- Moon Facts
Just some basic Moon facts---which, of course, beg for an explanation---and there is some explanation, but also some ``Just So.''_______________________________________________________________________________ Moon Facts _______________________________________________________________________________ Quantity Value _______________________________________________________________________________ Mean distance from Earth 384,400 km = 60.2684 R_Earth_equatorial Eccentricity of orbit 0.0549 Mean inclination to Ecliptic 5.145396 degrees Mean orbital period 27.32166 days Lunar Month 29.53059 days: this is also the lunar day Equatorial radius 1737.4 km = 0.2725 R_Earth_equatorial Mass 7.3483*10**22 kg = 1/81.3 Earth masses Mean density 3.36 g/cm**3 = about 3/5 Mean Earth density Surface gravity 0.167 Earth gravities Surface temperature -170 C at night to 130 C in the day _________________________________________________________________________ References: Cox-16,303,305; Se-445. _________________________________________________________________________
Though much smaller than the Earth, the Moon still ranks high among the rocky/icy bodies in the solar system.
Largest rocky/icy bodies in the solar system.
References: (Cox-295,305,306; HI-161; David Jewitt's Kuiper Belt site).
The Moon has geography, of course.
A map of the NASA and Soviet probe landing sites.
Luna (Soviet) and Surveyor (NASA) were uncrewed landers. The 6 Apollo landings were all crewed as everyone knows. 12 humans have walked on the Moon---but none since 1972!
The map table shows what the arrows mean. A pity NASA didn't bother to enlarge the names of the features. Recall the equatorial diameter of the Moon is 3476 km. This sets the scale.
The darker areas are the maria. Their names are almost legible.
The big rayed crater south of Mare Imbrium is Copernicus. East of Copernicus is a smaller rayed crater, Kepler. Tycho is the rayed crater that is hard to make out on this map, but Surveyor 7 landed on its rim.
Credit: NASA.
Some other facts:
- The Moon has virtually no atmosphere and thus is also soundless.
-
``In space, no one can hear you scream.''---ad for Alien
It's low gravity has been unable to hold a significant atmosphere over hundreds of millions of years. Some atmosphere from outgassing probably tried to develop up to about 2 Gyr ago when the Moon still had active volcanism (Se-455).
-
[There probably not nearly as much outgassing as on Earth
since Moon rocks
seem to be lacking in volatiles as we discuss below.
They are for example very nearly totally dry: i.e.,
lacking water, unlike Earth rocks
(Se-451).]
But there is always something if you look close enough.
The Moon does have a tenuous atmosphere and variable atmosphere with a pressure of order 3*10**(-15) atmospheres ( NASA's Moon fact sheet).
Helium, neon, H_2, argon, methane (CH_4), ammonia (NH_3), CO_2, etc. have been detected.
This minor atmosphere is probably produced by on-going outgassing due to meteoritic impact and accumulation from the solar wind???.
- The elemental composition of the Moon's surface is very close to
the elemental composition of the Earth's mantle, EXCEPT
the Moon is very depleted in volatiles
(HI-146).
For example, crustal Earth rocks contain about 1--2 % water either as molecules trapped in the rock or as molecules in some kind of chemical bond. But Moon rocks are very nearly totally dry (Se-451).
- The Moon has no large-scale magnetic field. It has small local
magnetic fields which are somewhat mysterious still
(SRJ-166) and which
we won't discuss further here.
Moon rocks tell us that the Moon earlier than 3 Gyr did have a field of about 4 % of Earth's current field. This suggests the Moon once had a molten iron core (HI-148).
- The Moon is 85 % covered by by lighter colored, heavily cratered
lunar upland dominated by ANORTHOSITE ROCKS
which are silicates rich in
calcium, aluminum, and oxygen
(HI-141,
Se-452).
Anorthosite rock is igneous rock. There is no sedimentary rock on the Moon. There is no evidence for there ever being large amounts of water on the Moon.
- The Moon is 15 % covered by MARIA (pronounced mar-eia)
(HI-140).
