Posts Tagged ‘GRAIL mission’

Extreme Worlds: The Moon’s Inner Core Revealed

Tuesday, February 22nd, 2011

By Todd Ratcliff

Apollo 15 and Apollo 11 retroreflectorsThe Apollo 15 (left) and Apollo 11 (right) lunar laser ranging retroreflector arrays. Image credit: NASA/D. Scott

Everyone knows that Earth’s Moon is a cold, dead chunk of rock, right? Hold on there, Sparky, not so fast! While it’s true that it was once thought that the moon was an inert, lifeless ball, scientists have known since the days of the Apollo astronauts that there’s much more going on inside our moon than meets the eye.

On Earth, seismology is one of the best ways to probe what’s happening inside our planet. We can “see” the layers and boundaries deep inside Earth, similar to how a sonogram lets us see the shape of a baby in a pregnant woman’s belly. Seismology is what lets us know that Earth’s core is divided into a solid inner core and liquid outer core.

So it only made sense for Apollo astronauts to place seismometers on the moon in order to measure the rumblings and grumblings of the lunar interior. Unfortunately, the locations of the moonquakes and positions of the seismometers didn’t allow for a nice “sonogram” of any possible lunar core. Despite almost a decade of measuring moonquakes, we never got a good peek at the core.

diagram of the moon's interior structureDiagram of the moon’s interior structure. Image credit: NASA

Another Apollo experiment left panels of retroreflectors — essentially giant mirrors (about a square meter, or about 11 square feet, in size) — on the moon’s surface. Observatories on Earth fire laser beams at the reflectors and measure how long it takes light to make the round-trip. Lunar Laser Ranging (LLR) gives us a remarkably accurate measure of the Earth-moon distance, good to within a couple of centimeters (a little less than an inch). LLR has also let us know that our moon probably isn’t completely solid. The moon most likely has a liquid iron core and possibly, like Earth, a solid iron inner core.

Now lunar scientists have revisited and applied modern seismic analysis techniques to the more than 30-year-old lunar seismic data set. Their analysis supports the idea that the moon does indeed have an inner core of solid iron surrounded by an outer core of liquid iron. Similar to (but very much smaller than) Earth’s core!

If all goes well, the soon-to-be launched GRAIL mission, whose twin orbiters will measure gravity in exacting detail, will shed even more light on the nature of our moon’s interior. Things are getting pretty interesting for this “cold, dead chunk of rock!”


Lunar Eclipse, the Moon’s Interior, and the Holy GRAIL

Wednesday, December 15th, 2010

By Sami Asmar

Earth's moon

In addition to the awesome views they offer, lunar eclipses have always provided scientific clues about the moon’s shape, location and even surface composition. Although there will continue to be opportunities for observers to examine and reflect on fundamental concepts about the moon, such as its origin and interior structure, more modern tools are aiding these observations.

When it comes to understanding what a moon or a planet is made of remotely — short of touching it or placing seismometers on its surface or probes below the surface — classical physics comes to the rescue. By measuring the magnetic and gravitational forces that are generated on the inside and manifested on the outside of a planet or moon, we can learn volumes about the structure of its interior.

A spacecraft in the proximity of the moon can detect these forces. In the case of gravity, the mass of the moon will pull on the spacecraft due to gravitational attraction. If the spacecraft is transmitting a stable radio signal at the time, its frequency will shift by an amount exactly proportional to the forces pulling on the spacecraft.

This is how we weigh the moon and go further by measuring the detailed distribution of the densities of mountains and valleys as well as features below the moon’s surface. This collection of information is called the gravity field.

In the past, this has lead to the discovery mascons on the moon, or hidden, sub-surface concentrations of mass not obvious in images or topography. If not accounted for, mascons can complicate the navigation of future landed missions. A mission, human or robotic, attempting to land on the moon would need to have a detailed knowledge of the gravity field in order to navigate the landing process safely. If a spacecraft sensed gravitational pull higher than planned, it could jeopardize the mission.

GRAIL spacecraft

The Gravity Recovery and Interior Laboratory (GRAIL) mission, scheduled to launch in September, is comprised of twin spacecraft flying in formation with radio links between them to measure the moon’s gravity field globally. This is because a single spacecraft with a link to Earth would be obstructed when the spacecraft goes behind the moon, leaving us with no measurement for nearly half of the moon, since the moon’s far side never faces the Earth. The GRAIL technique may also reveal if the Moon has a core with a fluid layer.

So as you go out to watch the lunar eclipse on the night of Dec. 20, think about how much we’ve learned about the moon so far and what more we can learn through missions like GRAIL. Even at a close distance from Earth, the moon remains a mystery waiting be uncovered.