Bridges, Gilmour, Sanders: Meeting report
New ideas on the early solar
Abstract
At the 12 October 2007 RAS Specialist
Discussion Meeting, the UK meteorites
and planetary science community
discussed a range of new and
existing ideas about how the earliest
planetesimals formed. The meeting
was held in honour of the late Dr Robert
Hutchison, a meteorite specialist and
Gold Medal winner of the RAS. Many of
the models that have been used over the
past 40 years are based on what we have
learnt about asteroids from primitive
meteorites. These are believed to be
fragments of what were originally small
planetary bodies (planetesimals) that
failed to aggregate into a large planet
between the orbits of Mars and Jupiter.
A
commonly held set of theories argues
that the most primitive meteorites
(chondrites) accreted cold from material in the dusty solar nebula. The main constituent of the most common types of chondrites
are chondrules: millimetre-sized silicate melt
droplets shown in figures 1 and 2. They may
have formed through flash melting of dust in
the solar nebula.
Supporting evidence for the nebula theory has
been taken from the bulk compositions of the
chondrites, which generally show limited fractionation from solar photosphere compositions,
for example in terms of Mg/Si and Ca/Al ratios
(Alexander 1994, 2007). The chondrite group
that lacks chondrules, CI, has a bulk chemical
composition closest to that of the volatile-free
Sun (Anders and Grevesse 1989). The presence
of interstellar grains in chondrites has also been
taken to indicate that the chondrite material
originated directly from a nebula mix of solar
and interstellar dust. Fine-grained matrices
in the least metamorphosed carbonaceous,
ordinary and enstatite chondrites contain diamond, silicon carbide and graphite that have
isotopic ratios or fractionated noble gases
which indicate a presolar origin (Huss and
Lewis 1995). It is clear that at least some of the
surviving matrix material escaped the heating
that formed chondrules. Thus nebula theories
largely replaced the older theories of planetary
origin for chondrules as that put forward by
cosmochemist and Nobel Laureate Harold Urey
in the 1950s (Urey 1959).
More recently, Hutchison et al. (2005)
showed that although bulk chondrites show
1.28
1: Sienna chondrite, showing chondrules:
millimetre-sized clumps of iron–magnesium silicates
from the early history of the solar system. (NHM)
little fractionation of their Ca/Al ratios from
solar photosphere abundances, their component
chondrules do contain significant Ca/Al fractionation. In 29 unclassified chondrules from
the primitive chondrite Semarkona, atomic
Ca/Al ratios range from 0.90–0.61 (Grossman
and Wasson 1983). Evaporation or condensation at high temperature from gas of solar composition cannot fractionate Ca from Al in the
presence of more volatile Fe, Mg and Si (Wood
and Hashimoto 1993). These results therefore
imply the alternative: elemental partitioning
between crystals and melt.
Model problems
Furthermore, in recent years the study of extinct
radionuclides and their decay products has
thrown up problems with the nebula model.
We now know that some achondrite meteorites
– that is, igneous meteorites formed on asteroids
– have crystallization ages that predate those
of chondrules. The Hf–W chronometer (based
on the decay of 182Hf to 182W with a half life of
9 Myr) can be used to show when planetesimal
cores formed (i.e. when lithophile Hf separated
from the siderophile W). Results suggest this
was within 0.5 Myr of the formation of the
refractory, high-temperature grains (Ca–Al rich
inclusions – CAIs) within carbonaceous chondrites, and was about 2 Myr before chondrule
formation (Burkhardt et al. 2007, Halliday and
Kleine 2006, Kleine et al. 2005). Kleine et al.
concluded that chondrites were the reaccreted
debris produced during collisional disruption of
first-generation planetesimals.
The CAIs have generally been considered to be
the oldest material within chondrites and their
age of formation has been taken as the age of
the solar system (4.57 Gyr). There is textural
evidence for old CAIs being trapped within
crystallizing chondrules to support this. The
I–Xe chronometer (based on the decay of 129I to
129
Xe with a half life of 16 Myr) has also shown
that the crystallization of chondrules occurred
at the same time as igneous activity on the earliest planetesimals (figure 3). This is the only
chronometer that has been applied to suites of
individual chondrules and that is capable of
resolving ages after the earliest ~4 Myr of solar
system history (Gilmour et al.2000, figure 3).
Modelling of the formation of the small planet­
esimals also indicates that they accreted from
the gas and dust of the nebula within 10 4 to
106 years (Kortenkamp et al. 2001). Thus, if
we trust radionuclide chronometers, the model
based on CAIs and chondrules forming in the
solar nebula followed by accretion into gradually growing planetesimals, followed in turn by
planetesimal melting and the creation of igneous asteroids and planets is no longer valid.
Instead, planetesimal melting occurred at the
same time or sometime before chondrule and
perhaps even CAI formation.
An alternative model for chondrule formation
presented at the 12 October RAS meeting concurs with the conclusions of Kleine et al. (2005).
It involves collisions of molten planetesimals
creating sprays of millimetre-sized melt droplets that rapidly accreted back onto cooling
planet­esimals (Sanders 2007, Sanders and Taylor 2005). If chondrule formation happened at
the same time as or after planetesimal melting
A&G • February 2008 • Vol. 49
Bridges, Gilmour, Sanders: Meeting report
ar system
2: Chondrules within primitive chondrites.
