As the first endeavour intended to study lunar gravity, its findings would help scientists better understand how rocky planets, like Earth, formed. That’s especially true since many theories suggest the moon was created when a cosmic collison lopped off a portion of the Earth. And they’re both made of the same rocky materials.
Since then, GRAIL has brought back some significant — and colourful — information. New findings are still being published, including recent details about the moon’s topography published in the May 2014 issue of Geophysical Research Letters.
Twin spacecraft, named Ebb and Flow — about the size of washing machines — chased each other around the moon for three months, from March to May 2012. Because different geographical formations, like mountains and valleys, exert stronger or weaker gravitational pulls, their distance apart varied as they traveled.
For example, if the first craft flew over a mountain — which has more gravitational pull because it has more mass — the first satellite would speed up, as shown (in exaggerated form) in the GIF to the left.
For a year after, data on the moon’s composition and gravity was collected from GRAIL. By analysing the two crafts’ distance apart, scientists could determine the moon’s gravity field, as well the topography and the make up of the rocks under the surface. In this way, GRAIL was vital to understanding how rocky plants, like Earth, Venus, Mercury, and Mars, formed.
And when Ebb and Flow had fulfilled their duties, NASA crashed them into a rocky hillside on the moon. These “psychedelic” maps and an ashy crater are all that remain.
NASA’s Scientific Visualisation Studio published the first map created using GRAIL’s data, shown below, in December 2012. The intense colouring , present in most gravity maps, indicates variations in gravitational pull across the moon’s surface.
Red marks areas of high gravity, while dark blue shows the lowest pull, measured in Radial Gravity Anomalies (mGALS).
Published in three papers in the journal Science in 2012, the detailed data reveals many never-before-seen features of the moon, such as tectonic structures and even dead volcanoes. The data also helped determine the moon’s crust is much thinner than previously imagined — between 21 and 27 miles thick, making it similar to Earth.
“This supports models where the moon is derived from Earth materials that were ejected during a giant impact event early in solar system history,” said Mark Wieczorek, GRAIL co-investigator at the Institut de Physique du Globe de Paris.
The map below also shows the moon’s gravity field but using a different calculation method. If the moon were a perfectly smooth sphere of uniform density, like a pool ball, its gravity map would show uniform colour. But like any terrestrial body, the moon’s surface has craters and mountains as well as variations under the surface that change its gravitational pull.
This creates a Bouguer gravity map, seen below, which subtracts the effect of the bumpy surface to show the lumpiness of the moon’s gravity underneath the surface, essentially, the gravity field created by the moon’s interior.
NASA also created a moon map based on GRAIL data to show the Moon’s surface topography, known as a Lunar Orbiter Laser Altimeter, or LOLA, map. It’s measured in kilometers, instead of mGALS. Red marks the tallest formations, while blue shows the lowest.
Finally, crustal thickness, shown below, can be inferred from the Bouguer map. Assuming the crust’s density is uniform, any variations in gravity visible on the Bouguer map can be explained by variations in the crust’s thickness, measured in kilometers, as well. Under the crust, also like Earth, lies a mantle, molten outer core and solid inner core, possibly made of iron.
This animated gif shows the stunning differences between the moon’s topography, on the left, and its gravity field, on the right.
Recently, NASA was able to increase the detail presents in these finding by combining the two measurements.
In the May 2014 issue of Geophysical Research Letters, a publication of the American Geophysical Union, the Scientific Visualisation Studio created the image below to correlate topographical formations with the gravity field, which previous maps had separated. It shows the moon’s surface in much greater detail than previous versions.
The paper adds more information to what we already know about the Moon’s interior and its thermal history.
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