IAL 12: The Moon and Mercury

Don't Panic

Sections

  1. Voyage to the Moon
  2. Moon Facts
  3. The Moon's Interior
  4. The Formation of the Moon
  5. The Evolution of the Moon I
  6. The Evolution of the Moon II
  7. Mountains on the Moon
  8. The Impact Cratering Process
  9. The Fate of the Moon
  10. Mercury: Closest to the Sun
  11. Mercury Facts
  12. The Spin-Orbit Resonance of Mercury
  13. Mercury's Interior
  14. Mercury's Surface
  15. The Fate of Mercury

  1. Voyage to the Moon

    2. In the IAL 3: The Moon: Orbit, Phases, Eclipses, and More, we considered the Moon as an astronomical object in the sense of old astronomy: an astronomical object to be watched on the sky: e.g., see the figure below/above (local link / general link: moon/afar/moon_stars.html).

    We now consider the     Moon as a physical body---we consider its nature, geology, origins, and fate.

    As a preview/review, see Moon videos below (local link / general link: moon_videos.html):

      EOF

  2. Moon Facts

    3. Just some basic Moon facts---which, of course, beg for an explanation---and there is some explanation, but also some "just so stories"---"just so stories" for us that is---somebody has thought of everything, I'd guess.

    1. Basic Moon Facts:

      First let's look at a table of Moon facts.

      Remember you look at tables NOT to try to memorize the numbers, but to contemplate what they mean while you look at them. 

      ________________________________________________________________________________________________________

Moon Facts
________________________________________________________________________________________________________

Quantity                      Value
________________________________________________________________________________________________________

Mean distance from Earth      384,400 km  =  60.2684 R_Earth_equatorial
                                Center-to-center distance as almost always in astronomy since 
                                that is the relevant distance for the graviational force
                                and center-of-mass motion and inertial frames.

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.
                                Volume and therefore mass tend to scale as the radius to power 3.
                                Since the Moon diameter is 
                                about 1/4 of the Earth diameter,
                                one would guess its mass would be (1/4)**3=1/64 of the 
                                Earth's mass.
                              The actual is small still since the Moon density is
                              less than the Earth density. 

Mean density                  3.36 g/cm**3 = about 3/5 Mean Earth density.
                                This is a clue:  it's much smaller than even uncompressed 
                                iron density 7.874 g/cm**3.  The Moon must have relatively less iron.
     

Equatorial surface gravity    0.1654 Earth gravities ≅ 1/6 Earth gravities.
                                The Moon mass is 
                                1/81.3 of the Earth's mass, but
                                the Moon's
                                gravity is NOT reduced by
                                the same factor, because the
                                Moon's radius is also smaller.
                                Mass and radius are both parameters of surface
                                gravity.
                                The lower gravity is actually vital for astronauts on the Moon
                                since the massive space suits they wear and heavy moving of equipment 
                                they do would be impossible without it.

Surface temperature           -170 C at night to 130 C in the day.
                                The lack of an atmosphere and oceans (which store 
                                heat energy and
                                insulate against heat
                                flows) cause the surface temperature to change rapidly between 
                                extremes with
                                heating by absorbing 
                                visible light
                                by day and cooling by emitting
                                IR by night.
                                The Moon
                                has a low thermal inertia:
                                i.e., a low resistance to temperature change.
                                The daytime and nighttime periods of about two weeks
                                are also a factor.
                                The astronauts on the Moon needed to be protected by their
                                space suits 
                                from hot surfaces, but NOT hot air since their is none.
________________________________________________________________________________________________________

References: Cox-16,303,305;
            Se-445.
________________________________________________________________________________________________________

      Now how does the Moon rank in the Solar System?

      Though much smaller than the Earth, the Moon still ranks high among the rocky/icy bodies in the Solar System---it's number 9---see the figure below (local link / general link: rocky_icy_body.html).

    2. Selenography:

      The Moon has geography, of course---with special-case name selenography.

      1. First, let's look at an image moon map of the near side of the Moon in figure below (local link / general link: moon_map_side_near.html).

      2. Second, let's look a topographic map of the near side of the Moon in the figure below. (local link / general link: moon_map_side_near_topographic.html).

      3. We've elucidated many aspects of the Moon by going there robotically and in person (see Wikipedia: Exploration of the Moon).

        The third moon map of near side of the Moon we will look at (see the figure below local link / general link: moon_map_side_near_exploration.html) is an exploration map that illustrates our "invasion"---the Selenites havn't even noticed.

      Enough of Moon maps.

    3. The Moon Has Almost No Atmosphere:

      The Moon has almost NO atmosphere, and thus is also soundless.

      "In space, no one can hear you scream."---ad for Alien (1979). The same is true for the Moon since for sound to propagate, your need transmission medium. See the figure below (local link / general link: alien_prototype_selenite.html).

      Why does the       Moon have no atmosphere?

      It's low gravity has been unable to hold a significant atmosphere over hundreds of millions of years.

