|
GRAIL - Science Reports
|
First GRAIL Science Data reveals new Feature of Lunar Geology
|
December 5, 2012
|
Image: NASA/JPL/Caltech
|
The first scientific findings of NASA’s GRAIL (Gravity Recovery and Interior Laboratory) Mission were presented on Wednesday as part of the fall meeting of the American Geophysical Union by members of the GRAIL Science Team. Data presented included the most refined gravity and Bouguer Gravity map of the Moon ever created which points to a previously unknown geological feature of the Moon revealing a new chapter of Lunar evolution.
GRAIL uses two spacecraft flying in close formation in Lunar orbit to measure the Moon’s gravity at an unprecedented accuracy. Measurements are made by using doppler shifting which utilizes changes in an orbiter’s radio frequency that changes as the spacecraft’s orbital velocity changes ever so slightly due to the gravitational field of the body it orbits. If a spacecraft flies over an area of concentrated gravity, it is pulled down and speeds up, and if it flies over an area where the gravitational field is weaker, it slows down. This can be tracked via doppler tracking using the spacecraft to emit radio waves that are measured by ground stations on Earth. |
Because the Moon only shows one side of it to Earth, two spacecraft are needed to generate a full gravity map, because a single spacecraft could not be tracked from Earth while in eclipse, hidden by the Moon. The two GRAIL spacecraft track the other’s relative velocity with an accuracy of 50 nanometers per second. These measurements can be converted into data on the Moon’s gravity which can then be used to examine a wealth of other properties of the Moon such as crustal thickness, crustal density, Bouguer Gravity and porosity as well as gravity anomalies which are of most interest to scientists. Data acquired by the GRAIL Spacecraft will provide better understanding of how Earth and other rocky planets in the solar system formed and evolved.
Data presented on Wednesday included the most accurate map of the Moon’s gravitational properties ever generated, even exceeding maps of Earth’s gravity. Below, two maps of Lunar gravity are shown: left is the map generated by the Lunar Prospector Mission in 1998/99 and on the right is the map generated with GRAIL data. The difference in quality is clearly visible. The two maps are calibrated, showing gravity in units of Milligalileos. Galileo is a unit of acceleration that is often used in gravimetry and is defined as 1 centimeter per second squared. In the two maps that are presented in Mercator projection, the far side of the Moon is shown in the center while the near side is at either side of the map.
Data presented on Wednesday included the most accurate map of the Moon’s gravitational properties ever generated, even exceeding maps of Earth’s gravity. Below, two maps of Lunar gravity are shown: left is the map generated by the Lunar Prospector Mission in 1998/99 and on the right is the map generated with GRAIL data. The difference in quality is clearly visible. The two maps are calibrated, showing gravity in units of Milligalileos. Galileo is a unit of acceleration that is often used in gravimetry and is defined as 1 centimeter per second squared. In the two maps that are presented in Mercator projection, the far side of the Moon is shown in the center while the near side is at either side of the map.
Lunar Prospector
Image: NASA/ARC/MIT
|
GRAIL
Image: NASA/ARC/MIT
|
"What this map tells us is that more than any other celestial body we know of, the moon wears its gravity field on its sleeve," said GRAIL Principal Investigator Maria Zuber of the Massachusetts Institute of Technology in Cambridge. "When we see a notable change in the gravity field, we can sync up this change with surface topography features such as craters, rilles or mountains."
With these gravity maps, the GRAIL science team was able to study the Moon’s crust, in particular its thickness, density and porosity.
"With our new crustal bulk density determination, we find that the average thickness of the moon's crust is between 21 and 27 miles (34 and 43 kilometers), which is about 6 to 12 miles (10 to 20 kilometers) thinner than previously thought," said Mark Wieczorek, GRAIL co-investigator at the Institut de Physique du Globe de Paris. "With this crustal thickness, the bulk composition of the moon is similar to that of 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."
With these gravity maps, the GRAIL science team was able to study the Moon’s crust, in particular its thickness, density and porosity.
