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19-1: From lower left to upper right: Mercury (a tiny almost invisible dot); Venus; Earth; Mars; Jupiter; Saturn; Uranus; Neptune; Pluto (also a tiny dot). BACK

19-2: Apollos 11 and 12 were in maria; Apollo 15 was on mare-type lavas locally within a segment of highlands; Apollos 16 and 17 were specifically in highlands; Apollo 14 set down on some of the ejecta from the crater Copernicus that was itself largely basaltic of mare character. BACK

19-3: Ultimately, this Section 19 would be about 3 pages shorter since the Apollo spacecraft, as designed, could not have safely landed. The surface was indeed covered with loose material but this contained a range of particle sizes that acted somewhat like gravel but had sufficient bearing strength to permit the heavy lunar landers to rest on firm ground. BACK

19-4: The Moon goes through its complete phase cycle in 29.5 days (lunar month). When the Moon lies directly between Earth and Sun, in its opening phase, it is said to be a New Moon, and appears dark (not visible) when viewed from the daylight side of Earth. As it waxes (right side becomes progressively illuminated), it becomes a waxing crescent (seen early around dusk and setting soon thereafter). The crescent enlarges with each successive day. When the Moon lies at 90 degrees to the Earth-Sun line of sight, it is in its First Quarter (its right half is illuminated). Thereafter, the side of the Moon facing Earth becomes progressively illuminated. When the Moon is along the line of sight but the Earth lies directly between it and the Sun, a Full Moon (completely illuminated) is visible from the night side of Earth. Then, as the Moon continues its revolution around Earth, a dark crescent first appears at its right limb. The Moon rises successively later each night, as the dark crescent grows until it reaches its Third Quarter (right side dark). Towards the end of the month, as the Moon comes up late at night, it darkens until only a light crescent appears around its left limb. It then passes again into total darkness and a new New Moon. Next time you have a period of extended insomnia, observe these different phases of the Moon if you have never done so yet. BACK

19-5: The Earth is a good example: It rotates on its axis every 24 hours. Each day it revolves a little bit about the Sun, at one focal point of its elliptical orbit, taking about 365.3 earth rotations (days) to return to an arbitrary starting point. Thus, rotation refers to the turning of a body around itself and revolution considers its rotation around another central body. BACK

19-6: To understand the Moon's inversion in the Southern Hemisphere, perform this simple experiment: Make a fist of your left hand - this is the Earth. Now, hold the thumb of your right hand upwards and the whole hand about 6 inches higher than the left hand - the thumbnail represents the Moon in its normal position as seen from the upper left fist (Earth's Northern Hemisphere). Then, swing the right hand and thumb downward along an arc until it is about 6 inches below the left fist. The back of the thumb now faces Earth. Rotate the entire right hand along the wrist 180° until the thumbnail faces the lower left fist. The thumb (and nail) = the Moon's frontside are now upside down and would appear that way to anyone in the Southern Hemisphere. A variant of this: imagine walking continuously from the central U.S. through Central America to northern Chile, without ever turning around. If that way in Chile, you would be facing away from the Moon and would need to turn around to see it, in its now inverted form. BACK

19-7: There are considerably fewer dark areas, those covered by mare lavas, on the farside; thus, most of the back of the Moon appears to be highlands. BACK

19-8: The lunar surface is a near-perfect vacuum (no atmosphere) and, thus unprotected, is being continually bombarded by the solar wind. If a gas of the appropriate composition comes to the lunar surface from a vent to the interior, this gas when excited by the solar wind could glow (much as argon or other gases in a fluorescent tube give off light as electrons pass across the tube), yielding a light illumination that might last for hours to days. BACK

19-9: Two origins of wrinkle ridges are considered likely. They may be caused by compression of the crust of cooling lavas in the maria or they may be extrusions of still mobile lava from below the surface crust that has developed elongate cracks through which the materials upwell. Sinuous rilles can be caused by erosion of a solid surficial crust by fluid lavas excaping from vents, much like water in a meandering stream erodes a surface; in some cases, the rilles could be collapsed lava tubes (under surface tunnels through which fluid lavas flow). All of these explanations are examples of comparative planetology, in which features on other planets are interpreted as analogs to similar features observed on Earth. BACK

