Lunar dynamo's lifetime extended by at least 1 billion years

New evidence from ancient lunar rocks suggests that an active dynamo once churned within the molten metallic core of the moon, generating a magnetic field that lasted at least 1 billion years longer than previously thought. Dynamos are natural generators of magnetic fields around terrestrial bodies, and are powered by the churning of conducting fluids within many stars and planets. In a paper published today in Science Advances, researchers from MIT and Rutgers University report that a lunar rock collected by NASA's Apollo 15 mission exhibits signs that it formed 1 to 2.5 billion years ago in the presence of a relatively weak magnetic field of about 5 microtesla. That's around 10 times weaker than Earth's current magnetic field but still 1,000 times larger than fields in interplanetary space today.

Full moon as seen from Earth's Northern Hemisphere, by Gregory H. Revera (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

Several years ago, the same researchers identified 4-billion-year-old lunar rocks that formed under a much stronger field of about 100 microtesla, and they determined that the strength of this field dropped off precipitously around 3 billion years ago. At the time, the researchers were unsure whether the moon's dynamo - the related magnetic field - died out shortly thereafter or lingered in a weakened state before dissipating completely.

The results reported today support the latter scenario: After the moon's magnetic field dwindled, it nonetheless persisted for at least another billion years, existing for a total of at least 2 billion years.

Study co-author Benjamin Weiss, professor of planetary sciences in MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS), says this new extended lifetime helps to pinpoint the phenomena that powered the moon's dynamo. Specifically, the results raise the possibility of two different mechanisms - one that may have driven an earlier, much stronger dynamo, and a second that kept the moon's core simmering at a much slower boil toward the end of its lifetime.

"The concept of a planetary magnetic field produced by moving liquid metal is an idea that is really only a few decades old," Weiss says. "What powers this motion on Earth and other bodies, particularly on the moon, is not well-understood. We can figure this out by knowing the lifetime of the lunar dynamo."

Weiss' co-authors are lead author Sonia Tikoo, a former MIT graduate student who is now an assistant professor at Rutgers; David Shuster of the University of California at Berkeley; Clément Suavet and Huapei Wang of EAPS; and Timothy Grove, the R.R. Schrock Professor of Geology and associate head of EAPS.

Since NASA's Apollo astronauts brought back samples from the lunar surface, scientists have found some of these rocks to be accurate "recorders" of the moon's ancient magnetic field. Such rocks contain thousands of tiny grains that, like compass needles, aligned in the direction of ancient fields when the rocks crystallized eons ago. Such grains can give scientists a measure of the moon's ancient field strength.

Until recently, Weiss and others had been unable to find samples much younger than 3.2 billion years old that could accurately record magnetic fields. As a result, they had only been able to gauge the strength of the moon's magnetic field between 3.2 and 4.2 billion years ago.

"The problem is, there are very few lunar rocks that are younger than about 3 billion years old, because right around then, the moon cooled off, volcanism largely ceased and, along with it, formation of new igneous rocks on the lunar surface," Weiss explains. "So there were no young samples we could measure to see if there was a field after 3 billion years."

There is, however, a small class of rocks brought back from the Apollo missions that formed not from ancient lunar eruptions but from asteroid impacts later in the moon's history. These rocks melted from the heat of such impacts and recrystallized in orientations determined by the moon's magnetic field.

Weiss and his colleagues analyzed one such rock, known as Apollo 15 sample 15498, which was originally collected on Aug. 1, 1971, from the southern rim of the moon's Dune Crater. The sample is a mix of minerals and rock fragments, welded together by a glassy matrix, the grains of which preserve records of the moon's magnetic field at the time the rock was assembled.

"We found that this glassy material that welds things together has excellent magnetic recording properties," Weiss says.

The team determined that the rock sample was about 1 to 2.5 billion years old - much younger than the samples they previously analyzed. They developed a technique to decipher the ancient magnetic field recorded in the rock's glassy matrix by first measuring the rock's natural magnetic properties using a very sensitive magnetometer.

They then exposed the rock to a known magnetic field in the lab, and heated the rock to close to the extreme temperatures in which it originally formed. They measured how the rock's magnetization changed as they increased the surrounding temperature.

"You see how magnetized it gets from getting heated in that known magnetic field, then you compare that field to the natural magnetic field you measured beforehand, and from that you can figure out what the ancient field strength was," Weiss explains.

The researchers did have to make one significant adjustment to the experiment to better simulate the original lunar environment, and in particular, its atmosphere. While the Earth's atmosphere contains around 20 percent oxygen, the moon has only imperceptible traces of the gas. In collaboration with Grove, Suavet built a customized, oxygen-deprived oven in which to heat the rocks, preventing them from rusting while at the same time simulating the oxygen-free environment in which the rocks were originally magnetized.