      The Moon experienced atmospheric escape that was rapid in comparison to larger astro-bodies such as the Earth, Venus, and, to a lesser degree, Mars.

      Some atmosphere from volcanic outgassing probably tried to develop up to about 2 Gyr ago when the Moon still had active volcanism (Se-455).

      Also, the Moon has no large-scale magnetic field to protect it from the solar wind. The solar wind can strip away gas. The Earth's magnetic field (the outer part being the Earth's magnetosphere) largely protects Earth from solar wind sputtering blast of the solar wind.

      But there is always something if you look close enough.

      The Moon does have a tenuous and variable atmosphere with a pressure of order 3*10**(-15) atmospheres (NASA's Moon fact sheet, 2021).

      Helium, neon, molecular hydrogen (H_2), argon, methane (CH_4), ammonia (NH_3), CO_2, etc. have been detected.

      This minor atmosphere is probably produced by ongoing outgassing from rock due to meteoritic impact and accumulation from the solar wind???.

      The solar wind blows gas away, but it can also add some.

      Atmospheric escape for the Solar System and in general is explicated in the figure below (local link / general link: solar_system/solar_system_atmospheric_escape.html).

    4. The Elemental Composition of the Moon:

      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).

      But NOT quite dry as we once thought (Wikipedia: Lunar water).

      Spectroscopic evidence suggests a microscopic trace of water everywhere on the surface. This is NOT water in macroscopic amounts, but water molecules bound to minerals.

      The water molecules may originate from solar wind hydrogen atoms chemically combining with rock oxygen atoms.

      Some sort of equilibrium between formation and break-up and escape of hydrogen atoms may occur.

      In addition, in the interior of polar lunar craters (where the Sun never shines), there may be accumulations of ice probably below a layer of regolith. There is significant evidence for such ice pockets, but it is inconclusive as of circa 2021 (see Wikipedia: Lunar water).

      The origin of this water ice may be impacting comets. They contain water ice that vaporizes on impact and then the water vapor collects and freezes in those locations where it can collect and freeze: the interiors of polar lunar craters where it is continuously cold enough for continuous ice.

      Lunar water is interesting just scientifically, but it could be of great importance to lunar colonies. Water and materials derived from it (oxygen and the hydrogen) would be of great utility.

    5. The Moon Has No Large-Scale Magnetic Field:

      The Moon has no large-scale magnetic field. It has small local magnetic fields which are somewhat mysterious still (SRJ-166) and which we will NOT discuss further here. See Wikipedia: The Moon's magnetic field.

      Moon rocks show that the Moon earlier than 3 Gyr did have a field of about 4 % of Earth's current field (see Wikipedia: Magnetic field of the Moon; HI-148). This suggests the theory that Moon once had larger and hotter molten outer iron core than at present. But there are other theories too. See Wikipedia: Magnetic field of the Moon.

    6. The Lunar Highlands:

      The Moon is 84 % covered by lighter colored, heavily cratered lunar highlands 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 lunar highlands are the original lunar crust formed before about 3.8 Gyr ago (see Wikipedia: Lunar geologic timescale: Lunar stratigraphy).

    7. The Lunar Maria:

      The Moon is about 16 % covered by maria (pronounced ma-ray-a) (see Wikipedia: Lunar mare). The singular of maria is mare (pronounced ma-ray), but sometimes people just use maria for the singular.

      Mare means sea in Latin. Galileo (1564--1642), himself suggested they were bodies of water though he probably eventually realized they had to be plains (HI-140). The names of the maria (at least the largest ones???) on the near side of the Moon were given by Giovanni Battista Riccioli (1598--1671) (see Wikipedia: Giovanni Battista Riccioli: Work concerning the Moon).

      The maria are less cratered than the lunar highlands and are dominated by basalt rocks: they are silicates rich in iron, manganese, and titanium (Se-451).

      The maria seem more extensive to Earthlings than they are because they are almost all on the near side of the Moon.

      The Mercator projection image Moon map in the figure below (local link / general link: moon_map_mercator.html) gives a better idea of the extent of the maria.

      The       far side of the Moon (which was NOT seen until the 1959) has NO large maria as seen in the figure below (local link / general link: moon_map_side_far.html).

  3. The Moon's Interior

    4. Just as with the Earth, we've never dug deep into the Moon's interior and must rely on seismology, density, composition, size, magnetic field, other evidence, and modeling principally to understand it.

    As far a direction observation goes, note that on the Earth, we have dug a few kilometers down, but on the Moon, no more than a few feet by the someone among the Apollo astronauts, I think???.

    1. 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 are typically of order 0.5 or 1.5 on the Richter scale: you would never notice them even standing near the epicenter (FK-218).

      Note the Richter scale is a logarithmic scale with each unit being a factor of 10**(3/2) ≅ 31.6 ≅ 30 in energy released by an earthquake (see Wikipedia: Richter scale: Details).

      There are three main classes of moonquakes (FK-218--219; HI-148; SRJ-166).

      1. The first class of moonquakes are correlated with the anomalistic month (the time from apogee to apogee or perigee to perigee). Most of these occur when the Moon is at perigee and apogee. These moonquakes are probably caused by the Earth's tidal force flexing the Moon.

        Yours truly guesses that tension in the inner layers of the Moon builds up during the flexing and sudden releases are cause of moonquakes.

        The epicenters are of order 1000 km down near the lower boundary of the rigid lunar upper mantle (lithosphere) (which is at a depth of about 1000 km).

      2. The second class of moonquakes may be caused by largish impactors.

      3. The third class are landslide moonquakes.

        These may often NOT be independent events: i.e., they may be induced by impactor or tidal-force moonquakes.

        However, a few independent landslide moonquakes probably do occur. The expansion and contraction of rock during the daily heating and cooling cycle can fracture rock which in turn can sometimes induce landslides and concomitant moonquakes.

      The seismology tells about the layering of the Moon and about the solid and liquid natures of the layers.

      See Seismic waves and earthquake videos for the Earth below (local link / general link: seismic_wave_videos.html):

        EOF

    2. Moon Density and Elemental 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 of the Earth's mean density.

      In uncompressed densities, the Moon mean density is about 0.75 of the Earth's (see Table Density Trends of the Rocky Bodies below).

      The Moon is probably very lacking in the densest, common refractory iron relative to the Earth.

      Thus, we don't expect a huge iron lunar core.

      But the past large-scale magnetic field of the Moon suggests that the lunar core must be iron-rich, and this is true (see below), but the lunar core is small relative to Earth's iron-rich core.

    3. 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 primordial heat from formation and past radioactive heat long ago.

      It still has radioactive isotopes in its interior just like the Earth and all rocky bodies, and so there is still some internal heating.

      But heat conduction is sufficiently rapid that primordial-radiogenic heat geology (see also Wikipedia: Earth's internal heat budget: Radiogenic heat: Primordial heat) is turned off or very nearly so.

    4. Modeling and Results:

      The observations discussed above and others plus computer modeling suggest the model of the internal structure of the Moon shown in cross section diagram in the figure below.

    5. So Much for ... :

      So much for a description of the interior of the Moon.

  4. The Formation of the Moon

    5. How did the Moon form? What is the origin of the Moon?

    1. Three Old Discarded Theories:

      There are 3 old discarded theories:

      1. Co-Accretion Theory:

        The Moon formed at the same time as the Earth as a separate accretion out of circumplanetary disk. Formation from circumplanetary disk is believed to be origin of at least the larger members of the Jupiter system of moons, Saturn system of moons, and Uranus system of moons (see Wikipedia: Moons of Jupiter: Origin and evolution; Wikipedia: Titan: Formation; Wikipedia: Protoplanetary disk: Planetary system).

        Problems: Why does the Moon then NOT have the same composition as the Earth? Why does it have a relative lack iron? Why isn't its orbit in equatorial plane of the Earth where a circumplanetary disk would tend to form.

      2. 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 mantle and why the Moon has relatively 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.

      3. Capture Theory:

        The Moon was formed elsewhere in the Solar System and captured by the Earth.

        Problems: Capture demands very improbable initial conditions to occur according to celestial mechanics. The theory still doesn't explain why the Moon material is like Earth's mantle, but has a relative lack of iron.

    2. Giant Impact Hypothesis:

      The problems with the old theories led to the giant impact hypothesis in the which is nowadays considered the overwhelmingly strong favorite. See the artist's conception of the giant impact in the figure below.

      The giant impact hypothesis was first introduced as a undeveloped theory in 1946. In the mid-1970s, the theory was revived and developed and quickly became the accepted theory of the origin of the Moon (see Giant impact hypothesis: History of model) and the figure below.

      The essence of the giant impact hypothesis is explained in the figure below (local link / general link: moon_formation.html).

    3. Where Did Theia Come From?

      Where did the hypothetical giant impactor---now usually called Theia---come from?

      One idea that has gained some traction is Lagrangian point theory which is illustrated by the animation in the figure below (local link / general link: moon_theia.html).

      Now for some exciting       Moon formation videos, see Moon formation and evolution videos below (local link / general link: moon_formation_evolution_videos.html).

        EOF
    4. So Much for ... :

      So much for the origin of the Moon.

      Now let us evolve the Moon.

  5. The Evolution of the Moon I

    6. After accretion phase (assuming the giant impact hypothesis is valid), the Moon was still very hot.

    It also contained radioactive isotopes that further heated the interior.

    Some chemical differentiation must have occurred creating a small iron-rich core.

    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 lunar crust (Se-453).

    There were a few very large impactors that created giant lunar basins which are impact craters greater than about 200 km in average diameter.

    Mostly in the period from about 3.5 to 3 Gyr ago, massive lava flows that flooded the giant basins on the Moon to create the lunar maria.

    Some maria are older than 4.2 Gyr and some seem to be as young as 1.2 Gyr (see Wikipedia: Lunar mare: Ages).

