The Earth and Moon May Have Formed Later Than Previously Thought

by Nancy Atkinson on June 8, 2010

Want to stay on top of all the space news? Follow @universetoday on Twitter

The Earth and Moon were created as the result of a giant collision between two planets the size of Mars and Venus. Until now it was thought to have happened when the solar system was 30 million years old or approximately 4.5 billion years ago. But new research shows that the Earth and Moon may have formed much later – perhaps up to 150 million years after the formation of the solar system.

“We have determined the ages of the Earth and the Moon using tungsten isotopes, which can reveal whether the iron cores and their stone surfaces have been mixed together during the collision,” said Tais W. Dahl, from the Niels Bohr Institute at the University of Copenhagen in collaboration with professor David J. Stevenson from the California Institute of Technology (Caltech).

The planets in the solar system were created by collisions between planetary embryos orbiting the newborn sun. In the collisions the small planets congealed together and formed larger and larger planets. When the gigantic collision occurred that ultimately formed the Earth and Moon, it happened at a time when both planetary bodies had a core of metal (iron) and a surrounding mantle of silicates (rock). But when did it happen and how did it happen? The collision took place in less than 24 hours and the temperature of the Earth was so high (7000º C), that both rock and metal must have melted in the turbulent collision. But were the stone mass and iron mass also mixed together?

The age of the Earth and Moon can be dated by examining the presence of certain elements in the Earth’s mantle. Hafnium-182 is a radioactive substance, which decays and is converted into the isotope tungsten-182. The two elements have markedly different chemical properties and while the tungsten isotopes prefer to bond with metal, hafnium prefers to bond to silicates, i.e. rock.

It takes 50-60 million years for all hafnium to decay and be converted into tungsten, and during the Moon forming collision nearly all the metal sank into the Earth’s core. But did all the tungsten go into the core?

“We have studied to what degree metal and rock mix together during the planet forming collisions. Using dynamic model calculations of the turbulent mixing of the liquid rock and iron masses we have found that tungsten isotopes from the Earth’s early formation remain in the rocky mantle,” said Tahl.

The new studies imply that the moon forming collision occurred after all of the hafnium had decayed completely into tungsten.

“Our results show that metal core and rock are unable to emulsify in these collisions between planets that are greater than 10 kilometers in diameter and therefore that most of the Earth’s iron core (80-99 %) did not remove tungsten from the rocky material in the mantle during formation” said Dahl.

The result of the research means that collision that created the Earth and the Moon may have occurred as much as 150 million years after the formation of the solar system, much later than the 30 million years that was previously thought.

The research results have been published in the scientific journal, Earth and Planetary Science Letters.

From a University of Copenhagen press release.

About 

Nancy Atkinson is Universe Today's Senior Editor. She also is the host of the NASA Lunar Science Institute podcast and works with Astronomy Cast. Nancy is also a NASA/JPL Solar System Ambassador.

Ravenas June 8, 2010 at 12:28 PM

I do not get it. Please explain. So how old is Earth and Moon then?

Kevin June 8, 2010 at 1:30 PM

120 million years later!!?!?!?!?!? This changes EVERYTHING!!!!!!@!!!111111

Torbjorn Larsson OM June 8, 2010 at 4:26 PM

It is a result from a model, and its later dates clashes with what we know. The first zircons measured at 4.4 Ga formed in an environment with a liquid water reservoir.

I’m sorry, but it just doesn’t seem very likely to me.

This changes EVERYTHING!!!!!!@!!!111111

Yes. Yes, it does. See above.

stuartajc June 9, 2010 at 4:03 AM

“It takes 50-60 million years for all hafnium to decay and be converted into tungsten”

Err, that’s not how half life works. It takes theoretically an infinite time for ALL of it to decay

Hf 182 has a half life of 9 million years, so most of it will have gone in 50-60 million years, but not all

Emilio June 9, 2010 at 4:40 AM

If entire life history of earth is equated to a year, moon formed on the 3rd day in January. The new study put it on the 12th day in January. And us, Homo habilis, did not appear until 5 hour before the last day of December!

neoguru June 9, 2010 at 5:36 AM

I’ve been very curious about the state of Venus and the apparent recent melt-down of it’s surface. Is there any evidence that it may have been due to a collision?

Quasy June 9, 2010 at 6:42 AM

I was wondering if this collision has anything to do with the diferent rotation speed of the iron core of Earth. If the collision was at a shallow angle, then it may have given a larger rotation speed to the molten earth. As the mantle cooled down, the rotation was slowed (and stabilised) by the moon, but the molten core continued to rotate at a different speed (giving us today the earth’s magnetic field).

Emilio June 9, 2010 at 7:01 AM

The collision may have affected earth spin. But have you try to spin a raw egg? It is difficult to do that because it is not a sold. So if earth was a puddle of molten object back then, collision may not have cause a spin.

Torbjorn Larsson OM June 9, 2010 at 10:36 AM

I’ve been very curious about the state of Venus and the apparent recent melt-down of it’s surface. Is there any evidence that it may have been due to a collision?

It appears that theory has been observationally challenged. “Map relations illustrate that the RTT displays planet-scale patterns that, together with altimetry, record a rich geologic history.” (But the work is behind a paywall, so I can’t say more.)

Interestingly, there is a collision proposal that predicts Venus’ slow retrograde rotation and absence of water and plate tectonics, without necessarily resulting in a moon. But it is an early accretionary planetesimal collision scenario.

the molten core continued to rotate at a different speed (giving us today the earth’s magnetic field).

Actually what gives our field is motions in the outer liquid FeS core, while the inner Fe cores appears solid in seismographic analysis. (Outer core has no shear waves, so liquid. Inner core has shear waves, so solid.)

There is no differential rotation measured of the inner core what I know of. But also no one seems to know how much it couples to the outer core and participate in the field.

The current theory seems to be that the inner core solidified, pushing out the sulfur from its FeS eutectic starting composition. (All this happened after the FeS melt differentiated into the core region of the protoplanet, see for example Sotin’s chapter on this in “Astrobiology”.)

The more viscous mantle started to convect after a transition period after the first cold plume from the surface reached it. No doubt the less viscous and putatively MHD coupled (so todays magnetic field) outer core could have started to convect earlier. Anyway, outer core (thermal and MHD) convection is the theory AFAIK.

Spoodle58 June 9, 2010 at 12:19 PM

That is correct stuartajc,
‘Nearly all’ would have been a better thing to say, with 98.4% of Hf 182 decayed to W 182 in 53.4 million years.

Torbjorn Larsson OM June 9, 2010 at 3:07 PM

D’oh! “its FeS eutectic starting composition” ; it’s tending to the eutectic; my bad.

IVAN3MAN_AT_LARGE June 9, 2010 at 4:13 PM

We seriously need a preview/edit facility here, don’t we, Torbjörn?

Torbjorn Larsson OM June 10, 2010 at 8:42 AM

YES, indeed, IVAN#MAN!!!

Right next to the vote box for good posts/suggestions.

Comments on this entry are closed.

Previous post:

Next post: