40 Encyclopedia of the Solar System
TABLE 1 Extinct RadionuclidesRadionuclide Half-life (million years) Ratio Initial Ratio Stable Daughter(^10) Be 1.5 (^10) Be/ (^9) Be 1 × 10 − (^310) B
(^26) Al 0.71 (^26) Al/ (^27) Al 6 × 10 − (^526) Mg
(^41) Ca 0.10 (^41) Ca/ (^40) Ca 1 × 10 − (^841) K
(^53) Mn 3.7 (^53) Mn/ (^55) Mn 6 × 10 − (^653) Cr
(^60) Fe 1.5 (^60) Fe/ (^56) Fe 1 × 10 − (^660) Ni
(^92) Nb 36 92 Nb/ (^93) Nb 3 × 10 − (^592) Zr
(^107) Pd 6.5 (^107) Pd/ (^110) Pd 9 × 10 − (^5107) Ag
(^129) I 15.7 (^129) I/ (^127) I1× 10 − (^4129) Xe
(^146) Sm 103 146 Sm/ (^144) Sm 0.008 (^142) Nd
(^182) Hf 9 182 Hf/ (^180) Hf 1 × 10 − (^4182) W
(^244) Pu 80 244 Pu/ (^238) U 0.007 131 , 132 , 134 , (^136) Xe
in which BSSI is the bulk solar system initial ratio,λ=
ln 2/half life is the decay constant (or probability of decay
in unit time), andtis the time that elapsed since the start of
the solar system. Using this method and the (^26 Al/^27 Al)BSSI
of∼ 6 × 10 –^5 (Table 1), it has been possible to demonstrate
that many chondrules formed 1–3 Ma after CAIs.
Over the past 40 years, scientists have found evidence
that about a dozen short-lived isotopes existed early in the
solar system. These isotopes are listed in Table 1. Other
isotopes such as^36 Cl and^205 Pb were probably present as
well, but their initial abundances are currently uncertain.
These short-lived isotopes can be broken down into three
types on the basis of their origin in the solar nebula:
1.The Sun and the other stars in its cluster inherited a
mixture of isotopes from their parent molecular cloud
that built up over time from a range of stellar sources.
2.Some short-lived isotopes were probably injected into
the Sun’s molecular cloud core or the solar nebula it-
self from at least one nearby star, possibly a supernova.
3.It is likely that some short-lived isotopes were also
generated in the innermost regions of the solar neb-
ula when material was bombarded with energetic par-
ticles from the Sun.
Determining the origin of a particular isotope and the
timing of its production is often difficult. Isotopes with half-
lives of less than 10^6 years must have come from a source
close to the solar nebula in order to have survived, while
isotopes with longer half-lives may have come from further
away. Irradiation in the solar nebula could have produced a
variety of light isotopes but the relative importance of local
production versus external sources is still unclear. Forma-
tion in the nebula appears to be the most promising source
for^10 Be. However, if all of the^26 Al had formed this way,
it seems likely that some of the other isotopes, especially
(^41) Ca, would have been more abundant than they actually
were. In fact, there is mounting evidence that many of the
short-lived isotopes were quite uniformly distributed in the
solar system, which is hard to explain if they formed in a
localized region close to the Sun.
Some of the heavier short-lived isotopes that existed in
the early solar system (e.g.,^107 Pd,^129 I) can only be produced
in large amounts in a massive star. For example, a large flux
of neutrons is required to produce^129 I, and this is achievable
during the enormously energetic death throws of a massive
star undergoing a type II supernova explosion. Many of
the isotopic ratios in Table 1 are similar, lying in the range
10 −^6 –10−^4 for isotopes with half-lives of 0. 7 × 106 to 30×
106 years. This is as expected if all of these isotopes were
synthesized in roughly similar proportions just prior to the
start of the solar system. Many of these isotopes have initial
abundances similar to those that would be formed by an
AGB star. However, models for AGB stars do not predict
the amounts of^53 Mn and^182 Hf that once existed. In fact,
(^182) Hf (half-life= 9 × 106 years) requires a large flux of
neutrons of the kind produced in the supernova explosion
of a much larger star. It is possible that more than one kind of
nucleosynthetic process gave rise to the short-lived isotopes
in the early solar system. At present, it seems likely that a
nearby supernova was involved because the abundance of
(^60) Fe, which has a fairly short half-life, is too high to be
explained by alternative sources. Some isotopes that may
have been present have yet to be found, including^126 Sn
and^247 Cm with half-lives of 0.3 and 16 Ma. These are both
r-process isotopes that should have been present in the early
solar system if a supernova occurred nearby. The fact that
(^247) Cm has not been detected places strong constraints on
a supernova source. Modeling these processes is complex,
but it appears that the supernova explosion of a 25 solar-
mass star may explain the correct relative abundances of
many of the short-lived isotopes, including^182 Hf , provided
that roughly 5 solar masses of material was left behind in
the form of a supernova remnant or a black hole.