Today in the history of astronomy, a map of the innermost planet reveals a surprise.
In this 1991 radar image of Mercury, red means more reflective while yellow and green are progressively less. The bright red dot at the top of the sphere indicates the presence of water at the planet’s north pole. Credit: Duane O. Muhleman (Caltech); NRAO/AUI/NSF
- In August 1991, radar observations using the Goldstone Deep Space Communications Complex’s 70-meter antenna and the Very Large Array targeted Mercury’s unmapped regions.
- These radar observations unexpectedly revealed highly reflective material at Mercury’s north pole, suggestive of water ice.
- This discovery was surprising given Mercury’s proximity to the Sun and high average surface temperature, but deep crater shadows could maintain sufficiently low temperatures for ice persistence.
- The presence of water ice at Mercury’s north pole was later confirmed by NASA’s MESSENGER spacecraft in 2012.
On Aug. 8 and Aug. 23, 1991, scientists from CalTech and the Jet Propulsion Laboratory turned the 70-meter dish antenna at the Goldstone Deep Space Communications Complex on Mercury. The goal was to produce a radar map of the portions of Mercury not photographed by Mariner 10 during its 1974-75 flybys. But to the researchers’ surprise, the radar image received by the Very Large Array revealed reflective material at the planet’s north pole, reminiscent of the polar caps on Mars and suggesting water ice. Although Mercury is the closest planet to the Sun and has an average temperature of 333 degrees Fahrenheit (167 degrees Celsius) with a maximum of 800 F (430 C), some craters rest in deep enough shadows for ice to survive – the axis on which Mercury spins is so tilted that in the polar regions, the Sun barely rises. Temperatures in those craters can be as low as -235 F (-148 C). In 2012, NASA’s MESSENGER spacecraft, which orbited Mercury, confirmed the presence of water ice.
