Wonderful Ice Halos

A bright moon halo surrounds the Moon on Dec. 11, 2011. © Jason Major

Have you ever seen a large ghostly disc around the Moon on a cool, calm, hazy night? If so, you have likely seen what is called an “Ice Halo” or “22° Halo.” Not only can the Moon display these ghostly rings of light, but the Sun does so in the day time too.

22° halos are visible all over the world and throughout the year; look for them whenever the sky is wispy or hazy with thin cirrus clouds – even in the hottest countries.

So what are they and why do they appear?

Ice halos or 22° radius Halos are in fact an optical illusion caused by 3 to 5 mile high, cold and very tenuous cirrostratus cloud, containing millions of tiny ice crystals.

The tiny ice crystals in the atmosphere create halos by refracting and reflecting light from the Moon. The halo is always the same diameter regardless of its position in the sky, though sometimes only parts of the circle are visible.

The much smaller coloured rings directly around the Moon or Sun are a corona produced by water droplets rather than ice crystals. They often form a rainbow effect or Moonbow.

Some people even believe they herald the onset of wet weather, but this has yet to be proved.

Moon Halo Imaged December '03 in Ontario, Canada by Lauri Kangas

The Thirty-Ninth Anniversary of the Last Moonwalk

Image Credit: NASA/Eugene Cernan


On December 13, 1972, Apollo 17 Commander Eugene A. Cernan and Lunar Module Pilot (LMP) Harrison H. “Jack” Schmitt made the final lunar EVA or moonwalk of the final Apollo mission. Theirs was the longest stay on the Moon at just over three days and included over twenty-two hours spent exploring the lunar surface during which they collected over 250 pounds of lunar samples.

To commemorate the thirty-ninth anniversary of this last EVA, NASA posted a picture of Schmitt on the lunar surface as its ‘Image of the Day.’ 

Apollo 17, the only lunar mission to launch at night. Image Credit: NASA/courtesy of nasaimages.org

Apollo 17 launched on a Saturn V rocket on December 7, 1972. Four days later on December 11, Cernan and Schmitt moved into the Lunar Module Challenger and descended to a touchdown in the Taurus-Littrow valley. Command Module Pilot Ron Evans, meanwhile, stayed in orbit aboard the Command Module America.

The Taurus-Littrow valley was chosen as the best landing spot to take advantage of Apollo 17’s capabilities. It was a “J mission,” one designed for extended EVAs that would take the astronauts further from the LM than any previous missions using the Lunar Rover. It was also a geologically interesting area. Here, the astronauts would be able to reach and collect samples from the old lunar highlands as well as relatively young volcanic regions. For this latter goal, Apollo 17’s greatest tool was its LMP, Schmitt.

When NASA began looking for its first group of astronauts in 1959, candidates had to be affiliated with the military, trained engineers, and have logged at least 1,500 hours of flying time in jets. The same basic criteria were applied to the second and third group of astronauts selected in 1962 and 1963 respectively.

Cernan's Apollo 17 lunar suit is currently on display at the Smithsonian National Air and Space Museum, just one of the 137 million Apollo-era artifacts in the museum's collection. Image Credit: National Air and Space Museum

The fourth group brought a change. In June 1965, six trained scientists joined NASA’s astronaut corps. For this group, PhDs were a necessity and the previous flight hours requirement was dropped. Three of the men selected were physicists, two were physicians, and one, Schmitt, was a trained geologist.

Schmitt had explored the geological possibilities of a a lunar mission as a civilian. Before he joined NASA, he worked with the U.S. Geological Survey’s Astrogeology Center in Flagstaff, Arizona. There he devised training programs designed to teach astronauts enough about geology as well as photographic and telescopic mapping to make their journeys to the Moon as fruitful as possible. He was among the astrogeologists that instructed NASA’s astronauts during their geological field trips.

