55-year old dark side of the Moon mystery solved

Jun 09, 2014 by A'ndrea Elyse Messer
This is a composite image of the lunar farside taken by the Lunar Reconnaissance Orbiter in June 2009, note the absence of dark areas. Credit: NASA

(Phys.org) —The Man in the Moon appeared when meteoroids struck the Earth-facing side of the moon creating large flat seas of basalt that we see as dark areas called maria. But no "face" exists on farside of the moon and now, Penn State astrophysicists think they know why.

"I remember the first time I saw a globe of the as a boy, being struck by how different the farside looks," said Jason Wright, assistant professor of astrophysics. "It was all mountains and craters. Where were the maria? It turns out it's been a mystery since the fifties."

This mystery is called the Lunar Farside Highlands Problem and dates back to 1959, when the Soviet spacecraft Luna 3 transmitted the first images of the "dark" side of the moon back to Earth. It was called the dark side because it was unknown, not because sunlight does not reach it. Researchers immediately noticed that fewer "seas" or maria existed on this portion of the moon that always faces away from Earth.

Wright, Steinn Sigurdsson, professor of astrophysics and Arpita Roy, graduate student in astronomy and astrophysics, and lead author of the study, realized that the absence of maria, which is due to a difference in crustal thickness between the side of the moon we see and the hidden side, is a consequence of how the moon originally formed. The researchers report their results in today's (June 9) Astrophysical Journal Letters.

The general consensus on the moon's origin is that it probably formed shortly after the Earth and was the result of a Mars-sized object hitting Earth with a glancing, but devastating impact. This Giant Impact Hypothesis suggests that the outer layers of the Earth and the object were flung into space and eventually formed the moon.

"Shortly after the giant impact, the Earth and the moon were very hot," said Sigurdsson.

The Earth and the impact object did not just melt; parts of them vaporized, creating a disk of rock, magma and vapor around the Earth.

"The moon and Earth loomed large in each others skies when they formed, " said Roy.

This is a composite image of the lunar nearside taken by the Lunar Reconnaissance Orbiter in June 2009, note the presence of dark areas of maria on this side of the moon. Credit: NASA

The geometry was similar to the rocky exoplanets recently discovered very close to their stars, said Wright. The moon was 10 to 20 times closer to Earth than it is now, and the researchers found that it quickly assumed a tidally locked position with the rotation time of the moon equal to the orbital period of the moon around the Earth. The same real estate on the moon has probably always faced the Earth ever since. Tidal locking is a product of the gravity of both objects.

The moon, being much smaller than Earth cooled more quickly. Because the Earth and the moon were tidally locked from the beginning, the still hot Earth—more than 2500 degrees Celsius—radiated towards the near side of the moon. The far side, away from the boiling Earth, slowly cooled, while the Earth-facing side was kept molten creating a temperature gradient between the two halves.

This gradient was important for crustal formation on the moon. The moon's crust has high concentrations of aluminum and calcium, elements that are very hard to vaporize.

"When rock vapor starts to cool, the very first elements that snow out are aluminum and calcium," said Sigurdsson.

Aluminum and calcium would have preferentially condensed in the atmosphere of the cold side of the moon because the nearside was still too hot. Thousands to millions of years later, these elements combined with silicates in the moon's mantle to form plagioclase feldspars, which eventually moved to the surface and formed the moon's crust, said Roy. The farside crust had more of these minerals and is thicker.

The moon has now completely cooled and is not molten below the surface. Earlier in its history, large meteoroids struck the nearside of the moon and punched through the crust, releasing the vast lakes of basaltic lava that formed the nearside maria that make up the man in the moon. When struck the farside of the moon, in most cases the crust was too thick and no magmatic basalt welled up, creating the dark side of the moon with valleys, craters and highlands, but almost no maria.

The NASA Astrobiology Institute and the Pennsylvania State Astrobiology Research Center supported this work.

