Meteorites and their Parent Planets

Seems like a slow news day, so let me share the fascinating highlights of my most recent read: Meteorites and Their Parent Planets. I typed up my favorite parts to help me remember them, as the 1999 book appears to be out of print.

“We have a curious need to understand our origin and our cosmic surroundings, and meteorites provide otherwise unobtainable information to help us in this quest.” (280)

“The Earth sweeps up 78 million kg of extraterrestrial material, most of which consists of micrometeorites, each year as it orbits about the sun.” (11)

“Twice as many fireballs are reported in the hours after midnight. This bias in time occurs because more meteoroids are encountered in the direction in which the Earth moves in its orbit, and the morning side faces the direction of the planet’s motion.” (15)

“Meteorites striking the ground may excavate small cavities, but typically they penetrate to depths nearly equal to their diameters. In fact, many meteorites are found practically sitting on the surface. This is, of course, due to the appreciable deceleration produced by atmospheric friction.” (19)

“Typical relative velocities for asteroids encountering other asteroids are of the order of 5km/s.” (233)

“Such accidental collisions liberate meteoroids from their parent bodies. As much as 10% of the original energy in cratering events is transmuted into energy of motion for the resulting debris.” (234)

“Many meteorite parent bodies are reaccreted piles of rubble produced during large impacts.” (234)

“The difference in destructibility may explain why meter-sized fragments of irons can persist in space for as long as a billion years, but only recently broken small pieces of stones complete the trip to Earth.” (247)

Near Earth Objects (NEO): “Many meteorites are undoubtedly derived from NEOs. However, NEOs are efficiently removed from the solar system by collisions or gravitational interactions with the planets on time scales of 10-100 million years, only a tiny fraction of the age of meteorites. We thus infer that today’s temporary NEO population must be continually resupplied from other sources. These sources are the parent bodies of the meteorites.” (33)

“The parent isotopes decay at fixed rates, allowing the age of any sample to be determined from analysis of the amount of parent isotope that has decayed or the new radiogenic isotope that has formed. Unfortunately, these radioactive clocks can be reset by any geologic events that cause heating.” (41) like volcanism

•Chondrites – pristine time capsules from our Solar System’s formation

“Chondrites are actually a kind of cosmic sediment, composed of diverse materials with varying origins. The clumping together of these dissimilar materials is called accretion, and it is a very important process about which we know very little. Tiny grains probably stuck together initially because of electrical charges on their surfaces, but the sticking agent for larger entities like chondrules is unknown. Each of the accreted components of chondrites contains a fossil record of some early solar system processes, but none are completely understood.” (51)

“Chondrites are the most thoroughly and accurately analyzed natural materials, and the list of precisely analyzed elements encompasses the entire Periodic table. Chondrites can be considered a sort of solar sludge, with compositions equivalent to the non-volatile portion of the Sun.” (47)

“Lithium and boron are utilized in fusion reactions that power the Sun. Their solar abundances have been reduced over the past 4.56 billion years, so in this way, the chondrites actually record the chemistry of the ancient Sun (hence the primeval solar system) even better than the present day Sun.” (48)

“There may have been a continuum of chondrite compositions in the early solar system, with different temperatures of formation and the resulting depletion patterns of volatile elements controlled by distance from the Sun. In this view, enstatite formed closest to the Sun, ordinary chondrites formed at intermediate distances, and carbonaceous chondrites formed farthest away.” (50)

“All of the asteroids taken together have only a fraction of the mass of the Moon.” (81)

‘The swarm of asteroids could never have accreted into a planet size body because of the perturbing effects of the planet Jupiter. The massive gravitational field of this giant neighbor would have ripped apart any larger planet within its sphere of influence as quickly as it formed.” (82)

“By preventing asteroids from assembling into a larger planet capable of geologic processes, Jupiter may have preserved chondrites in their present, relatively pristine states. However, the irregular shapes and the highly cratered surfaces of the few asteroids that have been examined at close range indicate that such bodies have been shaped and modified by repeated collisions.” (91)

“The organic compounds in chondrites are typically complexly branched hydrocarbons, probably crafted in asteroids from the deuterium-enriched materials originally made in molecular clouds.” (262)

