[meteorite-list] Triggering the Formation of the Solar System

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:25:42 2004
Message-ID: <200305212135.OAA26863_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/May03/SolarSystemTrigger.html

Triggering the Formation of the Solar System
Planetary Science Research Discoveries
May 21, 2003

     --- New data from meteorites indicates that
     formation of the Solar System was triggered by
     a supernova.

Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology

One of the most amazing discoveries in space science is the unambiguous
evidence from meteorites that the solar nebula (the cloud of gas and dust in
which the Sun and planets formed) contained radioactive isotopes with
half-lives so short that they no longer exist. These include isotopes with
very short half-lives, such as calcium-41, 41Ca, (100,000 years) and
aluminum-26, 26Al, (740,000 years), and those with longer half-lives such as
plutonium-244, 244Pu, (81 million years). The short-lived isotopes are
particularly interesting. If they formed in an exploding star, that
explosion might have triggered the collapse of the huge interstellar cloud
in which the Sun formed. On the other hand, if they formed in the solar
nebula by intense radiation close to the Sun, then it would prove some
hypotheses about the young Sun and jets of radiation from it.

As synthesized and lucidly explained by Ernst Zinner (Washington University
in St. Louis), recent data from ancient objects in meteorites point strongly
to the supernova trigger idea. K. K. Marhas and J. N. Goswami (Physical
Research Laboratory, Ahmedabad, India), and A. M. Davis (University of
Chicago) found clear evidence in meteorites that beryllium-10 (10Be), the
one isotope that everybody agrees can be produced by solar radiation, is not
accompanied by other short-lived isotopes as it would be if they were all
produced by radiation flowing from the young Sun. (10Be can also be made by
galactic cosmic rays in the interstellar molecular cloud from which the
solar system formed.) Two other research groups reported at the Lunar and
Planetary Science Conference (March, 2003) that unmetamorphosed ordinary
chondrites contained iron-60 (60Fe), an extinct isotope with a half-life of
1.5 million years. 60Fe cannot be produced by intense, energetic solar
radiation, so it must have been made before the Solar System began to form.
The best bet is that much of it was made during the supernova explosion that
triggered the formation of the Solar System.

     References:

     Zinner, Ernst (2003) An isotopic view of the early solar system.
     Science, v. 300, p. 265-267.

     Marhas, K. K., Goswami, J. M., and Davis, A. M. (2002) Short-lived
     nuclides in hibonite grains from Murchison: Evidence for solar system
     evolution. Science, v. 298, p. 2182-2185.

     Mostefaoui, S., Lugmair, G. W., Hoppe, P., and El Goresy, A. (2003)
     Evidence for live Iron-60 in Semarkona dn Chervony Kut: A nanosims
     study. Lunar and Planetary Science XXXIV, abstract #1585.

     Tachibana, S. and Huss, G. R. (2003) Iron-60 in troilites from an
     unequilibrated ordinary chondrite and the initial Fe-60/Fe-56 in the
     early solar system. Lunar and Planetary Science XXXIV, abstract # 1737.

             --------------------------------------------------

Short-Lived Isotopes: Gone Yet Useful

Some scientists analyze meteorites to determine the abundances of things
that no longer exist in the Solar System. Analyzing nothing might seem to be
impossible. Or perhaps it's a quixotic job taken on only by delusional
people. Neither is true. (Actually, I suppose it's possible that some of
those people are delusional, but not because they study things that are not
there.) Like all radioactive isotopes, short-lived ones decay to another
isotope. It is the distinctive nature of the daughter isotopes that record
the presence of the short-lived, extinct isotope. For example, 26Al decays
into magnesium-26, 26Mg. If present in a mineral grain that contains a small
amount of magnesium (most of which is in the form of non-radioactive 24Mg),
its decay leads to an anomalously high ratio of 26Mg to 24Mg. (In cases
where there is a lot of magnesium, the presence of 26Al cannot be
determined.) All short-lived isotopes are referenced to an appropriate
stable isotope. 26Al, for example, is referenced to 27Al, the only stable
(not radioactive) isotope of aluminum. Its initial abundance is given by the
ratio of 26Al to 27Al, which was 5 x 10-5 in the oldest meteoritic
materials. A certified list of short-lived isotopes is given in the table
below.

