Solar system's smallest planet Mercury getting smaller, studies reveal news
05 July 2008

Mercury, the smallest planet in the solar system and named after the fleet-footed Roman messenger god, seems to be getting smaller by the day, latest studies reveal. But astronomers and astrologers have no worry of the closest planetary neighbour to the sun disappearing from view any time soon the shrinking is happening on a planetary scale, or in other words, billions of years.

Of course, Mercury has not always been the smallest planet in the solar system. In fact, if human sentiments are concerned, it was not the smallest even two years back, until the International Astronomical Union reclassified then smallest planet Pluto as a ''dwarf planet'', or in other words not a planet at all. This decision handed Mercury the crown.

Back in January early this year, NASA had revealed images sent by its MESSENGER probe, when it snapped up some 1,200 photos taken from some 120 miles (170 kilometres) from the planet's surface. MESSENGER, the MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft launched by NASA in 2004, raced past Mercury on 14 January in the first visit in almost 33 years to the mysterious small planet, in the first of four sweeps over Mercury by the probe.

However, it is only now that the amazing results revealed by analyzing those photographs been revealed. In 11 papers published in scientific journal Science on 3 July, it has bee suggested that the innermost planet of our solar system has shrunk by more than a mile (1.4 kilometres) in diameter over its history.

Scientists believe the shrinkage is due to the planet's core slowly cooling. "Cooling of the planet's core not only fuelled the magnetic dynamo, it also led to contraction of the entire planet," said Principal Investigator Sean Solomon of the Carnegie Institution of Washington, US.

"And the data from the flyby indicate that the total contraction is at least one-third greater than we previously thought."

The analysis also indicates an abundance of silicon, sodium and sulfur in Mercury's core. Mercury currently the Sun once every 88 days, with a volume of 0.054 Earths and a surface of 0.108 Earths.

Scientists also revealed that they had found evidence of volcanic activity on the planet, previously hinted at by an earlier spacecraft. Further analysis of areas such as the Caloris basin, one of the solar system's largest and youngest impact basins, found volcanic vents and evidence of "pyroclastic" debris blown from the volcano as it erupted.

Other areas contained circular structures with wrinkled edges, similar to structures seen on the Moon and Mars. Scientists believe these are impact craters that have been filled with massive quantities of lava, possibly 2.7 kilometres (1.3 miles) deep.

"That's a lot of lava," said Dr James Head of Brown University. "It shows the planet was really active in its early history."

Researchers believe the peak of activity could have been three to four billion years ago.

Launched on a Boeing Delta II rocket, MESSENGER lifted off from Cape Canaveral Air Force Station, Florida 3 August 2004 with the goals to determine the chemical composition of Mercury's surface, its geologic history, the nature of the planet's magnetic field, the size and state of the core, the volatile inventory at the poles, and the nature of Mercury's exosphere and magnetosphere.

However, MESSENGER's most important mission is yet to come: its Mercury orbit insertion will be on 18 March 2011, beginning a year-long orbital mission which will see a lot more data sent to Earth.

The space probe is also interesting because its navigation team is lead by KinetX, the first private company to be responsible for navigation of a NASA deep space mission. Their experts are fully responsible for determining all trajectory adjustments throughout the probe's flight through the inner solar system ensuring that MESSENGER arrives at Mercury with the proper velocity for orbit insertion.

The robotic space probe Mariner 10 was the only spacecraft so far to approach Mercury, and managed to map about 40 per cent of its surface. Its mission ran between 1974 and 1975 and the probe was the first spacecraft to make use of an interplanetary "gravitational slingshot" maneuver.

Physically, Mercury is similar in appearance to the Moon. It is heavily cratered, has no natural satellites and no substantial atmosphere. It has a large iron core, which generates a magnetic field about 1 per cent as strong as that of the Earth.

It is an exceptionally dense planet due to the large size of its core. The surface temperatures on Mercury range from about 90 to 700 K (-183 C to 427 C), with the sub-solar point being the hottest and the bottoms of craters near the poles being the coldest.

Recorded observations of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes.

The English name for the planet comes from the Romans, who named it after the Roman god Mercury, which they equated with the Greek Hermes. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.

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Solar system's smallest planet Mercury getting smaller, studies reveal