After joining the astronaut corps, Schmitt spent 53 weeks catching up to his colleagues in flight proficiency. He also spent hundreds of hours learning to fly both the Lunar Module and the Command Module. All the while, he remained an integral part of the astronauts’ lunar geology training, often assisting crews in finding and collecting the right kinds of rocks from a control station in Houston during a lunar mission.

Schmitt’s lunar companion, Gene Cernan, was an Apollo veteran. As the LMP on Apollo 10, he had flown within eight miles of the lunar surface but didn’t have enough fuel — or NASA’s blessing — to actually land. As commander of Apollo 17, he spent more time on the Moon than any other man. As commander, he entered the LM after Schmitt at the end of their final moonwalk. His bootprints remain the most recent human-made mark on the lunar surface.

Cernan and Schmitt abord the LM Challenger during their Apollo 17 mission. Image Credit: NASA/courtesy of nasaimages.org

A Blood-Red Moon

December 10 lunar eclipse by Joseph Brimacombe


Photographer Joseph Brimacombe created this stunning image of a ruddy Moon made during the total lunar eclipse of December 10, 2011. Images taken during the penumbral and total phases of the eclipse were combined to create a full-face image of the Moon in color. Beautiful!

The red tint of the Moon during an eclipse is caused by sunlight passing through Earth’s atmosphere, in effect projecting the colors of all the world’s sunsets onto the Moon’s near face. The vibrancy and particular hue seen depends on the clarity of the Earth’s atmosphere at the time of the eclipse.

Joseph’s location in Cairns, Australia allowed for great viewing of the eclipse in totality, whereas many areas of North and South America and Europe missed the full eclipse event.

See more images by Joseph on Flickr.

Image © Joseph Brimacombe. All rights reserved. Used with permission.

Enceladus Gives Cassini Some Radar Love

New radar images from Encealdus' south pole show high amounts of surface texturing. NASA/JPL-Caltech/SSI.


Cassini’s done it again! Soaring over Saturn’s moon Enceladus back on November 6, the spacecraft obtained the highest-resolution images yet of the moon’s south polar terrain, revealing surface details with visible, infrared and radar imaging that have never been seen before.

Of particular interest are new image swaths acquired by the spacecraft’s synthetic-aperture radar (SAR) instrument, which has never before been used on Enceladus. The radar, which is highly sensitive to surface textures, reveals some extremely bright regions that have surprised scientists.

Detail of the radar-imaged area (enlarged). NASA/JPL-Caltech/SSI.

“It’s puzzling why this is some of the brightest stuff Cassini has seen,” said Steve Wall, deputy team lead of Cassini’s radar team based at NASA’s Jet Propulsion Laboratory in Pasadena. “One possibility is that the area is studded with rounded ice rocks. But we can’t yet explain how that would happen.”

The SAR images did not focus on the moon’s now-famous “tiger stripe” fractures (called sulci) which are the sources of its icy jets. Instead, Cassini scanned areas a few hundred miles around the stripes. These regions have not been extensively imaged before and this new data shows surface patterns and elevations that had been previously unknown.

Some of the steep grooves in the imaged areas were shown to be as deep as 2,100 feet (650 m), and 1.2 miles (2 km) wide.

Cassini passed by the 318-mile (511-km) -wide moon at 04:49 UTC on November 6, 2011. Cassini’s radar instrument was built by JPL and the Italian Space Agency, working with team members from the U.S. and several European countries. Previously used to image the surface of Titan, which is hidden from view by a thick atmosphere, this is the first time the instrument was used on Enceladus.

Here’s a video from the imaging team below:

See the news release on the NASA mission page here, or on the Cassini mission page maintained by JPL.

Life on Alien Planets May Not Require a Large Moon After All

Earth and Moon. Credit: NASA


Ever since a study conducted back in 1993, it has been proposed that in order for a planet to support more complex life, it would be most advantageous for that planet to have a large moon orbiting it, much like the Earth’s moon. Our moon helps to stabilize the Earth’s rotational axis against perturbations caused by the gravitational influence of Jupiter. Without that stabilizing force, there would be huge climate fluctuations caused by the tilt of Earth’s axis swinging between about 0 and 85 degrees.