"With our new crustal bulk density determination, we find that the average thickness of the moon's crust is between 21 and 27 miles (34 and 43 kilometers), which is about 6 to 12 miles (10 to 20 kilometers) thinner than previously thought," said Mark Wieczorek, GRAIL co-investigator at the Institut de Physique du Globe de Paris. "With this crustal thickness, the bulk composition of the moon is similar to that of 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."
|
The graphic to the right shows the bulk density of the Lunar highlands on the near (left) and far (right) side of the Moon. The map combines GRAIL’s gravity data and topographic data acquired by the Lunar Reconnaissance Orbiter, LRO. Red areas represent locations of higher than average densities while blue corresponds to lower than average densities. “The average bulk density of the lunar highlands crust is 2,550 kilograms per meter cubed, which is 12 percent lower than generally assumed,” a NASA statement noted. The image shows reduced crustal thickness in Lunar impact basins. The stars in the map mark Olivine-rich areas as detected by the Japanese Kaguya mission. These predominate in areas where the crust is absent and mantel material is exposed, for example as a result of impacts.
The map below shows the porosity of the Lunar highland crust based on GRAIL measurements and grain density measurements from NASA's Apollo moon mission samples as well as orbital remote-sensing data. |
Image: NASA/JPL-Caltech/IPGP
|
Red represents high porosities while blue corresponds to lower porosity levels. White regions contain mare basalts and were not analyzed. GRAIL determined the average porosity to be about 12%. Lunar porosity is the result of billions of years of impact cratering and associated fractures of lunar material and thus increasing porosity. It is believed that the Moon in its initial stage, after formation, had zero porosity because its outer layer was molten before becoming solid and so formed a non-porous surface layer, meaning that porosity is a feature of Lunar evolution. As the map shows, crustal porosities in the interiors of many impact basins are lower than their surroundings which is a consequence of higher temperatures experienced at the time of crater formation. Porosities measured in the direct vicinity of of many impact basins are higher than average, due to fracturing generated by the forces of impacts as well as the ejection of loose material during impacts.
GRAIL - Porosity Map
Image: NASA/JPL-Caltech/IPGP
One of the most important data sets provided by GRAIL is the Bouguer Gravity which represents gravity data with effective gravity removed. Effective gravity is caused by topographic features which can be seen on the surface of the Moon. Topographic data from LRO was used to calculate local topographic gravity to subtract these figures from GRAIL gravity to generate a Bouguer Gravity Map of the Moon. GRAIL data has shown that 98% of local gravity is caused by local topography while the remaining 2% are features of the crust. The Bouguer map shown below illustrates what remains from the gravity field when the attraction of surface topography is removed – showing mass anomalies inside the Moon, either due to variations in crustal thickness or mantle density. Red areas have stronger gravities than blue areas.
Clearly visible are local anomalies in the form of Mascons, mass concentrations hidden beneath the surface. Mascons have been known since the 1960 when teams observed a phenomenon of spacecraft suffering disturbances in their lunar orbits as a result of these hidden gravity features. Since then, Mascons have been of great interest and GRAIL data of Mascons will be heavily studied, especially data obtained during the extended mission which focused on local and regional gravity features.
Clearly visible are local anomalies in the form of Mascons, mass concentrations hidden beneath the surface. Mascons have been known since the 1960 when teams observed a phenomenon of spacecraft suffering disturbances in their lunar orbits as a result of these hidden gravity features. Since then, Mascons have been of great interest and GRAIL data of Mascons will be heavily studied, especially data obtained during the extended mission which focused on local and regional gravity features.
Image: NASA/JPL-Caltech/CSM
Comparison: Gravity (Left) and Bouguer Gravity (Right)
|
|
|
Gravity Gradient Maps
Image: NASA/JPL-Caltech/CSM
Image: NASA/JPL-Caltech/CSM
|
From these Bouguer Gravity Maps, scientists derived gravity gradient maps. To the left is a map of gravity gradients calculated by GRAIL. Red and blue represent stronger gravity gradients. The gravity gradients are given Eotvos which is a unit of acceleration divided by distance, in this case, it is defined as Galileo per Centimeter.
