"Given the trove of lunar samples in hand and the power of modern laboratory analyses, you'd think that by now geochemists should have completely nailed exactly how the Moon formed. But not so — in fact, there's still lots of debate on how Earth formed.
Here's the basic problem:
* about 30 years ago, dynamicists showed that a body roughly the mass of Mars could have struck Earth a glancing blow and ejected enough debris into orbit to collect into a Moon-size object. In virtually all of those simulations, most of what ends up in the Moon came from the impactor rather than from Earth.
* But the Apollo (and Luna) lunar samples, not to mention lunar meteorites, show that the Moon and
Earth have very similar compositions. Apart from their lack of iron and extreme lack of water, Moon rocks match Earth's isotopic ratios for the geochemically diagnostic elements titanium, calcium, silicon, and (especially) oxygen and tungsten. This really pins the dynamicists in a corner — only in rare cases, 1% or 2% of the time, do their simulations yield a Moon with an Earthlike composition. There's also a problem of fine-tuning the impact to yield the angular momentum of the current Earth-Moon system.
* In January 9th's Nature Geoscience, Israeli researchers Raluca Rufu, Oded Aharonson, and Hagai Perets argue that the notion of a single, giant impact is wrong. Instead, they propose that Earth endured dozens of lesser (but still potent) impacts with object ranging from 1% to 10% of its mass, each of which ejected debris into an orbiting disk. The rings quickly coagulated into moonlets, and tidal interactions with the young, mostly molten Earth then drove each of them outward. Over time they accumulated into the Moon. This approach yields a lunar composition that's an amalgam of many compositions, which eases the unyielding isotopic constraints. The most Earth-like contributions came from nearly head-on collisions that drilled deeply into our planet's mantle.
* The piecemeal assembly envisioned by the Israeli team would have taken a long time, perhaps even 100 million years — and that opens up another aspect of the lunar-formation debate. Some planetary scientists have indeed argued, mostly on geochemical grounds, that the Moon might have formed 150 to 200 million years after the beginning of the solar system. Others claim it showed up much sooner, within a few tens of millions of years.
* Another new analysis, published January 11th in Science Advances, maintains that the Moon came together in a hurry and had mostly solidified by 4.51 billion years ago, or 60 million years after the solar system's birth.
The evidence, say Mélanie Barboni (University of California, Los Angeles) and six colleagues, is found in eight tiny grains of the mineral zircon (ZrSiO4), collected by Apollo 14's astronauts, in which they found traces of uranium, lead, and hafnium used for isotopic age-dating.
* But that result had wide uncertainties, owing to the techniques used.
* As if the How and When of the Moon's formation weren't complicated enough, a third new analysis argues that — despite its extreme dryness today — the Moon likely contained a lot of water when it formed. In the same issue of Nature Geoscience, Yana Lin (Vrije Universiteit Amsterdam) and three others describe their experimental attempts to mimic how the Moon's magma ocean solidified. Lower density minerals would have floated to the top, forming a crust.
They find that the suite of minerals found in the lunar crust today — combined with its thickness — argue that water was part of the mix at a concentration of 270 to 1,650 ppm. This might not seem like much — but if proven true there'd be significant implications.
"A wet start of the Moon, coupled with the strong similarities between the composition of the Moon and the composition of the silicate Earth," Lin's team concludes, "suggests that equally high
concentrations of water were present in the Earth at the time of the Moon-forming event."
Sky&Telescope
RESPONSE: (Yana Lin is Getting Close)
Well, despite the nonsense of long age chronology amongst the bickering astronomers over who is right in how & when the moon formed---the closest to the truth appears to be Yana Lin for stating
..."suggests that equally high concentrations of water were present in the Earth at the time of the Moon-forming event".... And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters....And God made two great lights; the greater light to rule the day, and the lesser light to rule the night:
Genesis 1:2,16
Here's the basic problem:
* about 30 years ago, dynamicists showed that a body roughly the mass of Mars could have struck Earth a glancing blow and ejected enough debris into orbit to collect into a Moon-size object. In virtually all of those simulations, most of what ends up in the Moon came from the impactor rather than from Earth.
* But the Apollo (and Luna) lunar samples, not to mention lunar meteorites, show that the Moon and
* In January 9th's Nature Geoscience, Israeli researchers Raluca Rufu, Oded Aharonson, and Hagai Perets argue that the notion of a single, giant impact is wrong. Instead, they propose that Earth endured dozens of lesser (but still potent) impacts with object ranging from 1% to 10% of its mass, each of which ejected debris into an orbiting disk. The rings quickly coagulated into moonlets, and tidal interactions with the young, mostly molten Earth then drove each of them outward. Over time they accumulated into the Moon. This approach yields a lunar composition that's an amalgam of many compositions, which eases the unyielding isotopic constraints. The most Earth-like contributions came from nearly head-on collisions that drilled deeply into our planet's mantle.
* The piecemeal assembly envisioned by the Israeli team would have taken a long time, perhaps even 100 million years — and that opens up another aspect of the lunar-formation debate. Some planetary scientists have indeed argued, mostly on geochemical grounds, that the Moon might have formed 150 to 200 million years after the beginning of the solar system. Others claim it showed up much sooner, within a few tens of millions of years.
* Another new analysis, published January 11th in Science Advances, maintains that the Moon came together in a hurry and had mostly solidified by 4.51 billion years ago, or 60 million years after the solar system's birth.
The evidence, say Mélanie Barboni (University of California, Los Angeles) and six colleagues, is found in eight tiny grains of the mineral zircon (ZrSiO4), collected by Apollo 14's astronauts, in which they found traces of uranium, lead, and hafnium used for isotopic age-dating.
* But that result had wide uncertainties, owing to the techniques used.
* As if the How and When of the Moon's formation weren't complicated enough, a third new analysis argues that — despite its extreme dryness today — the Moon likely contained a lot of water when it formed. In the same issue of Nature Geoscience, Yana Lin (Vrije Universiteit Amsterdam) and three others describe their experimental attempts to mimic how the Moon's magma ocean solidified. Lower density minerals would have floated to the top, forming a crust.
They find that the suite of minerals found in the lunar crust today — combined with its thickness — argue that water was part of the mix at a concentration of 270 to 1,650 ppm. This might not seem like much — but if proven true there'd be significant implications.
"A wet start of the Moon, coupled with the strong similarities between the composition of the Moon and the composition of the silicate Earth," Lin's team concludes, "suggests that equally high
Sky&Telescope
RESPONSE: (Yana Lin is Getting Close)
Well, despite the nonsense of long age chronology amongst the bickering astronomers over who is right in how & when the moon formed---the closest to the truth appears to be Yana Lin for stating
..."suggests that equally high concentrations of water were present in the Earth at the time of the Moon-forming event".... And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters....And God made two great lights; the greater light to rule the day, and the lesser light to rule the night:
Genesis 1:2,16