    The lunar maria are usually in giant impact basins created by giant impactors. It may be that in many cases, the giant impactors weakened the lunar crust and initiated the massive lava flows, but this theory is still being debated circa 2021 (see Wikipedia: Lunar mare: Distribution of mare basalts).

    A theory for some lunar maria, in particular Oceanus Procellarum, is that they formed from lava flows from resembling rift valleys (see Wikipedia: Oceanus Procellarum: Origin; Gibney, E. 2014, Nature, Oct01, Moon's largest plain is not an impact crater), and NOT in giant impact basins. This theory is still being debated.

    The internal heat that caused the flooding was increasingly depleted and the flooding turned off.

    The flooded basins became the maria.

    But aside from the giant lava flows, here was much less volcanism on the Moon than on Earth.

    The unflooded, lunar highlands were never erased by volcanism: they only continued to suffer impacts at decreasing rate.

    The maria are sufficiently younger than the highlands that they missed the heavy bombardment or most of it.

    Most lunar maria probably formed more than 3 Gyr ago though some regions may be as young as 1.2 Gyr (see Wikipedia: Lunar maria: Ages).

    Hence the maria are much less cratered than the highlands.

    Now for some exciting Moon evolution videos, see Moon formation and evolution videos below (local link / general link: moon_formation_evolution_videos.html).

      EOF

  6. The Evolution of the Moon II

    7. After the maria stopped forming, the evolution of the Moon has been very slow.

    There are some moonquakes from the Earth's tidal force and from impacts as mentioned above.

    These can cause occasional landslides.

    The daily heating and cooling of rocks causes expansion and contraction can cause fracturing at a slow rate.

    But the main ongoing geological activity is continuing meteoritic impacts---at a much slower rate than in the heavy bombardment, of course. Thus the Moon has impact geology.

    The continuing bombardment has made a few recent large craters: e.g., Crater Tycho: see the figure below. (local link / general link: moon_map_composition_false_color.html).

    The continuing bombardment has also continuously fractured, fragmented, and blasted the surface. The blasting has been by the steady impacting     meteoroids, particularly of micrometeoroids (see Wikipedia: Regolith: Moon). The blasting process is called meteoritc weathering which is a special case of space weathering. See the figure below (local link / general link: space_weathering.html).

    Space weathering causes the surface of the Moon to be mostly covered by regolith.

    The regolith is a heterogeneous mixture rock fragments, pebbles, dust that reach maybe 3 to 30 meters in depth (HI-142).

    Only about 1 % of the regolith is meteoritic---most of it is fragmented lunar rock (Se-452).

    The uppermost few meters of regolith is mostly fine, glassy, slippery dust called lunar soil (Ze2002-177; HI-142).

    Before the first close up views of the lunar surface, people---and science and science fiction illustrators---often imagined the lunar surface as very rough. A jagged outcrops and peaks. Perhaps, they were thinking that a low-gravity, low erosion world would have many structures that on Earth would be rare. For an example of a 19th century artist's conception of the lunar surface, see the figure below.

    This old idea of the lunar surface is mostly wrong.

    To reiterate, Space weathering and fragmenting intermittantly by large impactors over billions of years has reduced it mostly to regolith.

    In most places at least, the lunar surface is smooth and powdery and slippery as illustrated in the figure below (local link / general link: moon_regolith_pete_conrad.html).

    There are, of course, larger rocks and even boulders around. See the figure below.

  7. Mountains on the Moon

    8. What are the lunar mountains?

    Well several kinds of lunar mountains exist, but there is some ambiguity in that some features are lunar mountains or NOT depending on what criteria are used to define lunar mountains.

    We adopt the rather broad and vague definition of anything that rises high above the surroundings, and thus include some features that some people would omit.

    The following is our list:

    1. Most of what we identify as lunar mountains are eroded large crater rims.

      Smaller impactors subsequent to the larger impactors that created the crater rims break the crater rims into lunar mountains.

      The giant impact craters (AKA basins) that flooded to become at least most of the maria are rimmed by the most obvious lunar mountains of the eroded crater rim kind.

      The eroded crater-rim lunar mountains form lunar mountain ranges.

      The figure below shows what is probably an eroded crater-rim lunar mountains---it would take an expert to know.

    2. A hypothesis from 2014 is that some of the lunar mountain ranges attributed to eroded crater rims around giant impact craters are NOT that.

      Instead as the Moon cooled some pieces of surface contracted unevenly and rifts formed between some of the pieces. Near the rifts, the surface crinkled like drying mud and the crinkles became lunar mountain ranges---that then suffered erosion by subsequent impactors.

      A contracted piece itself may have been covered by later lava flow and became some of the lunar maria.

      The contracted piece are in some respects like tectonic plates on Earth---but they arn't and there is no evidence that plate tectonics has ever occurred on the Moon.

      Let us call the scenario just sketched the lunar rifting process (see Gibney, E. 2014, Nature, Oct01, Moon's largest plain is not an impact crater).