                      Short lived, now extinct isotopes
                  proven to have been present in meteorites

     Radioisotope Half-life Daughter Reference Initial
                  (million years) isotope isotope ratio

     41Ca 0.10 41K 40Ca 1.5 x 10-8

     26Al 0.74 26Mg 27Al 5 x 10-5

     10Be 1.5 10B 9Be ~5 x 10-4

     60Fe 1.5 60Ni 56Fe ~10-6

     53Mn 3.7 53Cr 55Mn ~10-5

     107Pd 6.5 107Ag 108Pd 4.5 x 10-5

     182Hf 9 182W 180Hf 10-4

     129I 16 129Xe 127I 10-4

     244Pu 81 Fission Xe 238U (4 - 7) x 10-3

     146Sm 103 142Nd 144Sm (5 - 15) x 10-3

               From Zinner (2003) Science, v. 300, p.265-267.

These extinct isotopes are incredibly informative. For a long time
astrophysicists and meteorite experts agreed that the isotopes were produced
before collapse of the huge interstellar cloud of gas and dust in which the
Sun and perhaps other stars formed. Supernova explosions in the galaxy could
continuously produce the longer-lived ones, such as plutonium-244 ( 244Pu
and iodine-129 (129I). They would decay, but supernovae would continuously
make fresh batches. On the other hand, the short-lived isotopes are around
for too short a time to be replenished by supernova explosions or other
stellar element-forming processes, leading to low abundances than we observe
in meteorites. Their presence in our solar system indicates that cloud
collapse began within a few tens of thousand of years of isotope formation.
This led to the idea that a supernova explosion triggered the collapse of
the interstellar cloud, thereby causing formation of the Sun and planets.

                                                 This Hubble Space
                                                 Telescope mosaic gives us
                                                 a beautiful view of the
                                                 fertile star-forming
                                                 region "30 Doradus
                                                 Nebula." High-energy
                                                 ultraviolet radiation and
                                                 intense pressures of
                                                 stellar winds produced by
                                                 stars in the cluster (the
 [Image STScI-PRC2001-21 of star forming region] large blue blob left of
                                                 center) trigger the
                                                 collapse of parts of the
                                                 gas and dust clouds,
                                                 producing a new generation
                                                 of stars. Supernova
                                                 explosions might also
                                                 trigger the collapse of
                                                 interstellar clouds.[Click
                                                 the image to open a new
                                                 browser window with higher
                                                 resolution options.]

One of the characteristics of science is that someone always comes along to
challenge conventional wisdom. In this case, Frank Shu (University of
California, Berkeley, now President at the National Tsinghua University in
Hsinchu, Taiwan) and his coworkers suggested the renegade but imaginative
idea that short-lived isotopes were made near the Sun while it was
exhibiting a youthful exuberance by emitting vast amounts of radiation.
Shu's idea is that the short-lived isotopes 26Al, 41Ca, and 53Mn were made
by intense radiation by the "X-wind," a flow of material and radiation from
a region near the nascent Sun. The idea was shored up by the discovery by
Kevin McKeegan (UCLA) and his coworkers of beryllium-10, which everybody
agrees can be produced only by irradiation by energetic particles and was
not made in stars. Others liked the idea and calculated that the right
amounts of short-lived isotopes can be produced, though it requires a unique
combination of the compositions of the grains being irradiated and the
energy spectrum of the radiation (that is, the intensity of the radiation
has to vary with wavelength in a unique way).

