[X] My Assistant

[Guest_AlexBlackwell_*]

Moonquakes
Science@NASA
March 15, 2006

NASA astronauts are going back to the moon and when they get there they may need quake-proof housing.

[Guest_Richard Trigaux_*]

This story is a bit strange.

That there was many quakes on the Moon was already known. But that there was large ones, it is the first time I heard that. Do scientists not speak of something each time they don't understand it?


besides trite quakes due to temperature changes or meteorites, two kinds of quakes are interesting:

-deep ones at 700kms, explained as the effect of tides. This tells us that rocks are rigid until at least this depth. But what is under? Magma? If there are tides, and magma, there must be tidal heating. There was lenghty threads here speaking of tidal heating of Io, Europa, Enceladus, etc. Why the Moon would not have tidal heating? If so, it would have volcanoes. Why it don't have? (latest volcanic activity was 3.3 billion years ago).

-shallow quakes, the most powerfull (5.5 on the Richter scale, enough to be dangerous) take place at less than 35kms, but their exact location could not be traced. They say they could be landslides, but why so numerous? It is a bit strange, in such an old terrain, equilibrium slopes should be reached everywhere from long ago.

[Guest_Richard Trigaux_*]

Installing a network of seismometres in well separated parts of the Moon, a purpose fit for "my" concept of a
lunar rover

Such a rover could host an inhabited module (robots are still unable to install seismometres properly) and do the trip around the Moon in say three months, and two astronauts.

At last a serious motive to send again people on the Moon.

[dvandorn]

-deep (quakes) at 700 kms, explained as the effect of tides. This tells us that rocks are rigid until at least this depth. But what is under? Magma? If there are tides, and magma, there must be tidal heating. There was lenghty threads here speaking of tidal heating of Io, Europa, Enceladus, etc. Why the Moon would not have tidal heating? If so, it would have volcanoes. Why it don't have? (latest volcanic activity was 3.3 billion years ago).

Actually, there is good reason to believe that the youngest *sampled* lunar basalts are not the youngest basalts that actually exist there. Comparing the eastern Procellarum surface detail with that seen at the western Procellarum Surveyor I landing site suggests that the Surveyor I regolith is significantly younger and thinner than that at the Apollo 12 / Surveyor III site -- possibly only a billion to a billion and a half years old. And some small pockets of extremely dark, smooth mare may be less than a billion years old.

It is possible that the Moon may have retained, due to tidal heating, a molten or semi-molten magma layer that separates its solidified mantle and its now-believed-to-be-chondritic core -- some recent motion/gravity studies of the Moon have been suggesting that the Moon's core actually rotates at a slightly different speed, and along a slightly different axis, from its mantle and crust. That would require a pretty high degree of ductility, I would think -- a layer that is molten or something very close to it.

I grant you, it's been at *least* hundreds of millions of years since there has been significant volcanic activity on the surface of the Moon. But that does not necessarily preclude the possibility that molten or semi-molten rock does not still exist somewhere deep below the crust. It just means that it is *so* deep, and in such a relatively non-pressurized state, that it is no longer capable of reaching the surface.

-the other Doug

[Guest_Richard Trigaux_*]

Actually, there is good reason to believe that the youngest *sampled* lunar basalts are not the youngest basalts that actually exist there. Comparing the eastern Procellarum surface detail with that seen at the western Procellarum Surveyor I landing site suggests that the Surveyor I regolith is significantly younger and thinner than that at the Apollo 12 / Surveyor III site -- possibly only a billion to a billion and a half years old. And some small pockets of extremely dark, smooth mare may be less than a billion years old.

It is possible that the Moon may have retained, due to tidal heating, a molten or semi-molten magma layer that separates its solidified mantle and its now-believed-to-be-chondritic core -- some recent motion/gravity studies of the Moon have been suggesting that the Moon's core actually rotates at a slightly different speed, and along a slightly different axis, from its mantle and crust. That would require a pretty high degree of ductility, I would think -- a layer that is molten or something very close to it.

I grant you, it's been at *least* hundreds of millions of years since there has been significant volcanic activity on the surface of the Moon. But that does not necessarily preclude the possibility that molten or semi-molten rock does not still exist somewhere deep below the crust. It just means that it is *so* deep, and in such a relatively non-pressurized state, that it is no longer capable of reaching the surface.

-the other Doug

Thanks Doug for your comment.

Yes, to allow for the core to rotate at a different speed from the mantle, requires a nearby liquid layer (On earth, the iron layer ir thought to be as liquid as water, to allow for the core rotation).

