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Chris Evans is stepping down from his role as a presenter on Top Gear after just one season. He tweeted: "Stepping down from Top Gear. Gave it my best shot but sometimes that's not enough. The rest of the team are beyond brilliant, I wish them all the best."

In an official statement, Evans said: "I feel like my standing aside is the single best thing I can now do to help the cause. I remain a huge fan of the show, always have been, always will be. I will continue to focus on my radio show and the allied events that it encompasses.”

Evans' departure came just after the final episode of the current season of Top Gear aired on BBC2 on Sunday, which reached an average of just 1.9 million viewers. While that overnight figure does not yet include views on iPlayer or outside the UK, it still marks the lowest ever rating for the modern version of the show.

Mars may have once had a much larger moon that eventually fell into the red planet. (credit: Labex UnivEarths / Université Paris Diderot)

When a moon orbits a planet, everything is fine as long as the gravity holding the moon together exceeds the pull of the planet. However, if the moon gets too close, and the tidal forces of the planet exceed the gravitational bind of the moon, the satellite will break apart. This is known as the Roche limit. Fortunately for the Earth's Moon, this limit is just under 10,000km, and the satellite itself is nearly 385,000km away.

But this is not the case for the tiny Martian system of satellites. Phobos, the larger of the two Martian moons at 22km in diameter, is slowly falling toward Mars and will reach the Roche limit in about 20 million years. It will break apart thereafter into a spectacular ring. That will leave just Deimos, which is smaller and further out, as the last Martian satellite. It may be a lonely system then, but a new simulation suggests that Mars once had a very complex system of moons.

For a long time, scientists thought potato-shaped moons were probably captured asteroids, however their circular orbits at the equator argued in favor of another possibility—their formation from a giant impact billions of years ago. A very giant impact. The new research, published in Nature Geoscience, suggests a massive 2,000km proto-planet struck Mars in the past, resurfacing much of the red planet and kicking a mass of debris more than 100 times the mass of Phobos and Deimos into orbit.

Europol’s Internet Referral Unit (IRU) celebrated its first birthday at the weekend, but civil liberties organisations are worried that it goes too far in its efforts to keep the Web free from extremist propaganda.

The IRU has been up and running since July 2015 as part of the European Counter Terrorism Centre (ECTC) in the Hague. The unit is charged with monitoring the Internet for extremist propaganda and referring “relevant online content towards concerned Internet service providers” in particular social media. Much was made of how the IRU could "contact social network service provider Facebook directly to ask it to delete a Web page run by ISIS or request details of other pages that might be run by the same user."

Although companies are not required to take down the content, European Commission figures from April 2016 show that the IRU had an effective removal rate of 91 percent. At that time it had assessed more than 4,700 posts across 45 platforms and sent over 3,200 referrals for Internet companies to remove content. The totals now are closer to 8,000 and 7,000, and Europol told Ars it will publish full details in the coming days.