      [young star]
      This drawing depicts some of the processes that might have
      operated in the nebular disk surrounding the young Sun. It shows
      the jet flow model of the formation of calcium-aluminum-rich
      inclusions (CAIs) and chondrules. The yellow region near the Sun
      is very hot, which vaporizes all the dust falling into the
      nebula. The young Sun emits vast quantities of energetic
      particles, which create winds in the nebula. Rising plumes above
      the dashed lines are blown out to cooler parts of the disk. One
      hypothesis depicts CAIs forming closer the the Sun than
      chondrules, giving them higher initial abundances of short-lived
      isotopes like 26Al than chondrules, making them appear to have
      older ages. The competing hypothesis suggests that short-lived
      isotopes were distributed uniformly throughout the solar nebula.
      The higher 26Al in CAIs would then indicate that CAIs are older
      than chondrules.

             --------------------------------------------------

Age or Distance?

These two views affect whether we can use short-lived, extinct isotopes as
chronometers to distinguish events that took place 4.5 billion years ago.
For example, suppose 26Al was distributed uniformly throughout the solar
system. This implies that it formed in a supernova and that the supernova
debris was mixed uniformly in the material from which the solar system
formed. Now suppose that an isotopic expert measures its abundance in two
ancient objects from a meteorite and finds that the ratio of 26Al to 27Al
differs by a factor of two. That can be explained by one object being one
half-life (740,000 years) younger than the other. The difference in
26Al/27Al ratio between calcium-aluminum-rich inclusions (CAIs) and
chondrules in chondritic meteorites indicates an age difference of about 2
million years. Or does it? Not if Frank Shu is right. If the 26Al was made
near the Sun while it was spewing radiation, CAIs might have been closer to
the Sun than chondrules. So, instead of being 2 million years older than
chondrules, CAIs simply formed closer to the Sun. Which is it, age or
distance? Somebody had to figure out where the extinct isotopes were made.

[CAI] [chondrule]
LEFT: A calcium-aluminum-rich inclusion (CAI) in the carbonacious chondrite
Efremovka with anorthite (an), melilite (mel), and pyroxene (px). RIGHT: A
type I chondrule with olivine (ol), glass (gl), metallic rion (met), and
pyroxene (px). Ages determined by short-lived isotopes suggest that CAIs
either formed about 2 million years before chondrules or formed closer to
the Sun than did chondrules.

             --------------------------------------------------

Where Were Short-Lived Isotopes Made?

If the X-wind made short-lived isotopes, then 10Be, 26Al, and 41Ca should
all be present in the same object in a meteorite. Making this test requires
finding the right material--one that has low concentrations of the daughter
products of all three short-lived isotopes. The mineral hibonite, CaAl12O19
(though it can also contain magnesium and titanium), fills the bill. It is
low in beryllium and potassium, and does not contain too much magnesium to
determine if 26Al was present. Hibonite occurs in only a few special CAIs,
the ones that formed at the highest temperature in the solar nebula. [See
PSRD article: The First Rock in the Solar System.]

                      [M98-8 secondary electron image]

      A secondary electron image of a CAI, named M98-8, from the
      Murchison chondrite meteorite. Plates of hibonite, a crystal
      habit typical of this mineral, can be seen in the gap at the
      center of the photo (shown by the arrow).

K. Marhas and colleagues used an ion microprobe at the Physical Research
Laboratory in India to analyze several hibonite-bearing CAIs. They found
clear evidence for the presence (long ago) of 10Be, but not for 26Al or
41Ca. Whatever process made the beryllium-10 did not make the other
isotopes. Since 10Be can be made by solar irradiation but not by stellar
processes, this discovery favors the idea that is short-lived aluminum and
calcium isotopes were made in exploding stars. So, the answer to the
question "age or distance?" is age--the short-lived isotopes were made in
exploding stars and incorporated into the materials from which the Sun and
planets formed.

