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 that one can calculate 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 extinction of the parent, and the chronology is valid only if the initial distribution of the parent and daughter was homogeneous throughout the solar system. 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 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, inconsistencies still unresolved cast doubt on the assumption of homogeneity and the floating chronology built upon it (Kunihiro et al. 2004, Krestianinov et al. 2023).
If radioisotopes originated from reactions within the planets, becoming more common as a function of increasing heat and pressure, one would not expect homogeneity. In fact, chondrules show a 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 homogeneous and the ratio therefore not the same everywhere, one cannot calculate a universally valid initial 26Al/27Al ratio. That 26Mg was not homogeneously distributed is appearing increasingly likely, making attempts to save 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. It is not possible to use the 26Al-26Mg system to ascertain the fine chronology of chondrite formation. Most importantly, it is not possible to 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 size. If the planets were similar in size, their oldest dates will 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 inasmuch as the radioisotopes are not extinct and the two systems can each be used to cross-check the other, but one still has to assume that the initial 238U/235U ratio was the same everywhere, and 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), but the difference could be partly because the higher age assumes an invariant 238U/235U ratio, whereas in fact the ratio has been found to vary, 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 universe’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 in fact other reasons to be cautious, especially if the ages are taken as absolute ages. One is that evidence bearing on the timing of accretion leads to the improbable conclusion that CAIs persisted for up to 3.5 Ma before accreting with dust and chondrules. Another is that 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). This is known as the ‘storage problem’ (Desch et al. 2018). A third is that if chondrules are products of impacts, large protoplanetary bodies must have preceded their formation, consistent with the existence of differentiated parent bodies within 2 Ma of the oldest CAIs. How can those bodies have accreted and differentiated before chondrites even existed, if chondrites represent the most primitive of the meteorites? Indeed, given that some differentiated bodies started with below-par levels of 26Al and that 26Al-decay was the only heat source capable of melting them, some researchers have concluded that they must have accreted within 0.25 Ma of the oldest CAIs (Schiller et al. 2015). The discrepancies may be minor, but they point to something fundamentally wrong with the assumptions underlying the dating.
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 period in the solar system prior to isotopic closure is considered immaterial. However, it is now apparent ‘that chondritic meteorites represent the products of a complex, multi-stage history of accretion, parent body modification, disruption and re-accretion’ (Sokol et al. 2007). In the scenario proposed, the mineral began its existence in a created planet. Initially, there were no unstable isotopes. Rates of radioactive decay were vastly higher than today 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, which thereby obliterated their surfaces. Nothing is as it was when created. The whole solar system was given over to destruction, and year zero marks the beginning of that destruction.
Scientists send probes into space in the expectation that their preconceptions will be verified. Frequently, they are not verified. 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). However, rethinks that might involve a questioning of the solar nebula itself are taboo. There is complete freedom of scientific inquiry so long as every finding maintains the dogma that the universe created itself. Stopping short of a complete rethink, revision takes the form of ad-hoc surmises about multiple supernovas, about condensates and melt droplets lingering in separate parts of the nebula for millions of years, about separate isotopic reservoirs, about Jupiter itself performing a ‘grand tack’ of migration before settling into its present orbit. As with the Big Bang model of the universe and all the surgical operations that have been performed to keep that idea alive, the solar nebula ‘hypothesis’ continues on life support because there is no atheistic alternative.
