"The ice may be a time capsule from the same source that supplied the original water to Earth," said Matt Siegler at Southern Methodist University,. "This is a record we don't have on Earth. Earth has reworked itself so many times, there's nothing that old left here. Ancient ice from the moon could provide answers to this deep mystery."

Siegler and colleagues made the discovery while examining NASA data known to indicate lunar polar hydrogen. The hydrogen, detected by orbital instruments, is presumed to be in the form of ice hidden from the sun in craters surrounding the moon's north and south poles. Exposure to direct sunlight causes ice to boil off into space, so this ice -- perhaps billions of years old -- is a very sensitive marker of the moon's past orientation.

An odd offset of the ice from the moon's current north and south poles was a tell-tale indicator to Siegler and prompted him to assemble a team of experts to take a closer look at the data from NASA's Lunar Prospector and Lunar Reconnaissance Orbiter missions. Statistical analysis and modeling revealed the ice is offset at each pole by the same distance, but in exactly opposite directions.

In 1998 NASA's Lunar Prospector mission used the presence of hydrogen as a sign of potential ice deposits shown in the image above. As you can see in this video, Prospector data showed significantly more hydrogen at the south pole of the moon (areas colored blue).

This precise opposition indicates the moon's axis -- the imaginary pole that runs north to south through it's middle, and around which the moon rotates -- shifted at least six degrees, likely over the course of 1 billion years, said Siegler.

"This was such a surprising discovery. We tend to think that objects in the sky have always been the way we view them, but in this case the face that is so familiar to us -- the Man on the Moon -- changed," said Siegler, who also is a scientist at the Planetary Science Institute, Tucson, Ariz.

"Billions of years ago, heating within the Moon's interior caused the face we see to shift upward as the pole physically changed positions," he said. "It would be as if Earth's axis relocated from Antarctica to Australia. As the pole moved, the Man on the Moon turned his nose up at the Earth.

Very few planetary bodies known to permanently shift their axis. Planetary bodies settle into their axis based on their mass: A planet's heavier spots lean it toward its equator, lighter spots toward the pole. On the rare occasion mass shifts and causes a planet to relocate on its axis, scientists refer to the phenomenon as "true polar wander."

Discovery of lunar polar wander gains the moon entry into an extremely exclusive club. The only other planetary bodies theorized to have permanently shifted location of their axis are Earth, Mars, Saturn's moon Enceladus and Jupiter's moon Europa.

What sets the moon apart is its polar ice, which appears to effectively "paint out" the path along which its poles moved.

On Earth, polar wander is believed to have happened due to movement of the continental plates. Polar wander on Mars resulted from a heavy volcanic region. The moon's change in mass was internal -- the shift of a large, single mantle "plume." Ancient volcanic activity some 3.5 billion years ago melted a portion of the moon's mantle, causing it to bubble up toward its surface, like goo drifting upward in a lava lamp.

"The moon has a single region of the crust, a large basaltic plain called Procellarum, where radioactive elements ended up as the moon was forming," Siegler said. "This radioactive crust acted like an oven broiler heating the mantle below."

Some of the material melted, forming the dark patches we see at night, which are ancient lava, he said. "This giant blob of hot mantle was lighter than cold mantle elsewhere," Siegler said. "This change in mass caused Procellarum -- and the whole moon -- to move."

The moon likely relocated its axis starting about 3 billion years ago or more, slowly moving over the course of a billion years, Siegler said, etching a path in its ice. Over time, the axis shifted 125 miles or 200 kilometers -- about half the distance from Dallas to Houston, or equal the distance from Washington D.C. to Philadelphia.

Polar wander explains why the moon appears to have lost much of its ice. Siegler compares true polar wander to holding a glass filled with water. Most planets are like a steady hand holding a glass, their axis doesn't shift and the water stays put. A planet whose mass is changing is like a wobbly hand, causing its axis to shift and the water to spill out. Similarly, as Earth's moon changed its axis, much of its ice ceased to be hidden from the sun and was lost.

Co-author Richard Miller mapped the moon's remaining ice by using data from NASA's Lunar Prospector mission, which orbited the moon from 1998 to 1999. The presence of ice is inferred by measuring the energy of neutrons emitted from the lunar surface. Instruments on NASA's satellite, including a neutron spectrometer, measured neutrons liberated from the moon by a rain of stellar particles scientists call cosmic rays. Low energy neutrons indicate the presence of hydrogen, the dominant molecule in water and ice.

"The maps show four key features," said Siegler and his colleagues. "First, the largest quantity of hydrogen is offset from the current rotation axis of the moon by roughly 5.5 degrees. Second, the hydrogen enhancements are of similar magnitude at both poles. Third, the asymmetric enhancements do not correlate with expectations from the current thermal or permanently shadowed environment. And lastly, and most significantly, the spatial distributions of polar hydrogen appear to be nearly antipodal."

Siegler's discovery opens the door to further discoveries around an even deeper question -- the mystery of why there is water on the moon and on Earth. Scientific theory surrounding the formation of the solar system postulates water could not have formed much closer to the sun than Jupiter, Siegel said.

"We don't know where the Earth's water came from. It appears to have come from the outer solar system well after the Earth and moon formed," he said. "Ice on other bodies, like the moon or Mercury, might give us a clue to its origin." The fact lunar ice correlates so well with true polar wander implies that it predates this motion, Siegler said, making the ice very ancient.