But now that belief is being called into question thanks to newer research, which may mean that the number of planets capable of supporting complex life could be even higher than previously thought.

Since planets with relatively large moons are thought to be fairly rare, that would mean most terrestrial-type planets like Earth would have either smaller moons or no moons at all, limiting their potential to support life. But if the new research results are right, the dependence on a large moon might not be as important after all. “There could be a lot more habitable worlds out there,” according to Jack Lissauer of NASA’s Ames Research Center in Moffett Field, California, who leads the research team.

It seems that the 1993 study did not take into account how fast the changes in tilt would occur; the impression given was that the axis fluctuations would be wild and chaotic. Lissauer and his team conducted a new experiment simulating a moonless Earth over a time period of 4 billion years. The results were surprising – the axis tilt of the Earth varied only between about 10 and 50 degrees, much less than the original study suggested. There were also long periods of time, up to 500 million years, when the tilt was only between 17 and 32 degrees, a lot more stable than previously thought possible.

So what does this mean for planets in other solar systems? According to Darren Williams of Pennsylvania State University, “Large moons are not required for a stable tilt and climate. In some circumstances, large moons can even be detrimental, depending on the arrangement of planets in a given system. Every system is going to be different.”

Apparently the assumption that a planet needs a large moon in order to be capable of supporting life was a bit premature. The results so far from the Kepler mission and other telescopes have shown that there is a wide variety of planets orbiting other stars, and so probably also moons, which we are now also on the verge of being able to detect. It’s nice to think that more of the terrestrial-type rocky planets, with or without moons, might be habitable after all.

Consolation Prize for Phobos-Grunt? Experts Consider Possibilities for Sending Spacecraft to Moon or Asteroid

The Phobos-Grunt mission profile. Credit: Roscosmos


Editor’s note: Dr. David Warmflash, principal science lead for the US team from the LIFE experiment on board the Phobos-Grunt spacecraft, provides an update for Universe Today on the likelihood of saving the mission.

If communication with Russia’s troubled Phobos-Grunt is not established by November 21, the window for a trajectory to the Martian moon Phobos, will close, experts say. But this would not mean that the spacecraft could not travel to a different destination. In a statement published earlier today by the news and information agency Ria Novosti, Russian space expert Igor Lisov suggested that Phobos-Grunt could be sent to orbit the Moon – Earth’s Moon, that is – or may be even an asteroid, if communication is restored at any point before the 13-ton probe re-enters Earth’s atmosphere.

Evolution of Phobos-Grunt’s Orbit

Boosted into space by a Zenit 2 rocket last week, Phobos-Grunt entered into a low parking orbit, where she was supposed to wait only for 2.5 hours before the next booster stage, Fregat, would send her to a higher orbit and then on to Mars. Because the Fregat engine did not ignite, Grunt still orbits just above our heads. “Highly elliptical, with an initial altitude of 347 kilometers at apogee (the high point) and 207 kilometers at perigee (the low point), the orbit initially was predicted to decay by late November, causing the spacecraft to reenter the atmosphere and burn up. But while the apogee has been decreasing (down to 326 km today), the perigee actually has been increasing by about 0.5 kilometers per day (up to 210.2 km today), due to periodic maneuvering by way of the probe’s small thrusters. After it was realized that the first maneuvering episode had improved the orbit, the predicted reentry date was adjusted to mid January, and if the thrusting episodes continue we can expect the date of the probe’s demise to be moved back still more.

An artists concept of the Phobos-Grunt Mission. Credit: Roscosmos

Time for Trajectory to Phobos is Running Out

The improved orbit gives controllers at the Russian Space Agency, Roscosmos, several weeks –even more, if the perigee continues to get higher– to restore communication with Phobos-Grunt, allowing for the uploading of new commands. But, even if control is restored, a flight to Mars and Phobos will not be possible after Monday, November 21st, Lisov explained. Although the Fregat stage is loaded with fuel, to reach Mars, given Grunt’s orbit around Earth and the alignment between Earth and Mars after Monday, would require a higher change in velocity –what propulsion specialists call delta v – than the Fregat is capable of producing.