But if there is liquid, there must be tidal heating, and thus the bottom mantle must melt and become thinner, until it allows for volcanoes. Volcanism is rather the rule than the exception, so if there is not, we must invoke special conditions. For instance the magma is liquid, much denser than the mantle, and the mantle don't dissolves in it for whatever reason, so that the heat can escape only by conduction. But if so, why quakes at the bottom of the mantle?

[Guest_Richard Trigaux_*]

A possible explanation would be that volcanism would STILL occur on the Moon, but under an hidden form: basaltic flows filling under the regolithe layer, with no more visible surface effect than just a bulging. That would explain the shallow quakes, if such an eruption is taking place now somewhere. But the only way to know would be a network of seismometres, or a very precise altitude mapping allowing to show variations as small as 1 metre, or a very precise thermal mapping of the night side. As an indication, the (probable)filling of a magma chamber under the town of Pozzuoli (Italy) gave rise to a swarm of hundreds of small quakes looking like the Moon shallow quakes, with some minor damages to the town, and a 1-2m gain in altitude. The phenomenon lasted some weeks.

To allow for such hidden lava flows implies that there is very little or no gasses in them. A situation which never happens on Earth, as the mantle contains carbon dioxyd, sulphur and water.

[Bob Shaw]

A possible explanation would be that volcanism would STILL occur on the Moon, but under an hidden form: basaltic flows filling under the regolithe layer, with no more visible surface effect than just a bulging. That would explain the shallow quakes, if such an eruption is taking place now somewhere. But the only way to know would be a network of seismometres, or a very precise altitude mapping allowing to show variations as small as 1 metre, or a very precise thermal mapping of the night side. As an indication, the (probable)filling of a magma chamber under the town of Pozzuoli (Italy) gave rise to a swarm of hundreds of small quakes looking like the Moon shallow quakes, with some minor damages to the town, and a 1-2m gain in altitude. The phenomenon lasted some weeks.

To allow for such hidden lava flows implies that there is very little or no gasses in them. A situation which never happens on Earth, as the mantle contains carbon dioxyd, sulphur and water.

Richard:

Did I hear the word 'domes'?

Bob Shaw

[edstrick]

The locations of the shallow quakes were reasonably well triangulated over much of the moon from the Apollo seismic data. Locations are not precise to permit targeting high resolution imagery. That would require a global net of tens <?> of seismometers, at least. But they did not have any correlation with locations of regularly reported Transient Lunar Events or regions of young, low crater density, volcanism. A considerable number (if I recall) were in the southern highlands. Ability to detect and locate ones on the farside was poor.

[Guest_Richard Trigaux_*]

Richard:

Did I hear the word 'domes'?

Bob Shaw

Hmmmmm... domes, yeah, domes. I was rather speaking of bulging, understand any irregular form, when domes are regular (roounded). But why not.

Most of the domes are located on lava plains, that makes two explanations:
-very eroded anything, which takes a regular form when much spread
-small volcanoes which form on the top of certain lava flows (which the plains are). This can be found in France on a lava flow of a volcano called Tartaret: tens of small bonsaï volcanoes, some small enough to keep in a yard, but with all the features of volcanoes.

Bulging from underground flows can also explain these shapes.

I think this phenomenon could be common on the Moon. See the Schroeter valley: I wondered since my childhood how a lava flow could erode or melt such an incredible volume of rock. But it becomes realistic if we assume that the flow formed first under the regolite layer (such underground flows unearthed by erosion can be seen in Connecticut, Google Earth 41° 22' 30" N, 72° 58' W, altitude 34000ft) and then carried away the loose materials above it. The Schroeter flow was fairly big, but smaller could be still possible today, and Moon would be still active today, if we consider "active" one flow every 200 million years.

The locations of the shallow quakes were reasonably well triangulated over much of the moon from the Apollo seismic data. Locations are not precise to permit targeting high resolution imagery. That would require a global net of tens <?> of seismometers, at least. But they did not have any correlation with locations of regularly reported Transient Lunar Events or regions of young, low crater density, volcanism. A considerable number (if I recall) were in the southern highlands. Ability to detect and locate ones on the farside was poor.

Thanks for the localisation. I was not thinking to TLE, but the check was interesting to do.
Southern highlands? The oldest and most eroded terrains? This looks strange. Landslides? these regions have many craters with slopes, but most are old.

With my opinion, seismic study of the Moon is worth a landing. And manned, unless we continue to wait for nothing, untill robots will become able to install a seismometre.