Phobos in 2008, as seen by the Mars Reconnaissance Orbiter. Photo: NASA/JPL-Caltech/University of Arizona
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Mars' unusual moons may have been created by collision with Pluto-sized object

Mars' moons are unusual in the Solar System - for their size, shape and colour from their parent planet. Where did they come from? We've got some clues to work with.

Of the four terrestrial planets in the inner Solar System, Mars has most of the moons. Phobos and Deimos are unusual, however, or at least unusual to us because of how different they are to our own Moon. (Residents of the moons of Jupiter and Saturn would probably be nonplussed.)

They're both a lot smaller relative to Mars than the Moon is to Earth, and - in the case of Phobos in particular - much closer. Both are less spherical and more potato-shaped, because they aren't large enough to be rounded under the force of their own gravity. Deimos (on average, roughly 12km across) orbits Mars at a distance of around 24,000km; Phobos, which is an average of 22km across, orbits at a height of only 6,000km. In astronomical terms, that's extraordinary - it's closer than many artificial satellites orbit Earth. Phobos is so close, in fact, that it orbits Mars faster than Mars rotates. To anyone watching from the planet's surface, the moon will appear to rise and set three times a day. Oh, and they both contain gateways to Hell.

This setup has puzzled astronomers for the more than a century since the moons were first discovered, and, this being Mars, some truly fantastical theories have been put forward over the year. (The best of these, undoubtedly, was Soviet astronomer Iosif Samuilovich Shklovsky's belief that Phobos was a kind of Martian space station, though better data soon ruled this out. Sadly.) As we've sent more and more landers and orbiting probes to Mars, astronomers have managed to gather a significant amount of data on these two moons as well, leading to two main schools of thought: one that says that they're captured asteroids, and another that says that they're made up of the debris from a large impact many millions of years ago.

Phobos passes in front of Deimos, as seen from the surface of Mars by the Curiosity rover in August 2013. Image: Nasa

This second hypothesis is currently seen as the best explanation for how our own Moon formed - the proto-Earth would have been struck by another object roughly the size of Mars, blasting both bodies into pieces which reassembled into the Earth-Moon system we know today. (Though there are some slight issues that are still proving tricky to resolve, such as the differences in chemical composition between the two bodies.) And, according to a new paper published in Icarus, that kind of scenario also works for explaining the Martian system.

Planetary scientists Robert Citron of UC Berkeley, and Hidenori Genda and Shigeru Ida of the Tokyo Institute of Technology, performed some simulations of the debris patterns that would be expected from large impact events, and what that debris would subsequently be expected to do. The authors stress that it's still a preliminary study, but the results show that the impact theory seems relatively plausible.

Mars has a huge depression covering most of its northern hemisphere, known as the Vastita Borealis. When Mars was younger, and warmer, it's possible that it formed the basin for the planet's largest ocean, and illustrations of a wet Mars tend to show the Borealis as a huge polar ocean. The researchers note that the shape and formation of the Borealis is consistent with what would be expected with a large, ancient collision event, as well as the thickness of the planetary crust, so they primarily look into whether something of that scale would be enough to form both Phobos and Deimos, while adjusting variables like the collision object's mass, its speed, its angle of impact and so on to see what happened.

"Our simulations show that for Borealis-scale impacts, enough material is ejected into orbit to form accretion disks that could produce Martian satellites," the researchers write. They estimate that an object of 1.68 x 1022kg - or something roughly the size of Pluto - once smashed into Mars' polar region, ejecting a debris disc of roughly 5 x 1020kg. An event like this would have actually caused the creation of possibly hundreds of "moonlets", less than 100m across each, which would have either rained back down onto the Martian surface over time, or have been forcefully ejected fast enough to reach escape velocity, and left the Martian system altogether. This would explain many of the craters we've seen on Mars, which look "stretched" compared to, say, the neat ones we see on our own Moon - they're the result of orbiting moonlets falling to the surface at angles close to horizontal, like pebbles skimming across a pond.

This is still only theoretical though, of course - this paper is just an exploration of whether such an impact was a) possible, and b) would provide enough debris so that whatever wasn't lost to space or back to the surface would match the real masses of Phobos and Deimos. The conclusion is that "at least one" could have been formed this way, or maybe both. "While a Borealis-scale impact may generate sufficient debris to form both Phobos and Deimos, further studies of the debris disk evolution are necessary," they write. "Our results can serve as inputs for future studies of martian debris disk evolution."

Ian Steadman is a staff science and technology writer at the New Statesman. He is on Twitter as @iansteadman.

Alan Schulz
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An Amazonian tribe is challenging scientific assumptions about our musical preferences

The Tsimane’ – a population of people in a rural village in Bolivia – are overturning scientists' understanding of why humans prefer consonant sounds over dissonant ones.

It was 29 May 1913. Hoards of Parisians packed out the newly-opened Théâtre des Champs-Élysées. Messrs Proust, Picasso and Debussy were in attendance. Billed for the evening was the premiere of Le Sacre du PrintempsThe Rite of Spring, a ballet and orchestral work debuted by Russian composer Igor Stravinsky.

The attention and conjecture focused on the theatre that day meant expectations were high. However, within moments of the piece beginning, all preconceived notions held by the audience were shattered, as what was unfolding in front of them was a musical tragedy unlike anything they had ever witnessed.

