APOD: Lunar Farside (2016 Dec 30)

[APOD Robot]

Lunar Farside

Explanation: Tidally locked in synchronous rotation, the Moon always presents its familiar nearside to denizens of planet Earth. From lunar orbit, the Moon's farside can become familiar, though. In fact this sharp picture, a mosaic from the Lunar Reconnaissance Orbiter's wide angle camera, is centered on the lunar farside. Part of a global mosaic of over 15,000 images acquired between November 2009 and February 2011, the highest resolution version shows features at a scale of 100 meters per pixel. Surprisingly, the rough and battered surface of the farside looks very different from the nearside covered with smooth dark lunar maria. The likely explanation is that the farside crust is thicker, making it harder for molten material from the interior to flow to the surface and form the smooth maria.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

[Guest]

I would think the far side is also more marked gets bartered with more stuff from space.

[Ann]

Craters, craters, craters. Craters. Look, over there are a few craters! And some more craters. And some craters.

And a mare! A real small one! And look! There is another one! Even smaller! Two maria!

Our Moon is a very polite satellite. Instead of showing us Earthlings its craters, craters, craters, craters, craters, craters, craters, craters, craters, craters, craters, craters, craters and craters - and two miniscula maria - it always shows us its best face.

Ann

[neufer]

I would think the far side is also more marked gets bartered [sic] with more stuff from space.

The Earth only fills up only 0.015% of the Moon's near side sky
...hence it physically blocks only 0.015% of the near side 'stuff from space.'

[jargalarn]

!!!!WOW!!!! Thanks for this incredible image!

[Chris Peterson]

In fact this sharp picture, a mosaic from the Lunar Reconnaissance Orbiter's wide angle camera, is centered on the lunar farside.

I think the term "mosaic" is used pretty loosely here. What's been done is that a great many images made from a low lunar orbit have been mapped onto a mathematical model of the lunar surface (this is also what Google does when you look at their satellite images of Earth), and then an orthographic projection has been synthesized and rendered, showing the Moon as it would appear to an observer infinitely far away from it, thus allowing an entire hemisphere to be seen (something which is not possible in any direct image).

[Fred the Cat]

When I saw the teaser yesterday I was hoping to see another farside.

[Guest]

I would think the far side is also more marked gets bartered [sic] with more stuff from space.

The Earth only fills up only 0.015% of the Moon's near side sky
...hence it physically blocks only 0.015% of the near side 'stuff from space.'

I would have thought that the depth of Earths gravity well would be a big factor in protecting the near face of the Moon. Anything coming from space would be pulled in and hit us saving the completion of the man in the moon. I don't understand this 'double crust thickness' theory tho. A body old enough to become tidally locked, should be well formed by that time, and the structure (including the crust) should be reasonably consistent.

[rstevenson]

In fact this sharp picture, a mosaic from the Lunar Reconnaissance Orbiter's wide angle camera, is centered on the lunar farside.

I think the term "mosaic" is used pretty loosely here. What's been done is that a great many images made from a low lunar orbit have been mapped onto a mathematical model of the lunar surface (this is also what Google does when you look at their satellite images of Earth), and then an orthographic projection has been synthesized and rendered, showing the Moon as it would appear to an observer infinitely far away from it, thus allowing an entire hemisphere to be seen (something which is not possible in any direct image).

And, if you want to see similar orthographic projections centered on other parts of the Moon, the "highest resolution version" link takes one to a page where there are 6 different views. Fill yer boots!

Rob

[Chris Peterson]

The Earth only fills up only 0.015% of the Moon's near side sky
...hence it physically blocks only 0.015% of the near side 'stuff from space.'

I would have thought that the depth of Earths gravity well would be a big factor in protecting the near face of the Moon. Anything coming from space would be pulled in and hit us saving the completion of the man in the moon.

The Earth doesn't cause a very large deviation in most of the (solar orbiting) material that passes near it. And to the extent it does, it's almost as likely to deviate a body in such a way that it misses the Moon as to hit it.