(The singular is mare: pronounced ma-ray.)
Mare means sea in Latin. GALILEO, himself suggested they were bodies of water though he probably eventually realized they had to be plains (HI-140).
The maria are less cratered than the uplands and are dominated by BASALT rock: they are silicates rich in iron, manganese, and titanium (Se-451).
The MARIA seem more extensive to Earth observers than they are because they are almost all on the near side of the Moon.
Mercator Moon map.
All the big maria are on the near side.
Oceanus Procellarium is the big mare. Orientale Basin is to the south-west of Oceanus Procellarium. Mare Tranquillitatis is hind end (but not the tail or feet) of the rabbit. Mare Moscoviense is due east of Mare Tranquillitatis and rather inconspicuous.
One can see the maria cover only a small part of the lunar surface: i.e., about 15 % (HI-140).
Credit: JPL's map page; download site: a hidden ??? JPL page.
The far side of the Moon was not seen until the first Soviet probe orbited the Moon in 196???. Because the Soviets got the first glimpse of the far side, they named all the big features for Soviet things: e.g., Mare Moscoviense (Sea of Muscovy) which is the largest ??? far side mare---and it's not very large.
-
Question: Why wasn't the far side seen until 196???.
- It is always the night side of the Moon, and so isn't clearly
seen.
- Tidal locking long ago caused the lunar day to become
as long as the lunar month. Thus, the Moon always turns
nearly the same face towards the Earth. The far side is the
``far side'': the side we don't see from Earth.
Answer 2 is right.The Moon wobbles a bit, and so over time a bit more than half can be seen from Earth.
- It is always the night side of the Moon, and so isn't clearly
seen.
- There are no obvious traces of volcanoes on the Moon.
There may be some obscured ones or ones may have existed
in the past.
There are no fold or fault mountains. There is no evidence of tectonic plates.
All the lunar mountains seem to be crater rims or parts thereof or central crater peaks.
- The Moon's Interior
Just as with the Earth, we've never dug into the Moon's interior and must rely on seismology, density, composition, size, magnetic field, and modeling principally to understand it.
SEISMOLOGY.The Apollo missions left seismographs on the Moon that continued working for several years (FK-219).
There are of order 3000 MOONQUAKES per year, but they are all very tiny: the strongest were typically of order 0.5 or 1.5 on the Richter scale: you would never notice them even standing near the epicenter (FK-218).
-
There are two main classes of MOONQUAKES
(FK-218--219;
HI-148;
SRJ-166) and
probably a third minor class:
- One class of moonquakes are correlated with the lunar month: most of
occur when the Moon is at perigee and apogee.
These moonquakes are probably caused by the Earth's tidal force flexing the
Moon.
The epicenters are of order 1000 km down near the lower boundary of the Moon's rigid lithosphere (which is at a depth of about 1000 km).
- The other moonquakes may be caused by meteoritic impacts.
- Landslide moonquakes must occur too, but
they may often not be independent events: i.e.,
they may be induced by impact or tidal force moonquakes
usually.
However, a few independent landslide moonquakes probably
do occur.
The expansion and contraction of rock during the
daily heating and cooling cycle can probably induce landslides
and concomitant moonquakes
sometimes.
DENSITY AND COMPOSITION.Well the elemental composition of the surface rocks of the Moon are much like those of the Earth's mantle.
But the Moon mean density is only about 3/5's of the Earth's mean density.
In uncompressed densities, the Moon mean density is about 0.8 of the Earth's. (See Densities Table below).
The Moon is probably very lacking in the densest, common refractory: IRON.
Thus, we don't expect a huge iron core.
But the past large-scale magnetic field of the Moon suggests there must be some iron core, but that it is probably no longer molten (HI-148) or only partially molten.
SIZE.The Moon is much smaller than the Earth.
It is only about 1/4 of the Earth in diameter.
Thus, it probably lost most of its residual heat from formation and past radioactive heat long ago.
It still has radioactive elements in its interior, and so there is still some interior heat.