19-10: Each of the strips is about 1/10th of a mile wide; some crater diameters are at least as wide as a strip (thus, over 500 feet from rim to rim and perhaps 50 feet deep). There are a few areas that have mostly smaller craters. But, in the writer's opinion, landing in such a heavily cratered mare terrain seems risky because the LM isn't all that maneuverable as it descends and if set-down is on a crater wall, the tilt of the space vehicle could be a problem. BACK

19-11: Orientale is bigger than any known impact structure on Earth by at least a factor of 2. But, such large structures probably existed on Earth during its early history after a thick crust formed about 4 billion years ago. They have since been obliterated by erosion or burial. BACK

19-12: The floor has oval upbulges or domes made of lava. On Earth, these are known as "tumuli" and form when fluid lava forces up the thin surface crust. A tumulus can split open allowing the lava beneath to spill out. There are also a few small pressure ridges on Tycho's floor. BACK

19-13: The most obvious conclusion, that none of these rock types were present at the Surveyor VII site, proved valid. Later, A.Turkevich and others proposed that the highlands were composed mostly of the igneous rock type Anorthosite, which is made up primarily of plagioclase feldspar. Its composition was closer to the VII results, especially the similarity of Ca and Na, which are both high in plagioclase. The Apollo 11 sample returns strongly supported this conclusion. This analysis is a good example of the chemical aspect of comparative planetology. BACK

19-14: The Ranger program showed in the higher resolution (closer to surface) images that there were many craters of small size, demonstrating that the lunar surface contains a wide range of crater sizes as would be expected from the impact model; Orbiter gave a large number of high quality and resolution images that displayed better than before crater morphology, ejecta deposits, volcanic features (domes and flows), crustal deformation features, presence of layered units, and a good look at much of the farside; Surveyor confirmed that the lunar surface could support landers, revealed a fragmental rock-strewn surface, and provided meaningful chemical analyses of the lunar soil. BACK

19-15: The surface of the highlands becomes saturated for craters of this size and smaller. Thus, any new craters in this size range will not change the surface appearance since there are already as many in this range as the surface can record. But the highlands could "accept" craters of larger sizes because such big ones don't occur everywhere (not so many as to have them touching or interfering with each other). BACK

19-16: The description for the first dashed line at lower left describe the oldest and most degraded stage in the history of a crater. Going successively towards the upper right, each description refers to a progressively lesser degree of crater modification and destruction, until in the upper right, the appearance of a fresh, young crater is given. BACK

19-17: At least these (not in the order of their relative ages): 1) the very dark mare lavas; 2) large, somewhat isolated blocks that resemble hills (probably ejecta from a distant excavated basin; 3) an ejecta apron around the smaller crater near the upper right; 4) the crater walls of Copernicus; 5) the floor of the Copernicus crater; 6) the widespread light ejecta blanket coming from Copernicus and from small craters on the mare. BACK

19-18: Note that most of the dates are associated with lavas. These rock ages must be determined on the rock samples themselves and represent the age of formation (cooling into hard rock). Breccia samples cannot be dated as such as they are composed of fragments of different rocks formed at different times. Meaningful dates are possible only for those rocks at Apollo sites that are local bedrock or fragments thereof. BACK

19-19: Apollo 13 is perhaps the most famous "failure" in the space program (beating even the disastrous Challenger Space Shuttle accident, in that it is highlighted by a successful "rescue" shown dramatically in the movie "Apollo 13"). About three quarters to the Moon, there was a sudden explosion in the Service Module (SM) when an electrical short circuit caused a malfunction that led to pressure buildup of the cryogenic oxygen; the failure blew off the outer panel and disabled some systems. The crew, of course, being in severe danger had to abort the lunar landing. Power in the Command Module (CM) was turned way down to conserve it; the crew moved to the small Landing Module (LM) and existed on its power and oxygen until after the spacecraft had circled behind the Moon and, with a rocket firing, headed back towards Earth. Although crises ensued and were solved with courage and innovation, as they approached Earth, they jettisoned the damaged SM and then the LM, returning safely in the CM to splash down in the Pacific, as the world watched and prayed in wonder. BACK

19-20: The photo shown doesn't reveal the two sets of lineations visible in telephoto lens pictures of the Apennines taken during Apollo 15. One set appears to be layering which may be typical of the lunar crust. When the Imbrium basin was formed, it was probably multi-ringed. The Apennines then are one of the crustal rims in this ring series, thrust up by the cratering process. BACK

19-21: The photo with House Rock has the North massif in the background; the photo with the LM and Rover has the South massif in the distance. BACK

19-22: The diagram shows that the range of potassium in lunar rocks is much greater than most igneous rocks on Earth. Many lunar rocks have low potassium contents, in the ranges characteristic of chondritic and carbonaceous chondritic meteorites. The low K/U values suggest that U is relatively enriched in the lunar rocks compared with meteorites and terrestrial rocks. A few lunar rocks have a composition approaching that of granite. The group of rocks called KREEP basalts (for potassium [K], rare earth elements [REE], phosphorus [P], plus barium and uranium) are believed to derive from melts originating in the lunar mantle. The loss of water and volatile elements left a higher relative proportion of refractory elements such as aluminum, magnesium, titanium. The observed lunar composition, being different from crustal rocks on Earth would seem to argue against the Moon being derived from Earth as a huge mass ejected by a great impact in early Earth history; however, that Earth stage would be prior to the emergence of the later crust, so that the Moon's composition may in fact represent the composition of the pre-crustal Earth only a few hundred years after its first organization as a spherical body (rocks older than 4 billion years). Note that the group called Eucrites has a composition similar to moon rocks; this group refers to a class of meteorites that are now believed to have been cast off the Moon during impacts. Note, too, that neither the Moon nor Earth are similar to the meteorite groups Chondrites and Carbonaceous Chondrites, which are probably the parent materials from which the terrestrial planets were formed. BACK

19-23: An (incomplete) list is: The Moon was derived by impact from Earth; it melted early and formed a differentiated crust rich in feldspar; the Moon is depleted in volatile elements and enriched in refractory elements; water is absent in the Moon rocks (but a small amount of degassed water has accumulated at the poles); Moon rocks are mostly very old (3.2 to 4.1 b.y.); the wide range of volcanic processes observed on Earth are largely absent or localized on the Moon (except features associated with basaltic volcanic flows); the lunar surface everywhere is covered by a thin regolith (fragmental rock produced by impact); the Highlands are covered by 1-3 kilometers of ejecta; moonquakes are uncommon but do occur; shock metamorphism proves the widespread effects of impact. BACK

19-24: The actual number of rings is debatable. The inner ring, or basin rim, is obvious. Much farther out is a semi-circular thick curved group of cratered and knobby hills that might be termed a ring but not a rim; it may be ejecta deposits from Caloris. In between are several curved, narrow ridges in segments that suggest some ringlike rises in the Smooth Plains terrain they form in. Generally, investigators concede only two well-developed rings occur around the Caloris excavation. BACK

19-25: Mercury lacks a distinct highlands terrain, although its average albedo of 12% suggests its volcanics are more silicic than typical dark basalts, perhaps being anorthositic. Its Intercratered Plains is probably the equivalent of the lunar highlands and its Smooth Plains terrain is somewhat mare-like. There appear to be less large impact basins on Mercury, but Caloris is roughly the size of the Moon's Orientale basin. Owing to stronger gravity, the ejecta deposits around mercurian craters does not move as far out from the rim as on the Moon. Mountainous terrain is less common on Mercury. Compressional effects on Mercury (thrust faults), probably the result of late cooling that induced contraction, have no evident counterpart on the Moon. Mercury's core is larger than the small one indicated for the Moon and controls the weak magnetism associated with the planet. BACK

19-26: The blackish rock is characteristic of basalt. Basalts seem to be a (the) common type of igneous lava found on non-continental primary crust of the inner planets. BACK

19-27: The numerous close-spaced, parallel ridges that do curve or change orientation over mainly the right half of the radar image. These include Maxwell Montes. They appear to be folds of some kind (if the main surface rock is basalt, this type of folding would be unusual) produced by compression. The central crater, Cleopatra, resembles a large (100 km) volcanic caldera more than an impact crater (it lacks obvious ejecta deposits even though it appears "fresh" (young). BACK

19-28: This broad strip is Aphrodite Terra. The nearly circular depression (in blue, upper right) is Atalanta Planum. BACK

19-29: This time the center of the hemisphere is the western end of Aphrodite Terra. The two light orange areas are Ovda Regio and Thetis Regio. To the south of the latter is a broad curved trench (nearly circular) that is Artemis Chasma. BACK