Explore further: A wet Moon

More information: Paper: iopscience.iop.org/2041-8205/788/2/L42/article

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julianpenrod
1.3 / 5 (21) Jun 09, 2014
If this was the case, Mercury should be nearly in a molten state constantly. Concrete buildings near blast furnaces should melt. In fact, there could never be so much heat, even from a molten earth, located even 10,000 miles away, that it would keep the moon molten. Besides, if that were the case, the entire near side should be a "sea". Also, the "seas" formed when the moon was cool enough not to have remelted after they formed. They are considered the result of a form of flooding by magma that didn't occur on the other side, likely the result of magma lying just under the surface. This is consistent more with a gravitational or tidal displaying of molten material toward the earth.
rockwolf1000
4.4 / 5 (21) Jun 09, 2014
If this was the case, Mercury should be nearly in a molten state constantly. Concrete buildings near blast furnaces should melt. In fact, there could never be so much heat, even from a molten earth, located even 10,000 miles away, that it would keep the moon molten. Besides, if that were the case, the entire near side should be a "sea". Also, the "seas" formed when the moon was cool enough not to have remelted after they formed. They are considered the result of a form of flooding by magma that didn't occur on the other side, likely the result of magma lying just under the surface. This is consistent more with a gravitational or tidal displaying of molten material toward the earth.


As always, you're attempts at debunking are.... minimal.
douglaskostyk
4.1 / 5 (9) Jun 09, 2014
dark side of the Moon


The FAR side of the moon.
rockwolf1000
3.8 / 5 (13) Jun 09, 2014
dark side of the Moon


The FAR side of the moon.


Dark Side

From the article:
"It was called the dark side because it was unknown, not because sunlight does not reach it."
RealityCheck
1.6 / 5 (10) Jun 09, 2014
If the Calcium/Aluminium compounds solidified first and other compounds still molten while the Moon got tidally locked as it is now, then the molten material would have all flowed from the far side to the near side and formed a 'molten tidal bulge' facing the Earth. Such flows from far to near side would have 'uncovered' the farside solidified calcium/aluminium-compound containing 'structures' (peaks and valleys and craters etc), while on the nearside the molten material 'tidally locked' BULGE would have 'covered' much of the the calcium/Aluminium-compound structures formed there.

Result? The nearside surface material is mostly later-solidified material; farside surface material is earlier-solidified material.

Note: since the moon is tidally locked, its 'spin rate' is once per month, so no 'farside bulge' of molten material would have formed due to 'centrifugal forces' like those on Earth (there is a bulge on Earth on BOTH sides, ie, one nearest Moon, other furthest from moon). :)
Torbjorn_Larsson_OM
4.5 / 5 (8) Jun 10, 2014
Ah, finally! Seems the 'new' Moon, with dating from Earth's late bombardment resupplying its litophile elements later, isotopic differences in both Moon and an incompletely molten Earth and now this irradiance phenomena shakes out well. That the Earth was only partially molten doesn't change the irradiance effect much, the contribution didn't need to be large as the moon started out molten.

@julianpenrod: Handwaving means nothing. Mercury is not tidally locked. (And is not subjected to heat irradiance like the initial here. "Mercury's surface experiences the greatest temperature variation of all the planets, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day at some equatorial regions." [ http://en.wikiped...(planet) ])

Yes, your idea is a hypothesis too. But you can't discredit the new idea without presenting calculations showing numerical errors et cetera.
Jonseer
1.9 / 5 (9) Jun 10, 2014
Oh please, this will go down as one of the more absurd hypothesis proposed to explain what we see.

The idea that the Earth was so hot that it acted like a miniature sun keeping the near side of the moon molten is patently absurd.