A Consolation Prize

While cautioning that the idea of sending Phobos-Grunt somewhere other than Phobos falls into the realm of wishful thinking, Lisov urged that efforts to reconnect with the spacecraft continue in full force as long as the craft is in space. Despite several failures of lunar missions, the former Soviet space program did succeed in returning samples from the lunar surface to Earth in the 1970s. Thus, re-purposing the current mission as “Luna-Grunt” or something of that nature is not likely to have the same appeal as Phobos-Grunt has among Russians. Nor could the Grunt landing craft, designed to scoop a surface sample into a capsule that would return to Earth, even set down on the lunar surface. But other components of the science payload might be useful. Though built to observe Mars,China’s Yinghuo-1 orbiter might be able to do something interesting from lunar orbit. Instruments that were to remain on the Phobosian surface might be useful as well.

Then, there is the issue of avoiding reentry. Experts at Roscosmos are confident that the many tons of nitrogen teroxide and hydrazine in Grunt’s fuel tanks will burn up high in the atmosphere if the probe reenters. But people around the planet are scared, and thus might prefer that the fuel be used, even for a one-way mission with undefined science objectives. More importantly, achieving in a partial victory by sending the spacecraft anywhere but back to Earth could give rise to an Apollo 13-like milieu that might reinvigorate the Russian planetary program.

Millions of Tiny Passengers

The Planetary Society’s Living Interplanetary Flight Experiment (LIFE) capsule, on board the Phobos-Grunt spacecraft. Credit:The Planetary Society

As I’ve discussed in a previous update, to be useful scientifically, the Planetary Society’s Living Interplanetary Flight Experiment (LIFE) rides inside the capsule that was designed to return the Phobosian sample to Earth. The point of the experiment is to test the effects of the space environment on several different types of organisms. Because the Moon orbits Earth far outside the Van Allen radiation belts, the radiation received per time by organisms on lunar flights is the same as that received during flights to Mars. If the capsule could be sent into lunar orbit, our millions of passengers would be like organisms traveling inside a meteoroid from Mars. Then perhaps some future mission could recover the capsule some day, and we could study the organisms, as we planned to do upon their return from Phobos.

A Possible Asteroid Mission

Lisov also speculated about sending the Grunt spacecraft to an asteroid instead of the Moon. Various asteroids travel fairly close to Earth, and it’s plausible that a Grunt probe revived after November 21 would have enough delta v to reach one of them. Unlike Earth’s Moon, whose gravity the Grunt lander was not designed to withstand, many asteroids are small. Theoretically, Grunt’s lander could set down on any celestial body with a gravitational force similar to that of Phobos. If any such asteroid candidate exists –and this is a big if– the ascent engine, designed to propel the Grunt return capsule back to Earth might be utilized to deliver a sample of the asteroid, along with the LIFE experiment.

Do-It-Yourself Guide to Measuring the Moon’s Distance

The Moon. Photo credit: NASA.


When the distance from the Earth to the Moon comes up, the common figure thrown around is 402,336 km (or 250,000 miles). But have you every wondered how astronomers got that figure? And how exact it really is? There are a couple of ways you can measure the distance of the Moon that don’t require lasers or any instruments. All you need are your eyes, a clear sky, and someone else willing to stand outside all night with you. 

There are two ways to measure the distance from the Earth to the Moon on your own: using a Lunar eclipse and using parallax. Let’s look at eclipses first.

The phases of a Lunar eclipse. Photo credit: Keith Burns for NASA/JPL

The Ancient Greeks used Lunar eclipses – the phenomena of the Earth passing directly between the sun and the Moon – to determine the distance from the Earth to its satellite. It’s a simple matter of tracking and timing how long it takes the Earth’s shadow to cross over the Moon.