The Daily Galaxy via Southern Methodist University and http://nature.com

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Where did the two natural satellites of Mars, Phobos and Deimos, come from? For a long time, their shape suggested that they were asteroids captured by Mars. However, the shape and course of their orbits contradict this hypothesis. Two independent and complementary studies provide an answer to this question. One of these studies conducted by researchers from the CNRS and Aix-Marseille Université, rules out the capture of asteroids, and shows that the only scenario compatible with the surface properties of Phobos and Deimos is that of a giant collision.

In the second study, a team of French, Belgian, and Japanese researchers used cutting-edge digital simulations to show how these satellites were able to form from the debris of a gigantic collision between Mars and a protoplanet one-third its size. This research, which is the result of collaboration between researchers from Université Paris Diderot and Royal Observatory of Belgium, in collaboration with the CNRS, Université de Rennes 12 and the Japanese Institute ELSI.

The origin of the two Martian moons, Phobos and Deimos, remained a mystery. Due to their small size and irregular shape, they strongly resembled asteroids, but no one understood how Mars could have « captured » them and made them into satellites with almost circular and equatorial orbits. According to a competing theory, toward the end of its formation Mars suffered a giant collision with a protoplanet: but why did the debris from such an impact create two small satellites instead of one enormous moon, like the Earth's? A third possibility is that Phobos and Deimos formed at the same time as Mars, which would entail that they have the same composition as their planet, although their low density seems to contradict this hypothesis. Two independent studies have now solved the puzzle: the Martian moons must have arised from a giant collision.

In one of these studies, a team of Belgian, French, and Japanese researchers offers, for the first time, a complete and coherent scenario for the formation of Phobos and Deimos, which would have been created following a collision between Mars and a primordial body one-third its size, 100 to 800 million years after the beginning of the planet's formation. According to researchers, the debris from this collision formed a very wide disk around Mars, made up of a dense inner part composed of matter in fusion, and a very thin outer part primarily of gas. In the inner part of this disk formed a moon one thousand times the size of Phobos, which has since disappeared. The gravitational interactions created in the outer disk by this massive star apparently acted as a catalyst for the gathering of debris to form other smaller, more distant moons.

After a few thousand years, Mars was surrounded by a group of approximately ten small moons and one enormous moon. A few million years later, once the debris disk had dissipated, the tidal effects of Mars brought most of these satellites back down onto the planet, including the very large moon. Only the two most distant small moons, Phobos and Deimos, remained. See image below:

Mars is struck by a protoplanet one-third its size (1). A debris disk forms within a few hours. The elementary building blocks of Phobos and Deimos (grains smaller than a micrometer) condense directly from gas in the outer part of the disk (2). The debris disk soon produces a moon near Mars that moves further away and propagates its two areas of dynamical influence like ripples (3), which over the course of a few thousand years causes the accretion of more dispersed debris into two small moons, Phobos and Deimos (4). Under the effect of the tidal pull of Mars, the large moon falls back to the planet within approximately five million years (5), while smaller Phobos and Deimos take up their current positions in the ensuing billions of years (6).

Due to the diversity of physical phenomena involved, no digital simulation is able to modelize the entire process. Pascal Rosenblatt and Sébastien Charnoz's team thus had to combine three successive cutting-edge simulations in order to provide an account of the physics behind the giant collision, the dynamics of the debris resulting from the impact and its accretion to form satellites, and the long-term evolution of these satellites.

In a second study, researchers from the Laboratoire d'astrophysique de Marseille (CNRS/Aix-Marseille Université) ruled out the possibility of a capture on the grounds of statistical arguments based on the compositional diversity of the asteroid belt. They moreover show that the light signature emitted by Phobos and Deimos is incompatible with that of the primordial matter that formed Mars (meteorites such as ordinary chondrite, enstatite chondrite and/or angrite). They therefore support the collision scenario. From this light signature they deduced that the satellites are made of fine-grained dust (smaller than a micrometer ).

Yet the very small size of grains on the surface of Phobos and Deimos cannot, according to the researchers, be solely explained as the consequence of erosion from bombardment by interplanetary dust. This means that the satellites were from the beginning made up of very fine grains, which can only form by gas condensation in the outer area of the debris disk (and not from the magma present in the inner part). Both studies are in agreement on this point. Moreover, the formation of Martian moons from these very fine grains could also be responsible for a high internal porosity, which would explain their surprisingly low density.

The theory of the giant collision, which is corroborated by these two independent studies, could explain why the northern hemisphere of Mars has a lower altitude than the southern hemisphere: the Borealis basin is most probably the remains of a giant collision, such as the one that in fine gave birth to Phobos and Deimos. It also helps explain why Mars has two satellites instead of a single one like our Moon, which was also created by a giant collision. This research suggests that the satellite systems that were created depended on the planet's rotational velocity, because at the time Earth was rotating very quickly (in less than four hours), whereas Mars turned six times more slowly.

New observations will soon make it possible to know more about the age and composition of Martian moons. Japan's space agency (JAXA) has decided to launch a mission in 2022, named Mars Moons Exploration (MMX), which will bring back samples from Phobos to Earth in 2027. Their analysis could confirm or invalidate this scenario. The European Space Agency (ESA) has planned a similar mission in 2024 in association with the Russian space agency (Roscosmos).

This research received support from IPGP, the Labex UnivEarthS, ELSI, Kobe University, the Royal Observatory of Belgium and Idex A*MIDEX.

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