Meteorite Meteoroid

Meteoroid - the name given to a meteorite or a meteor before it enters the Earth s (or any other planetary) atmosphere. 

Meteorite A meteoroid entering the atmosphere that survives the journey through the atmosphere to land on the ground and become a find . 

Meteoroid A particle of interplanetary debris that can enter the atmosphere of a planet to become either a meteor or a meteorite. 

Cosmic-ray exposure ages are determined from spallation-produced radioactive nuclides. Cosmic-ray irradiation normally occurs while a meteoroid is in space, but surface rocks unshielded by an atmosphere may also have cosmogenic nuclides. These measurements provide information on orbital lifetimes of meteorites and constrain orbital calculations. Terrestrial ages can be estimated from the relative abundances of radioactive cosmogenic nuclides with different half-lives as they decay from the equilibrium values established in space. These ages provide information on meteorite survival relative to weathering. 

Meteorite A meteoroid that has survived impacting the Earth s surface. 

Bloch, M. R., Fechtig, H., Gentner, W., Neukum, G., Schneider, E. (1971) Meteorite impact craters, crater simulations, and the meteoroid flux in the early solar system. Proc. Second Lunar Science Conf., 3, 2639-52. 

Meteorites are meteoroids that survive their fiery flight and hit the ground. They are classified by their mineral composition and texture. Thousands of meteorites have been identified on Earth and there are undoubtedly many more yet to be discovered. 

Some meteoroids shatter upon impact, while others remain intact. Those that break when they hit the ground may be found in fragments around the site of impact. Some of these fragments may weigh a couple of tons All of the meteorites from a single fall are given the same name. Thus, the many hundreds of meteorites that have been found at Meteor Crater in Arizona are called by the name Canyon Diablo. 

Voshage H. and Feldmann H. (1979) Investigations on cosmic-ray-produced nuclides in iron meteorites 3. Exposure ages, meteoroid sizes and sample depths determined by mass-spectrometric analyses of Potassium and rare gases. Earth Planet. Sci. Lett. 45, 293-308. 

The classic idea of a cosmic-ray exposure (CRE) age for a meteorite is based on a simple but useful picture of meteorite evolution, the one-stage irradiation model. The precursor rock starts out on a parent body, buried under a mantle of material many meters thick that screens out cosmic rays. At a time fj, a collision excavates a precursor rock—a meteoroid. The newly liberated meteoroid, now fully exposed to cosmic rays, orbits the Sun until a time ff, when it strikes the Earth, where the overlying blanket of air (and possibly of water or ice) again shuts out almost all cosmic rays (cf. Masarik and Reedy, 1995). The quantity ff — h is called the CRE age, f. To obtain the CRE age of a meteorite, we measure the concentrations in it of one or more cosmogenic nuclides (Table 1), which are nuclides that cosmic rays produce by inducing nuclear reactions. Many shorter-lived radionuclides excluded from Table 1 such as Na (ff/2 = 2.6 yr) and °Co ty = 5.27 yr) can also furnish valuable information, but can be measured only in meteorites that feu within the last few half-Uves of those nucUdes (see, e.g., Leya et al. (2001) and references therein). 

CRE ages have implications for several interrelated questions. From how many different parent bodies do meteorites come How well do meteorites represent the population of the asteroid belt How many distinct collisions on each parent body have created the known meteorites of each type How often do asteroids collide How big and how energetic were the collisions that produced meteoroids What factors control the CRE age of a meteorite and how do meteoroid orbits evolve through time We will touch on these questions below as we examine the data. 

The production rate of each nuclide, i, depends on numerous factors unique to each meteorite. At any time t, the full expression for the production rate at a location with coordinates x, y, z in the meteoroid is given by... 

Other authors give values of the constant between 425 and 433 (Lavielle et al., 1999 Terribilini et al., 2000a Leya et al., 2000). For the few meteorites with very short exposure ages, this equation must be solved iteratively. Most meteoroids, however, orbit in space for times long compared to the half-life of C1, —300 kyr. Thus, e Ms negligible for ages >1.5 Myr and we have... 

Pair refers paired meteorites from the same locality. Mass is the recovered mass. Z)2a-is the depth at which irradiation on the Moon took place. 72 is the duration of the lunar irradiation. / 4 isthe radius of the meteoroid while in transit to Earth. 74 is the duration of transit to Earth. Ti is the terrestrial age. (i) Greshake et al. (2001) note similarities to MAC 88104/5. (ii) Assume density 2.7 g cm. (iii) T2tt before compaction, (iv) Full model has three stages on Moon. References (a) Eugster and Lorenzetti (2001). (b) Nishiizumi and Caffee (2001a). (c) Warren (1994). (d) Nishiizumi and Caffee (2001b). (e) Shukolyukov et al. (2001). (f) Scherer et al. (1998). (g) Nishiizumi et al. (1998). 

Despite their brecciated nature, only two mesosiderites—Veramin (Begemann et al., 1976) and Eltanin (Nishiizumi et al., 2000b)— seem to contain appreciable concentrations of solar wind or SEP gases. The meteorite Eltanin deserves special consideration. Fragments of this object were recovered from deep-ocean sediments beneath stormy southern seas. The meteoroid was probably enormous (Gersonde et al., 1997). Silicates dominate and metal is rare in the material recovered. The silicates seem to be intermediate in character between those of eucrites and of mesosiderites (Kyte et al., 2000). Eltanin s exposure age of 20Myr lies close to the CRE ages of several eucrites. 

Figure 20 CRE ages of various meteorites versus aphelion of parent body or meteoroid.

Leya L, Lange H.-J., Neumann S., Wider R., and Michel R. (2000) The production of cosmogenic nuclides in stony meteoroids by galactic cosmic ray particles. Meteorit. Planet. Sci. 35, 259—286. 

Voshage H., Feldmann H., and Braun O. (1983) Investigations of cosmic-ray-produced nuclides in iron meteorites 5. More data on the nuclides of potassium and noble gases, on exposure ages and meteoroid sizes. Z. Naturforsch. 38a, 273-280. 

Welten K. C., Nishiizumi K., Caffee M. W., and Schultz L. (2001a) Update on exposure ages of diogenites the impact history of the HED parent body and evidence of space erosion and/or collisional disruption of stony meteoroids. Meteorit. Planet. Sci. 36, A2Ti. 

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