This website uses cookies
Find out more
Product has been added to the basket

In Brief

Venusian volcanicity - are ancient continents and plate tectonics on Venus a supposition too far?


Geoscientist 20.01 January 2010


VenusVenus enjoys scorching temperatures, crushing pressures, and a toxic atmosphere of carbon dioxide and sulphuric acid clouds, yet suggestions are being aired that its past was very different1. ‘Venus Express’ has charted the first map of Venus’ southern hemisphere at infrared wavelengths. Nils Müller2 of the University of Münster’s Joint Planetary Interior Physics Group, suggests the new map hints at an Earth-like system of continents, plate tectonics and even an ocean of water. The lowlands are believed to be basaltic, but the highlands are believed by this team (because their low radiation of heat, captured by the Visible and Infrared Thermal Imaging Spectrometer [VIRTIS]) to be granitic. Though eight Russian landers only found basaltic rocks in the highlands, the Phoebe and Alpha plateaux in the new map are reportedly light-coloured. The team makes the rather surprising claim: “if there is granite on Venus, there must have been an ocean and plate tectonics in the past”.

Müller admits that this is not proof, but says that it is ‘consistent’ and urges sending a lander to search for it. But to invoke plate tectonics is surely a speculation too far at this early stage. Highly silicic rocks may not necessarily be granitic – many small bodies are evident in the radar imagery of Venus that look like silicic volcanic ‘tholoids’ – and the nature of Venus is strongly against plate tectonics ever having operated there. Taylor and McLennan conclude that the Earth is the only planet in the solar system with a ‘Tertiary’ (in their classification) continental crust3,4. Venus has almost no magnetic field, for which reason it is supposed to have lost its hydrogen, an essential ingredient for water, stripped away by solar wind1. Water is surely necessary for plate tectonics to operate. Also, Venus rotates very slowly (1 Venus day = 243 Earth days!). This would prevent the formation of an Earth-like dynamo generated by circulation of the liquid-metal core1; though the planet may once have rotated faster. However there is nothing in the radar-generated imagery of Venus that even hints at the pre-existence or existence of continents5.

I have lately suggested that not only is the Earth the only planet in the solar system that has developed plate tectonics, but also that plate tectonics is a secondary complication of an early development (probably as far back as the Hadean) of a sialic crust, dominated by alkaline feldspars, unlike the early thick anorthositic crust of the Moon. Plate tectonics only initiated at the end of the Archaean, when the crust cooled sufficiently for deep fracturing to allow plates to separate and move laterally along with Mantle convection6 – whatever the mechanism that moves the plates, which I consider to be as yet unresolved.

Müller2 notes that the infra-red observations are sensitive to temperature and the plateau rocks yield temperatures far too low for them to be products of recent volcanic activity. Ivanov and Head7make no mention of active volcanism, and despite past eruptions, no active eruptivity has been detected.

Refs

  1. Gramling, C. 2009. Venus’ gentler Earth-like past. Earth 54(10), 16.
  2. http://www.esa.int/esaMI/Venus¬_Express/SEMUQCLXOWF_0.html 
  3. Taylor, S.R., McLennan, S.R. 2008. Planetary Crusts. Cambridge University Press, 378pp.
  4. McCall, G.J.H. 2009. Planetary Crusts (Review). Geoscientist 19.10 p9.
  5. Mojzsis, S.J. 2005. Atmosphere evolution. In: Selley, R,C., Cocks, L.R.M. & Plimer, I.R., eds., The Encyclopedia of Geology, Elsevier, Amsterdam, 197-207.
  6. McCall, G,J.H. (in press) 2009. A new paradigm for a single-plate early Earth. Australian Journal of Earth Sciences.
  7. Ivanov, M.A., Head, L. W. 2005. Solar System: Venus. In: Selley, R,C., Cocks, L.R.M. & Plimer, I.R., eds., The Encyclopedia of Geology, Elsevier, Amsterdam, 244-264.


Down-under doubts - Pacific sea levels may not be rising as they should.


Geoscientist 20.01 January 2010


It has been proposed that sea-levels on Australia’s eastern seaboard are actually rising at less than a third of the rate that the NSW government is predicting, as it overhauls its planning laws and bans thousands of landowners from developing coastal sites1. The Rees government has warned that coastal waters will rise by 40cm on 1990 levels by 2050, with potentially disastrous effects. However, the Bureau of Meteorology’s National Tide Centre reported in June an average yearly increase of only 1.9 mm in the combined net rate of relative sea level at Port Kembla, south of Sydney - a figure which is consistent with a historical global sea level rise during the 20th Century of about 20cm (or an average of 1.7mm per year). This means that it will take to 2200 to achieve the predicted effect.

The figures have been greeted in two different ways. Bill Kininmonth, former head of the National Climate Centre, has doubts about the modelling: he believes that only a thin layer of the ocean is actually warming – about 200mm – making it unlikely that the oceans are expanding to any degree. He said there was little evidence that the polar caps were melting, causing sea level to rise. Computer models tend to underestimate the way evaporation regulates temperature, thereby exaggerating future temperature predictions. He sees little reason to think that the little bit of extra heat generated by greenhouse gases will make a difference.

On the other hand CSIRO’s John Church, an expert on sea level rise, remains convinced that the rise on Australia’s eastern seaboard will be in line with global averages predicted. He also noted that the Australian continent was rising slightly – about 0.3-0.4mm per year around Sydney, which would partially offset increases in sea-levels.

It is apparent that a considerable number of reputable scientists in Australia are raising doubts about climate change models, which are now widely accepted in political circles. Ian Plimer’s recent controversial book2 stating the opposing view has done little to clarify the position, being long-winded and repetitive – and according to many experts, at variance with well established fact. What is now needed is a concise statement of the reasons for their disquiet, and a concise reply from an acknowledged expert.

Refs

  1. Warne-Smith, D, Madden, J. 2009. Science is in on climate change sea-level-rise: 1.7 mm. From The Australian November 7.
  2. Plimer, I.R. 2009. Heaven & Earth: global warming the missing science. Taylor Trade Publishing. Lanham, New York, Boulder, Toronto, Plymouth (UK), 504 pp.



And finally… retro-exo


Geoscientist 20.01 January 2010


There are an estimated 10 billion planetary systems in our galaxy alone, and WASP (Wide Angle Search for Planets) is a method of detection based on transits, which dim a star’s luminosity1. This method, operated by a consortium of British universities, and involving a battery of cameras that monitor thousands and thousands of stars, has so far discovered 17 exoplanets.

Close-orbiting gas giants known as “Hot Jupiters” are easiest to detect. A South African University has lately discovered such a planet, named WASP-17, 1000 light years away, estimated at twice the size of Jupiter, but only half its mass2. It is the least dense planet so far discovered. The big surprise, however, is that it orbits in the reverse direction to all other known planetary bodies.

Planets are born from the same ball of rotating gas that creates the parent star and spin in the same direction as the star. Venus does spin backwards (very slowly – see above) but still does not have a retrograde orbit. Such aberrations are attributed to collisions or near-collisions; but, in truth, the more we study the universe, the more surprising anomalies are apparent. Our understanding of the processes of the universe is far from complete, and there may be another explanation of this extraordinary discovery?

One more thing to bear in mind: WASP observations require very careful checking by the sensitive spectrographs of the Geneva Planet Search before they can be confirmed, because there are a large number of phenomena that can change stellar brightness.

Refs

  1. http://www.superwasp.org/how.htm 
  2. http://www.nydailynews.com/news/national/2009/08/13/2009-08-13_scientists_discover_new_planet_wasp17_that_orbits_backwards.html