Start with the few knowns. We know, as did the Ancient Greeks, that the Moon travels around the Earth at a constant speed – about 29 days per revolution. The diameter of the Earth is also known to be about 12,875 km or 8,000 miles.By tracking the movement of the Earth’s shadow across the Moon, Greek astronomers found that the Earth’s shadow was roughly 2.5 times the apparent size of the Moon and lasted roughly three hours from the first to last signs of the shadow.

From these measurements, it was simple geometry that allowed Aristarchus (c. 270 BC) to determined that the Moon was round 60 Earth radii away (about 386,243 km or 240,000 miles). This is quite close to the currently accepted figure of 60.3 radii.

You can follow Aristarchus’ method in your own backyard if you have a clear view of a Lunar eclipse. Track the movement of the Earth’s shadow on the Moon by drawing the changes and time the eclipse. Use your measurements to determine the Moon’s distance.

Lunar parallax: the moon as observed from Italy and China at the same time during a lunar eclipse. Photo credit: measurethemoon.org/wordpress

For the second method, you’ll need a friend to help out. The Ancient Greeks also knew about parallax, an object’s apparent change in position when seen from two different viewpoints. You can experience parallax by holding a pen out at arm’s length and looking at it with one eye at a time. As you switch between your left and right eye, the pen will appear to move back and forth.

The same thing can be seen on a giant scale. Two observers in different parts of the world (at least 3,200 km or 2,000 miles apart) will see the Moon’s position as different from where calculations say it should be in the night sky.

To find the distance of the Moon from the Earth, you and a friend stand 3,200 km apart and each take a picture of the Moon at exactly the same time. Then, compare your images. The Moon will be in a different spot, but the background stars will be in the same place. What your images have given you is a triangle. You know the base (the distance between you and your friend), and you can find the angle at the top (the point of the Moon in this triangle). Simple geometry will give you a value for the distance of the Moon.

It might be a little more labour intensive than searching the internet, but determining the Moon’s distance yourself is sure to be more fun! If you really want to get involved, check out International Measure the Moon Night on Dec. 10, 2011. Join participants around the world who register their own events and share their images and observations!

A graph showing which parts of the world have the best chance of measuring the moon's distance using these two methods. Regions in red can see full eclipses while regions covered in red bars are best suited to measurements using parallax. Photo credit: measurethemoon.org/wordpress

The Moon as You’ve Never Seen It Before

Lunar Reconnaissance Orbiter Wide Angle Camera color shaded relief of the lunar farside (NASA/GSFC/DLR/Arizona State University).


You’re looking at a brand new view of the lunar farside, as never seen before. The team from the Lunar Reconnaissance Orbiter has released the first version of a topographic map of nearly the entire Moon, from data from the Wide Angle Camera (WAC) on the spacecraft.

“This amazing map shows you the ups and downs over nearly the entire Moon, at a scale of 100 meters across the surface, and 20 meters or better vertically,” said principal investigator Mark Robinson, writing on the LROC website. “Despite the diminutive size of the WAC (it fits in the palm of one’s hand), it images nearly the entire Moon every month.”

Every month? So why is this a “new” map since LRO has been in lunar orbit since mid-2009?

Robinson said that each month the Moon’s lighting changes, so the WAC methodically builds up a record of how different rocks reflect light under different conditions, and adds to the LROC library of stereo observations.

“The WAC really is the little camera that could!” Robinson said.

Left: LROC Wide Angle Camera attached to a test setup shortly before mounting on the spacecraft. Right: WAC being handed up to engineers for integration with LRO. Photos courtesy Mark Robinson, via the LROC website.

It is very similar to the MARCI camera (Mars Color Imager) on the Mars Reconnaissance Orbiter, another wide-angle, low-resolution camera specially built for orbital observations; both cameras were built by Malin Space Science Systems.

Topographic maps provide a detailed and accurate graphic representation of natural features on the ground, and Robinson this new map of the Moon will help both lunar scientists and future explorers on the Moon.