(a): Compound chondrule from the Bovedy
(L3) primitive chondrite. The multiple layers
of the chondrule are the result of separate
melt droplets coalescing, possibly from a
spray that resulted from the collision of two
molten planetesimals. The textural type of this
chondrule is barred olivine. Plane polarized light.
(b): Chondrule from chondrite Frontier
Mountains 90045. This is a porphyritic olivine
textural type and contains crystals of Mg–Fe
silicate (olivine) with glassy mesostasis. Crosspolarized light.
(c): Chondrule from chondrite Frontier
Mountains 90045. This chondrule is a poikilitic
olivine–pyroxene textural type and contains
olivine grains surrounded by pyroxene. Crosspolarized light. The chondrules crystallized
rapidly from melt droplets. The compositions of
chondrules vary, e.g. Ca/Al ratios and iron metal
contents, suggesting that their precursors
underwent crystal fractionation rather than
differentiation based on the relatively volatility
of the elements as would be expected if they
formed in the nebula. Despite the chemical
fractionation between different chondrules,
the bulk compositions of the chondrites that
contain them have preserved compositions
for non-gaseous elements similar to the solar
photosphere. The field of view for each thin
section is 1.5 mm.
and core formation, then we would expect to
find relicts of igneous rocks and planetesimal
cores in chondrites. Robert Hutchison identified
just such material in the form of rare, chemically differentiated clasts and iron-metal rich
chondrules. Fragments of igneous rock occur in
members of many chondrite groups: for example CV (Kennedy and Hutcheon 1992), H/L, L
and LL (Hutchison 1992, Kennedy et al. 1992,
Bridges et al. 1995).
Hydrothermal action
Hydrothermal action is also now known to have
been widespread on the chondrite parent bodies.
Evidence for this includes the growth of carbonate (Hutchison et al. 1987, Ellen et al. 2007)
and occasional halite (Bridges et al. 2004) and
the resetting of some oxygen isotope compositions (Bridges et al. 1999, Lyons and Young
2005), but this does not seem to have altered
the bulk compositions of chondrite parent bodies, perhaps because of the low perm­eability of
chondrite parent bodies inhibiting large-scale
redistribution of elements (Bland et al. 2007).
The extinct radionuclide 26Al (with a half
A&G • February 2008 • Vol. 49 John Bridges, Jamie Gilmour and Ian Sanders consider developments
on understanding the role of planetesimal formation in the early solar
system, highlighting the work of the late Robert Hutchison.
(a)
Robert Hutchison
(b)
(c)
life of 0.73 Myr, decaying to 26Mg) is now
considered to have been present in sufficient
concentrations in early planetesimals to cause
melting. Figure 4 shows that 26Al at the time
of CAI formation accounted for about 8 kJ of
radioactive energy per gram of “dry” primitive dust (figure 4, Sanders 2007). Widespread
distribution of this heat source is confirmed
by the uniform excess, over the initial ratio in
CAIs, of 26Mg/24Mg in the Earth, Moon, Mars
and asteroids (Thrane et al. 2006). Since only
1.6 kJ g–1 is needed to completely melt primitive dust from cold, the presence of 8 kJ g–1 of
energy implies that a planetary body melted if
its radius grew to more than about 30 km during
its first 1.5 million years (Hevey and Sanders
2006). This is based on a widely accepted value
for 26Al/27Al of 5.9 × 10 –5 in the earliest stages of
the solar system (Thrane et al. 2006).
Consistent with 26Al heating, early molten
bodies were abundant; 108 out of 135 inferred
meteorite parent bodies melted (Meibom and
Clark 1999). The evidence for this is the presence
in meteorite collections of 108 distinct, differentiated meteorite types. The number of separate
Robert Hutchison (1938–2007) was a
leading figure in meteoritics, spending
much of his career at the Natural History
Museum in London. He was especially
interested in chondrites and inclusions
in meteorites, with implications for
the roles of planetary and nebular
processes in the early solar system.
He was responsible for the NHM’s
meteorites, but as well as curating this
internationally significant collection,
he contributed to it by organizing
expeditions to Antarctica to find more
samples. In addition, he recognized
the unusually young age of the Nakhla
meteorite, heralding the study of martian
meteorites. But perhaps more than all
this, he was an inspiration to generations
of researchers, both directly, with those
with whom he worked at the NHM and
visitors from across the world, and
indirectly through his published work
and popular books.
meteorite types has been established through a
combination of mineralogy and oxygen isotope
signatures (e.g. Greenwood and Franchi 2006).
Thus it is no longer realistic to think of the solar
nebula as a disc of gas and dust devoid of planetary bodies; the latter were aggregating by
mutual collision. It is likely that the dust in the
disc included a great deal of impact ejecta from
these collisions between molten or partially molten bodies, including splash droplets that may
have formed chondrules (Sanders and Taylor
2005). If the ejecta accreted before 1.5 Myr, the
parent bodies probably remelted, destroying the
1.29
Bridges, Gilmour, Sanders: Meeting report
John Bridges, Space Research Centre, Dept of
Physics and Astronomy, University of Leicester, LE1
7RH; [email protected]. Jamie Gilmour, School of
Earth, Atmospheric and Environmental Sciences,
University of Manchester, M13 9PL. Ian Sanders,
Dept of Geology, Trinity College Dublin, Ireland.
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d
4555
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4: At the start of the solar system, when CAIs
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