Leiden University, the Netherlands

A theoretical physicist journeys to a hairy black hole's horizon.

Rumour has it that Steven Spielberg is producing the ultimate science fiction movie, using state-of-the-art general-relativity simulations to create a realistic image of the warped space-time near a black hole. But wouldn't it be great to see such worlds in real life? In fact, you can: by extending your eyesight with 'AdS/CFT', a mathematical result of string theory that describes a 'through the looking-glass' experience that would embarrass the imagination of Lewis Carroll.

AdS/CFT states that information about the strange world of the black hole is, in a very indirect way, encoded in or 'imaged' by the properties of certain quantum-weird forms of matter. Scientists realized recently that these 'quantum critical' states of matter are routinely produced in condensed-matter laboratories. But a particular prediction of AdS/CFT made the string theorists nervous: the event horizon of the special black hole that is imaged by the quantum critical electrons seems to imply that the latter should show a macroscopic entropy at zero temperature. It has further been predicted that the black hole would be unstable and would eventually suck up 'stuff' from its surroundings, covering its horizon with 'hair' (S. A. Hartnoll et al. J. High Energy Phys. 2008, 015; 2008). In the electron system, out of the blue and at a quite low temperature, some unexpected order will set in that removes the ground-state entropy, giving it a unique ground state.

Intriguingly, I learned the other day that condensed-matter experimentalists, unaware of the string theorists' nervousness, are now in the grip of the same idea. The latest thermodynamic experiments on quantum-critical electrons are suggestive (albeit inconclusive) of a developing zero temperature entropy — for the experimentalists, a catastrophe — interrupted at a very low temperature by the onset of an exotic quantum liquid crystalline order (Z. Fisk Science 325, 1348–1349; 2009). It may be that we don't need spacecraft or Spielberg to visit black holes, just a little patience with the condensed-matter experimentalists.

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Washington University, St Louis, Missouri

A planetary scientist has big hopes for a little world.

Right now, the most exciting object in the Solar System is Saturn's diminutive moon Enceladus. Its deformed south polar region emits copious amounts of heat along the length of several young, active ridges and fractures, as well as plumes of tiny ice particles, water vapour and other chemicals.

The Cassini spacecraft — equipped with plume-gas and particle analysers and clever imaging gadgetry — is currently in the neighbourhood. Seizing this opportunity, Gabriel Tobie of the University of Nantes in France and his colleagues have incorporated some of its recent measurements into theoretical models of tidal heat production on Enceladus (G. Tobie et al. Icarus 196, 642–652; 2008). Only the ebb and flow of tides could properly account for such prodigious geological activity on an icy moon that measures just 500 kilometres in diameter.

The authors start with the generally accepted idea that Enceladus has differentiated into a rock core and an icy mantle. They then show that the size of the tidal motion of the mantle is inadequate to generate the observed thermal emission, so there must be a fluid ocean sandwiched between the two solid layers. This is no great surprise, but Tobie et al. go further, showing that even if the mantle is made soft and deformable over the southern polar region (as the ice would be if it were relatively warm), a sandwiched, liquid ocean must reach at least as far as around the entire southern hemisphere.

The team imagines that, below Enceladus's south pole, tidal heating concentrates in warm, upwelling, convectively mobile ice. This, in turn, causes the cold, brittle surface layer to rupture — and the exposed warm ice sublimates, releasing trapped gases. It is a compelling picture, and one that promises to help unlock the internal activity of other icy satellites.

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Ecole Normale Supérieure, Lyon, France

A geochemist wonders about the Solar System's true age.

Scientists have long looked at the constituent elements of meteorites to find out how old the Sun and its planets are. The most perfect example of the oldest meteorites — those that formed at the same time as the planets — broke up and fell as a large shower near Pueblito de Allende in Mexico in 1969. This was named the Allende chondrite, and was recently the subject of a study by Jim Connelly of the University of Texas at Austin and his colleagues.