19-30: Stratocones are generally composed of volcanic rocks that are higher in silica, such as andesites. These are the dominant type present in the Pacific Ring of Fire (volcanoes in the U.S. and Alaska, in the South American Andes, in Japan, and elsewhere) which develop in the tectonic plate that rests on top of an active subduction zone. Their presence on Mars would disclose that plate tectonics is an operating process and would further imply that melts have differentiated into dacites, granites and other types that are higher in silica. The absence on Venus in fact supports that the volcanism in principally that which is associated with more fluid basalts and only shield volcanoes and calderas developed in that rock type are to be expected, as is the case. BACK

19-31: Fractures would develop around the outer boundaries of the dome. These would likely by utilized during any eventual collapse (as lava escapes, leaving a void) and thus further emphasized. The arching of the domical roof would place it into tension which would be relieved by radial fracturing. BACK

19-32: This is still debatable, but one view holds it to be a volcanic caldera of greater size than any known in the Solar System. It lacks an evident ejecta apron (although if this is an older impact crater, subsequent lavas may have covered those deposits) but does seem to show local outflows. The inner rim most closely resembles that of calderas. Also, for an impact crater of this size, a central peak would be expected; its apparent absence seems to rule out impact unless the peak were submerged by lavas. However, the impact theorists argue that it is an old impact structure (supported by its size) that has been modified by subsequent volcanism in both interior and exterior. BACK

19-33: The blackish areas would make one think first of basaltic rocks. The red is typical of extreme iron oxide "rust" of the variety that doesn't contain water (which usually produces yellows and browns), probably owing to oxidation of Fe in the basalts, implying that at one time oxygen was a significant component in the martian atmosphere (it isn't now). The white could be bright desert sand but in fact is polar ice. BACK

19-34: The upper view gives almost no impression of a volcanic structure with side walls. The lower view is so exaggerated in relief that these walls seem to be the dominant characteristic (not so!). BACK

19-35: The carbon dioxide-rich atmosphere here is layered, with long straight wispy clouds (condensed water?). The image suggests a yellowish tint in the air which is almost certainly dust carried upwards by blowing winds deflating the martian surface fines that are enriched in iron oxides. BACK

19-36: The Big Island of Hawaii, capped by Mauna Loa, is a close counterpart. BACK

19-37: The martian atmosphere has a very low density compared with Earth. It seems unexpected that the thin-air winds could be so effective in picking up and transporting the fine-grained materials, even though they can attain high velocities. One would suspect that the dunes are not made up of sand but of silt- or even clay-sized particles. They are probably vulnerable to further wind interactions that cause the dunes to migrate or even be destroyed. The writer's guess is that they are relatively young and transient, so that dune formation may be monitorable from spacecraft orbiting over periods of a few years. BACK

19-38: For the images above this last one, this explanation is likely: The polar ice caps grow or shrink (depending on hemisphere) during the martian year. Ice is continuously being deposited. Meanwhile, the winds blow fiercely at the poles (dunes are common beyond polar peripheries) and as the season changes may then deposit their particle loads. The layering is probably alternating dust and ice deposits. However, in this last case, this high resolution Mars Orbiter image provides a strong impression that the dark bands are shadows associated with ledges or cliffs in the flowing ice.BACK

19-39: Valles Marineris extends into the Tharsis region. That region is marked by extensive volcanism that has caused that part of the martian crust to upswell. This bulging puts the crust into tension. The Valles formed along a vast array of normal faults so that it is now a rift zone with a major downdropped interior. Mass wasting (landslides), and possible water action, have modified and enlarged this zone into a huge canyon. On Earth, the East African Rift and Red Sea are similar structural troughs and are in the same size range. BACK

19-40: It would be most unusual if this steep cliff which runs fully around Olympus Mons is a fault scarp. But it could be an erosional scarp. One speculation is that there was once a martian ocean and these are wave cut cliffs. The problem with that is that no other such marine cliffs are found elsewhere on Mars. The question has not been fully resolved. BACK

19-41: The large structure has steep slopes running outward from the rim. This is a volcano, with a big central caldera. The two smaller structures cutting the rim are impact in origin as indicated by their central peaks, terraces, and ejecta. This is a beautiful example of the two major types of craters occurring actually superimposed, so that their differences, as well as some similarities, can be compared. BACK

