Exploring Mercury

The earliest mentions of Mercury come from the 3rd millennium BCE, when it was known to the Sumerians of Mesopotamia. The Babylonians (2000–500 BC) Discovered the planet before the Sumerians, and early Babylonians may have recorded observations of the planet but none these have survived, later Babylonian records from the 7th century BC refer to much earlier records. The Babylonians called the planet Nabu or Nebu after the messenger to the Gods in their mythology. The ancient Greeks gave the planet two names: Apollo when it was visible in the morning sky and Hermes when visible in the evening. However, Greek astronomers came to understand that the two names referred to the same body, with Pythagoras being the first Greek to propose the idea. The first telescopic observations of Mercury were made by Galileo in the early 17th century. Although he observed phases when he looked at Venus, his telescope was not powerful enough to see the phases of Mercury. In 1631 Pierre Gassendi made the first observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by Johannes Kepler. In 1639 Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited around the Sun. A very rare event in astronomy is the passage of one planet in front of another (occultation), as seen from Earth. Mercury and Venus occult each other every few centuries, and the event of May 28, 1737 is the only one historically observed, having been seen by John Bevis at the Royal Greenwich Observatory. The next occultation of Mercury by Venus will be in 2133. Mercury has been far less studied than the other planets. In 1800 Johann Schröter made observations of surface features, but wrongly estimated the planet’s rotational period at about 24 hours. In the 1880s Giovanni Schiaparelli mapped the planet more accurately, and suggested that Mercury’s rotational period was 88 days. In June 1962 Soviet scientists at the Institute of Radio-engineering and Electronics of the USSR Academy of Sciences lead by Vladimir Kotelnikov became first to bounce radar signal off Mercury and receive it, starting radar observations of the planet. Three years later radar observations by Americans Gordon Pettengill and R. Dyce using 300-meter Arecibo Observatory radio telescope in Puerto Rico showed conclusively that the planet’s rotational period was about 59 days. Italian astronomer Giuseppe Colombo noted that this value was about two-thirds of Mercury’s orbital period. Data from Mariner 10 then confirmed this view. In 2000, high-resolution lucky imaging from the Mount Wilson Observatory 1500 mm telescope provided the first views that resolved some surface features on the parts of Mercury which were not imaged in the Mariner missions. Later imaging has shown evidence of a huge double-ringed impact basin even larger than the Caloris Basin in the non-Mariner-imaged hemisphere. It has informally been called the Skinakas Basin. Most of the planet has been mapped by the Arecibo radar telescope, with 5 km resolution, including polar deposits in shadowed craters of what may be water ice. Ground-based telescopes have detected the bright rays around some radar-mapped craters.