      The largest region of maria (which includes the Oceanus Procellarum, the largest mare) may have formed at least in part by the lunar rifting process as mentioned above (see Wikipedia: Oceanus Procellarum: Origin; Gibney, E. 2014, Nature, Oct01, Moon's largest plain is not an impact crater).

      But Oceanus Procellarum is the only region for which the lunar rifting process has been invoked so far.

      The evidence for the lunar rifting process is from gravimetric measurements by the GRAIL mission.

      Time will tell if the lunar rifting process theory remains viable.

    3. Large, not-heavily-uneroded crater rims can be regarded as circular lunar mountains.

      As an example, see Crater Copernicus in the figure below (local link / general link: mare_imbrium.html).

    4. The central peaks of central-peak craters form isolated lunar mountains.

    5. There are a few volcanoes on the Moon.

      But they are a small proportion of all lunar mountains and are NOT obvious at first examination of lunar imagery.

      The volcanoes are called lunar domes and are a form of shield volcano.

      They are probably all extinct and probably have been so at least 1 Gyr ???? since the youngest lunar maria are from 1.2 Gyr BP (see Wikipedia: Lunar mare: Ages). The youngest lunar maria are an indication of the last lava flows to the lunar surface.

      Lunar volcanoes are a very subordinate feature of the present-day Moon.

      There may be many more in the early days of the Moon but they probably have been largely erased by impact geology.

      The figure below shows some lunar domes.

      There are also irregular mare patches which relativly small round mounds. They are found in lunar maria and are typically of order 500 m wide. It has been suggested that they originate in lava flows that only a few tens of millions years old (see Wikipedia: Irregular mare patch: Origin). That there could be lava flows as young as a few tens of millions years is debated.

    6. Lunar lobate scarps: relatively small ridges extending only a few miles with heights up to 100 m.

      Stretching a point, one might call them lunar mountains. But probably most people would NOT call them lunar mountains.

      The lunar lobate scarps seem to have formed within the last few hundred million years.

      As the lunar interior cools and contracts a little??? over geologic time, the Moon, and consequently the rigid lunar surface cracks and buckles to form the lunar lobate scarps.

      Mercury has similar, but much larger, Mercurian lobate scarps.

    On the Moon, there are no fold mountains nor fault-block mountains at least as due to plate tectonics.

    Such mountains may never have existed on the Moon or they may have been totally erased by impact geology.

  8. The Impact Cratering Process

    9. What causes lunar craters?

    Short story: impactors.

    Now the long story.

    1. The Discovery of Lunar Craters:

      Yours truly doesn't think obviously recognizable lunar craters can be seen with the naked eye under any circumstances.

      So obviously recognizable lunar craters, were only discovered with the invention of the telescope in the early 17th century.

      The first telescopic maps of the Moon were made Thomas Harriot (c. 1560--1621) (see figure below: local link / general link: thomas_harriot.html) from 1609 Jul26 on.

      Harriot beat Galileo Galilei (1564--1642) to the punch in making Moon maps, but he, Harriot, never published his Moon maps, and so Galileo, who published promptly in 1610, got all the credit---which is just since what good is a discovery that no one knows about.

      Galileo: scientist, musican, artist---he was a Renaissance man after all For his moon maps, see the figure below (local link / general link: galileo_moon_map.html).

      By the later       19th century, quite good Moon maps existed for the near side of the Moon showing the most prominent lunar craters---see the figure below (local link / general link: moon_map_1881_point_inverted.html).

    2. Lunar Crater Theories:

      No one had a plausible scientific hypothesis about the nature of lunar craters until the geologists became knowledgeable about volcanic craters.

      The plausible hypothesis after volcanology was sufficiently developed was that lunar craters were volcanic craters. In fact, up until the mid-20th century, most scientists thought that the lunar craters were volcanic craters (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 lunar craters thought to be VOLCANIC?

      Well most CRATERS on Earth are volcanic craters after all.

      But lunar craters are, in fact, rather different from volcanic craters.

      In contrast to volcanic craters, lunar craters:

      1. are NOT on top of mountains.
      2. can be very large.
      3. have wide rims compared to their bases.
      4. tend to have interiors below the surrounding plain.

      We will now compare a volcanic crater and an impact crater:

      1. First, a volcanic crater in the figure below.

      2. Next, a typical largish impact crater in the figure below (local link / general link: crater_keeler.html).

      As you see from the above images, volcanic craters and impact craters are NOT alike.

      So why were lunar craters (which are impact craters) thought to be volcanic for so long by most scientists (i.e., until the mid-20th century: (FMW-173)?

      There was a major obstacle to the idea of IMPACT CRATERING being common on the Moon.

        Question: The seeming obstacle was:

        1. the age of the craters.
        2. the roundness of the craters.
        3. the color of the craters.











        Answer 2 is right.

      Yes, the ROUNDNESS.

      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 the high speed of impactors.

      This high speed means huge kinetic energy, much of which becomes heat energy on impact. Becoming heat energy on impact is no distinction from the sandbox case, but the amount of heat energy is explosive for large impactors. By explosive, we mean the hot vapor exerts strong pressure forces. The explosions strongly tend to be azimuthally symmetric about the point at some depth in the impact site.