Meteorites are often dated by measuring how much aluminium-26 they contain. This isotope decays at a rate that allows researchers to tell when one meteorite is older than another, but too fast to work out these rocks' absolute ages. For the relative ages to be accurate, however, aluminium-26 must have been spread evenly among the protoplanetary debris from which meteorites were born. If it was not, this isotope would reflect where they formed as well as when they formed, and meteorite chronologies would be higgledy piggledy.

But true ages can be calculated from lead isotopes. Until recently, lead had not been measured in both of two common parts of the oldest meteorites — chrondules and calcium–aluminium-rich inclusions — for any one rock, and there was no way of telling whether different rocks formed in the same bit of the nascent Solar System. But the lead isotopes in both chrondules and calcium–aluminium-rich inclusions can be counted in Allende.

Connelly and his team have confirmed an age difference between the chondrules and calcium–aluminium-rich inclusions that had been inferred from aluminium-26 measurements. This means that the relative meteorite chronologies are correct, and that aluminium-26 was indeed evenly distributed when the Sun ignited (J. N. Connelly et al. Astrophys. J. 675, L121–L124; 2008). If this is right, then the Solar System must be 4,567.5 million years old.

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Imperial College London, UK

A planetary scientist learns how comet dust gets from the inner to the outer Solar System.

I was lucky enough to be part of a team studying the grains of comet dust collected last year by NASA's Stardust mission. Comets are primitive, pristine objects, and the Stardust samples are changing the way we think about how our Solar System formed.

Among many surprising findings, perhaps the most significant is that a large fraction of the dust grains are minerals formed at high temperatures — temperatures expected only in the inner Solar System. How did this stuff get out to where the comet began its life, in the cold, outer regions of the Solar System?

At the recent Lunar and Planetary Science Conference in Houston, Texas, I learned about a numerical simulation that potentially offers a neat solution (F. J. Ciesla and J. N. Cuzzi, abstract here).

Observations of dusty disks around young stars show an inward flow towards the central star. Ciesla and Cuzzi's simulation suggests that this inward transport is confined to the top and bottom of the disk. It predicts that there is a narrow region near the disk's midplane where dust flows outwards — a flow sufficient to account for the Stardust results.

So now we know that comets contain a mixture of stuff from the inner Solar System, and we have a physical model that can explain how it got there. But we're still left with one question.

Virtually everything in the inner Solar System — Earth, Mars, the Moon, almost all meteorites — is depleted in volatile elements, which can't condense at high temperatures. But the cometary dust grains don't show this depletion signature. Why not? It'll be fun finding out.

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University of Oklahoma, Norman, Oklahoma, USA

An astronomer invites you to contemplate the history of some of the oldest stars in the Universe.

Much of my work is an attempt to determine what kinds of stars formed and what types of element synthesis occurred when our Galaxy was very young. This means that I am particularly interested in a class of stars referred to as 'carbon-enhanced metal-poor'.

The composition of a star reflects the properties of the interstellar medium at the time it formed, which evolves as generations of stars come and go. Metal-poor stars were born early in the history of our Galaxy, before dying stars enriched the interstellar medium with heavy elements. The fact that some of these metal-poor stars are carbon-enhanced provides insight into the types of stars that came before them.

Recently, one group reported that around 20% of metal-poor stars are carbon-enhanced (S. Lucatello et al. Astrophys. J. 652, L37–L40; 2006). A previous study had produced a lower figure (J. Cohen et al. Astrophys. J. 633, L109–L112; 2005), prompting a battle between the competing groups. But both papers agree that more metal-poor stars are carbon-enhanced than are younger, high-metallicity stars.

To me, this is one of the most interesting and compelling results to come from the study of such stars. Massive stars — with at least ten times the mass of the Sun — produce carbon efficiently, so this gives us a clear indication that massive stars, although very rare today, were much more common early in the history of the Universe.

The differences between the studies' numbers may lie in how the authors define a carbon-enhanced star, or could be a matter of statistics. I look forward to future papers that address these issues — and perhaps continuing the controversy.

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