Combing data from the WAC along with the LRO Lunar Orbiter Laser Altimeter (LOLA), the scientists are able to provide a topographic map of nearly the entire Moon. Due to persistent shadows near the poles it is not possible to create a complete WAC stereo map at the very highest latitudes, but LOLA provides a very high resolution topographic model of the poles.

How is a digital topographic map created from stereo images? The WAC stereo images were compared one against another by pattern-matching a moving box of pixels until the best fit was found between two images with different viewing angles. The new topographic model was constructed from 69,000 WAC stereo models.

Robinson and his team are already looking towards improvements they can make with subsequent versions of their topographic maps.

“The current model incorporates the first year of stereo imaging, and there is another year of data that can be added to the solution,” he said. “These additional stereo images will not only improve the sharpness (resolution) of the model but also fill in very small gaps that exist in the current map. The LROC team has made small improvements to the camera distortion model, and the LOLA team has improved our knowledge of the spacecraft position over time. These next generation steps will further improve the accuracy of Version 2 of the LROC GLD100 topographic model of the Moon.”

You can see the “zoomable” full resolution versions of the new map for both the far and near side at this link.

Source: LROC website

Europa’s Hidden Great Lakes May Harbor Life

Chaos terrain on Europa points to subsurface lakes, new research suggests. (NASA/JPL/Ted Stryk)


New research on Jupiter’s ice-covered moon Europa indicates the presence of a subsurface lake buried beneath frozen mounds of huge jumbled chunks of ice. While it has long been believed that Europa’s ice lies atop a deep underground ocean, these new findings support the possibility of large pockets of liquid water being much closer to the moon’s surface — as well as energy from the Sun — and ultimately boosting the possibility it could contain life.

During a press conference today, November 16 at 1 p.m. EST, researchers Britney Schmidt, Tori Hoeler, Louise Prockter and Tom Wagner presented new theories concerning the creation of “chaos terrain” on Europa.

Chaos terrain is exactly what it sounds like: irregularly-shaped landforms and surface textures on a world. In the case of Europa, the terrain is made of water ice that evidence shows has been loosened by the motion of liquid water beneath, expanded, and then has refrozen into hills and jagged mounds.

Topographic data shows the chaos terrain elevations above the surrounding surface. Reds and purples are the highest elevations. Credit: NASA

These mounds are visible in topographic data acquired by the Galileo spacecraft in 1998.

During the presentation a good analogy for the processes at work on Europa was made by Britney Schmidt, a postdoctoral fellow at the Institute for Geophysics, University of Texas at Austin and lead author of the paper. She demonstrated the formation of Europa’s “mosh pit of icebergs” using a drinking glass partially filled with ice cubes. When water was added to the glass, the ice cubes naturally rose up and shifted orientation. Should the water beneath them refreeze, as it would in the frigid environments found in the Jovian system, the ice cubes would be held fast in their new expanded, “chaotic” positions.

“Now we see evidence that it’s a thick ice shell that can mix vigorously, and new evidence for giant shallow lakes. That could make Europa and its ocean more habitable.”

– Britney Schmidt, lead author

Similar processes have also been seen occurring on Earth, both in Antarctica along the edges of ice shelves and in Greenland, where glaciers continually break apart and flow into the sea – often rolling over themselves and each other in the process.

Europa's "Great Lake." Scientists speculate many more exist throughout the shallow regions of the moon's icy shell. Image Credit: Britney Schmidt/Dead Pixel FX/Univ. of Texas at Austin.

The importance of these findings is that scientists finally have a model that demonstrates how Europa’s deep liquid ocean interacts with the ice near its surface in such a way as to allow for the transportation of energy and nutrients.

“This is the first time that anyone has come up with an end-to-end model that explains what we see on the surface,” said APL senior planetary scientist Louise Prockter.

With such strong evidence for this process, the likelihood that Europa could harbor environments friendly to life goes up dramatically.

“The potential for exchange of material between the surface and subsurface is a big key for astrobiology,” said Wes Patterson, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., and a co-author of the study. “Europa’s subsurface harbors much of what we believe is necessary for life but chemical nutrients found at the surface are likely vital for driving biology.”

