Donald Argus with researchers from JPL’s satellite orbit determination team, the University of Nevada, the University of Montana and Germany’s Helmholtz Centre for Geosciences.
Satellites orbit Earth according to its gravitational field, which is governed by the planet’s center of mass. Small changes in the distance between satellites and ground stations can therefore reveal movement in that reference point.
The approach expands on satellite laser ranging, in which ground stations measure the positions of the LAGEOS 1 and LAGEOS 2 satellites using reflected laser pulses. Those satellites, launched in 1976 and 1992, carry reflective prisms and are tracked by stations in more than 20 countries.
The new method adds GPS measurements and orbital data from several low-Earth-orbit satellites. It also accounts for the way the weight of water and ice bends Earth’s crust, allowing the movement of the ground stations themselves to be included in the calculation. This addresses a limitation of laser ranging: ground stations are not evenly distributed around the planet.
How the seasons shift Earth’s mass
The researchers grouped the seasonal sources of movement into oceans, atmosphere and continental water. Continental water includes land ice, snow, lake and river water, soil moisture and groundwater.
Snow accumulation in North America and Eurasia reaches a maximum in March. The researchers estimate that it shifts Earth’s center of mass about 3 millimeters toward the North Pole. In April, rainwater in the Amazon basin peaks at about 2,400 gigatons, moving the center of mass an estimated 2.2 millimeters toward South America.
Monsoon water in Southeast Asia reaches a maximum of about 600 gigatons in November and adds a smaller contribution to the yearly oscillation. Between August and October, meltwater and rain increase the mass of the oceans, with the largest effect occurring in the South Pacific. Seasonal changes in several other seas, including the Mediterranean, Red, North, Baltic and Barents seas, contribute smaller shifts.
Atmospheric mass matters as well. Using a model from the European Centre for Medium-Range Weather Forecasts, the researchers found that dense winter air shifts the balance toward Arabia, Asia and northern Africa around Dec. 21, and toward South America and South Africa around June 21.
Why millimetres matter
The mass calculations are consistent with observations from the Gravity Recovery and Climate Experiment Follow-On, or GRACE-FO, mission. Its twin satellites measure monthly changes in Earth’s gravitational pull, primarily caused by water moving above and below the surface. When one satellite passes over a region with more mass, such as a swollen river basin, the resulting gravitational tug changes the distance between the two spacecraft.
The NASA-German Research Centre for Geosciences mission has been operating since its 2018 launch. NASA says the follow-on GRACE-Continuity mission is targeting a launch in late 2028 to extend the nearly 25-year GRACE data record.
The JPL team says better estimates of the geocenter can support more accurate reference systems for mapping and navigation. Those systems underpin applications ranging from global shipping logistics to precision agriculture, where millimetre-scale changes in positioning can affect measurements and operations.