"In this way, we finally have gotten an accurate measurement of the lunar field," Weiss says.

From their experiments, the researchers determined that, around 1 to 2.5 billion years ago, the moon harbored a relatively weak magnetic field, with a strength of about 5 microtesla - two orders of magnitude weaker than the moon's field around 3 to 4 billion years ago. Such a dramatic dip suggests to Weiss and his colleagues that the moon's dynamo may have been driven by two distinct mechanisms.

Scientists have proposed that the moon's dynamo may have been powered by the Earth's gravitational pull. Early in its history, the moon orbited much closer to the Earth, and the Earth's gravity, in such close proximity, may have been strong enough to pull on and rotate the rocky exterior of the moon. The moon's liquid center may have been dragged along with the moon's outer shell, generating a very strong magnetic field in the process.

It's thought that the moon may have moved sufficiently far away from the Earth by about 3 billion years ago, such that the power available for the dynamo by this mechanism became insufficient. This happens to be right around the time the moon's magnetic field strength dropped. A different mechanism may have then kicked in to sustain this weakened field. As the moon moved away from the Earth, its core likely sustained a low boil via a slow process of cooling over at least 1 billion years.

"As the moon cools, its core acts like a lava lamp - low-density stuff rises because it's hot or because its composition is different from that of the surrounding fluid," Weiss says. "That's how we think the Earth's dynamo works, and that's what we suggest the late lunar dynamo was doing as well."

The researchers are planning to analyze even younger lunar rocks to determine when the dynamo died off completely.

"Today the moon's field is essentially zero," Weiss says. "And we now know it turned off somewhere between the formation of this rock and today."

This research was supported, in part, by NASA.

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Moon has a water-rich interior

A new study of satellite data finds that numerous volcanic deposits distributed across the surface of the Moon contain unusually high amounts of trapped water compared with surrounding terrains. The finding of water in these ancient deposits, which are believed to consist of glass beads formed by the explosive eruption of magma coming from the deep lunar interior, bolsters the idea that the lunar mantle is surprisingly water-rich.

Scientists had assumed for years that the interior of the Moon had been largely depleted of water and other volatile compounds. That began to change in 2008, when a research team including Brown University geologist Alberto Saal detected trace amounts of water in some of the volcanic glass beads brought back to Earth from the Apollo 15 and 17 missions to the Moon. In 2011, further study of tiny crystalline formations within those beads revealed that they actually contain similar amounts of water as some basalts on Earth. That suggests that the Moon's mantle - parts of it, at least - contain as much water as Earth's.

"The key question is whether those Apollo samples represent the bulk conditions of the lunar interior or instead represent unusual or perhaps anomalous water-rich regions within an otherwise 'dry' mantle," said Ralph Milliken, lead author of the new research and an associate professor in Brown's Department of Earth, Environmental and Planetary Sciences. "By looking at the orbital data, we can examine the large pyroclastic deposits on the Moon that were never sampled by the Apollo or Luna missions. The fact that nearly all of them exhibit signatures of water suggests that the Apollo samples are not anomalous, so it may be that the bulk interior of the Moon is wet."

Full Moon photograph taken 10-22-2010 from Madison, Alabama, USA. By Gregory H. Revera (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

The research, which Milliken co-authored with Shuai Li, a postdoctoral researcher at the University of Hawaii and a recent Brown Ph.D. graduate, is published in Nature Geoscience.

Detecting the water content of lunar volcanic deposits using orbital instruments is no easy task. Scientists use orbital spectrometers to measure the light that bounces off a planetary surface. By looking at which wavelengths of light are absorbed or reflected by the surface, scientists can get an idea of which minerals and other compounds are present.

The problem is that the lunar surface heats up over the course of a day, especially at the latitudes where these pyroclastic deposits are located. That means that in addition to the light reflected from the surface, the spectrometer also ends up measuring heat.

"That thermally emitted radiation happens at the same wavelengths that we need to use to look for water," Milliken said. "So in order to say with any confidence that water is present, we first need to account for and remove the thermally emitted component."

To do that, Li and Milliken used laboratory-based measurements of samples returned from the Apollo missions, combined with a detailed temperature profile of the areas of interest on the Moon's surface. Using the new thermal correction, the researchers looked at data from the Moon Mineralogy Mapper, an imaging spectrometer that flew aboard India's Chandrayaan-1 lunar orbiter.

The researchers found evidence of water in nearly all of the large pyroclastic deposits that had been previously mapped across the Moon's surface, including deposits near the Apollo 15 and 17 landing sites where the water-bearing glass bead samples were collected.

"The distribution of these water-rich deposits is the key thing," Milliken said. "They're spread across the surface, which tells us that the water found in the Apollo samples isn't a one-off. Lunar pyroclastics seem to be universally water-rich, which suggests the same may be true of the mantle."