Although the research favors the existence of these lakes, however, confirmation of such has not yet been found. That will require a future mission to Europa and the direct investigation of its icy surface – and what lies beneath.

Luckily a Europa mission was recently rated as one of the highest priority flagship missions by the National Research Council’s Planetary Science Decadal Survey and is currently being studied by NASA.

“If we’re ever to send a landed mission to Europa, these areas would be great places to study,” Prockter said.

Read more about this discovery in the Johns Hopkins University Applied Physics Laboratory press release, or in the NASA news release here. Also, watch the full conference recorded on Ustream below:

How the Moon Became Magnetized

astronauts faced possible radiation dangers on the Moon.
Apollo 17 astronaut Harrison "Jack" Schmitt at Tracy Rock on the lunar surface. If a solar storm had hit the Moon while the astronauts were on the surface exploring, it could have been a disaster. Credit: NASA.


It’s been a mystery ever since the Apollo astronauts brought back samples of lunar rocks in the early 1970s. Some of the rocks had magnetic properties, especially one collected by geologist Harrison “Jack” Schmitt. But how could this happen? The Moon has no magnetosphere, and most previously accepted theories state that it never did. Yet here we have these moon rocks with undeniable magnetic properties… there was definitely something missing in our understanding of Earth’s satellite.

Now a team of researchers at the University of California, Santa Cruz thinks they may have cracked this enigmatic magnetic mystery.

In order for a world to have a magnetic field, it needs to have a molten core. Earth has a multi-layered molten core, in which heat from the interior layer drives motion within the iron-rich outer layer, creating a magnetic field that extends far out into space. Without a magnetosphere Earth would have been left exposed to the solar wind and life as we know it could may never have developed.

Apollo 17 lunar rock sample

Simply put, Earth’s magnetic field is crucial to life… and it can imbue rocks with magnetic properties that are sensitive to the planet-wide field.

But the Moon is much smaller than Earth, and has no molten core, at least not anymore… or so it was once believed. Research of data from the seismic instruments left on the lunar surface during Apollo EVAs recently revealed that the Moon may in fact still have a partially-liquid core, and based on a paper published in the November 10 issue of Nature by Christina Dwyer, a graduate student in Earth and planetary sciences at the University of California, Santa Cruz, and her co-authors Francis Nimmo at UCSC and David Stevenson at the California Institute of Technology, this small liquid core may once have been able to produce a lunar magnetic field after all.

The Moon orbits on its axis at such a rate that the same side always faces Earth, but it also has a slight wobble in the alignment of its axis (as does Earth.) This wobble is called precession. Precession was stronger due to tidal forces when the Moon was closer to Earth early in its history. Dwyer et al. suggest that the Moon’s precession could have literally “stirred” its liquid core, since the surrounding solid mantle would have moved at a different rate.

This stirring effect – arising from the mechanical motions of the Moon’s rotation and precession, not internal convection – could have created a dynamo effect, resulting in a magnetic field.

This field may have persisted for some time but it couldn’t last forever, the team said. As the Moon gradually moved further away from Earth the precession rate slowed, bringing the stirring process – and the dynamo – to a halt.

“The further out the moon moves, the slower the stirring, and at a certain point the lunar dynamo shuts off,” said Christina Dwyer.

Still, the team’s model provides a basis for how such a dynamo could have existed, possibly for as long as a billion years. This would have been long enough to form rocks that would still exhibit some magnetic properties to this day.

The team admits that more paleomagnetic research is needed to know for sure if their proposed core/mantle interaction would have created the right kind of movements within the liquid core to create a lunar dynamo.

“Only certain types of fluid motions give rise to magnetic dynamos,” Dwyer said. “We calculated the power that’s available to drive the dynamo and the magnetic field strengths that could be generated. But we really need the dynamo experts to take this model to the next level of detail and see if it works.”

In other words, they’re still working towards a theory of lunar magnetism that really sticks.


Read the article by Tim Stephens on the UCSC website.