        Note that momentum is conserved.

        The impactor momentum gets deposited to the Moon as a whole and that hardly moves the Moon---unless the impactor is giant impactor---but there have been none of those since the heavy bombardment.

        The explosion momentum is small since the explosion is nearly isotropic. Individual bits of ejecta from the explosion have high momentum, but momentum is vector and the isotropy leads to cancellation.

        Note also that impactor energy grows relative to momentum magnitude with impactor speed since KE/p=(1/2)mv**2/(mv)=(1/2)v.

    3. The Cratering Process:

      Let us look at the cratering process:

      1. First in cartoon in the figure below:

      2. Then in videos:

        See Impactor videos below (local link / general link: impactor_videos.html):

          EOF

      3. Finally in words (Se-447, FMW-174):

        1. 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 
            
  By conservation of mechanical energy

  (1/2)mv_infinity**2=(1/2)mv_impact**2-GMm/r
                   
  v_infinity**2=v_impact**2-2GM/r

  v_infinity**2=v_impact**2-v_escape**2

  v_impact**2=v_infinity**2 + v_escape**2  .

          The impactor thus has tremendous energy. We can do a rough estimate of the KINETIC ENERGY PER KILOGRAM: 

            kinetic energy per kilogram 

  = (1/2)mv**2/m = about ( 10 km/s * 10**3 m/km )**2 = 10**8 J/kg  .

          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 altogether.

        2. A fast impactor creates a DEEP IMPACT.

          It tends to penetrate the surface to 2 or 3 times its own diameter (FMW-174) and all its kinetic energy is turned into kinetic energy of rebounding parts of the surface and heat energy.

          The momentum of the interaction is conserved.

          But a large amount of the mass must act as the effective impacted object.

          Compared to the impactor this mass may be close to infinity.

          An infinite mass violates momentum conservation on collisions. It acts as a momentum sink and source. This is the correct limiting behavior with infinite mass objects. Really momentum is conserved, but the very massive object acquires very little velocity and in the infinite mass limit none at all.

          The infinite-mass effect is at the basis of causing an oblique impactor to result in a nearly circular impact explosion.

          The momentum of the impactor gets spread throughout a large region of the impactee, but the energy of the impactor results in a rather localized round explosion.

          This is handwaving, but correct handwaving I think.

        3. The impactor and some of its surroundings are VAPORIZED by the heat.

        4. The vapor is rapidly expansive (i.e., explosive) and hurls up and out in all directions ROUGHLY EQUALLY ejecta from the impact site.

          The ejecta exceeds the impactor mass typically by a factor of 10.

          Some of the impactor kinetic energy probably directly becomes rebound kinetic energy of the ejecta. Maybe a significant amount. I'm NOT sure.

        5. The SHOCK WAVES from the explosion compress the surface below and around the impactor.

          The shock is basically azimuthally symmetric.

        6. The compression downward can cause a REBOUND that forms a central peak. This tends to happen only for bigger impactors.

        7. The shock way acting along the ground level pushes up the crater RIM.

        8. EJECTA falls back in the crater and around it often creating a flat crater floor and an apron of material around the rim that may be twice the rim's diameter.

        9. Thus, the region excavated and shaped by the impactor is essentially CIRCULAR.

        10. Bomb and shell craters on Earth are formed in a similar fashion---they are roughly circularly symmetric explosions---and are also essentially circular (FMW-174).

        11. For large impactors, ejecta thrown far beyond the crater rims can create concentric rings and rays such as for Crater Tycho as seen in the figure (local link / general link: moon_map_composition_false_color.html).

          Also           secondary craters can be created by fragments thrown out by the impactor.

        12. The impactor can cause moonquakes that may initiate landslides in older craters.

        13. As the craters ages, the wall can slump from landslides due to erosion and moonquakes and then TERRACED WALLS are created as we see in the image of Crater Keeler in the figure below (local link / general link: crater_keeler.html).

      The cratering process just described is for moderately big impacts. Smaller impacts result in bowl-shaped craters (HI-140).

      Secondary craters caused by ejected fragments from the impactor just falling under near-surface gravity (and therefore do NOT reach very high velocities) can indeed be non-circular and have elongated shapes like stones in a sandbox ??? (HI-141).

      See the probable small elongated secondary craters in the figure below (local link / general link: mare_imbrium.html). We'd need a impact crater expert to now for sure that they are secondary craters.

    4. The Creation of the Lunar Maria:

      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 (or multi-ring basins) 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. See the figure below.

      The formation of the Orientale Basin is explicated in the figure below (local link / general link: moon_orientale_basin_formation.html).

  9. The Fate of the Moon

    10. Tidal-force geology, space weathering, and occasionally large impact events will continue to slowly evolve the Moon.

    But since its surface looks much like it did 3 Gyr ago when the maria mostly stopped forming, one imagines the Moon will look much the same some gigayears in the future.

