A group of scientists has made a mirror by coating a liquid with silver. The mirror could form the heart of a telescope on the Moon capable of detecting infrared light from stars a thousand times dimmer than those resolvable today.
Liquid mirrors are not new. The surface of the liquid metal mercury, when rotated at constant speed in a gravity field, becomes a parabola that is perfect for focusing a telescope. For a given diameter, it is a hundred times cheaper to construct than a mirror made of ground glass. Such liquid mirrors can be made very large 鈥� up to 6 metres across 鈥� and are in use in a number of astronomical telescopes around the world.
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But until now, it has been impossible to make a liquid mirror suitable for the Moon, as temperatures there fall as low as -147掳 Celsius 鈥� far below mercury鈥檚 freezing point of -38掳 C.
鈥淎nd the density of mercury means it鈥檚 just too heavy to get enough of it there anyway,鈥� says Ken Seddon, a chemistry expert at Queen鈥檚 University in Belfast in the UK. Mercury is also toxic, making it a severe risk for technicians to handle.
Now, he and researchers led by Ermanno Borra at Laval University in Quebec, Canada, have come up with a liquid mirror concept for the Moon that does not rely on mercury.
鈥楿gly鈥� ions
Their concept uses so-called 鈥渋onic liquids鈥� 鈥� complex salts composed entirely of large, unwieldy ions that Seddon dubs 鈥渦gly鈥�. These clunky ions do not easily form neatly stacked crystal structures as they cool. That means they have very low freezing points and do not easily evaporate 鈥� the key requirements for any liquid meant to be used on the Moon.
By taking a highly viscous salty liquid called imidazolium ethylsulphate, which freezes at -98掳 C, and depositing infrared-reflecting silver nanoparticles on its surface in a vacuum, the researchers have made a robust mirror that has continued working after several months.
鈥淣obody had ever tried to deposit a metal on a liquid before,鈥� says Seddon, adding that they do not yet know why the silver sticks so well to the liquid.
鈥楶ainting on air鈥�
鈥淥ur results show it is not only possible but an extremely good answer 鈥� we could almost go to the Moon with it now,鈥� he told New 女生小视频. 鈥淏ut we want to lower the freezing point by another 50 degrees and improve the infrared reflectivity first.鈥�
Indeed, NASA and the Canadian Space Agency are funding feasibility studies of a project called the Lunar Liquid Mirror Telescope. They hope to establish a liquid telescope at one of the lunar poles by 2018.
But rather than spinning the bucket of liquid using a conventional motor 鈥� which could get gummed up by lunar dust 鈥� the plan is to use a superconducting magnet.
鈥淭hese mirrors are like painting on air,鈥� says Borra. 鈥淏ut we鈥檝e shown the magic works.鈥�
Journal reference: Nature (vol 447, p979)
