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Following the recent theme of science-hubbub in the popular press, a story has been making its rounds for the past couple of weeks concerning a paper by Matese and Whitmire (2011) (pdf on arxiv.org) proposing the existence of Tyche: a gas-giant planet 1-4 times the size of Jupiter orbiting around the Sun in the far-off reaches of our Solar System.

(I should note that I caught wind of the story via a friend on Facebook – JurassicMatt on Twitter – in confirmation of the valauble role social networking can play in dissemninating information; as if revolting Egyptians on Twitter wasn’t enough.)

Pretty wild, eh? You can be forgiven if you are skeptical. Contrary to popular reports, Tyche is not confirmed, but should be visible in the data from the Wide-field Infrared Survey Explorer (WISE). The excellent Bad Astronomy blog offers a more in-depth analysis on this theme.

What interests me in this story is the resurgence of the idea that a 26 million-year mass extinction cycle on Earth is evidence for Tyche’s existence. Following the discovery by Raup and Sepkoski (1984) (pdf) of a ~26 million-year periodicity, Whitmire and Jackson (1984) (abstract) along with Davis et al. (1984) (abstract) proposed an extraterrestrial origin for these extinction events. This came to be known as the Nemesis hypothesis: a black-dwarf companion star to the Sun that periodically disrupted comets and caused Earth-crossing orbits, resulting in impacts and mass extinctions. The problem, however, is that the original idea is unsupported by the evidence in the geological record, and applying the reasoning to Tyche is misguided at best.

First, Matese and Whitmire (2011) do not invoke the extinction periodicity as evidence for Tyche. They argue on the basis of an anomalous concentration of comets in the outer Oort cloud, which they attribute the gravitational effects of a planetary body over the weak stellar impulse.  From my limited understanding of physics, I speculate that the mechanism of the described phenomenon is related to the Roche-limit segregation of debris orbiting Saturn into its characteristic rings. Speculation aside, the point remains clear: if one of the original authors of the Nemesis hypothesis is not referencing his previous work, then what justifcation do bystanders have for doing so?

Second, the reported 26 million-year periodicity in mass extinctions only covers the past 250 million years. Fossil evidence of macroscopic animal life extends back to ~580 Ma, so what about the previous 330 million years? I haven’t checked if updated research has extended the periodicity back prior to the Permian-Triassic boundary (ca. 250 Ma), but neither have the Tyche-extinction proponents. Additionally, the vast majority of extinction events depicted in the figure below barely register as “elevated” after Keller (2008) (abstract), and some, such as the events in the Tertiary, plot near/within “background” levels. Only the end-Permian and end-Cretaceous events rate as “major” mass extinctions, so any argument of a clear extinction periodicity is hardly convincing.

Raup and Sepkoski( 1984)

Adapted from Raup and Sepkoski (1984)

Third, the current state of research is skeptical about the role of bolide impacts on known mass extinctions. Apart from the K-T event that knocked out the dinosaurs (which is still controversial), major mass extinctions are linked to other causes such as flood basalt volcanism, ocean anoxia, and climate change.

To date, characteristic evidence for high-velocity impactors (regardless of composition) such as spherule layers, crater structures, turbidites, carbon mats, microdiamonds, and Ni/Cr anomalies is conspicuously absent in the record. Furthermore, ideas of antipodal or depressurized impact-volcanism relationships are soundly refuted. Indeed, the lack of evidence is not evidence itself, but just as the lack of evidence for you being a serial killer is not reason to believe you are, pursuing the impact-extinction idea without research to back it up is a logically-bankrupt position.

So with three strikes on the Tyche-extinction speculation, is there any reason to persist with the notion? I like the idea of comets and asteroids destroying all life as much as the next Hollywood fanatic, but let’s be honest here – the research is not favourable to the idea and there is no good reason to perpetuate it at this time.

Further reading available from various links:

Bailer-Jones (2009) – The evidence for and against astronomical impacts on climate change and mass extinctions: A review 

Arens and West (2008) – Press-pulse: a general theory of mass extinction?

White and Saunders (2005) – Volcanism, impact and mass extinctions: incredible or credible coincidences?

Keller (2005) – Impacts, volcanism and mass extinction: random coincidence or cause and effect?

Wignall (2001) – Large igneous provinces and mass extinctions

The February issue of Nature Geoscience features a couple of interesting articles on the origin of water in the Earth-Moon system. By way of an introduction, Robert (2011) reviews the known ratios of deuterium (heavy hydrogen; one proton, one neutron) to hydrogen (one proton) of various planetary bodies in the solar system: the proto-Sun, Earth, and Moon, along with carbonaceous chondrite meteorites and comets (Fig. 1).

Robert (2011)

Fig. 1. Deuterium/hydrogen ratios of the proto-Sun (peach), Earth (blue), Moon (red), carbonaceous chondrites (black), and comets (green). The D/H ratio is multiplied by 10^6 reflecting parts-per-million quanities of deuterium with respect to hydrogen. Adapted from Robert (2011).

As you can see, there are a couple of interesting isotopic associations. The Earth overlaps strongly with carbonaceous chondrites, while the Moon spans a range of D/H values, potentially indicating a significant affinity with comets.

Why might this disparity between the Earth and Moon exist? The prevailing hypothesis for the formation of the Moon is the impact of Theia with Earth, so within this framework, it stands to reason that water on Earth appeared after the formation of the Moon via significant contributions of water from carbonate chondrites.

By comparison, Greenwood et al. (2011) discovered that abundant lunar water exists bound up within the hydrous mineral apatite, which represents a mafic phase within the mare basalts and anorthositic highlands. The interesting thing about the Moon, however, is that a number of sources are identified including solar protons, the lunar mantle, and comets (D/H ratios increasing respectively, with the solar fraction as the lightest). These different sources potentially explain the wide range of D/H ratios observed in lunar rocks.

This appears to be a tidy hypothesis, but as Robson (2011) hints, how do you explain the prominent influence of comets on lunar water, and its apparent absence in terrestrial water? The Earth and Moon are next-door neighbours in the context of the solar system, and Greenwood et al. (2011) predict a likewise cometary bombardment of the Earth at this time.

So where is the terrestrial D/H isotopic signature reflecting this cometary bombardment interval? Or is it there, but just obscured by the lighter, carbonaceous chondrite fraction? That may be the case following the research of Kulikov et al. (2006) which indicates that the relatively high D/H ratio on Venus arises from the equivalent loss of a terrestrial ocean; something which most certainly did not occur on Earth.

References

Greenwood, J.P., Itoh, S., Sakamoto, N., Warren, P., Taylor, L., and Yurimoto, H., 2011: Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon. Nature Geoscience, vol. 4, p. 87-92.

Robert, F., 2011: Planetary science: A distinct source for lunar water? Nature Geoscience, vol. 4, p. 74-75.

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