It's idiotic also because few astronomers believe today that planetary formation involves a stage of global melting into a magma planet as the Earth would be in this silly hypothesis.
Torbjorn_Larsson_OM
4.2 / 5 (5) Jun 10, 2014
@Reality_Check: A tidal bulge is a response to forces, so show the hydrostatic surface. If it moves, it would have induced some redistribution flows but not enough to collect material. I think the model relies on a) rapid tidal lock has happened before crust formation ("Since the accreting Moon rapidly achieved synchronous rotation, a surface and atmospheric thermal gradient was imposed by the proximity of the hot, post-giant impact Earth." [abstract] and b) the described thermal gradient, "a primordial bulk chemical inhomogeneity that seeded the crustal asymmetry" [abstract] and convection to redistribute the early forming refractory material. Like how an iron melt can form and distribute slag on top.

Interestingly in this context, recent LRO (IIRC) data has been claimed to show how today's bulge moves, very slightly, as the Moon wobbles. The mantle/busted up crust has still enough "bend" in it.
Torbjorn_Larsson_OM
5 / 5 (6) Jun 10, 2014
@julianpenrod, Jonseer: Don't forget that Sigurdsson, that blogs a lot, is one of the more cautious and competent astrophysicists out there. There is no need to suggest that the hypothesis doesn't work off the bat. You need to show _how_ it fails, with numbers and all.

Also, there is a misunderstanding here. The heat flow wasn't huge, melting the Moon. It wasn't even necessary medium sized, keeping the Moon asymmetrically molten by supplying a heat differential. It was enough that the flow set up a guiding gradient, making the refractories appear asymmetrically: "This gradient guided condensation of atmospheric and accreting material, preferentially depositing crust-forming refractories on the cooler farside, resulting in a primordial bulk chemical inhomogeneity that seeded the crustal asymmetry." [abstract]
Torbjorn_Larsson_OM
4.2 / 5 (5) Jun 10, 2014
[ctd] Earth was indeed molten, if not fully so, else you can't predict the seen isotope inhomogenities: "a major part of the impacted hemisphere would probably have been completely vaporised, but the opposite hemisphere would have been partly shielded, and would not have undergone complete melting." [ http://phys.org/n...rth.html ]
Jonseer
2 / 5 (4) Jun 10, 2014
[ctd] Earth was indeed molten, if not fully so, else you can't predict the seen isotope inhomogenities: "a major part of the impacted hemisphere would probably have been completely vaporised, but the opposite hemisphere would have been partly shielded, and would not have undergone complete melting." [ http://phys.org/n...rth.html ]


No doubt about that, but that is very different from being a molten planet. What I find odd is the quick acceptance of a hypothesis as if proven. A far better one explains the difference as due to a slow motion impact of a smaller body that shared the moon's orbit while it formed, as many objects did. That one did a far better job of explaining what we see without such absurd assumptions.
Teper
Jun 10, 2014
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Teper
Jun 10, 2014
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Teper
Jun 10, 2014
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yyz
5 / 5 (5) Jun 10, 2014
A (non-paywalled) preprint of the paper is available here: http://arxiv.org/...2020.pdf
larreey69
4.2 / 5 (5) Jun 10, 2014
Dark Side. Best album ever.
AeroSR71
3 / 5 (4) Jun 10, 2014
Just let the scientists do their jobs ffs. I come to read intelligent discourse in every article I'm interested in, yet all I read is a bunch of nonsense from uneducated twats that clearly have never picked up a science textbook. Their should be a strict policy in the comment section stating that every time you post a bunch of speculative nonsense, that you must back up your statements with peer reviewed sources.
antigoracle
1 / 5 (2) Jun 12, 2014
Well... this calls for a little Floyd
yaridanjo
1 / 5 (5) Jun 12, 2014
The Moon's rotation has been such that one side always faces the Earth for a long time now. The Earth serves a shield to prevent a lot of the comets etc from hitting the side facing Earth. Very few comets pass thru the Earth on their way to strike the side of the Moon's surface that always faces Earth. BTW, the Earth Moon system formed as a double planet and not via a collision with a Mars size planet. If there had been such a collision, I think the Moon's orbit about the Earth would be very elliptical and there would be an 'asteroid like' ring around the Earth. Other planetary rings formed by 'exploding' ultra-tiny satellites
antialias_physorg
5 / 5 (6) Jun 12, 2014
Here's an image of Earth and Moon that shows you how big they are compared to the distance they are apart. As you can easily see: Earth is not gonna shield anything. The effective surface (proportion of a sphere centered around the Moon with a Moon-Earth radius) it covers is negligible
http://imgur.com/pjnFP