I don't understand this 'double crust thickness' theory tho. A body old enough to become tidally locked, should be well formed by that time, and the structure (including the crust) should be reasonably consistent.

Bodies don't get more uniform as they get older. In fact, they're probably at their most uniform when they're created. Then they begin differentiating, and many things can occur that result in asymmetries of density.

[Guest]

The Earth only fills up only 0.015% of the Moon's near side sky
...hence it physically blocks only 0.015% of the near side 'stuff from space.'

I would have thought that the depth of Earths gravity well would be a big factor in protecting the near face of the Moon. Anything coming from space would be pulled in and hit us saving the completion of the man in the moon.

The Earth doesn't cause a very large deviation in most of the (solar orbiting) material that passes near it. And to the extent it does, it's almost as likely to deviate a body in such a way that it misses the Moon as to hit it.

I don't understand this 'double crust thickness' theory tho. A body old enough to become tidally locked, should be well formed by that time, and the structure (including the crust) should be reasonably consistent.

Bodies don't get more uniform as they get older. In fact, they're probably at their most uniform when they're created. Then they begin differentiating, and many things can occur that result in asymmetries of density.

So as I understand your explanation, the moon remained molten until it after it became tidally locked to the Earth and the molten material then differentiated? Or did solid rock move thru other solid rock to change the structure of the moons crust? I'm not sure there is a third alternative.

[Chris Peterson]

Bodies don't get more uniform as they get older. In fact, they're probably at their most uniform when they're created. Then they begin differentiating, and many things can occur that result in asymmetries of density.

So as I understand your explanation, the moon remained molten until it after it became tidally locked to the Earth and the molten material then differentiated? Or did solid rock move thru other solid rock to change the structure of the moons crust? I'm not sure there is a third alternative.

What do you mean by "molten". The deep interior of the Moon remains liquid. It's been cooling for more than four billion years. Differentiation started immediately, and the primary differentiation of iron towards the center would have occurred over a short time- not more than a few million years. Crust would have formed quickly, and over time got thicker. Density variation in the crust are part of what led to the Moon's tidal lock with Earth.

[starbrush]

I should have been quicker to notice, that the direction of lighting varies: sometimes from the left, sometimes the right. A lovely portrait, that does things that a simple photograph could not (and that might not occur to an artist to do!).

[Chiming In]

Bodies don't get more uniform as they get older. In fact, they're probably at their most uniform when they're created. Then they begin differentiating, and many things can occur that result in asymmetries of density.

So as I understand your explanation, the moon remained molten until it after it became tidally locked to the Earth and the molten material then differentiated? Or did solid rock move thru other solid rock to change the structure of the moons crust? I'm not sure there is a third alternative.

What do you mean by "molten". The deep interior of the Moon remains liquid. It's been cooling for more than four billion years. Differentiation started immediately, and the primary differentiation of iron towards the center would have occurred over a short time- not more than a few million years. Crust would have formed quickly, and over time got thicker. Density variation in the crust are part of what led to the Moon's tidal lock with Earth.

So wouldn't that mean that the heavier, more dense crust would be on the Earth side, instead of the other way around?

[life]

Yesterday there was something unexpected, or the day before, and a bunch of people addressed it. seems like odd things ought to get mention. in today's lunar picture there is also an oddity which it seems ought to have been mentioned - the rather dark area toward the upper left.

[neufer]

Density variation in the crust are part of what led to the Moon's tidal lock with Earth.

So wouldn't that mean that the heavier, more dense crust
would be on the Earth side, instead of the other way around?

---

Tidal forces of the Earth on the Moon are quadrupole not dipole .

The thick far crust and the dense basaltic near crust produce a quadrupole deformation that must line up radially vs-a-vis the Earth but it doesn't matter which side is the near side and which is the far.

[Catalina]

What causes a body/moon to become tidally locked? What affect of an elliptical orbit, such as our moon's, have on it being tidally locked? Are the moons around the other planets of the solar system all locked in such a way, as well?

[RocketRon]

What do you mean by "molten". The deep interior of the Moon remains liquid.

