The relative ages of meteorites are determined by calculating how long it took for a radioactive isotope (e.g. 26Al, one neutron lighter than the natural isotope 27Al) to decay into its stable daughter (e.g. 26Mg, two neutrons heavier than the most abundant isotope 24Mg). The calculation requires three things: (i) the decay rate of the radioactive element, (ii) the initial ratio of the radioactive element to its stable counterpart (e.g. 26Al/27Al) and (iii) a determination of the initial amount of daughter isotope that was already present (e.g. 26Mg/24Mg) so as to determine the proportion attributable to in-situ decay (the excess 26Mg). The date so arrived at then marks the point at which cooling sealed the system, preventing the isotopes from exchanging with their surroundings. Without such isolation radioisotope dating would give a false result.
In order to arrive at an absolute date, intervals based on short-lived radioisotopes have to be linked to a chronology based on long-lived isotopes. Short-lived isotopes such as 26Al provide only a floating chronology, terminating with the parent’s extinction, and the chronology is valid only if the distribution of parent and daughter at the start was homogeneous. Dating studies assume that the initial 26Al/27Al in the nebula was fifty-two 26Al atoms per million 27Al, written as 5.2 x 10-5. Thus if an achondrite cooled from a magma with an initial 26Al/27Al ratio of 4.1 x 10-7 (as calculated from its 26Mg content), equal to 1/128th the nebula’s initial ratio, it can be inferred to have crystallised 7 half-lives of 26Al, or 5 Ma, after the start of the nebula. However, unresolved discrepancies cast doubt on the assumption of homogeneity and the chronology built upon it (Kunihiro et al. 2004, Krestianinov et al. 2023).
If, on the other hand, radioisotopes originated from reactions within the planets themselves, becoming more common as a function of increasing heat and pressure, one would not expect homogeneity. Chondrules in fact show a wide span of ages. CAIs encompass a narrower span, perhaps because they represent a smaller depth-range within the planet.
The same issue concerns the initial 26Mg/24Mg ratio of the nebula. If the nebula was not well mixed and the ratio not the same everywhere, there was no universally valid initial 26Al/27Al ratio. That possibility increasingly seems likely, making the nebula concept ever more complicated (Larsen et al. 2020). In the alternative scenario, 26Mg was synthesised within the planet itself, and as with 26Al, the amount synthesised depended on the heat and pressure at a particular depth. One cannot use the 26Al-26Mg system to ascertain the fine chronology of chondrite formation. Most importantly, one cannot use radioisotope dating to determine which of the two missing planets exploded first. Each would have had its own maximum 26Al/27Al ratio immediately prior to exploding, depending on the planet’s mass. If the planets were similar in mass, their oldest dates would also be similar.
Ostensibly, dates based on long-lived radioisotopes such as 235U, which decays into 207Pb, and 238U, which decays into 206Pb, are more reliable, since the radioisotopes are not extinct and the systems can each cross-check the other, but one still has to assume that the initial 238U/235U ratio was the same everywhere. Inconsistencies remain. One study claims that the age of the oldest CAI is 4567.3 Ma (Connelly et al. 2012), another, 4568.2 Ma (Bouvier & Wadhwa 2010). Some of the discrepancy could be because the higher age assumes an invariant 238U/235U ratio, whereas in fact the ratio varied, from 1/137.41 to 1/137.89 (Brenneka et al. 2009). In particular cases correction of the canonical ratio of 1/137.88 can shift published ages by up to 3 Ma. The initial ratio on Earth also varies, from 1/137.74 to 1/138.49.
The adjustment is of course minor in relation to the billions of years attributed to the solar system’s entire history. Nonetheless the variability is unexpected, and possibly the range of CAI and chondrule ages is not as wide as assumed. There are also other reasons to be cautious, especially if the ages are taken as absolute ages. For one thing, evidence bearing on the timing of accretion suggests that CAIs persisted for up to 3 Ma before accreting with dust and chondrules, which is not at all plausible. Some chondrules in a given chondrite group show a spread in Pb–Pb ages, from 0 to 4 Myr, whereas Al–Mg ages remain relatively constant (Kruijer & Borg 2019). Gas drag should have caused the particles to spiral into the Sun within a few tens of thousands of years (Bizzarro et al. 2004, Rudraswami & Goswami 2007), a difficulty known as the ‘storage problem’ (Desch et al. 2018), for the hypothesis requires much more time. And if chondrules are products of impacts, large protoplanetary bodies must have preceded their formation, consistent with the existence of differentiated parent bodies within 1–2 Ma of the oldest CAIs. How can those bodies have even existed, let alone accreted and differentiated, if chondrites represent the most primitive of the meteorites? Indeed, some researchers have concluded that they must have accreted within 0.25 Ma of the oldest CAIs (Schiller et al. 2015). While some discrepancies may seem minor, they point to something fundamentally wrong with the dating method.
A radioisotope date generally signifies the point when cooling sealed the mineral containing a radioactive element. Isotopic closure does not signify year zero since there must have been a period of existence prior to that point, and how long that was is a matter of interpretation. In the nebula scenario the dated mineral originated when a nearby star went supernova and exploded; the residence period in the nebula prior to isotopic closure is considered immaterial. In the alternative scenario the mineral began its existence in a created planet. Initially, there were no unstable isotopes. Rates of radioactive decay were vastly higher and exponentially decreased over time. Mineral cooling began only when the planet exploded. Planets larger than Earth and the original Mercury exploded entirely; others were blanketed by erupting magma, obliterating their original surfaces. Nothing is as it was at the Creation. The whole solar system was given over to destruction, and year zero marks the beginning of that destruction.
Scientists send probes into space expecting that their preconceptions will be verified. Frequently, they are not. The recognition that asteroids do not form an orderly compositional gradient across the main belt resulted in ‘major changes in the interpretation of the history of the Solar System’ (DeMeo & Carry 2014). Analysis of the comet Wild 2’s composition resulted in a ‘complete revision of our understanding of early-stage processes in the solar nebula’ (Michel 2014). Rethinks that would involve a questioning of the nebula itself, however, are taboo. Revision takes the form of ad-hoc surmises about multiple supernovas, melt droplets and condensates lingering in separate parts of the nebula for millions of years before combining, separate isotopic reservoirs persisting for millions of years, Jupiter performing a ‘grand tack’ of migration to get to its present orbit. As with the Big Bang model of the universe and all the surgical operations performed to keep that idea alive, the solar nebula ‘hypothesis’ carries on because there is no atheistic alternative.