Their amino acids have “a slight preference for left-handed molecules. Organic molecules formed in interstellar space might result from exposure to circularly polarized light emitted from neutron stars. If this is correct, these organic compounds were inherited directly from molecular clouds, without processing in meteorite parent bodies. These molecules would thus qualify as intact interstellar matter.” (265)

“Carbonaceous chondrites contain as much as 20% water, so a late-accreting veneer of such material might account for some fraction of our planet’s oceans” (274)

“The H chondrite parent asteroid appears to have been destroyed by a massive impact and then gravitationally reassembled from the resulting fragments. This conclusion comes from a study of a cooling rate speedometer based on metal grains.” (107)

“The thermal model predicts an asteroid diameter of 175km, which is in good agreement with the 185km size of 6 Hebe, a possible H-chondrite parent body.” (103) 6 Hebe is also close to a secular resonance with Jupiter, that its ejecta could become Earth crossing.

“The L-chondrite parent body was apparently also catastrophically disrupted by impact, but unlike the H-chondrite asteroid, it probably did not reaccrete. All heavily shocked L-chondrites have gas retention ages of approximately 340 million years, marking the time of this event. Asteroid 3628 Boznemkova, which has a spectral signature similar to that of L6 chondrites, may be a relatively small fragment surviving from this collision.” (109)

“The H and LL chondrites show ranges of isotope ages, implying a series of smaller, less disruptive impacts extending to recent times. These events may have liberated chondritic samples from their asteroidal sources.” (235)

• Achondrites (sourced from planets and planetoids)

“Meteorites that form by crystallization of magma are called achondrites and those that are residues from partial melting are termed primitive achondrites.” (118)

“Achondrites elicit wonder (at least in those who know what they are), because they also are the geologic products of other worlds.” (149)

“Rocks consist of mixtures of minerals that melt over a range of temperatures, generally a few hundred degrees. Partial melting produces most magmas, and complete melting is rarely achieved. Magmas form by melting only a modest fraction (commonly less than 25%) of the mantle source materials. Only a small fraction of magmas actually erupt; most stall within the crust and solidify as plutons.” (121)

• Bang your HED, from asteroid Vesta

“Eucrites, like the other members of the HED clan, contain no water, whereas terrestrial basalts contain minor amounts of hydrated minerals or dissolved water in glass. Eucrites have basaltic compositions, indicating they were once liquids and thus could have erupted onto the surface of their parent body. These are fine-grained rocks, sometimes with small, interlocking crystals that resemble those in terrestrial volcanic flows.” (128)

“In contrast, diogenites consist of larger, interlocking crystals, as appropriate to plutonic rocks.” (129)

“Achondrite regolith breccias containing both eucrite and diogenite clasts are called howardites.” (129)

• MARS
+ Earlier post on my collection of Martian rocks

Basaltic shergottites “are relatively fine grained and apparently formed as volcanic flows or shallow intrusions on their parent body. Their elongated pyroxene crystals commonly have preferred orientations, probably aligned by magma flow.” (131)

“Shergottite magmas contain at least a small amount of water.” (131)

“Lherzolitic shergottites are related to the basaltic shergottites, but their coarse grain sizes mark them as plutonic rocks.” (131)

“The SNC parent body must be a geologically complex body, characterized by multiple periods of igneous activity. Isotopic data indicate it was differentiated 4.5 billion years ago, and the mantle thus formed had a nonchondritic composition. This source region was remelted approximately 1.3 billion years ago and again more recently to produce shergottite magmas.” (136)

Mg/Si and Al/Si “element ratios illustrate that the peculiar compositions of SNC meteorites are also seen in rocks and soils analyzed by the Viking landers and the Mars Pathfinder rover.” (178)

“Trapped gasses in the EET79001 Antarctic shergottite link the SNC meteorites to Mars. Pockets and veins of shock melt in this meteorite formed by impact, which also implanted atmospheric gasses in the liquid before it solidified as glass. The abundance of carbon dioxide, nitrogen, and various nonradiogenic isotopes of gaseous argon, neon, krypton, and xenon in the glass are identical to those measured in the Martian atmosphere by Viking spacecraft.” (179)

“The ancient highlands of Mars were once scoured by torrents of running water, producing branching networks of valleys and huge outflow channels like that onto which the Mars Pathfinder spacecraft landed. Today, however, liquid water is not stable anywhere on the Martian surface.

