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News in Brief - January 2009


Joe McCall's eye is caught by matters extraterrestrial and dinosaurian

Geoscientist 19.1 January 2009

New views of Titan


This view of Titan’s south pole reveals the intriguing dark feature named Ontario Lacus and a host of smaller features dotting the south polar region. . Image: NASA, Cassini Imaging TeamWith the passage of time, scientists often come to believe that the hypotheses they themselves have created are ‘gospel truth’. But science progresses very much by revisionism and nowhere is this more true than in planetology. Titan, Saturn’s huge and haze-obscured satellite (radius ~2575km, only just exceeded by Jupiter’s Ganymede), discovered by Huygens in 1655, was long thought to posses global oceans of light hydrocarbons1; but Cassini, on its 38th close flyby, has now shown that no such oceans exist. Nevertheless Titan does have features that are analogous to terrestrial lakes, and one 235km long, “Ontario Lacus”2, has been recognised by radar and infra-red spectroscopy near 78°N and 250°W.

Titan possesses a nitrogen and methane atmosphere and the Visual and Infra-Red Mapping Spectrometer on Cassini has produced strong evidence that this lake contains a solution of methane, nitrogen and low molecular mass hydrocarbons. The reflectance evidence strongly suggests the presence of ethane; propane and butane are also indicated. Liquid methane is difficult to detect because of methane’s abundance in the atmosphere, but it is almost certainly present. Other possibilities are water ice, ammonia and carbon dioxide; but they are not favoured by the evidence and would be solid at Titan’s temperatures. If present their albedo would not fit with observations by Cassini.

Titan has long been thought likely to harbour life, and Ontario Lacus could well harbour some strange life form, living off the organic soup identified in this research. The new research article is very carefully written, detailing highly technical evidence and lines of argument, but this does not mean that another Cassini flyby may not, yet again, require a completely fresh revision of our view of Titan.

Refs


  1. Lunine, J I, Stephenson, D J & Yung, YL 1983. Ethane ocean on Titan. Science 222; 1229-1230.
  2. Brown, R H, Soderblom, L A, Soderblom, J M et al. 2008. The identification of liquid ethane in Titan’s Ontario Lacus. Nature 454; 607-610.

Metallic helium?


It has long been recognised that hydrogen would take on a metallic state at high enough pressures1. Helium, the other most abundant planetary constituent of the universe, is generally thought to be a reluctant partner of hydrogen at high pressure because its expected metallisation pressure at low temperature is very high2. Helium does not combine with other elements - or even with atoms of itself and it exhibits almost no chemical bonding at all3. The reason for this is that at low pressure it is an electrical insulator with a wide energy gap between occupied and unoccupied electron orbitals. This gap is only predicted to close with metallisation at a pressure of ~100Mbar - a pressure greater than at the centre of Jupiter - and this is really what this discussion is about. It has previously been supposed that helium’s solubility in metallic hydrogen was limited, and that therefore for gas giants like Jupiter and Saturn to possess a core consisting of mixed metallic hydrogen and metallic helium was unlikely.

However, new research by Stixrude and Jeanloz3 shows that band closure in pure helium occurs at lower pressures than previously thought, provided high temperature is also taken unto account. This suggests that helium does behave as a metal, at least in the high pressures and temperatures encountered in the core of Jupiter and perhaps over a wider range of pressures in many of the often much hotter planets of this mass and larger, now evidently quite common in the universe2. This is obviously an area for ongoing research, but these initial findings are highly significant. Still not understood is the surprising absence of helium in both Jovian and Saturnian atmospheres3.

Refs

  1. Chabrier, G, 1992. The molecular-metallic transition of hydrogen and the structure of Jupiter and Saturn. Astrophys. J., 391; 817-826
  2. Stevenson, D J, 2008. Metallic helium in massive planets.
  3. Stixrude, L, Jeanloz, R, 2008. Fluid helium at conditions of giant planetary interiors. PNAS 105(32); 11071-11075

Cretaceous dinosaurs


It has long been debated whether dinosaurs were part of the Cretaceous terrestrial revolution (KTR) from 125-80Ma “when flowering plants, herbivorous and social insects, squamates, birds and mammals all underwent rapid expansion”1. Dinosaur diversity is supposed to have undergone an explosion in the mid-Cretaceous, many new groups emerging. Yet the erection of a new “supertree” of dinosaurs does not support this view at all1. The authors of this new study1 attribute this misconception to a sampling artefact.

These authors find that diversification shifts are concentrated in the lower part of the dinosaur tree, in the Late Triassic-Early Jurassic (230-175Ma). Geometric arguments might suggest that it is inevitable that the majority of the diversification shifts will occur low in the phylogenetic tree. These authors find that there is limited evidence that angiosperm diversification drove Cretaceous diversification of dinosaurs. Dinosaurs were not, they conclude, part of the KTR, which was the key event from which modern continental ecosystems developed.

This is a highly complex statistical analysis, the details and supporting figures of which cannot be reproduced here. This is clearly a seminal publication on the dinosaurs, though the methods and interpretations may come in for some future dispute. It must also have a significant bearing on the much media-publicised supposed instantaneous obliteration of the dinosaurs by a bolide. Hallam2 found little evidence for bolide involvement in mass extinctions outside the K/T boundary, and the present author has argued that the recent evidence of Keller and others3, more accurately dating the Chicxulub event to 300,000 years prior to the K/T event, has left very little evidence for bolide involvement at all4.

Refs

  1. Lloyd, G T, Davis, K E, Pisani, D, Tarver, J E, Ruta, M, Sakamoto, M, Hone, D W E, Jennings, R, Benton, M J 2008. Dinaosaurs and the Cretaceous Terrestrial Revolution. Proceedings of the Royal Society B 275, 2483-2490.
  2. Hallam, A 2004. Catastrophes and lesser calamities. Oxford University Press; 274 pp.
  3. Keller, G, Stinnesbeck, W, Adaite, T, Holland, B, Steuben, D, Harting, M, De Leon, C, De LasCruz, J 2003. Spherule deposits in Cretaceous boundary sediments in Belize and Guatemala. J. Geol. 160; 1-13.
  4. McCall, G J H (in press) . Half a century of progress in research on terrestrial impact structures. Earth Science Reviews (Elsevier).