    What about the Moon's orbit?

    Well, the Moon is slowly spiraling away from the Earth at a current rate of 4 cm/yr (Ni-78). This is due to the tidal force of the Earth on the Moon. The rate of separation will NOT stay constant, but nevertheless gigayears in the future the Moon will be significantly farther away. In fact, calculations suggest that in ∼ 50 Gyr, the Moon will have an orbital period or ∼ 47 days, much larger than the (sidereal month (27.321661554 days: J2000)) and the Moon and Earth will be mutually tidally locked (see Wikipedia: Orbit of the Moon: Tidal evolution).

    But this theoretical tidal locking will NEVER happen. The Moon will be vaporized along with the Earth in the post-main-sequence evolution of the Sun in ∼ 5 Gyr (see Wikipedia: Sun: After core hydrogen exhaustion). For the details of the Earth-Moon system doom, see file sun_red_giant.html.

    "So the glory of this world passes away: sic transit gloria mundi."

  10. Mercury: Closest to the Sun

    11. Mercury is in many respects similar to the Moon.

    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.

    See Mercury in slar transit in the figure below.

    In fact, until 2008 the only close-up views of Mercury came from the Mariner 10 spacecraft that did 3 flybys in the period 1973--1974.

    It's NOT that Mercury is uninteresting, it's just that there always seems to be more interesting Solar System things to study.

    Mariner 10 was in a heliocentric retrograde orbit (NASA: Mariner 10).

    Only about half of the surface was imaged by Mariner 10 (HI-160).

    But starting in 2008 the NASA MESSENGER spacecraft (launched 2004aug03) began giving us new close-up images and other data for Mercury and other Solar System data too.

    Here's a portrait of our Solar System from MESSENGER.

    Initially, MESSENGER just did 3 flybys in 2008--2009, but on 2011mar18, it went into orbit around Mercury.

    Here's double image of Mercury from MESSENGER.

    Now that MESSENGER is orbiting Mercury, we will soon know a lot more about Mercury: maybe too much for this course.

    One new thing is new named impact craters. For some reason, it was decided that craters on Mercury should be named for artists of all genres and few others too. The US Geological Survey (USGS) keeps a Mercury crater list, where you can see if your favorite artist has made the cut---along with Boethius (c. 480--524 or 525) (Boethius), Katsushika Hokusai (1760--1849) (Hokusai), and Pablo Neruda (1904--1973) (Neruda).

    Some new names are shown in the figure below.

    In this lecture, we will just do a quick run through on Mercury emphasizing how it differs from the Moon.

  11. Mercury Facts

    12. Just some basic Mercury facts---which, of course, beg for an explanation---and there is some explanation, but also some "Just So". 

    _______________________________________________________________________________

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/ex-planets.

Inclination to Ecliptic       7.00487 degrees :  Again only Pluto has a 
                                           larger one among the planets/ex-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---see the figure below (local link / general link: rocky_icy_body.html).

  12. The Spin-Orbit Resonance of Mercury

    13. It was long suspected that Mercury would be tidal locked to the Sun: i.e., that Mercurian year and day were the same length, and thus Mercury always turned the same face to the Sun.

    Tidal locking is very common for moons in the Solar System where most significant moons are tidal locked to their parent planets (e.g., the Moon). For those moons that are NOT, see Wikipedia: Tidal locking: Occurrence: Moons. Recall we discussed tidal locking in IAL 3: The Moon: Orbit, Phases, Eclipses, and More: Lunar Rotation and Tidal Locking.

    1. Tidal Locking Recapitulated:

      We can recapitulate an explanation of tidal locking.

      Say you have two gravitationally bound astro-bodies: a primary (whose name means the more massive of the two) and a secondary (whose (whose name means the less massive of the two).

      The tidal force of primary slows/speeds up the secondary into a tidal locked state. The more massive the primary and the closer the secondary, stronger the tidal locking effect and the faster tidal locking is brought about..

      The primary's tidal force stretches the secondary and then acts differentially on the tidal bulges to bring about the tidal locking of the secondary.

      The secondary can also tidal lock the primary, but this takes longer. It may NOT happen at all if the secondary has to compete with other astro-bodies. The Moon is trying to tidal lock the Earth, but this will take a long time because the Earth has so much rotational inertia (i.e., resistance to rotational acceleration) and because the Sun is trying to tidal lock the Earth. The Sun's tidal locking effect is smaller and the Sun will lose out to the Moon, but it will delay the Moon's victory.

      Among the planets, only ex-planet Pluto and its biggest moon Charon are mutually tidally locked (see Wikipedia: Tidal locking: Occurrence).

      Tidal locking is never exactly perfect. Small perturbations keep trying to desynchonize the tidal locked astro-body. But the tidal locking effect keeps acting bring the astro-body back toward the perfectly tidal locked state.

    2. The Spin-Orbit Resonance:

      Giovanni Schiaparelli (1835--1910) in the 1880s thought he had evidence for the tidal locking of Mercury to the Sun from observations of features he could barely discern (Se-458).