19-42: The terminus or beginning point of each channel does not have any kind of volcanic structure at its tip. Absence of a visible volcanic source feature argues against volcanism as a factor. These features look like classic headwaters stream branches of a fluvial nature. BACK

19-43: The atmosphere (at least now) is too tenuous (non-dense) to be able to sculpt out such a distinct form, even given millions of years of wind blowing. Also, the winds are likely not to blow from the same direction, as would be the case if aerodynamic forces were to be effective. BACK

19-44: Knobby terrain. BACK

19-45: The Viking search was ambiguous and generally negative. The meteorite observations also are open to challenge and are not conclusive. The best evidence would be living life detected while some spacecraft were actually on the martian surface. Next best would be living forms found in samples returned to Earth, provided the possibility of contamination is ruled out with certainty; or clear cut extinct life forms found in the samples. Of course, the very best proof would result from landing astronauts on Mars.BACK

19-46: The layers appear thick and tend to alternate in degree of darkness, suggesting different materials. This is not a shadowing effect, since the shadows on the right side of other features indicates solar illumination from the left. BACK

19-47: Alba Patera, a volcanic bulge some 1600 km in its longest dimension rises to only 7 km. Olympus Mons, about 600 km in diameter, rises to 17 km (more than 10 miles). BACK

19-48: Wind pitting, the result of deflation. BACK

19-49: You should have noticed that the highest pyroxene content corresponds to the black areas. These are almost certainly basic rocks (akin to basalts) with a minimum of iron rust; that rust elsewhere along the traverse masks the response of the (presumed) basalt underneath. Pyroxene is a major constituent of basalt. BACK

19-50: Barnacle Bill contains about 50% orthopyroxene, which is a dark mineral, so this rock will be darker than Yogi which contains a lot of quartz and feldspar. We have been specifying basalt as a likely principal rock type on the Martian surface because of the dark color and the shield-type volcanoes. Andesites usually build up as stratocones. Andesites usually are lighter in color but dark varieties, those rich in amphiboles and/or pyroxenes, are known. BACK

19-51: Oxygen - the air on Mars just hasn't enough unless some efficient device to extract the (0.2% of this element in the atmosphere can be invented; water - so far, it is found mainly near the poles in sparse amounts but more may exist under the martian surface in enough quantity to be extracted by some device that needs to be developed (this will become a priority if the next group of probes being sent discover water as subsurface ice). BACK

19-52: The two white dots are saturnian satellites. The black dot on Saturn's surface is the shadow of a satellite. BACK

19-53: The GRS has some attributes that seem to allow it to be called a hurricane. Chief among these is the counterclockwise wind circulation. But, compared with hurricanes on Earth, it is huge, it lacks a central eye, and it persists for centuries. BACK

19-54: No obvious impact craters are observable. Some features suggest volcanic processes. BACK

19-55: The material being ejected is sent outwards in broad arching arcs into the vacuum around Io. On Earth, most large eruptions send clouds of steam and particles primarily upwards in great plumes in part because of the influence of the atmosphere. BACK

19-56: The light (whitish) apron changes in size, shape and location. It may be ash or a chemical sublimate. In the lower right, a new flow of very dark lava has emerged. BACK

19-57: A cue ball used in billiards (or pool). Old, well-used ones develop cracks and tend to turn a creamy-yellow to light brown. This is even more realistic than the pizza comparison for Io. BACK

19-58: There are very few large impact craters, meaning that the present surface hasn't been around long enough to record the collisions, which would be few anyway owing due to the greatly decreased flux of meteorites and asteroids moving haphazardly around the Solar System in the last billion years. BACK

19-59: What is especially remarkable about the dark terrain units, which are heavily crater, are that many of the individual large pieces have such straight boundaries. Some larger patches have rounded edges.This would seem to rule out impacts breaking up earlier crusts. Some kind of tectonic action involving stress that causes breaks along long straight lines is probably involved. Ice can crack up along straight lines. BACK

