Moon 'too dry to have life', say scientists

By Katia Moskvitch
Science reporter, BBC News

Image caption, Scientists have been searching for water on the Moon for years

Contrary to recent reports about water content in lunar rocks, the Moon may be quite dry, say scientists.

A study by US researchers, published in Science, analysed chlorine isotopes of the much-studied samples, brought to Earth by the Apollo space missions.

They added that there was no or very little hydrogen in the magma ocean during the Moon's formation.

And that would mean the Earth's natural satellite may have always been too dry to host life.

Zachary Sharp from the University of New Mexico led the study.

Moon magma

According to one of the theories of the Moon's formation, a Mars-size object collided with the young Earth billions of years ago. As a result, our planet's satellite was formed.

It crystallised and cooled shortly thereafter, about 4.5 billion years ago.

Before it cooled, there was a so-called magma ocean on the Moon's surface - molten rocks, capable of retaining high quantities of water.

"As the Earth has cooled and crystallised, there were gases from volcanoes coming to the surface and the steam from them has probably formed the majority of the oceans. Our oceans came from water dissolved in rocks," said Dr Sharp.

"The same thing may have happened on the Moon, except that the Moon is too small; gravity is too weak to retain this water so it would have been lost to space."

In their quest for water on our planet's satellite, researchers have had to rely on the analysis of lunar rocks that US Apollo space missions brought to Earth in the late 1960s and early 1970s.

The main goal has always been to find hints that the rocks were hydrated when they cooled, said the scientist.

Chlorine analysis

Dr Sharp and his colleagues decided to analyse an extremely hydrophilic element of the Apollo samples - chlorine.

It is a very sensitive indicator of hydrogen levels - and where there is hydrogen, there is water.

"Chlorine loves water. And we wanted to see if the ratio of chlorine 37 to chlorine 35 was similar to Earth's or not," he said.

"We very absolutely shocked when we found that not only it's not similar, but unlike the Earth where every sample is essentially the same, here we're getting these enormous differences. They were so big that we thought at first that we had maybe some analytical errors, that we were doing something wrong."

He explained that on Earth, the chlorine 37 to chlorine 35 ratio is pretty much constant - it varies only by about 0.1%.

Image caption, Scientists say that at first, there was a "magma ocean" on the Moon

But the team found that on the Moon it was about 25 times that of the terrestrial chlorine isotope ratio.

Dr Sharp said that if lunar rocks had initial hydrogen contents anywhere close to those of terrestrial rocks, the chlorine isotope ratios would not have so much scatter.

"It was just unheard of, it was inexplicable. We went back, re-analysed things and checked that everything was OK.

"We then came up with the idea that in order for this to happen, to have these huge variations, the [Moon] must have been dry when these basalts crystallised from magma. The lavas that must have poured out onto the surface had no water dissolved in them."

Previous studies

When geologists first studied the Apollo samples, they found no evidence of water and declared the Moon dry.

Image caption, The only way for scientists to look for water on the Moon is by analysing lunar rocks

But over the last few years, researchers re-examined the rocks and suggested otherwise.

Earlier in 2010, a US-led team led by Francis McCubbin looked at the mineral apatite in lunar samples and discovered that there was at least 100 times more water in the Moon's minerals than previously believed.

Dr Sharp said that he realised the results of his team's research clashed with previous studies.

A radar experiment aboard India's Chandrayaan-1 lunar spacecraft also found thick deposits of water-ice near the Moon's north pole. But these deposits probably came from comets that slammed into the Moon's surface, said Dr Sharp.

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