All these evidences plus computer MODELING suggest the following interior model.
A cartoon of a model of the interior of the Moon.
The Moon model is much less certain than that of the Earth.
The past magnetism of the Moon suggests that something like an iron core or iron-sulfide core exists, but how large it is uncertain (SRJ-166).
The LUNAR LITHOSPHERE is certainly very thick though: about 1000 km deep as suggested by seismology (HI-147--148).
Recall the EARTH'S LITHOSPHERE is only about 100 km thick.
The thick lithosphere is consistent with no active RESIDUAL/RADIOACTIVE-HEAT GEOLOGY.
The heat from the interior cannot drive magma through the thick lithosphere. It can only escape by the slow and gentle process of HEAT CONDUCTION.
Note that the crust of the Moon is about 60 km on the near side and about 100 km or more on the far side.
Why this is so has not been fully elucidated.
But the early major impacts that led to the near side maria (see below) may have weakened the near side crust (Se-455). If so, then the cause is merely the random pattern of impactors.
- The Formation of the Moon
How did the Moon form?There are 3 old theories:
- CO-ACCRETION THEORY
The Moon formed at the same time as the Earth as a separate accretion out of circum-Earth disk. We believed that the larger moons of Jupiter and Saturn formed this way.
Problems: Why does the Moon then not have the same composition as the Earth? Why does it lack iron?
- FISSION THEORY
The early earth spun so fast that a part of the mantle broke off after CHEMICAL DIFFERENTIATION had occurred. This explains why the Moon surface material has an elemental composition similar to the Earth's and why the Moon has little iron.
Problems: Why was the early Earth spinning so fast? Why is the Moon orbit not in the equatorial plane of the Earth where a fission would tend to put it. The Moon orbit is much closer to being in the Ecliptic Plane.
- CAPTURE THEORY
The Moon was formed elsewhere in the solar system and captured by the Earth.
Problems: Capture seems very hard to arrange to the celestial dynamicists. This theory still doesn't explain why the Moon material is like the Earth's mantle, but lacks iron.
GIANT IMPACTOR THEORY (Se-456).
Some millions of years after the early Earth formed and chemically differentiated, an impactor of order the size of Mars hit the Earth at a glancing angle.
A cartoon of one version of the giant impactor theory
(Se-457).
The impactor combined with Earth and significantly increased its size. It is possible that very dense materials like gold and platinum were spread in the Earth's crust by the impactor (SRJ-168). It has been wondered how these materials are abundant as they are in the crust:
Ejecta made of mantle material was thrown off.
Some of this ejecta fell back to Earth and some escaped Earth altogether.
But some of the ejecta went into orbit and began to accrete under gravity to form the proto-Moon within a few days (PF-105).
The heat of the impacted ejecta material caused them to outgas almost all their volatiles leaving them volatile-depleted as Moon rocks are today (HI-150).
The GIANT IMPACTOR THEORY is consistent with our overall theory of the early solar system with many leftover protoplanets still swarming around.
Computer simulations show that the GIANT IMPACTOR THEORY is possible.
But, of course, they don't tell us exactly what the impactor was or how it hit. All we can do is create plausible scenarios and calculations that lead to what we find today.
- The Evolution of the Moon I
After accretion phase (assming the GIANT IMPACTOR THEORY theory is valid), the Moon was still very hot.It also contained radioactive elements that further heated the interior.
Some CHEMICAL DIFFERENTIATION must of occurred probably creating a small iron core (HI-148).
In the period from about 4.6 to 3.8 Gyr ago, the heavy bombardment occurred.
The Moon was heavily cratered. Most of the large craters we see today date from the heavy bombardment.
There was cratering on cratering fragmenting the crust (Se-453).
There were a few very large impactors that created GIANT BASINS.
In the period from about 4 to 2 Gyr ago, the weakened crust in the giant basins permitted massive LAVA FLOWS that flooded the basins (Se-455).
The internal heat that caused the flooding was increasing depleted and the flooding turned off.
