254 Encyclopedia of the Solar System
FIGURE 2 From nebula to meteorite: genetic processes and the
corresponding age determinable for each process. Nuclides of
nearly all elements were formed by nuclear reactions in interiors
of large stars, which then ejected them in very energetic
supernova events. Ejected nebular gas and dust subsequently
nucleated, condensed, and accreted into primitive bodies. Source
bodies for most meteorites were heated, causing solid-state
metamorphism or, at higher temperatures, differentiation
involving separation of solids, liquids, and gases. As a body
evolved, it suffered numerous impacts, and, if atmosphere-free,
its surface was irradiated by solar and galactic particles that
embedded in the skins of small grains and/or caused nuclear
reactions. Larger impacts ejected fragments that orbited the
Sun. Subsequently, orbital changes caused by large-body
gravitational attraction placed meteoroids into Earth-crossing
orbits allowing their landing and immediate recovery (as a fall) or
later (as a find). Each process can alter elemental and/or isotopic
contents. Which of these processes affected a given meteorite
and the time elapsed since it occurred are definable.
the hundred or so known meteoritic minerals generally hav-
ing some chemical compositional range, reflecting its for-
mation and/or subsequent alteration processes. Important
episodes during meteorite genesis are in Fig. 2.
1.2 From Parent Body to Earth
To arrive on Earth, a meteoroid (meteorite-to-be) must be
excavated and removed from the gravitational field of its
parent body by an impact. This impact can generate short-
lived but intense shocks, which provides the impulse nec-
essary for the meteoroid to exceed the parent body’s escape
velocity. In general, the higher the shock pressure acting
upon matter, the higher its ejection velocity and tempera-
ture, both the shock temperature derived from passage of
the pressure wave and the postshock residual temperature
(from compressional, nonadiabatic heat) after decompres-
sion. Residual temperatures as high as 1250◦C, have been
recorded in stony meteorites and correspond to pressures
>57 GPa or 570,000 atm (570,000 times the Earth’s sea
level pressure). Significantly higher temperatures (pres-
sures) would vaporize matter, so there is a limit to the
shock-induced ejection velocity of survivable meteoroids
(i.e., Mars’ escape velocity, 5.4 km/s).
In very special scenarios, ejecta can be accelerated by
impact-jetting—especially during oblique impacts—thus
acquiring a velocity higher than expected from the degree
of shock-loading. At least some martian meteorites, the
7 nakhlites, are not heavily shocked and may signal this un-
usual case. In general, however, a parent body much larger
than Mars is unlikely to provide meteorites to Earth.
The overwhelming majority of meteorites, those of as-
teroidal origin, seemingly sample a few hundred dominant
asteroids, not the thousands known. These may include the
near-Earth asteroids (NEA) already in Earth-crossing or-
approaching orbits, ejected from Kirkwood Gap regions
by chaotic motion and gravitational effects of Jupiter [see
Main-BeltAsteroidsand Near-EarthObjects]. As
discussed later, some types of meteorites and asteroids can
be linked. The nine meteorite falls whose orbits were deter-
mined photographically seem NEA-like (Fig. 3). Some evi-
dence suggests that co-orbital streams of meteorites and/or
asteroids exist—perhaps arising from meteoroids’ gentleFIGURE 3 Orbits determined from overlapping camera
coverage for nine recovered chondrite falls: Pr-Pribram (H5, 7
Apr. 1959); LC—Lost City (H5, 3 Jan. 1970); In—Innisfree (L5,
5 Feb. 1977); Pe—Peekskill (H6, 9 Oct. 1992); TL—Tagish Lake
(C, 18 Jan. 2000); Mo—Moravka (H5-6, 6 May 2000);
Ne—Neuschwanstein (EL6, 6 Apr. 2002); PF—Park Forest (L5,
26 Mar. 2003); Vb—Villalbeto de la Pe ̃na (L6, 4 Jan. 2004). The
orbits shown are projections onto the ecliptic plane (orbits of the
terrestrial planets and Jupiter in color are included withγ, the
vernal equinox). Pribram and Neuschwanstein had identical
orbits, but are of different chondritic types.