Its only a recent realization/discovery that the moon has a partially liquid core (2010) ?
(Re-analysing old data)

And, it has been mentioned that its due to earth's gravity ??

So, all theories/comments/suggestions of the moons structure/geology/history prior to then should be read with that in mind....

[heehaw]

Interesting discussion about the deep interior of our Moon! Much harder to understand than the deep interior of the Sun, because the latter is gaseous from the very center to the outermost edge. The discussion makes me think of Pluto, which has turned out to be so active - to our astonishment! Way out there in the sticks, for goodness' sakes! Yet bubbling and boiling!

[neufer]

What causes a body/moon to become tidally locked?

A liquid interior and a planet that looms large in the sky.

What affect of an elliptical orbit, such as our moon's, have on it being tidally locked?

An elliptical orbit helps to maintain a liquid interior.

However, there is also the real danger of rotation becoming chaotic like Hyperion (eccentricity = 0.123).

Are the moons around the other planets of the solar system all locked in such a way, as well?

Code: Select all

Parent Body 	Tidally-locked Satellites
---------------------------------------------------------
Earth 	        Moon

Mars            Phobos · Deimos

Jupiter 	     Metis · Adrastea · Amalthea · Thebe · Io · Europa · Ganymede · Callisto

Saturn 	      Pan · Atlas · Prometheus · Pandora · Epimetheus · Janus · Mimas 
                Enceladus · Telesto · Tethys · Calypso · Dione · Rhea · Titan · Iapetus
                              
Uranus 	      Miranda · Ariel · Umbriel · Titania · Oberon

Neptune 	     Proteus · Triton

Pluto 	       Charon 

[ta152h0]

FAR OUT !!!

[Ann]

Yesterday there was something unexpected, or the day before, and a bunch of people addressed it. seems like odd things ought to get mention. in today's lunar picture there is also an oddity which it seems ought to have been mentioned - the rather dark area toward the upper left.

I mentioned it. It is a mare, plural maria. They are the flat smooth dark areas that exist in large numbers on the near side of the Moon. They are the dark markings that make us see "the man in the moon".

Maria are (or so I believe) basalt, which is the stiffened remnant of liquid magma from the Moon's interior. The maria are very old, and it is extremely unlikely that liquid magma from the lunar interior will break the lunar crust again, forming new maria.

Don't ask me why the maria exist in such numbers on the near side, where they also are so big, while they are so tiny and few on the far side of the Moon.

Ann

[neufer]

Yesterday there was something unexpected, or the day before, and a bunch of people addressed it. seems like odd things ought to get mention. in today's lunar picture there is also an oddity which it seems ought to have been mentioned - the rather dark area toward the upper left.

I mentioned it. It is a mare,

• When the moon hits your eye like a big pizza pie
  That's a mare,

[Sam Brewster]

Given how battered the moon, might we infer that planet earth has received that many more hits in its 5 billion years? And given the wealth of these moon hits, what might be the best estimate of measurable frequency for both locations....once per century? More? Less?

[Chris Peterson]

Given how battered the moon, might we infer that planet earth has received that many more hits in its 5 billion years? And given the wealth of these moon hits, what might be the best estimate of measurable frequency for both locations....once per century? More? Less?

Impact cratering follows a power law- the larger the impact, the less frequent. Most of the cratering on the Moon was formed during the Late Heavy Bombardment, when the entire inner solar system was being impacted. But since this happened about 4 billion years ago, virtually all geological evidence of those impacts on the Earth have been removed. After that, the impact rate declined significantly.

On the Moon, the rate of readily detectable impacts is several per day. Impacts large enough to create craters visible from Earth with careful telescopic observation might occur every few years. Impacts large enough to create craters visible to the naked eye probably only occur on a scale of thousands of years or longer. Some bodies (in the 10 m diameter range) will create small craters on the Moon but would not be capable of cratering the Earth, because they'd break up in the atmosphere. 50-100 m bodies that can create small (km scale) craters on Earth occur every few tens of thousands of years, similar to the lunar frequency.