-
Question: What became of the lava flooded basins?
- They sunk into the Moon and were never seen again.
- They became the maria.
- They became the green cheese.
Answer 2 is right.But beside from the giant lava flows, here was much less volcanism on the Moon than on Earth.
The unflooded, uplands were never erased by volcanism: they only continued to suffer impacts at decreasing rate.
The MARIA are sufficiently younger than the uplands that they missed the heavy bombardment or most of it.
Hence the MARIA are much less cratered than the uplands.
- The Evolution of the Moon II
After the MARIA stopped forming, the evolution of the Moon has been very slow.There are some MOONQUAKES from the Earth tidal force and from impacts as mentioned above.
These can cause occasional land-slides.
The daily heating and cooling of rocks causes expansion and contraction can cause fracturing at a slow rate.
But the main on-going geological activity is continuing meteoritic impacts---at a much slower rate than in the heavy bombardment, of course. Thus the Moon has METEORITIC GEOLOGY.
The continuing bombardment has made a few recent large craters: e.g., Tycho.
The Galileo probe on its way to Jupiter produced this false color image.
The false colors emphasize the maria and craters and other features. The credit reference gives color meanings in terms of composition. North is at the top.
The Crater Tycho is near the south. It is not the biggest crater on the Moon, but it is very recognizable because of the rays that radiate from it.
The rays were produced by the impact: they filaments of ejecta thrown out. They extend halfway around the Moon. These rays would have been erased by subsequent impacts if Tycho were old.
Tycho may be only of order 100 Myr (Se-448).
Credit: NASA.
The continuing bombardment has also continuously fractured, fragmented, and sandblasted the surface. The sandblasting has been by microscopic meteorites (HI-142).
The result is that the surface of the Moon is mostly covered by regolith.
The regolith is rocks and pebbles that reach maybe 3 to 30 meters in depth (HI-142).
Only about 1 % of the regolith is meteoritic: most of it is pounded up lunar rock (Se-452).
The uppermost few meters of regolith is mostly fine, glassy, slippery dust (Ze2002-177; HI-142).
-
[Glass is mostly silica (SiO_2) like rock, but it does NOT
have a crystalline structure. It has an amorphous
structure. The atoms are arranged randomly.
Glass is created by quick cooling of molten silicates.
Small, lumps of molten or vapor silicate ejecta from an impact site is likely to cool very rapidly and become small glassy spheres (Ze2002-177).]
But in many places at least the surface is smooth and powdery and slippery.
Pete Conrad of Apollo 12 (1969nov20) finds Surveyor 3 (1967apr20).
Notice the regolith dust. This powdery dust has been created by sandblasting by small meteorites. The dust (not the regolith) is perhaps a few centimeters thick (Se-445.) The dust is quite glassy and slippery (Ze2002-177).
The astronauts always landed in the daytime, but the sky looks black because there is no atmosphere to scattering sunlight around.
Recall the lunar day is as long as the lunar month (29.53059 days on average), and so lunar daytime is about 14.7 days.
Because of the Moon's small size the horizon seems very near. Perhaps this is not obvious in images, but the astronauts were quite struck by it (PF-103).
Credit: NASA.
There are, of course, larger rocks and even boulders around.
Jack Schmitt, his ``dune buggy,'' regolith, and Crater Shorty's edge at
the right.
Notice the regolith dust. This powdery dust has been created by sandblasting by small meteorites. The dust (not the regolith) is perhaps a few centimeters thick (Se-445.) The dust is quite glassy and slippery (Ze2002-177).
The sandblasting has given much (most???) of the Moon's surface a smooth and soft appearance. There are a few rocks and outcroppings about though. The Moon is not mostly the jangle of broken-up rock pictured by old science fiction stories.
The astronauts always landed in the daytime, but the sky looks black because there is no atmosphere to scattering sunlight around.
Recall the lunar day is as long as the lunar month (29.53059 days on average), and so lunar daytime is about 14.7 days.
Note this is a color picture, but the Moon is not colored: it's black, white, and shades of gray mostly.
Credit: NASA.
-
Question: What is different about the lunar
terrain compared to arid
terrain on Earth such as in the American south-west?
- It is hilly.
- It is ochre.
- There are no gullies or water channels.
Answer 3 is right.Most arid terrain on Earth still shows water run-off features. On the Moon there are NO WATER CHANNELS AND GULLIES. The Moon does not have these: no significant atmosphere, no water probably for gigayears.
I don't think the Moon looks ochre. Sort of grey mostly.
-
[Ochre is the color pale-yellow to orange or reddish in hue or
pigments of this color made of a mixture of hydrated iron oxide
and earthy materials
(Ba-838).]
- The Cratering Process
Up until the mid-20th century, most scientists thought that the lunar craters were VOLCANIC (FMW-173).We know now that they are all or almost all IMPACT CRATERS.
-
[Crater comes from the ancient Greek word KRATER meaning a mixing
bowl for wine and water
(Ba-283).]
Why were the craters thought to be VOLCANIC?
Well most craters on Earth are VOLCANIC after all.
But lunar craters are, in fact, rather different from volcanic craters.
In contrast to volcanic craters, lunar craters:
- are NOT on top of mountains;
- can be very large;
- have wide rims compared to their bases;
- tend to have interiors below the surrounding plain.
We can compare a volcanic crater and an impact crater.
Mt. Cotopaxi in Ecuador. A false color constructed model.
Mt. Cotopaxi is the highest active volcano. It is 5897 meters above sea level and is more than 3000 meters higher than the surroundings.
It's base is about 23 km. The outer crater at the top is 800 X 650 meters.
Mt. Cotopaxi is a dangerous active volcano.
Credit: NASA.
Crater 302: a terrraced-walled crater. Apollo 10, 1969may01.
Crater 302 is the largest crater in the image and it must be pretty big since the curvature of the Moon's limb is visible.
Crater 302 is a typical large terraced-walled crater.
- There is a central peak formed by rebound.
- Back falling ejecta provided a flatish floor.
- A rim wall produced by excavation plus shock pushing up the rim.
- The rim shows terraces which are created by landslides.
Even on the Moon, slow erosion from meteoritic impacts, moonquakes,
and heating-and-cooling expansion and contraction will eventually
lead to landslides and slumps.
- Ejecta also falls outside the rim to about twice the crater
diameter producing an apron which possibly can be descryed.
- There may be secondary impacts from the ejecta, but that is
is not obvious here. So many later or earlier impacts have riddled
the ground.
See FMW-174 for more details on impacts.
Credit: NASA.
There was a major obstacle to the idea of IMPACT CRATERING being common on the Moon.
-
Question: The seeming obstacle was:
- the age of the craters.
- the roundness of the craters.
- the color of the craters.
Answer 2 is right.Surely impact craters should usually have odd shapes depending on the ANGLE OF IMPACT. When you throw a stone in a sandbox if it hits obliquely it creates grooved path.
But almost all the lunar craters are ROUND.
But modern computer modeling and experiments have resolved the ROUNDNESS PROBLEM.
The essential distinction of space impactors from stones in sandboxes is their high speed.
Let us look at the CRATERING PROCESS first in cartoon and then in words (Se-447, FMW-174).
A cartoon of the cratering process.
- The impactor hits at very high speeds.
Recall that the orbital speed of the Earth is about 30 km/s.
Any impactor in the vicinity of the Earth is in an orbit with a speed of order 30 km/s.
The relative speed on collision might be lower depending on the direction of impact, but is still likely to be of order 10 km/s. Much more for a head-on impact.
Also the Moon's gravity will accelerate the impactor by an amount up to 2.4 km/s which is the Moon's escape speed.
-
[For a radial impact where v_infinity is the speed at infinity
v_impact**2=v_inf**2 + v_escape**2by conservation of energy.]
- The Moon has virtually no atmosphere and thus is also soundless.