Models of the bulk composition of Mars based on the SNC meteorites curiously indicate a planet with a low water abundance but high concentrations of other volatile elements. This might be explained by reaction of water and metallic iron in accreted materials, stripping the oxygen from water to form iron oxide with the resultant loss of hydrogen from the planet. The interior of Mars would thus be dry and highly oxidized. Partial melting of the Martian mantle produced the magmas that crystallized to form SNC meteorites. These magmas were rich in oxidized iron but poor in water. Consequently, the amount of water outgassed from the Martian interior over time has probably been modest. Expressed as a global ocean of uniform depth, the outgassed water on Mars may amount to no more than a few hundred meters, compared to 2.7 km for the Earth.” (274)

“Rocks launched from Mars take significantly longer to reach the Earth than do lunar rocks, simply because their orbits do not initially cross that of our planet. These objects, like those in the asteroid belt, are subject to resonances related to other planets, and over a few millions of years, their orbits are perturbed so as to become Earth crossing. Most that fall to Earth will do so within 10 million years or so, with much of the remainder eventually being driven into the sun.” (244)

• Moon
+ Earlier post on my moon rocks

“Calculations suggest that more than a billion grams of impact ejecta might be lost from the Moon each year and possibly a hundredth of that should be swept up by the Earth.” (137)

“Iron-rich anorthosites form a crustal layer that is of the order of 50km thick on the near side and as much as 86km thick on the far side. This crust formed when the Moon was extensively melted to form a magma ocean in its earliest history. This global magma body experienced fractional crystallization as less dense plagioclase crystals floated to the top and solidified to form the feldspar-rich highlands. The difference in thickness of the anorthosite crust on the near and far sides may be related to the gravitational pull of the Earth acting continuously on one face. The magma ocean stage of lunar history had ended by approximately 4.4 billion years ago. Soon thereafter, another suite of plutons invaded the anorthosite crust. Samples of these rocks, called the magnesian suite, vary in age from 4.4 to 4.2 billion years. They too are cumulate rocks, but they contain appreciable quantities of olivine or pyroxene, producing rocks called troctolite and norite, respectively.” (156)

“It is rare to find well-preserved pieces of the ancient lunar crust in the rock collections brought back from the Moon, because meteorite bombardment has broken most of them into tiny fragments.” (157)

“The lunar surface area bounded by all of the Apollo and Luna landing sites is only a small fraction, less than 9%, of the total lunar surface. Odds are high that the lunar achondrites sampled regions outside this limited area and they probably were derived from the far side. “ (161)

“The highlands sampled by Apollo are atypical of the entire moon’s crust. The average Iron composition of the lunar highlands meteorites provides a closer match to the crustal composition than does the average of the Apollo samples” (161)

“From approximately 4.0 to 3.2 billion years ago, and perhaps even later, the gigantic basins were filled with vast outpourings of basaltic magma that crystallized to form the maria. These mare basalts contain no water and were formed under highly reducing conditions. Mare basalt magmas are thought to have formed by partial melting of the olivine and pyroxene-rich cumulate that settled from the magma ocean as a complement to the anorthosite crust. The evidence for this assertion comes from the measured rare-earth-element patterns for these rocks.” (158)

“Low-titanium mare basalts formed by melting at deeper levels than their high-titanium relatives.” (160)

“The Apollo samples are strongly biased toward high-titanium basalts, whereas most orbital measurements and the few known lunar basaltic achondrites are low-titanium varieties.” (162)

“The absence of mare basalts on the lunar far side probably reflects the greater thickness of the anorthosite crust that had to be traversed.” (160)

“Rocks from the moon and Mars are ejected as small objects that make the journey to Earth in relatively short periods. Lunar meteorites generally have ages of less than a million years (often far less), and Martian meteorites fall into groups with cosmic-ray exposure ages of approximately 3 million and 12 million years, with a few older stones.” (247)