"We used gradients of the gravity field in order to highlight smaller and narrower structures than could be seen in previous datasets," said Jeff Andrews-Hanna, a GRAIL guest scientist with the Colorado School of Mines in Golden. "This data revealed a population of long, linear gravity anomalies, with lengths of hundreds of kilometers, crisscrossing the surface. These linear gravity anomalies indicate the presence of dikes, or long, thin, vertical bodies of solidified magma in the subsurface. The dikes are among the oldest features on the moon, and understanding them will tell us about its early history." Below, the left are the same gravity gradient maps with linear gravity anomalies being highlighted. These anomalies appear as long blue streaks in the map. Teams found at least 22 anomalies with a combined length of more than 4,800 Kilometers. Individual features are as long as 500 Kilometers and have a width of up to 40 Kilometers. |
Teams identified these features as dikes which are sheet intrusions that refer to geologic bodies that cut discordantly across rock formations. Dikes can be of magmatic or sedimentary origin. The dikes found on the Moon are solidified magma filled cracks that forms beneath the surface.
Dikes have not been identified on the Moon before, making them a discovery purely made by the GRAIL mission. The presence of these dikes confirms a chapter of early Lunar evolution that was previously predicted by theoretical models. As the Moon formed, a phenomenon called thermal inversion occurred. Material was being deposited onto the outer layer of the forming body after a Mars-sized object crashed into Earth to start the formation of our Moon. As a result of its formation, the Moon was warmer on its exterior than on its inside. As a result, the inside started to heat up before reaching thermal equilibrium. This caused the Moon to expand, increasing its radius by as much as 5 Kilometers. During this expansion, the surface cracked open and these cracks were filled in with volcanic material, forming the dikes that were now observed by GRAIL.
Dikes have not been identified on the Moon before, making them a discovery purely made by the GRAIL mission. The presence of these dikes confirms a chapter of early Lunar evolution that was previously predicted by theoretical models. As the Moon formed, a phenomenon called thermal inversion occurred. Material was being deposited onto the outer layer of the forming body after a Mars-sized object crashed into Earth to start the formation of our Moon. As a result of its formation, the Moon was warmer on its exterior than on its inside. As a result, the inside started to heat up before reaching thermal equilibrium. This caused the Moon to expand, increasing its radius by as much as 5 Kilometers. During this expansion, the surface cracked open and these cracks were filled in with volcanic material, forming the dikes that were now observed by GRAIL.
|
The process of Lunar expansion had a duration of the first 500 million to one billion years of the Moon’s history. In this early stage, during which the crust became solid, the dikes were the first geological features to be formed on the Moon.
This is confirmed by looking at areas where impact craters and dikes intersect. In the image to the right, a gravity gradient map and a topographic map of the Crisium basin is shown. Gravity gradient data shown in the left map reveal a linear anomaly which represents a dike while the right image of the same region does not show a sign of the feature. The fact that the deep impact basin has broken up the gravity anomaly confirms that the dike was present when the impact occurred. |
Image: NASA/JPL-Caltech/CSM
|
Left: NASA/JPL-Caltech/CSM, Right: Photo Copyright © Louis Maher
This image shows a profile across one of the linear gravity anomalies found by GRAIL, confirming that the anomalies have a higher gravity than their surroundings which is typical for dikes. In the bottom left, the measured gravity profile is shown in red with a model of the gravity properties of a dike shown in white color – confirming a good match. In the upper left, the lunar gravity gradient map is shown with the location of the measured anomaly in the center. To the right is an image of a dike on Earth, located at Ship Rock, New Mexico. The lunar dikes discovered by GRAIL are 50 times longer and 1,000 times wider than the dike seen here.
Lunar Dike Map
Image: NASA/JPL-Caltech/CSM
|
Dike on the Far Side
Image: NASA/JPL-Caltech/CSM
|
 |
