14 | NewScientist | 18 May 2013

OUR planet has a heart of iron, but it’s not as tough as we thought.

Earth’s molten outer core and solid inner core are made mostly of iron, with a touch of nickel. To probe the inner core’s properties, Arianna Gleason of Stanford University in California and colleagues subjected a piece of iron foil to pressures of about 200 gigapascals. That’s 2 million times atmospheric pressure, but

still lower than in Earth’s centre.The researchers measured the

speed at which deforming waves travel in the foil, and its density, from which they calculated the iron’s strength. They then extrapolated this result to conditions in Earth’s core (Nature Geoscience, doi.org/mhg). “Iron is actually weaker than expected under these extreme pressures and temperatures,” says Gleason.

Sun power made Mercury dense

STARLIGHT’s heat could be why Mercury, and the inner planets around other stars, are so dense.

When gas molecules collide with a hot dust grain, they pick up heat, bouncing off faster than they approached. This gives the grain a little shove. Gerhard Wurm of the University of Duisburg-Essen in Germany and colleagues calculated how this would affect dust grains swirling around a star.

A metallic grain conducts heat, so its temperature is evenly spread and it will be shoved from all sides and won’t be pushed far from the star. But insulating grains, such as less-dense silicates, have a hot, sun-facing side where departing gas gives a bigger shove than on the cold side.

This process will leave metals close to the star, and push less-dense particles away. Planets form from these grains, so this could explain why inner planets are so dense (arxiv.org/abs/1305.0689).

Feathery cirrus clouds are fans of heavy metal

ETHEREAL they may look but cirrus clouds form around metals and minerals.

Clouds form when the temperature is low enough for water vapour in the air to condense into droplets or turn into ice crystals, depending on the altitude. Often this happens when the vapour collects around “seeds” such as bacteria, soot or other particles suspended in the air.

This has been well studied for the low-hanging clouds, such as cumulus, but what goes on in cirrus clouds has been more of a mystery because the altitudes they form at are often higher than those at which planes fly.

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Planet Earth is a bit of a softie at heart That could help explain an odd property of the inner core – it isn’t uniform. Seismic waves that pass right through Earth travel about 3 per cent faster moving from pole to pole than they do along the equatorial plane. “That’s really strange,” says Gleason.

One possible explanation is that the grains of iron in the inner core may be aligned. If the core is weaker than we thought, then it is easier for it to deform, allowing the grains to line up.

But this hasn’t deterred all researchers. A team headed by Daniel Cziczo of the Massachusetts Institute of Technology has compiled data from four series of research flights sampling cirrus over North and Central America and the nearby oceans (Science, doi.org/mg4).

They found that 61 per cent of ice crystals form around two types of seed that account for less than 1 per cent of the particles found at the altitudes where cirrus forms. These are the fine mineral dust from sources such as deserts, and specks of metals like lead, zinc, tin, copper and silver. Few cirrus ice crystals condense around the abundant sulphates and soot that are common seeds for clouds at lower elevations. Cziczo says this could be because the crystalline structures of the metals and minerals are full of cracks where ice crystals form easily.

OUR australopith ancestors heard their world differently from modern humans.

Rolf Quam at Binghamton University in New York State and colleagues have discovered rare middle ear bones from two extinct southern African hominins – Australopithecus africanus and Paranthropus robustus. A combination of ape-like and human-like features in the bones indicate some australopiths lacked sensitivity to the midrange frequencies that modern humans use for speech (PNAS, DOI: 10.1073/pnas.1303375110).

“Anthropologists are in general agreement that these early hominins likely did not possess spoken language,” says Quam – the new findings back that claim.

Early ancestors had shoddy hearing

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