A Unique Metal. Mercury, aka quicksilver, is the only metallic element known to be liquid at Standard Temperature and Pressure. STP is defined by 0 degrees Celsius (32 degrees Fahrenheit) and an absolute pressure of 1 bar (about 14.5 pounds per square inch).
Mercury occurs in deposits throughout the world, mostly as cinnabar, mercury sulfide. (The red pigment vermillion comes from grinding cinnabar.)
With a density of 13.534 g/cc at STP, Mercury is only moderately heavy among metals. Osmium tops the list at 22.59 g/cc. Platinum, Gold, and Tungsten are each more dense than 19 g/cc. Light metals include Titanium (4.5 g/cc), Aluminum (2.7 g/cc), and Magnesium (1.74 g/cc). Lithium is the lightest metallic element at 0.53 g/cc.
Telescope Mirrors. A traditional telescope mirror begins with a cast blank of glass. Then it is painstakingly ground into a parabolic surface and coated with reflective aluminum.
By contrast, a Mercury mirror needs nothing more than gravity, carefully controlled spinning, and centrifugal force. Physics does the job of its forming a perfect parabola.
Science’s Clery reports, “…the 4-meter-wide International Liquid Mirror Telescope (ILMT) is the first one to be purpose-built for astronomy, at the kind of high-altitude site observers prize—the 2450-meter [8038-ft.] Devesthal Observatory in the Himalayas.
Clery notes, “ILMT was originally dreamt up in the late 1990s. The dish-shaped vessel that holds the Mercury was delivered to India in 2012, but construction of the telescope enclosure was delayed. Then researchers found they didn’t have enough Mercury. As they waited for more, the COVID-19 pandemic struck, making travel to India impossible. Finally, in April, the team set 50 liters of Mercury spinning, creating a parabolic layer 3.5 millimeters thick. After such a long gestation, ‘we’re all very happy,’ says team member Paul Hickson of the University of British Columbia, Vancouver.”
A Vertical View. “Staring straight up,” Clery says, “the rotating mirror will see a swath of sky almost as wide as the full Moon while Earth’s rotation scans it across the heavens from dusk to dawn. ‘You just turn it on and let it go,’ Hickson says.”
Clery continues, “Objects appear as long streaks in the image; the separate pixels can be added together afterward to create a single long exposure. Because the telescope sees roughly the same strip of sky on successive nights, exposures from many nights can be added together to get extremely sensitive images of faint objects.”
“Alternatively,” Clery notes, “one night’s image can be subtracted from the next’s to see what has changed, revealing transient objects such as supernovae and quasars, the bright hearts of distant galaxies that wax and wane as supermassive black holes consume matter.”
Cost-effective Viewing. Clery describes, “Although astronomers must satisfy themselves with only looking straight up, the $2 million instrument, built by a consortium from Belgium, Canada, and India, is much cheaper than telescopes with glass mirrors. A stone’s throw from ILMT is the 3.6-meter, steerable Devasthal Optical Telescope (DOT)—built by the same Belgian company at the same time—but for $18 million.”
While pushing around those little blobs of Mercury, we never thought of telescopes. ds