Is the Earth growing or shrinking? The change may be small but the effects large long term. Since Charles Darwin's time, scientists have speculated that the solid Earth might be expanding or contracting. That was the prevailing belief, until scientists developed the theory of plate tectonics, which explained the large-scale motions of Earth's lithosphere, or outermost shell. Even with the acceptance of plate tectonics half a century ago, some Earth and space scientists have continued to speculate on Earth's possible expansion or contraction on various scientific grounds. Now a new NASA study, published recently in Geophysical Research Letters, has essentially laid those speculations to rest. Using a cadre of space measurement tools and a new data calculation technique, the team detected no statistically significant expansion of the solid Earth.
Is the Earth growing or shrinking? The change may be small but the effects large long term. Since Charles Darwin's time, scientists have speculated that the solid Earth might be expanding or contracting. That was the prevailing belief, until scientists developed the theory of plate tectonics, which explained the large-scale motions of Earth's lithosphere, or outermost shell. Even with the acceptance of plate tectonics half a century ago, some Earth and space scientists have continued to speculate on Earth's possible expansion or contraction on various scientific grounds. Now a new NASA study, published recently in Geophysical Research Letters, has essentially laid those speculations to rest. Using a cadre of space measurement tools and a new data calculation technique, the team detected no statistically significant expansion of the solid Earth.
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Before the concept of plate tectonics, global cooling was a reference to a geophysical theory by James Dwight Dana, also referred to as the contracting earth theory. It suggested that the Earth had been in a molten state, and features such as mountains formed as it cooled and shrank. As the interior of the Earth cooled and shrank, the rigid crust would have to shrink and crumple. The crumpling could produce features such as mountain ranges. After radioactive decay was discovered, it was realized it would release heat inside the planet. This undermines the cooling effect upon which the shrinking planet theory is based.
Knowing how the Earth expands or contracts provides a frame of reference for scientists to extrapolate from and removes uncertainty.
To make these measurements, the global science community established the International Terrestrial Reference Frame. This reference frame is used for ground navigation and for tracking spacecraft in Earth orbit. It is also used to monitor many aspects of global climate change, including sea level rise and its sources; imbalances in ice mass at Earth's poles; and the continuing rebound of Earth's surface following the retreat of the massive ice sheets that blanketed much of Earth during the last Ice Age.
But measuring changes in Earth's size hasn't exactly been easy for scientists to quite literally "get their arms around." After all, they can't just wrap a giant tape measure around Earth's belly to get a definitive reading. Fortunately, the field of high-precision space geodesy gives scientists tools they can use to estimate changes in Earth's radius. These include:
Satellite laser ranging -- a global observation station network that measures, with millimeter-level precision, the time it takes for ultrashort pulses of light to travel from the ground stations to satellites specially equipped with retroreflectors and back again.
Very-long baseline interferometry -- a radio astronomy technology that combines observations of an object made simultaneously by many telescopes to simulate a telescope as big as the maximum distance between the telescopes.
Global Positioning System -- the U.S.-built space-based global navigation system that provides users around the world with precise location and time information.
Doppler Orbitography and Radiopositioning Integrated by Satellite -- a French satellite system used to determine satellite orbits and positioning. Beacons on the ground emit radio signals that are received by satellites. The movement of the satellites causes a frequency shift of the signal that can be observed to determine ground positions and other information.
Scientists use all these techniques to calculate the International Terrestrial Reference Frame. Central to the reference frame is its point of origin: the precise location of the average center of mass of the total Earth system (the combination of the solid Earth and the fluid envelope of ocean, ice and atmosphere that surrounds it, around which all Earth satellites orbit). Scientists currently determine this origin point based on a quarter century of satellite laser ranging data, considered the most accurate space geodetic tool for this purpose.
The team applied a new data calculation technique to estimate the rate of change in the solid Earth's average radius over time, taking into account the effects of other geophysical processes. The previously discussed geodetic techniques (satellite laser ranging, very-long baseline interferometry and GPS) were used to obtain data on Earth surface movements from a global network of carefully selected sites. These data were then combined with measurements of Earth's gravity from NASA's Gravity Recovery and Climate Experiment (GRACE) spacecraft and models of ocean bottom pressure, which help scientists interpret gravity change data over the ocean.
The result? The scientists estimated the average change in Earth's radius to be 0.004 inches (0.1 millimeters) per year, or about the thickness of a human hair, a rate considered statistically insignificant.
For further information: http://www.nasa.gov/topics/earth/features/earth20110816.html
