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.

Lifestage
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Everything that is wrong with the app Facebook doesn't want over 21s to download

Facebook's new teen-only offering, Lifestage, is just like your mum: it's trying too hard to relate and it doesn't care for your privacy.

Do you know the exact moment Facebook became uncool? Designed as a site to connect college students in 2004, the social network enjoyed nearly a decade of rapid, unrivalled growth before one day your mum – yes, your mum – using the same AOL email address she’s had since her dial-up days, logged on. And then she posted a Minion meme about drinking wine.

Facebook knows it’s uncool. It had a decline in active users in 2014, and in 2015 a survey of 4,485 teens discovered it came seventh in a ranking of ten social apps in terms of coolness. In fact, only 8 per cent of its users are aged between 13 and 19. This is the main reason the corporation have now created Lifestage, an app specifically for under-21s to “share a visual profile of who [they] are with [their] school network”.

Here’s how it works. After signing up and selecting their school, users are prompted to create a series of short videos – of their facial expressions, things they like, and things they dislike – that make up their profile. Once 20 people from any school sign up, that school is unlocked, meaning everyone within it can access one another’s profiles as well as those from nearby schools. Unlike Snapchat (and truly, this is the only thing that is unlike Snapchat) there is no chat function, but teens can put in their phone number and Instagram handles in order to talk. Don’t worry, though, there are still vomit-rainbows.

But with this new development, rather than hosting your mum, Facebook has become her. Lifestage is not only an embarrassing attempt to be Down With The Kids via the medium of poop emoji, it is also an invasive attempt to pry into their personal lives. Who’s your best friend? What do you like? What’s not cool? These are all questions the app wants teens to answer, in its madcap attempt to both appeal to children and analyse them.

“Post what you are into right now – and replace the video in that field whenever you want,” reads the app description on the iTunes store. “It's not just about the happy moments – build a video profile of the things you like, but also things you don’t like.” They might as well have written: “Tell us what’s cool. Please.”

Yet this is more than an innocent endeavour to hashtag relate, and is a very real attempt, like Facebook’s many others, to collect as much data on users as possible. Teens – no matter how many hot pink splashes and cartoon toilet rolls are used to infantilise them – are smart enough to have figured this out, with one of the 16 reviews of the app on the iTunes store titled “Kinda Sorta Creepy”, and another, by a user called Lolzeka, reading:

“I don't like how much information you have to give out. I don't want my phone number to be known nor do I want everyone to know my Instagram and Snapchat. I could not figure out how to take a picture or why my school was needed. Like I said, I don't want all my information out there.”

But Facebook already knows everything about everyone ever, and it’s not this data-mining that is the most concerning element of the app. It is the fact that – on an app specifically designed for children as young as 13 to share videos of themselves – there is no user verification process. “We can't confirm that people who claim to go to a certain school actually go to that school,” Facebook readily admits.

Although the USP of this app is that those over the age of 21 can only create a profile and aren’t allowed to view others, there isn’t a failsafe way to determine a user’s age. There is nothing to stop anyone faking both their age and the school they go to in order to view videos of, and connect with, teens.

Yet even without anyone suspicious lurking in the shadows, the app’s privacy settings have already come under scrutiny. The disclaimer says all videos uploaded to Lifestage are “fully public content” and “there is no way to limit the audience of your videos”. Despite the fact it is designed to connect users within schools, videos can be seen anyone, regardless of their school, and are “viewable by everyone”.

Of course none of this matters if teens don’t actually bother to use the app, which is currently only available in the US. Lifestage’s creator, 19-year-old Michael Sayman, designed it as a “way to take Facebook from 2004 and bring it to 2016”. Although he has the successful app 4Snaps under his belt, there is no guarantee Lifestage will succeed where Facebook’s other app attempts (Notify, Facebook Gifts, Poke) have not.

There are a few tricks Facebook has put in place to prompt the app to succeed, including the fact that users are ranked by how active they are, and those who don’t post enough updates will be labelled with a frowning or (here we go again) poop emoji. Still, this hardly seems enough for an app whose distinguishing feature is “Privacy? Nah.” 

Only time will tell whether the app will appeal to teens, but one thing is certain: if it does, your mum is totally downloading it.

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