The case for a pre-solar source for short-lived isotopes (except for
beryllium) is also supported by the agreement between the 26Al clock and
other dating techniques. See, for examples, the PSRD articles Dating the
Earliest Solids in our Solar System and Using Aluminum-26 as a Clock for
Early Solar System Events. These age techniques do not depend on short-lived
isotopes, so the agreement provides strong evidence for the veracity of the
aluminum chronometer and for the production of most short-lived isotopes in
other stars.

             --------------------------------------------------

The Trigger

The relatively high abundance of 60Fe isotopes in primitive meteorites
points to a supernova as the source for most short-lived isotopes and as the
trigger for solar system formation. Measurements suggested that the
60Fe/56Fe ratio was quite low and could be explained by continuous
production of 60Fe throughout the region where the solar system formed.
However, recent measurements reported at the Lunar and Planetary Science
Conference indicate that troilite (FeS) in primitive, unheated chondrites
("nicknamed unequilibrated ordinary chondrites") has a relatively high ratio
of 60Fe to 56Fe. Teams at Arizona State University (S. Tachibana and G.
Huss) and at the Max-Planck Institute for Chemistry in Mainz, Germany (S.
Mostefaoui, G. Lugmair, P. Hoppe, and A. El Goresy) made the measurements
using secondary-ion mass spectrometry. The indicated amount of 60Fe is much
too high to record continuous production and decay. A large amount of it
must have been manufactured right before the solar system formed. This
points to a supernova explosion forming the 60Fe (and other short-lived
isotopes) and also triggering the collapse of the interstellar cloud. Other
stellar sources do not seem to be capable of making enough 60Fe, so a
supernova seems to be the best bet. The problem is not quite solved,
however. Astrophysicists need to improve models of element formation in
stars before we can confidently conclude that the source of short-lived
isotopes and the triggering mechanism for formation of our solar system was
a supernova. For example, some scientists argue that 60Fe can be produced by
stars that have left the main branch and have arrived at the asymptotic
giant branch (AGB). Most stars do this after their main hydrogen fusion
phase has ended, becoming cool, luminous, and pulsating red giant stars.
Such stars, if the right size, might spew enough 60Fe to produce the high
ratio of 60Fe/56Fe observed in the unequilibrated chondrites. Clearly, more
research is needed before we can decide if supernova explosions are needed
or if AGB stars can produce enough 60Fe.

           Polished piece of the meteorite Bishunpur, as viewed in
           reflected light. Kamacite, labeled Kam, is metallic iron-nickel.
 [troilite]Tr denotes troilite (FeS). Dark gray areas are silicates.
           Troilite contains only tiny amounts of nickel, making it ideal
           for detecting the presence of 60Fe which decays to 60Ni.

This research is a good example of how meteorite studies overlap with
astronomical and astrophysical studies. Studies of pre-solar grains also
provide this link. Meteoriticists use microscopes, electron microscopes, ion
microprobes, and other high-tech gizmos to study bits of stardust and the
isotopic remains of catastrophic explosions. Astronomers use telescopes,
spectrographs, and nuclear physics. Both address the same basic questions:
How do stars form? How did the solar system form?

             --------------------------------------------------

ADDITIONAL RESOURCES

     Hubble Space Telescope

     Marhas, K. K., Goswami, J. M., and Davis, A. M. (2002) Short-lived
     nuclides in hibonite grains from Murchison: Evidence for solar system
     evolution. Science, v. 298, p. 2182-2185.

     Mostefaoui, S., Lugmair, G. W., Hoppe, P., and El Goresy, A. (2003)
     Evidence for live Iron-60 in Semarkona dn Chervony Kut: A nanosims
     study. Lunar and Planetary Science XXXIV, abstract #1585.

     Tachibana, S. and Huss, G. R. (2003) Iron-60 in troilites from an
     unequilibrated ordinary chondrite and the initial Fe-60/Fe-56 in the
     early solar system. Lunar and Planetary Science XXXIV, abstract # 1737.

     Zinner, Ernst (2003) An isotopic view of the early solar system.
     Science, v. 300, p. 265-267.
Received on Wed 21 May 2003 05:35:19 PM PDT