NASA Missions Reveal Lunar Tidal Bulge From Space
Alan McStravick for redOrbit.com – Your Universe Online
Growing up, we all learned about how the tides of our oceans, seas and large lakes are caused by the moon. The explanation given us had to do with the effect of gravity that the heavenly body had on the fluid surfaces of our planet. It would make sense that Earth’s gravity would also have an effect on the moon, save its solid surface. In fact, there is such thing as a lunar body tide. Today, thanks to the combination of observations by two NASA missions, we are able to see this phenomenon for the first time.
The first mission, begun in 2009, was NASA’s Lunar Reconnaissance Orbiter (LRO). The second mission, called GRAIL (Gravity Recovery and Interior Laboratory) was instrumental in helping us to visualize the lunar body tide. As NASA noted, because both missions were orbiting spacecraft, Earth-bound scientists were able to take the entire moon into account. Observation of the moon from Earth is limiting because the moon only shows us one side.
“The deformation of the moon due to Earth’s pull is very challenging to measure, but learning more about it gives us clues about the interior of the moon,” said Erwan Mazarico, a scientist with the Massachusetts Institute of Technology who works at NASA’s Goddard Space Flight Center.
As noted above, the moon and the Earth are engaged in a perpetual gravitational tug-of-war that slightly deforms both orbs. As the researchers explain, the roundness of the objects is not as true as one might believe. In fact, both celestial bodies are pulled slightly by gravity causing the ends to resemble eggs rather than circles with the pointed ends pointing at one another.
Onboard the LRO is the Lunar Orbiter Laser Altimeter, or LOLA, which is key to being able to map height features on the moon’s surface. The LRO team selected lunar locations that the spacecraft had passed over repeatedly, albeit from different flight paths. In total, 350,000 locations were mapped using LOLA. Locations were on both the far and near sides of the moon.
Once the measurements were collected, the team calculated whether or not the height had risen or fallen from one pass to the next. If there was a change in height, it was determined that there had been a shift in the location of the moon’s bulge.
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On its own, the LRO would not have been able to provide visual proof of the lunar body tide. The GRAIL mission was required to help pinpoint the exact altitude of the LRO above the lunar surface. GRAIL was able to provide a detailed map of the moon’s gravity field.
“This study provides a more direct measurement of the lunar body tide and much more comprehensive coverage than has been achieved before,” said John Keller, LRO project scientist at Goddard.
“This research shows the power of bringing together the capabilities of two missions. The extraction of the tide from the LOLA data would have been impossible without the gravity model of the moon provided by the GRAIL mission,” said David Smith, the principal investigator for LRO’s LOLA instrument and the deputy principal investigator for the GRAIL mission. Smith is affiliated with Goddard and the Massachusetts Institute of Technology.
We now know the lunar body tide is created by a gravitational force that raises the surface of the moon at its bulge by approximately 20 inches on both the near and far sides.
Additionally, the missions showed the bulge is not location-permanent on the moon’s surface. It shifts a few inches over time. This is because the Earth, when viewed from the moon’s surface, moves around in a small patch of sky. Therefore, the moon’s bulge is responsive to where in space the Earth and its gravitational pull come from.
“If nothing changed on the moon – if there were no lunar body tide or if its tide were completely static – then every time scientists measured the surface height at a particular location, they would get the same value,” said Mike Barker, a Sigma Space Corporation scientist based at Goddard and co-author of the new study, which is available online in Geophysical Research Letters.