THREE missions have been gathering a treasure trove of data on the nature of space rocks in the solar system, making unexpected findings that may shed new light on how Earth formed.
Earlier this year, NASA鈥檚 New Horizons probe flew by MU69, the most distant rock we have visited. This tiny world is made up of two lobes, and looks a bit like two pancakes stuck edge to edge. Results presented at the Lunar and Planetary Science Conference in Texas last week show that both lobes are covered in organic compounds that give them a red hue.
Their surfaces also show signs of water ice and methanol, a compound of methane and oxygen. The shape of the final object, with both lobes in the same plane, suggests that they were separate rocks that orbited one another closely in a slow dance before merging.
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A lack of large cracks or rubble on MU69 suggests that when the two objects merged, they probably hit each other at just 2 or 3 metres per second. 鈥淚f you take a brisk walk into a wall, you will find out what that [sort of collision] is like,鈥� says team member Bill McKinnon at Washington University in Missouri.
Elsewhere, NASA鈥檚 OSIRIS-REx spacecraft has spotted the near-Earth asteroid Bennu spewing out dust and rocks on 11 separate occasions in just a few weeks, which took us by surprise. For the most part, Bennu is as we predicted before OSIRIS-REx arrived in December: shaped like a spinning top and rotating once every 4.3 hours. It isn鈥檛 very dense and its surface is one of the darkest in the solar system.
The lumps seen ejected from it range from a few centimetres to tens of centimetres in size. At least four appear to have ended up in orbit around Bennu, forming what are essentially miniature moons around the asteroid. 鈥淲e have had spacecraft around other asteroids, and nothing like this was ever reported,鈥� says Andrew Rivkin at Johns Hopkins University in Maryland. 鈥淭he question is, why is this asteroid different?鈥�
One major distinction is that, unlike the other asteroids we have been to, Bennu is full of hydrated minerals that have water locked into their molecular structure. So there is a chance it also has ice under its rocky surface, in which case heat from the sun could be turning the ice into gas and blowing rocks away, says Rivkin.
Efforts to learn more about Bennu may be hampered by another surprise: its surface is covered in large boulders, which could make it more difficult for OSIRIS-REx to descend and grab a sample from it.
Instead of smooth areas coated in dust, the surface has thousands of small boulders and more than 200 that are in excess of 10 metres across. We only expected one rock this big from images taken before the spacecraft orbited Bennu.
The mission was designed to pick up samples from dusty regions, not rocky ones. The team has found only a few areas that will work, but it is confident it will still be able to get a sample of dust.
OSIRIS-REx is set to return to Earth in 2023. Because Bennu is a relic from the age of planet formation in the early solar system, material from it may help us understand how planets like Earth formed and where Earth got its water and complex chemistry.
Finally, Japan鈥檚 Hayabusa 2 spacecraft has sent back its first data on the near-Earth asteroid Ryugu, revealing another surprise: it appears to have virtually no water at all.
The asteroid鈥檚 surface is extremely dark, reflecting less than 2 per cent of the light that hits it. Comparisons with meteorites we have collected on Earth 鈥� which were heated as they passed through our atmosphere 鈥� show the colours match. This suggests the asteroid was heated in the past, perhaps when it was chipped off from a larger object.
鈥淎steroid Bennu has been spotted spewing out rocks, forming what are in effect miniature moons鈥�
That could also explain why there seems to be very little water on the asteroid. Given Ryugu probably formed in the water-rich asteroid belt, it is unlikely that it was dry from the start, says team member Seiji Sugita at the University of Tokyo. 鈥淥ur preferred scenario is that the parent body acquired water and then subsequently re-expelled most of the water,鈥� he says.
Because Ryugu鈥檚 surface is so homogeneous, this indicates its parent body may have lost its water through heating from radioactive materials rather than being hit by other objects, as collisions wouldn鈥檛 affect the entire asteroid equally.
Researchers have long thought that asteroids like Ryugu may have brought water and other essential ingredients for life to Earth in the distant past. If it turns out that many of them, like Ryugu, don鈥檛 have much water, we may have to think again and look for these things elsewhere.
