杏吧原创

Water has recently been found on the lunar surface, but researchers are divided about how much water lies inside the moon 鈥� a new study suggests the moon was very dry when it formed 4.5 billion years ago

The moon鈥檚 interior may not be that wet after all, despite some recent studies that have suggested otherwise. A new analysis of Apollo rocks backs the old idea of a waterless world.

For decades after the Apollo astronauts touched down on the desolate lunar surface, the moon was considered to be parched. But that view began to change in 2008, when researchers found water inside tiny spheres of lunar volcanic glass at concentrations calculated to be similar to those found in some terrestrial volcanic rocks.

Now, researchers led by at the University of New Mexico in Albuquerque say measurements of chlorine in a dozen Apollo samples suggest that the moon鈥檚 interior has always been extremely dry, containing 10,000 to 100,000 times less water than Earth鈥檚.

Water-loving element

Chlorine comes in two stable isotopes 鈥� ³⁵Cl and ³⁷Cl, which has two more neutrons.

Sharp鈥檚 team found the heavier version is relatively more abundant in the moon samples than on Earth, suggesting the moon rocks formed in a very dry environment.

That is because hydrogen atoms contained in water readily bond with the heavy chlorine isotope to form hydrochloric acid gas, which then leaks away into space 鈥� leaving more of the lighter isotope behind.

The moon sample measurements suggest that water was present at concentrations of 180 parts per billion in the lunar mantle. That鈥檚 in line with water concentrations in a lunar mineral called apatite by of the Carnegie Institution of Washington in Washington DC. 鈥淭hese water contents [are] very dry in comparison to Earth and Mars,鈥� he says, suggesting that previous studies that argued for Earth-like water concentrations 鈥渁re likely extrapolating their data a bit too far鈥�.

What鈥檚 the norm?

Those studies may have based their conclusions on very unusual lunar samples, Sharp says. He says the moon may have formed with very little water 鈥� and that no additional water from comets is necessary to explain the suite of current observations.

The moon is thought to have formed from the shrapnel of a collision between a Mars-sized object and the infant Earth 4.5 billion years ago. As the initially molten moon crystallised into rock, Sharp says minute amounts of water would have become more and more concentrated in ever-shrinking amounts of liquid magma.

This water-rich magma eventually would have erupted onto the surface because it was rich in volatiles, and the Apollo astronauts may have collected it in the form of the volcanic glass beads. 鈥淲hether these volatile-rich magmas or glasses are representative of the moon or not 鈥� that鈥檚 the question,鈥� Sharp told New 杏吧原创.

鈥楤lind men and the elephant鈥�

It鈥檚 too soon to settle that question, says James Greenwood of Wesleyan University in Middletown, Connecticut, whose own isotopic studies of lunar apatite suggest that comets delivered water to the early moon. He says different regions of the moon may have different amounts of water because the roiling magma ocean that enveloped the early moon may have solidified before water from impacting comets got mixed through it evenly.

鈥淚 think what we鈥檙e really doing is the case of the three 鈥� they鈥檝e looked at samples and that鈥檚 what they find, and we find what we find,鈥� he told New 杏吧原创.

McCubbin says studying more samples, particularly those collected from areas not yet explored by Apollo or robotic missions, will be key to piecing together the history of lunar water. Water can change the force needed to bend or break rocks, and affect what minerals are formed when magma crystallises, he says: 鈥淭he question of a wet or dry moon matters because water plays such an important role in geologic processes.鈥�

Journal reference: