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_______________________________________________________________________________ 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:
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).
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???.
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).
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).
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.
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.
The Moon wobbles a bit, and so over time a bit more than half can be seen from Earth.
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 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).
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).
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.
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.
There are 3 old theories:
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?
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.
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.
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.
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.
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 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.
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.
We know now that they are all or almost all IMPACT CRATERS.
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:
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.
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.
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.
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.
v_impact**2=v_inf**2 + v_escape**2
by conservation of energy.]
The impactor thus has tremendous energy. We can do a rough estimate of the KINETIC ENERGY PER KILOGRAM:
kinetic energy per kilogram
= about ( 10 km/s * 10**3 m/km )**2 = 10**8 J/kg
where recall the kinetic energy formula is (1/2)*mv**2.
For comparison, a car of mass 1000 kg moving at 50 m/s (i.e., 112 mi/hr) has kinetic energy of only about 10**6 J.
It tends to penetrate the surface to 2 or 3 times its own diameter (FMW-174) and all its kinetic energy is turned into shock and heat energy.
The ejecta exceeds the impactor mass typically by a factor of 10.
The shock is basically circularly symmetric.
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.
Also SECONDARY CRATERS can be created by fragments thrown out by the first impactor.
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.
See FMW-174 for more details on impacts.
Credit: NASA.
The CRATERING PROCESS we described is for moderately big impacts. Smaller impacts can be bowl-shaped (HI-140).
Secondary impacts where the impactor is just falling under local gravity can indeed be non-circular and have elongated shapes like stones in a sandbox ??? (HI-141).
Crater Copernicus.
Crater Copernicus has a rim diameter of about 90 km and is one of the largest craters on the Moon.
It is south of Mare Imbrium which is partially seen at the top of image.
The impactor must have been a few kilometers in diameter (HI-141).
There is a surrounding of secondary craters. These had lower impact speeds and are sometimes elongated along the direction away from the main crater??? or so it seems to my eye.
Credit: NASA; download site the Copernicus site of the Lunar Orbiter Photographic Atlas of the Moon. These guys have copyrighted their site but the original images are public domain surely.
The GIANT IMPACTORS of the HEAVY BOMBARDMENT (circa 4.6--3.8 Gyr ago) are believed to have created the major basins in which MARIA formed when lava flowed into them.
Some of the giant impactors at least created MULTI-RING CRATERS which in many cases have be highly eroded by subsequent impacts.
The ORIENTALE BASIN is a fairly clear example of a MULTI-RING CRATER with a small mare in the inner part.
The Galileo probe on its way to Jupiter produced this color image.
This image shows the western hemisphere of the Moon which is the eastern side as seen from Earth (HI-141,143). The widest ring, which one can sort of see, is about 1000 km in diameter. Recall the Moon's equatorial diameter is only 3476 km.
The largest impactors created weak spots in the lithosphere of the Moon through which lava flowed up to create the maria: i.e., the lava plains. The Orientale Basin is contains a small mare.
The rings may have been produced by waves that froze in place (SRJ-159).
The impact probably shook the whole Moon substantially and probably created converging shock waves at the antipodal point where a jumbled WEIRD TERRAIN has been noticed (HI-454).
Credit: NASA.
A cartoon of the formation of the Orientale Basin and its
antipodal weird terrain (Se-454,460).
But since its surface looks much like it did 2 Gyr ago when the MARIA stopped forming, one imagines the Moon will look much the same some gigayears in the future.
The Moon is slowly spiraling away from the Earth at a current rate of 4 cm/yr (Ni-78).
This is due to a tidal effect of the Earth on the Moon.
So gigayears in the future the Moon will be significantly farther away.
If the SUN swallows the Earth in its red giant phases as discussed in IAWL Lecture 9: The Life of the Sun, then the Moon gets swallowed and vaporized too.
If not, then the Moon and Earth may continue as a bound pair in some orbit around the SOLAR WHITE DWARF for hundreds of gigayears into the future.
It is a small, rocky, airless body that is heavily cratered and has lava-flow plains.
It is less well studied than the Moon.
It is much farther away and it's hard to get close-up views of it.
Mercury transiting the Sun in 1999.
Johannes Kepler missed the chance of discovering sunspots pretelescopically because he misinterpreted a sunspot for Mercury in transit (Ca-167).
Credit: Bill Livingston, NSO/AURA/NSF
In fact, the only existing close-up views of Mercury come from the Mariner 10 probe that did 3 flybys in the period 1974--1975.
Mariner 10 was in a retrograde heliocentric orbit (HI-160).
Mercury false-color mosaic from Mariner 10.
This mosaic shows about all of Mercury that Mariner 10 was able to image. The other side of Mercury is unknown as is that cowlick that comes over the limb.
Credit: NASA; download site ????.
It's not that Mercury is uninteresting, it's just that there always seems to be more interesting solar system things to study.
NASA's Messenger (launched 2004aug03) will go to Mercury. It is scheduled to do 3 flybys in 2008 and 2009 and settle into orbit in 2011 ( NASA: Messenger).
After Messenger gets to Mercury, we will know a lot more about Mercury than we do now: maybe too much for this course.
In this lecture, we will just do a quick run through on MERCURY emphasizing how it differs from the MOON.
_______________________________________________________________________________
Mercury Facts
_______________________________________________________________________________
Quantity Value
_______________________________________________________________________________
Mean distance from the Sun 0.387 astronomical units
Eccentricity of orbit 0.2056 : Only Pluto has a larger one among
the planets.
Inclination to Ecliptic 7.00487 degrees : Again only Pluto has a
larger one among the planets.
Orbital period 87.969 days
Rotational Period 58.646 days =2/3 of orbital period nearly exactly
Mercurian day 175.938 days =2 of orbital period nearly exactly
Equatorial radius 2439 km = 0.382 R_Earth_equatorial
Mass 3.31*10**23 kg = 0.0558 Earth masses
Mean density 5.44 g/cm**3 = about Mean Earth density
Uncompressed mean density 5.4 g/cm**3 which is an estimate and
is larger than the Earth's
estimated uncompressed mean density
of 4.2 g/cm**3
Surface gravity 0.38 Earth gravities
Surface temperature -173 C at night to 330 C in the day
The daytime high is due to the closeness
to the Sun, of course.
At night the surface quickly radiates
infrared radiation to space and
cools off. Remember space is cold.
_________________________________________________________________________
Sources: Cox-294,295,
Se-418, 459.
_________________________________________________________________________
MERCURY is not very large compared to the other ROCKY PLANETS and it is even surpassed by two of the large moons in the solar system: it comes in as only the 6th largest rocky/icy body.
Largest rocky/icy bodies in the solar system.
References: (Cox-295,305,306; HI-161; David Jewitt's Kuiper Belt site).
This SYNCRHONOUS TIDAL LOCKING is very common in the solar system.
The Earth's Moon exhibits SYNCRHONOUS TIDAL LOCKING and so do many other planets' moons. In those cases, the tidal locking is to the parent planet, not the Sun, of course.
The tidal force force stretches the second body and then acts differentially on the tidal bulges to bring about SYNCRHONOUS TIDAL LOCKING.
SYNCRHONOUS TIDAL LOCKING is established more quickly, the smaller the second body.
But in 1965 radar reflections off Mercury showed that its mean revolution and rotation periods relative to the FIXED STARS were 87.969 days and 58.646 days, respectively (Se-458).
The ratio of mean rotation period to mean revolution period is 2/3 as exactly as can be measured???.
Thus 3 rotation periods (about 176 days) equals 2 revolution periods (about 176 days).
Thus we have the following conversion factors:
1 = 3 rotations / 2 revolutions converts revs to rots.
1 = 2 revolutions / 3 rotations converts rots to revs.
The exact ratio tells us there is tidal locking, but NOT synchronous tidal locking.
Somehow---which celestial dynamicists may understand, but I don't---the tidal force of the Sun drove Mercury into this stable NONSYNCHRONOUS TIDAL LOCKING.
The upshot of this unusual NONSYNCHRONOUS TIDAL LOCKING is that the Mercurian day is TWICE the Mercurian year (i.e., the revolution period).
The following figure illustrates the situation, where we've made use of the conversion factors given just above.
Mercury's nonsynchronous tidal locking illustrated.
In fact there is a GENERAL DAY FORMULA that allows you to calculate the DAY OF ANY PLANET given its revolution period P_rev and its rotation period relative to the fixed stars P_rot provided the axial tilt from the perpendicular to the orbital plane is NOT extreme.
NO probe has ever landed on Mercury and nothing has every come
back from there.
Even the
Messenger when in gets to Mercury to stay in 2011
(if all goes well) will ONLY be an orbiter.
Meteorites from the Moon and Mars have been recognized, but
none so far as I am aware from Mercury.]
The EARTH'S density is slightly larger at 5.51 g/cm**3
(Cox-240).
The MOON'S density is distinctly smaller at 3.36 g/cm**3
(Se-445).
But these are densities where much of the matter is under compression
due to high internal pressure.
It is possible to estimate the uncompressed densities of the
rocky bodies:
Note: The uncompressed densities are model results, and so can be used
as input data only in heuristic arguments or calculations.
In compressed density, MERCURY is the 2nd densest
of the bodies in the table. And because of low gravity one
does not expect great compression even before modeling for
uncompressed density.
The conclusion that one draws is that MERCURY must be richest in the
densest abundant refractory: IRON.
On the other hand, the surface of Mercury appears to be ordinary
SILICATE ROCK.
But recall no one has actually seen a sample up close.
The conclusion is that Mercury must have an IRON CORE that
relative to Mercury's size is larger than the Earth's iron core.
And, of course, Mercury's iron core must be much larger than the Moon's.
The core formed through CHEMICAL DIFFERENTIATION when
Mercury was young and hot.
We don't know how big the Mercurian IRON CORE is really, but
the rocky surface may be a fairly thin layer relative to Mercury's
radius
(Se-459).
Why should Mercury have should a relatively large amount of iron?
Well it formed out of the primordial solar nebula closer to the Sun than
the other rocky bodies.
This was hotter than the location of the Earth and a relatively smaller
amount of less-refractory silicates may have condensed there than
in the Earth's neighborhood.
But the people who do the calculations think that even this temperature
effect is not enough to explain Mercury's high iron content.
A giant impactor is invoked
(Se-462).
(When you can't explain something invoke a giant impactor.)
It is only 0.5 % as strong as the Earth's
(Se-462).
But it seems odd that there should be any significant field.
To the Mercury modelers, it seems that Mercury's core should not
be molten now---it should have cooled off due to Mercury's small size
(Se-462).
Mercury's magnetic field remains a bit of puzzle.
Because of its small size and lack of significant current outgassing
Mercury is essentially airless just like the Moon.
The north pole is near the top and the equator is about 2/3 of the way
down.
The resolution is about 2 km.
The large MULTI-RINGED CALORIS BASIN is half visible
at the terminator on the left.
It is just above the middle of the image.
In the upper right is the obvious rayed Crater Degas which has
a diameter of about 45 km.
The rays indicate that Degas is comparatively young.
By the time NASA got to Mercury they were down to 19th century artists.
Mercury does have lava plains similar to the lunar maria, but they
arn't so different in color from the other regions and so are hard
to pick out.
Credit:
NASA.
Some not downloadable??? maps of Mercury are a
ESO.
Although not obvious to eye, MERCURY
is somewhat LESS cratered than
the Moon and there are relatively fewer small craters
(Ze2002-179).
This distinction may be due to the SUN whose strong gravity may
have affected the distribution of impactors.
But also Mercury is somewhat larger than the Moon and may have
had active RESIDUAL/RADIOACTIVE-HEAT GEOLOGY for a bit longer.
Some evidence for this is shown by the Mercurian uplands.
They show somewhat more LAVA PLAINS than
the lunar uplands
(HI-165).
The Mercurian LAVA PLAINS are much the same color as the rest of
Mercury (i.e., gray) and so don't stand out as well as the
lunar maria (Se-462).
But they can be noticed somewhat in a Mercator mosaic.
Only about 50 % of Mercury was images which accounts for the blank
regions. Only 10 to 180 degrees west longitude is available, but
I don't know how the prime meridian is defined or why there is
strip on the right.
Credit: NASA and USGS which put the mosaic together;
download site
Views of the Solar System by Calvin J. Hamilton.
Mercury---like the Moon---shows MULTI-RINGED BASINS
formed by giant impactors followed by lava floods.
The main example is the CALORIS BASIN.
Unfortunately, the terminator bisects the basin. However, the
long shadows do tend to emphasize contrasts in altitude.
The outermost rings are about 1300 km in diameter and the highest
features are about 3 km.
The Caloris Basin is vast multi-ring impact basin like
the Orientale Basin on the Moon.
(HI-141,143).
The largest impactors created weak spots in the lithosphere of bodies
like Mercury and the
Moon through which lava flowed up to create the maria: i.e., the
lava plains. The Caloris Basin has lava filling too, but not
as dark for some reason as the lunar maria.
The rings may have been produced by waves that froze in place
(SRJ-159).
The impact probably shook the whole planet substantially and probably
created converging shock waves at the antipodal point where
a jumbled WEIRD TERRAIN has been noticed
(HI-454).
Credit:
NASA.
In the image north is up. The large, flat-floored crater at the left
has a diameter of about 35 km.
To experts who have studied a lot of Mercurian landscape this
region is WEIRD TERRAIN.
To them it looks jumbled and chaotic.
The region is antipodal to the Caloris Basin.
It is thought that
seismic waves generated by the Caloris Basin impactor were focussed
at the antipodal point and created the WEIRD TERRAIN.
Credit:
NASA.
Just a reminder from above of how WEIRD TERRAIN is thought to form.
One striking feature of MERCURY
not seen on the Moon are LOBATE SCARPS.
A common geological feature of Mercury, not seen on the Moon, are lobate
scarps (or faults).
These are giant curved cliffs that rise as high as 3 km and
can stretch over hundreds of kilometers
(Se-460,
HI-165).
They are believed to have formed when Mercury, already with
a rigid lithosphere, cooled and became contractable internally.
The lithosphere under its own gravity collapsed inward slowly compressing
the inside of Mercury, but crinkling or faulting on the outside
to make the scarps.
It is thought that Mercury contracted by a few kilometers overall
(Se-460).
Mercury's lithosphere when it was still quite on hot on the inside
was probably rather thin compared to the Moon.
The Moon's thicker and, perhaps stronger, lithosphere didn't
compress and fault when the inside cooled.
Credit:
NASA
Slow meteoritic evolution followed by vaporization in the
expanded solar atmosphere of the Sun in its red giant phase in
7--8 Gyr (FK-493).
First, say that counterclockwise motion is positive, and count as
negative the periods of any clockwise motion.
The general day formula for non-extreme axial tilt is
P_rev x P_rot
P_day = ---------------------- .
P_rev - P_rot
In the case of Mercury, P_rot = (2/3) P_rev and both are positive.
Thus
(2/3) P_rev**2
P_day_Mercury = ------------------------- = 2 x P_rev .
P_rev - (2/3) P_rev
The derivation is given as a homework problem.
Unlike the Earth and the Moon, for Mercury we have NO seismology
and NO rocks in our hands to examine.
[It is possible that impactors on Mercury have knocked off
material that later found its way to Earth and touched down
as a meteorite.
But we do know MERCURY'S MEAN DENSITY from knowing its size and
from knowing its mass (from ???).
[Actually, it is a bit tricky to know the mass of a planet without
a moon. Perhaps, we know the mass from studying how Mercury perturbs
the orbits of comets, asteroids, and human-made probes.]
This density is 5.44 g/cm**3.
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Mean Densities and Estimated Uncompressed Mean Densities
Of the Rocky Planets and the Moon
__________________________________________________________________________
Body Mean Density Uncompressed Mean Density Estimate
(g/cm**3) (g/cm**3)
_________________________________________________________________________
Mercury 5.44 5.4
Venus 5.24 4.2
Earth 5.51 4.2
Moon 3.36 3.35
Mars 3.93 3.3
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A cartoon of the relative sizes of iron cores
(Se-459;
SRJ-166
Ze2002-160).
[Silicates condense out at about 1200 K and iron at about 1300 K and
iron oxides at about 1500 K
(Se-418).]
With a smaller fraction of silicates condensing out, the iron fraction
would be higher.
Question: What extra effect is invoked to explain the high
iron content of MERCURY?
MERCURY does have a global dipole magnetic field like the Earth,
and unlike the Moon.
Answer 3 is a viable explanation---not a ``right'' explanation note.
MERCURY'S SURFACE at first glance is much like the Moon's: heavily
cratered with no current RESIDUAL/RADIOACTIVE-HEAT GEOLOGY.
Mercury from a Mariner 10 mosaic from 1974mar30 on its 1st flyby.
There are some differences from the Moon.
A Mercator mosaic of mercury based on Mariner 10 images from its three
1974-1975 flybys.
The Caloris Basin from a Mariner 10 mosaic from 1975mar16 on its 3rd and
final flyby.
[WEIRD TERRAIN has become the accepted name for
this jumbled landscape it seems.]
The Orientale Basin and has similar antipodal weird terrain
(Se-454,460).
Mariner 10 image of the weird terrain antipodal to the Caloris Basin.
A cartoon of the formation of the Orientale Basin and its
antipodal weird terrain (Se-454,460).
Mariner 10 image of a lobate scarp.
Scenario for the formation of Mercury's lobate scarps
(HI-165).
To be laconic about it: