"It's a bumpy world"!!
This is a three dimensional rendering of the Earth's geoid magnified a thousand times, with the portion due to the Earth's oblateness removed. The magnification highlights smaller scale variations in gravitational potential that are caused by the uneven distribution of mass over the surface of the Earth. The GRACE mission will measure these fluctuations with unprecedented precision. (Image courtesy of Jet Propusion Laboratory.)
Why A Mission To Study Gravity?
If Earth were a smooth sphere composed of similar elements or ingredients, there would be no need for a GRACE mission; the assumption made in most introductory physics courses that the acceleration due to Earth's gravitational field has a constant value would indeed be correct - end of story. However, previous observations have clearly demonstrated that our Earth isn't smooth and homogeneous and it really isn't even a sphere! What's more, the images shown above are just instantaneous snapshots from one moment in time. The reality is that the gravity field is continually changing, mostly due to variations in water content as it cycles between the atmosphere, oceans, continents, glaciers, and polar ice caps. By far the largest "lump" is the flattening observed at the poles - the Earth's oblateness. The above profiles have removed the portion of the response due to oblateness in order to focus on the smaller anomalies that exist. GRACE will reveal the broad features of the Earth's gravitational field overland and sea; it will also allow for these smaller scale features to be identified and studied with unprecedented accuracy, and it will show how the Earth's gravity field varies with time.
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