The idea that the interior of the Moon is water-rich raises interesting questions about the Moon's formation. Scientists think the Moon formed from debris left behind after an object about the size of Mars slammed into the Earth very early in solar system history. One of the reasons scientists had assumed the Moon's interior should be dry is that it seems unlikely that any of the hydrogen needed to form water could have survived the heat of that impact.

"The growing evidence for water inside the Moon suggest that water did somehow survive, or that it was brought in shortly after the impact by asteroids or comets before the Moon had completely solidified," Li said. "The exact origin of water in the lunar interior is still a big question."

In addition to shedding light on the water story in the early solar system, the research could also have implications for future lunar exploration. The volcanic beads don't contain a lot of water - about .05 percent by weight, the researchers say - but the deposits are large, and the water could potentially be extracted.

"Other studies have suggested the presence of water ice in shadowed regions at the lunar poles, but the pyroclastic deposits are at locations that may be easier to access," Li said. "Anything that helps save future lunar explorers from having to bring lots of water from home is a big step forward, and our results suggest a new alternative."

The research was funded by the NASA Lunar Advanced Science and Exploration Research Program (NNX12AO63G).

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On this day in history - the moon landing goal was announced

In 1961, the formal announcement of an American lunar landing was made by President John F. Kennedy speaking to the Congress: “I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to the Earth. No single space program in this period will be more impressive to mankind, or more important in the long-range exploration of space; and none will be so difficult or expensive to accomplish.” 

Since, a total of twelve men have landed on the Moon. This was accomplished with two US pilot-astronauts flying a Lunar Module on each of six NASA missions across a 41-month time span starting on 20 July 1969 UTC, with Neil Armstrong and Buzz Aldrin on Apollo 11, and ending on 14 December 1972 UTC with Gene Cernan and Jack Schmitt on Apollo 17. Cernan was the last to step off the lunar surface.

Lunar crater Daedalus on the Moon's far side

All Apollo lunar missions had a third crew member who remained on board the Command Module. The last three missions had a rover for increased mobility.

The atmosphere of the moon

The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than 10 metric tons (9.8 long tons; 11 short tons). The surface pressure of this small mass is around 3 × 10−15 atm (0.3 nPa); it varies with the lunar day. Its sources include outgassing and sputtering, the release of atoms from the bombardment of lunar soil by solar wind ions. Elements that have been detected include sodium and potassium, produced by sputtering, which are also found in the atmospheres of Mercury and Io; helium-4 and neon from the solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle.

The absence of such neutral species (atoms or molecules) as oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in the regolith, is not understood. Water vapour has been detected by Chandrayaan-1 and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from the sublimation of water ice in the regolith. These gases can either return into the regolith due to the Moon's gravity or be lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind's magnetic field.

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Tim Peake: How the UK astronaut gets to space and back

UK astronaut Tim Peake will travel to the International Space Station (ISS) on 15 December. Since the space shuttle's retirement, the Russian Soyuz launch system is now the only way for crew members to get to the ISS.

The basic design for the Soyuz capsule was laid down as far back as the 1960s. It was originally intended to serve as the craft that would carry cosmonauts to the Moon.

When the US beat them to the lunar surface in 1969, the USSR's lunar programme was scrapped. But the Soyuz was retained, and became the Soviet - and subsequently Russian - vehicle of choice for launching humans to low-Earth orbit.

It was the craft that carried the first crew to the International Space Station in 2000, and has been the only craft ferrying humans to the orbiting outpost since the retirement of the US space shuttle in 2011.

"International Space Station after undocking of STS-132" by NASA/Crew of STS-132 -Licensed under Public Domain via Commons 

The current version, known as the Soyuz-TMA, can transport up to three cosmonauts and a limited amount of cargo to and from the ISS. At least one Soyuz is docked to the space station at all times to be used as a lifeboat in an emergency.

At one end of the spacecraft is the spherical orbital module. It's about the size of a large van and provides extra living space for the crew during flight. It can be used to store supplies and other cargo, such as experiments, and there's also a toilet.

The orbital module contains the mechanism used to dock with the space station and the hatch that allows crew members to enter the ISS.

The craft's mid-section is known as the descent module, and is where crew members sit during launch and the journey back to Earth. It contains the spacecraft's controls and displays, including a periscope that allows the crew to see the docking target on the ISS.

The seats have custom-fitted liners, individually moulded to each person's body. This is designed to help cushion the crew members when they land on Earth after a mission.

The third module is known as the instrument module. It contains the thrusters, oxygen and propellant tanks, communications equipment and the onboard computer.