A bassoon hummed into the ether before ballet dancers stomped on stage; the music, unpredictable with its experimental edge, drove forth the onstage narrative of a young girl whose selection during a pagan ritual saw her sacrificially dance towards death. Stravinsky’s composition and the ensemble of the night caused the room to descend from laughter and disruption to chaos and uproar.

The employment of dissonance – sharp, unstable chords – largely contributed to the audience’s disturbed reaction. Dissonant chords create a tension, one which seeks to be resolved by transitioning to a consonant chord – for example an octave or perfect fifth. These musical intervals sound far calmer than the chords which riveted the audience of The Rite of Spring.

Dissonant and consonant intervals find themselves as binary opposites; the frequencies at which notes played together vibrate determine whether an interval is consonant or dissonant. Consonant intervals have simple mathematical relationships between them, but greater digression from that simplicity makes an interval increasingly dissonant.

It’s long been believed  both experimentally and anecdotally – that the preference among Westerners for consonant chords highlights a universal, perhaps biologically-rooted, leaning among all humans towards consonant sounds. If you were present at the introduction of Stravinsky’s The Rite of Spring on that night of furore in Paris, you’d find it hard to disagree.

There is, however, a growing movement against this consensus. Ethnomusicologists and composers alike argue that favouring consonance may just be a phenomenon that has evolved from Western musical culture. And following the visit of a group of researchers to a remote Amazonian society, these claims could well be grounded in scientific evidence.

Led by Josh McDermott, an MIT researcher who studies how people hear, the group travelled to a village in the Amazon rainforest called Santa Maria. It’s populated by the Tsimane’ – a group of native Amazonians whose rural abode is inaccessible by road and foot, and can be reached only by canoe. There are no televisions in Santa Maria and its inhabitants have little access to radio, meaning exposure to Western cultural influences is minimal.

The researchers were curious to see how the Tsimane’ would respond to music, in order to determine whether they too had a preference for consonant sounds over dissonant ones. To everyone’s surprise, the Tsimane’ showed no preference for consonance; the two different sounds, to the Tsimane’ at least, were equally pleasant.

Detailing their research in a paper published by Nature, the group explains how the Tsimane’ people’s indifference to dissonance is a product of their distance from Western culture and music, removing any purported notion that humans are hard-wired to praise perfect fifths and fourths.

McDermott tells me that the Western preference for consonance may just be based on familiarity. “The music we hear typically has more consonant chords than dissonant chords, and we may like what we are most exposed to,” he says. “Another possibility is that we are conditioned by all the instances in which we hear consonant and dissonant chords when something good or bad is happening, for example in films and on TV. Music is so ubiquitous in modern entertainment that I think this could be a huge effect. But it could also be mere exposure.”

To fully gauge the Tsimane’ responses to the music, 64 participants, listening via headphones, were asked to rate the pleasantness of chords composed of synthetic tones, and chords composed of recorded notes sung by a vocalist. At a later date, another 50 took part in the experiment. They had their responses compared to Bolivian residents in a town called San Borja, the capital city La Paz, and residents in the United States – locations selected based on their varying exposures to Western music.

What made the Tsimane’ particularly interesting to McDermott and his group was the absence of harmony, polyphony and group performances in their music. It was something the researchers initially thought may prevent an aesthetic response from forming, but the worry was quickly diminished given the Tsimane’ participants’ measure of pleasantness on the four-point scale they were provided.

Unsurprisingly, the US residents showed a strong preference for consonance – an expected preference given the overrunning of Western music with consonant chords. Meanwhile, the San Borja and La Paz residents demonstrated inclinations towards consonant sounds similar to the US residents. The implication of these results – that consonance preferences are absent in cultures “sufficiently isolated” from Western music – are huge. We most probably aren’t as polarised by consonance and dissonance as we assume; cultural prevalence is far more likely to have shaped the consonant-dominant sounds of Western music.

McDermott raised the question about why Western music may feature certain intervals over others to begin with:

“One possibility is that biology and physics conspire to make conventionally consonant and dissonant chords easy to distinguish, and so that distinction becomes a natural one on which to set up an aesthetic contrast even if the preference is not obligatory. We have a little evidence for this in that the Tsimane' could discriminate harmonic from inharmonic frequencies, which we believe form the basis of the Western consonance/dissonance distinction, even though they did not prefer harmonic to inharmonic frequencies.”

There has been some criticism of this. Speaking to The Atlantic, Daniel Bowling from the University of Vienna said:

“The claim that the human perception of tonal beauty is free from biological constraint on the basis of a lack of full-blown Western consonance preferences in one Amazonian tribe is misleading.”

Though the results from the Amazonian tribe demonstrate a complete refutation of previous assumptions, people's musical preferences from other cultures and places will need to be analysed to cement the idea.

With research beginning to expand beyond WEIRD people – those from a Western, Educated, Industrialised, Rich and Democratic background – the tastes in music of people the world over may continue to surprise, just as the Tsimane’ did.

The Rite of Spring, which was met with ridiculing reviews has now been canonised and is considered to be one of the most important pieces of music of the twentieth century. A Tsimane’ crowd on that tender night a century ago in Paris may have responded with instant praise and elation. With further research, the imagined Bolivian adoration of a Russian composer’s piece in the French city of love may prove music to be the universal language after all.