19-60: The densities of Ganymede and Callisto, 1.93 and 1.83 gm/cm3 respectively, require that some low density material make up a large part of these satellites (a mix of ice at 0.99 and rock at 2.8 gm/cm3 will give these intermediate densities). The fact that there are no mountains on either satellite higher than a kilometer or so indicates that these rises are made up of a material (ice) that flows under its own weight owing to the plasticity of this material; craters also tend to lose their rims from flow. Also, many of the surface features (interlocking blocks; pressure ridges, etc.) on the satellites are observed on smaller scales in terrestrial ice packs. Heat from the interior probably converts the lower part of the outer ice crust (the cryosphere) into water, so that much of the inner part, above the silicate core, is fluid. If that fluid reaches the surface, the intense cold would freeze water almost immediately. BACK

19-61: Crushed ice takes on a white tone (think of a smashed ice cube); also the ice in the crater rims may be recrystallized (like lake ice) which also is rendered white rather than colorless. The dark uncratered surface is probably ice with rock impurities. BACK

19-62: Some of the rings are discontinuous (in arc segments) but there appear to be 6 for sure. BACK

19-63: The particles are not likely to accrete into a new satellite because they lie so close to the Roche limit and thus are being subjected to strong tidal forces. Collisions can do two things: large blocks that hit other blocks will likely disrupt the colliders if the impact is big enough; large blocks can also sweep up smaller dust causing them to grow. Gravitational forces may eventually bring all particles into Saturn's atmosphere but the larger satellites can slow this process down. BACK

19-64: Smooth terrain is dominant on the right limb; cratered plains are present in the south central region; cratered terrain is evident at the left limb; ridged terrain is present in the center and north; and grooved terrain occurs in the lower hemisphere. BACK

19-65: Keep in mind that the profusely cratered hemisphere is not the same as the one facing Jupiter. Only half of that face will show high crater density. The cratered hemisphere is that which is following the direction of orbit at a right angle to a line from satellite to Jupiter's center. As it moves, it tends to encounter and sweep up objects in its path; the opposing hemisphere is largely sheltered from this bombardment. BACK

19-66: The chemical conditions on and under the Titan surface seem especially favorable for the generation of a number of organic molecules. The possibility of primitive life cannot be ruled out. There is a proposed future mission, called Huygens, to penetrate into Titan's outer regions in search for these compounds. BACK

19-67: From left to right: Mimas; Enceladus; Tethys; Dione; Rhea; Titan; Hyperion; Iapetus; and Phoebe. BACK

19-68: The atmospheres of Uranus and Neptune lack any appreciable ammonia compounds, including those which contain sulphur that produces the distinctive banding on Jupiter. Hydrogen and helium gases are colorless, so the blue color results from the methane in the atmosphere. BACK

19-69: None of the four satellites looks as though it had broken up and reorganized, as does Miranda. All appear old but Umbriel and Titania have a greater density of craters and thus may retain a pristine surface that dates back to the time of their formation. Ariel and Oberon, less cratered, either formed later or have undergone some resurfacing before the present population of craters was imposed. BACK

19-70: One can go either way on this concept. If an impact completely disrupted an earlier satellite, chances are it would be fragmented mainly into small pieces so that the corona terrains would not be as large as they are. But, these terrains may have been spallation blocks that stayed intact. Just how they then would reassemble as we seen them is not clear. BACK

19-71: Both satellites have an atmosphere comprised mainly of nitrogen and methane. On Triton, the amount of methane is higher but the density is lower and clouds are sparse. On Titan, nitrogen dominates, the atmospheric pressure at the surface is higher, and there is a nearly continuous cloud deck plus photochemical smog at higher altitudes. BACK

19-72: Most of the oval features are very similar in size. An impact-cratered surface should show a wide range of crater diameters. These features are more likely to have an internal origin, being structural and/or ice-volcanic in nature. BACK

19-73: The martian satellites, Phobos and Deimos, look just like asteroids, as do many of the smaller satellites imaged during spacecraft visits to the Outer Planets. Most planetary scientists suspect that these bodies, especially those near Mars, are captured asteroids. BACK

19-74: Only you can answer this. BACK

19-75: In Man's history, comets were for millennia considered to be omens or portents of something great or terrible likely to happen in the near future. In Earth history, comets may have been a prime source of organic molecules that entered primitive oceans to become the building-blocks of life. BACK

19-76: A comet collision on the Earth's land surface would strike solid rock. An impact on Jupiter's atmosphere strikes material of low density, so the effects of shock waves and direct displacement will travel unimpeded over a very wide radius, reaching Earth scale proportions. BACK