      But in 1965, radar reflections off Mercury showed that its mean orbital and axial rotation periods relative to the observable universe were different. The modern values are 87.9691 days and 58.646 days, respectively (see Wikipedia: Mercury; Wikipedia: Mercury: Spin-orbit resonance;

      The ratio of mean axial rotation period to mean orbital period is 2/3 to very high accuracy.

      Why do we have this situation? Mercury's orbit and spin show what is called 3:2 spin-orbit resonance brought about by the tidal force of the Sun. Celestial mechanics can explain how the 3:2 spin-orbit resonance arose.

        The varying eccentricity of Mercury's orbit over millions of years due to perturbation of the other planets may cause Mercury to get trapped in the odd 3:2 spin-orbit resonance (Tytell, D. 2004, Sky & Telescope, November, 22; Correia, A. C. M. 2004, Nature, June 24). We won't go into details.

      There must be a slight variation in the 3:2 ratio due to perturbations, but these are damped out by the whatever forces the 3:2 spin-orbit resonance. So the 3:2 spin-orbit resonance is STABLE at least over human time scales.

      The 3:2 spin-orbit resonance means that 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 upshot of this unusual 3:2 spin-orbit resonance is that the Mercurian day is TWICE the Mercurian year (i.e., the revolution period). The figure below (local link / general link: mercury_3_2_spin_orbit_resonance.html) explicates the Mercurian day.

      Mercury's 3:2 spin-orbit resonance and its Sun-on-sky behavior are illustrated in the video Mercury tidal lock - double sunrise phenomenon | 1:27 given in Mercury videos below (local link / general link: mercury_videos.html).

        EOF
    3. The Synodic Day Formula:

      In fact, there is a formula that allows you to calculate the DAY (i.e., the synodic day) of planet given its orbital period and its axial rotational period, both relative to the observable universe, with the simplyfing assumption of uniform circular motion in a common plane. The synodic day formula is given in the figure below (local link / general link: synodic_period_day.html).

      Now       Mercury satisfies the simplyfing assumption of the synodic day formula well enough to be approximately correct.

      Applying the formula to Mercury gives 

        P_day = P_orbital*P_axial/(P_orbital - P_axial) = (2/3)*P_orbital**2/[P_orbital-(2/3)*P_orbital]
        = 2*P_orbital 2 * 87.9691 days = 175.9382 days
      which is indeed approximately correct (see above local link / general link: mercury_3_2_spin_orbit_resonance.html and Wikipedia: Mercury: Spin-orbit resonance).

  13. Mercury's Interior

    14. Unlike the Earth and the Moon, for Mercury we have NO seismology and NO rocks in our hands to examine.

    NO spacecraft has ever landed on Mercury and nothing has every come back from there. MESSENGER is just an orbiter

    But we do know Mercury's MEAN DENSITY from knowing its size and from knowing its mass (from ???).

    This density is 5.44 g/cm**3.

    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 calculate by modeling the uncompressed densities of the rocky bodies: see the table just below.

    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). See the cartoon illustrating the relative sizes of iron cores of Earth, Moon, and Mercury in the figure below.

    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.

    With a smaller fraction of silicates condensing out, the iron fraction would be higher.

    But the people who do the calculations think that even this temperature effect is NOT enough to explain Mercury's high iron content.

    Mercury does have a global dipole magnetic field like the Earth, and unlike the Moon.

    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.

  14. Mercury's Surface

    15. Mercury's SURFACE at first glance is much like the Moon's: heavily cratered with no current primordial-radiogenic heat geology.

    Because of its small size and lack of significant current volcanic outgassing Mercury is essentially airless just like the Moon.

    The surface of Mercury is shown in the figure below.

    There are some differences from the Moon.

    Although NOT obvious to eye, Mercury is somewhat LESS cratered than the Moon and there are relatively fewer small crater (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 primordial-radiogenic heat geology for a bit longer.

    Some evidence for this that lava plains on Mercury cover a larger fraction of Mercury than the lunar maria do on the Moon (HI-165).

    The Mercurian lava plains similar to the lunar maria. However, they 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).

    The Mercurian lava plains can be noticed somewhat in a Mercator mosaic. See the figure below.

    Mercury---like the Moon---shows multi-ring basins formed by giant impactors followed by lava flows.

    The main example is the Caloris Basin. See the figure below.

    See the weird terrain at the antipodal point to Caloris Basin in the figure below.

    See the figure below (local link / general link: moon_orientale_basin_formation.html) for a reminder of how weird terrain is thought to form.

    One striking feature of     Mercury are lobate scarps.

    The figure below (local link / general link: mercury_lobate_scarps.html) explicates lobate scarps.

  15. The Fate of Mercury

    16. To be laconic about it: slow evolution caused by tidal-force geology (due to the Sun's tidal force), space weathering, and occasionally large impact events followed by vaporization in the expanded solar atmosphere of the Sun in its red giant phase in 5--6 Gyr (FK-493).