Also Earth's gravity doesn't help 'suck in' asteroids that would otherwise hit the Moon. (For comaprison: recent simulations of Jupiter show that it doesn't protect Earth from comets/asteroids (it flings as many away as it redirects other ones towards us that would have otherwise missed). In any case that would be an effect that would equally affect all of the Moon - not just the 'dark side'.
yaridanjo
1 / 5 (3) Jun 12, 2014
The Earth is the biggest gravity source in the Earth Moon system by a factor of 81. It does not protect the near side of the Moon from impacts, but it just bias those that hit the back side vs the front side. Thus, the front side has less craters.

The Moon won't protect Earth much, the Earth will protect the Moon more.

Jupiter is too far away to do us much good, but every little bit helps.
Jantoo
3 / 5 (2) Jun 12, 2014
Earth is not gonna shield anything. The effective surface (proportion of a sphere centered around the Moon with a Moon-Earth radius) it covers is negligible
With the same logics we could say, that the Earth is radiating anything, because it covers 1/20000 of the sky at the Moon. Even if Earth would glow at 2500 °C, it would heat the Moon surface just by 1/20000 i.e. less than 0.1 °C and the above theory would be nonsensical. But this theory considers too, that the distance of Moon was lower than 20.000 km at its very beginning (Roche limit) and under such a situation even the shielding effect of Earth has some merit.
Jantoo
3 / 5 (2) Jun 12, 2014
The massive objects are shielding falling bodies like antenna, which cover larger area, than it corresponds its actual size. The falling bodies are trapped with gravity field of nearby massive bodies and the result is similar.
Uncle Ira
5 / 5 (3) Jun 12, 2014
The massive objects are shielding falling bodies like antenna, which cover larger area, than it corresponds its actual size. The falling bodies are trapped with gravity field of nearby massive bodies and the result is similar.


Hey now, are the Socratic-Skippy that used to be the Zephir-Skippy changing your name again? What is wrong with you Cher? I am starting to worry for you podna.
rockwolf1000
5 / 5 (4) Jun 12, 2014
Earth is not gonna shield anything. The effective surface (proportion of a sphere centered around the Moon with a Moon-Earth radius) it covers is negligible
With the same logics we could say, that the Earth is radiating anything, because it covers 1/20000 of the sky at the Moon. Even if Earth would glow at 2500 °C, it would heat the Moon surface just by 1/20000 i.e. less than 0.1 °C and the above theory would be nonsensical. But this theory considers too, that the distance of Moon was lower than 20.000 km at its very beginning (Roche limit) and under such a situation even the shielding effect of Earth has some merit.


I do see your point but by your own reasoning if the earth only represents 1/20,000 of the sky from the moon and it is somehow intercepting asteroids etc, why would the differential of cratering be more than a factor of 1/20,000 which it clearly is?

It appears to be more than 1/4 so even if I halved your figure over 5 times it still seems way off.
Jantoo
1.3 / 5 (3) Jun 12, 2014
why would the differential of cratering be more than a factor of 1/20,000 which it clearly is
It's explained for example here, more technically here. With respect to statistics of high number of impacts, the gravity field of massive bodies behaves like the similar lens, like the gravitational lens for photons. Under certain circumstances this shielding effect may lead into focusing of impactor paths and increased frequency of impacts instead.
adam_russell_9615
Jun 14, 2014
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