• Aubrites – huge white crystals of ejected magma

“The aubrites are achondrites composed primarily of iron-free magnesium pyroxene (enstatite), in contrast to the pyroxene compositions in other igneous meteorites. They also contain a variety of exotic minerals formed under extremely reducing conditions. These minerals decompose rapidly by reacting with oxygen in water or even air. Most aubrites are marred by brownish spots that result from oxidation of sulfides during their residence on our planet.” (141)

“Clasts within the breccias are commonly composed of enormous enstatite crystals, some as long as 10cm” (141)

“Basaltic magmas may have erupted explosively as sprays of droplets accelerated by expanding volatiles. If the volatile content was high enough, most of the erupted droplets would have escaped the gravitational hold of the aubrite parent body and have been lost to space.” (141) … and arriving here as meteorites.

Cosmic-ray exposure ages of “aubrites are old as compared to chondrites and other achondrite classes. Derivation from a long-lived source body, perhaps the near-Earth asteroid 3103 that is spectrally similar to aubrites, can explain their longevity. The orbit of this particular asteroid has high inclination, so it may experience less-frequent collisions with other bodies that lie mostly within the elliptic plane. Impact-derived fragments from this asteroid would presumably inherit the same highly inclined orbit and thus last longer in space.” (247)

• Ureilites — a banger from bizarro world

“The ureilites are arguably the most bizarre and perplexing of all meteorites. Filling the spaces between the larger silicate grains is a matrix of graphite or diamond. The coarse-grained size of the ureilites suggest they formed in the deep interior of their parent body. These crystals typically meet in triple junctions and have curved boundaries. They also show preferred orientations” (145)

“One of the most interesting characteristics of ureilites is that they have experienced variable but typically intense shock metamorphism… 4.0 billion years ago.” (147)

• Angrites — ancients enriched in calcium

“The textures of angrites are variable, but all indicate crystallization from basaltic magmas. They have very ancient ages, approximately 4.56 billion years.” (148)

• Messy Mesosiderites
“Mesosiderites can be viewed as mixtures of core and crustal materials , but without samples of the intervening mantle that would have separated these components in any plausible parent body.” (211)

“An obvious way to accomplish mixing is by a collision between two already differentiated asteroids, allowing the still liquid core of one body to mix with the solidified crust of the other.” (212)

“The mixing of metal with silicates seems almost accidental in mesosiderites and likely occurred when iron meteorite collided with a Vesta-like surface.” (225)

“The cooling rates for mesosiderites measured from nickel profiles in taenite are exceptionally slow, less than a half degree per million years. No satisfactory explanation for how these meteorites could have cooled so slowly has been offered, unless they formed within a very large asteroid.” (212)

• BIG IRONS — the exposed molten core of busted asteroids

“The cooling-rate data suggest that iron meteorites must have formed as cores in relatively small bodies or near the surfaces of relatively large bodies.” (217)

“Powdered regolith is an effective insulator, holding in the heat. It may cool as much as 10 times slower than a body without a regolith.” (217)

“Most irons and pallasites formed within bodies that had diameters of less than 100km.” (218)

“Each iron group represents a distinct chemical system, presumably the core of a different asteroid.” (218) There are 13 groups. “Anomalous irons that are unique or have only a few recognized members expand the number of possible parent bodies to sixty.” (221)

“M-type asteroids, such as 16 Psyche, are characterized by sloping spectra that resemble those of iron meteorites.” (224)

Canyon Diablo: “Meteorite specimens found on the crater rim contain small diamonds, whereas those collected farther from the crater do not. The diamonds were produced from graphite by shock in fragments that may have been spalled off the rear of the impacting projectile.” (28)

• Ataxites
“The finest octahedrites are graded into meteorites with no obvious structure. These irons, which have the highest nickel contents, are called ataxites. Such meteorites consist almost entirely of taenite.” (196)

• Pallasites – the prettiest of them all

“Similarities in metal composition suggest that the IIIAB iron core was rimmed by main group pallasites.” (219)

“The tendency for metal and silicate to separate is demonstrated by the fact that most differentiated meteorites contain either less than 1% or greater than 99% metal by weight. Pallasites are unstable mixtures frozen in place.” (220)

“Potential parent bodies for pallasites have been identified as A-type asteroids, like 246 Asporina” (225)