The Origin of the Solar System 39

FIGURE 14 Most solar system nuclides heavier than hydrogen
and helium were produced in stars over the history of our galaxy.
This schematic figure shows the difference between nuclides
that are stable, those that have very long half-lives (such as^238 U
used for determining the ages of geological events and the solar
system itself), and those that have short half-lives of< 108 years,
assuming all were produced at a constant rate through the
history of the galaxy. The short-lived nuclides decay very fast and
provide crucial insights into the timescales of events, including
planet formation, immediately following their incorporation into
the solar nebula.

a rapid burst of neutrons). Such extremely high fluxes of
neutrons are generated in supernova explosions.
The composition of the Sun and solar system represents
the cumulative∼8 Ga history of such stellar processes in
this portion of the galaxy prior to collapse of the solar neb-
ula (Fig. 14). It is unknown how constant these processes
were. However, the isotopes of some elements in meteorites
provide evidence that stellar nucleosynthesis was still going
on just prior to the collapse of the solar nebula. In fact,
the formation of the solar system may have been triggered
by material being ejected from a massive star as it was ex-
ploding, seeding the solar nebula with freshly synthesized
nuclides.
Chondrites show evidence that they once contained
short-lived radioactive isotopes probably produced in mas-
sive stars shortly before the solar system formed. As already
pointed out most stable isotopes are present in the same
ratios in the Earth, the Moon, Mars, and different groups
of meteorites, which argues that material in the solar neb-
ula was thoroughly mixed at an early stage. However, a few
isotopes such as^26 Mg are heterogeneously distributed in
chondrites. In most cases, these isotopes are the daughter
products of short-lived isotopes. In other words the excess

(^26) Mg comes from the radioactive decay of (^26) Al. Every atom
of^26 Al decays to a daughter atom of^26 Mg; therefore,
( 26
Mg
)
today=
( 26
Mg
)
original+
( 26
Al
)
original (1)
Because it is easier to measure these effects using isotopic
ratios rather than absolute numbers of atoms, we divide by
another isotope of Mg:
( 26
Mg
(^24) Mg
)
today

( 26
Mg
(^24) Mg
)
original

• 
  

  ( 26
  Al
  (^24) Mg
  )
  original
  (2)
  However, the^26 Al is no longer extant and so cannot be
  measured. For this reason, we convert Eq. (2) to a form
  that includes a monitor of the amount of^26 Al that would
  be determined from the amount of Al today. Aluminum has
  only one stable nuclide^27 Al. Hence, Eq. (2) becomes
  ( 26
  Mg
  (^24) Mg
  )
  today

  
  

  ( 26
  Mg
  (^24) Mg
  )
  original

  
  


• 
  

  {(
  (^26) Al
  (^27) Al
  )
  original
  ×
  ( 27
  Al
  (^24) Mg
  )
  today
  }
  (3)
  which represents the equation for a straight line (Fig. 15). A
  plot of^26 Mg/^24 Mg against^27 Al/^24 Mg for a suite of co-genetic
  samples or minerals will define a straight line the slope of
  which gives the^26 Al/^27 Al at the time the object formed. This
  can be related in time to the start of the solar system with
  Soddy and Rutherford’s equation for radioactive decay:
  ( 26
  Al
  (^27) Al
  )
  original

  
  

  ( 26
  Al
  (^27) Al
  )
  BSSI
  ×e−λt (4)
  FIGURE 15 The decay of a short-lived nuclide such as^26 Al
  generates excess^26 Mg in proportion to the elemental ratio
  Al/Mg. The data here were produced for a CAI from the Allende
  meteorite. The slope of the line corresponds to the^26 Al/^27 Al at
  the time of formation of the object. See text for discussion.
  (Based on a figure in T. Lee, D.A. Papanastassiou, and G. J.
  Wasserburg, 1976,Astrophys. J. 211 , L107.)