Launched from Baikonur Cosmodrome in Kazakhstan, the 50m-high launcher consists of three sections, or stages. The first stage consists of four identical liquid-booster rockets. These are strapped around the core, or second, stage. The third, or upper, stage carries the Soyuz spacecraft.

The vehicle uses refined kerosene and liquid oxygen as fuel and can deliver payloads of more than seven tonnes - about the weight of a small lorry - into orbit.

Crew members enter the spacecraft two-and-a-half hours before launch to prepare it. At T-minus zero, the four boosters and core engine ignite, propelling the rocket into the air. About two minutes into the flight, the four booster rockets are jettisoned.

The core stage keeps firing, until it too separates at about 4 minutes 48 seconds after launch. A third stage engine then propels the Soyuz to its desired orbit at an altitude of some 220km. During the nine-minute sequence, crew members have to withstand forces up to three-and-a-half times their bodyweight.

The spacecraft then has to perform five engine burns in order to catch up with the ISS. This generally takes six hours, but if things don't go as planned, mission control may decide to fall back to an alternative two-day transfer mode.

Rendezvous and docking with the space station is automated by the onboard computer. It keeps track of the positions of the Soyuz and ISS using measurements from mission control and a radar system called Kurs. However, crew members closely monitor the process and have the ability to intervene or take over manual control if required.

During the final approach, a docking probe on the end of the Soyuz inserts into a cone on the ISS. Once "capture" is confirmed, the docking probe retracts, bringing the two vehicles together. A series of hooks and latches then close over, securing the Russian capsule to the ISS.

Once a tight seal is confirmed, the air pressure in the Soyuz is equalised with that of the ISS and the hatch is opened, so the new arrivals can enter the station.

When crew members are ready to return to Earth, a command is given to start opening the hooks and latches that hold the Soyuz to the ISS. The spacecraft then separates from the space station at a graceful speed of 10cm/s (4ins/s). Once the Soyuz has reached a distance of 20m (66ft) the Soyuz fires its thrusters for 15 seconds.

When the capsule reaches a distance of 19km (12mi) from the ISS, the Soyuz makes its main "de-orbit burn", firing the engines for 4 minutes, 21 seconds to begin the return to Earth. The descent module carrying the crew separates from the empty orbital module which burns up in the atmosphere.

About 15 minutes before landing, the capsule deploys a drogue parachute to slow its descent speed from 230m/s (755 ft/s) to 80m/s (262 ft/s). The main parachute is then released, cutting the capsule's speed to 7 m/s (24ft/s) and shifting it to a vertical position.

Six engines fire on the underside of the capsule to cushion the craft just before it thuds down on the Kazakh steppe.

A recovery and rescue team then arrives to extract the crew members.

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On this day - April 11th 1970

Apollo 13 was launched on April 11, 1970, at 13:13 CST from the Kennedy Space Centre, Florida.  It was the seventh manned mission in the American Apollo space program and the third intended to land on the Moon. The lunar landing was aborted after an oxygen tank exploded two days later, crippling the Service Module (SM) upon which the Command Module (CM) depended. Despite great hardship caused by limited power, loss of cabin heat, shortage of potable water, and the critical need to jury-rig the carbon dioxide removal system, the crew returned safely to Earth on April 17.

Apollo 13 launches from Kennedy Space Center, April 11, 1970

A film was made regarding the Apollo 13 mission in 1995 directed by Ron Howard who went to great lengths to create a technically accurate movie, employing NASA's technical assistance in astronaut and flight controller training for his cast, and even obtaining permission to film scenes aboard a reduced gravity aircraft for realistic depiction of the "weightlessness" experienced by the astronauts in space.

Three days into the mission, the crew send a live television transmission from Odyssey, but the networks, believing the public now regards lunar missions as routine, decline to carry the broadcast live. Astronaut Swigert is told to perform a standard housekeeping procedure of stirring the two liquid oxygen tanks in the Service Module. When he flips the switch, one tank explodes, emptying its contents into space and sending the craft tumbling. The other tank is soon found to be leaking, prompting Mission Control to abort the Moon landing.

The crew is soon subjected to freezing conditions.  When the carbon dioxide exhaled by the astronauts reaches the Lunar Module's filter capacity and approaches dangerous levels, an engineering team quickly invents a way to make the Command Module's square filters work in the Lunar Module's round receptacles.  The crew eventually get the Odyssey going and make a safe return to earth.

The Apollo 13 Command Module

The Apollo 13 incident gave rise to the famous line 'Houston, We've Got a Problem'.

Further Apollo missions continued which resulted in successful manned landings on the moon.  There are a number of proposed future missions to the moon - most of them robotic or remote missions using rovers.  However the Russians did announce in 2007 their intention to send cosmonauts to the moon by 2025 and establish a permanent robotically operated base there in 2027–2032.

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