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.

Show Hide image’s censorship story is a striking insight into Google’s unchecked power

The founder of the community-driven website claims its advertising revenue was cut off for five weeks.

When Microsoft launched its new search engine Bing in 2009, it wasted no time in trying to get the word out. By striking a deal with the producers of the American teen drama Gossip Girl, it made a range of beautiful characters utter the words “Bing it!” in a way that fell clumsily on the audience’s ears. By the early Noughties, “search it” had already been universally replaced by the words “Google it”, a phrase that had become so ubiquitous that anything else sounded odd.

A screenshot from Gossip Girl, via

Like Hoover and Tupperware before it, Google’s brand name has now become a generic term.

Yet only recently have concerns about Google’s pervasiveness received mainstream attention. Last month, The Observer ran a story about Google’s auto-fill pulling up the suggested question of “Are Jews evil?” and giving hate speech prominence in the first page of search results. Within a day, Google had altered the autocomplete results.

Though the company’s response may seem promising, it is important to remember that Google isn’t just a search engine (Google’s parent company, Alphabet, has too many subdivisions to mention). Google AdSense is an online advertising service that allows many websites to profit from hosting advertisements on its pages, including the New Statesman itself. Yesterday, Drew Curtis, the founder of the internet news aggregator, shared a story about his experiences with the service.

Under the headline “Google farked us over”, Curtis wrote:

“This past October we suffered a huge financial hit because Google mistakenly identified an image that was posted in our comments section over half a decade ago as an underage adult image – which is a felony by the way. Our ads were turned off for almost five weeks – completely and totally their mistake – and they refuse to make it right.”

The image was of a fully-clothed actress who was an adult at the time, yet Curtis claims Google flagged it because of “a small pedo bear logo” – a meme used to mock paedophiles online. More troubling than Google’s decision, however, is the difficulty that Curtis had contacting the company and resolving the issue, a process which he claims took five weeks. He wrote:

“During this five week period where our ads were shut off, every single interaction with Google Policy took between one to five days. One example: Google Policy told us they shut our ads off due to an image. Without telling us where it was. When I immediately responded and asked them where it was, the response took three more days.”

Curtis claims that other sites have had these issues but are too afraid of Google to speak out publicly. A Google spokesperson says: "We constantly review publishers for compliance with our AdSense policies and take action in the event of violations. If publishers want to appeal or learn more about actions taken with respect to their account, they can find information at the help centre here.” has lost revenue because of Google’s decision, according to Curtis, who sent out a plea for new subscribers to help it “get back on track”. It is easy to see how a smaller website could have been ruined in a similar scenario.

The offending image, via Fark

Google’s decision was not sinister, and it is obviously important that it tackles things that violate its policies. The lack of transparency around such decisions, and the difficulty getting in touch with Google, are troubling, however, as much of the media relies on the AdSense service to exist.

Even if Google doesn’t actively abuse this power, it is disturbing that it has the means by which to strangle any online publication, and worrying that smaller organisations can have problems getting in contact with it to solve any issues. In light of the recent news about Google's search results, the picture painted becomes more even troubling.

Update, 13/01/17:

Another Google spokesperson got in touch to provide the following statement: “We have an existing set of publisher policies that govern where Google ads may be placed in order to protect users from harmful, misleading or inappropriate content.  We enforce these policies vigorously, and taking action may include suspending ads on their site. Publishers can appeal these actions.”

Amelia Tait is a technology and digital culture writer at the New Statesman.