Mercury, the innermost planet of our Solar System, is less than half as distant from the Sun as the Earth. Because of this proximity, parts of Mercury's surface are heated to temperatures nearing 425 degrees Celsius (800 degrees Farenheit). Thus, it was long considered one of the least likely places to find ice.
Though Mercury is one of Earth's closest planetary neighbors, we know less about it than we do many of the more-distant planets. In 1974 and 1975, the Mariner 10 spacecraft made three passes by Mercury, sending back photos of 45 percent of the planet's surface. The rest of Mercury is little known, however, so ground-based observers continue to study the planet.
In 1991, planetary scientists Duane Muhleman and Bryan Butler from Caltech and Martin Slade from the Jet Propulsion Laboratory, studied Mercury using a radar system consisting of a 70-meter (230-foot) dish antenna at Goldstone, CA, equipped with a half-million-watt transmitter, and the VLA as the receiving system. The beam of 8.5-GHz microwaves sent from Goldstone bounced off Mercury and was collected at the VLA to produce a radar image of the planet. The researchers used the Goldstone-VLA radar system to look at the side of Mercury that was not photographed by Mariner 10.
The resulting radar image, shown here, contained a stunning surprise. In this image, red indicates strong reflection of the radar signal and yellow, green, and blue, progressively weaker reflection. The bright red dot at the top of the image indicates strong radar reflection at Mercury's north pole. In fact, it resembles the strong radar echo seen from the ice-rich polar caps of Mars.
"Normal" ice, such as that found on Earth, absorbs radio waves, but ice at very low temperatures is a very effective reflector of radio waves. The strong reflection seen on Mercury is too large to be caused by a momentary "glint" off a crater wall, and when studied in more detail shares the characteristics of reflections from the water ice seen on Mars and the icy moons of Jupiter.
Scientists now believe that the ice resides on the floors of craters at Mercury's north pole, where it can remain permanently shaded from the Sun and reach temperatures as low as 125 degrees Kelvin (-235 degrees Farenheit).
The VLA, with its great angular resolution, or ability to see fine detail, was crucial to this discovery. It was able to provide sufficient detail of small regions (down to 100 meters in this observation) to reveal the ice reflections. Other analytical capabilities of the VLA helped to further confirm the discovery. In 1994, the same observing team discovered a similar radar reflection from Mercury's south pole. In sum, the VLA's capabilities provided major new insight into the nature of this planet.