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Friday, 04 July 2008 18:35

Our MESSENGER to Mercury sends home important data

The NASA spacecraft MESSENGER flew by Mercury on January 14, 2008. It reported back to scientists on Earth some information they did not know about its volcanic history and the source of its magnetic field.

NASA reported these discoveries from the planet Mercury on Friday, July 3, 2008, based on the mapping by MESSENGER of about 30% of the planet.

For over thirty years, since the NASA Mariner 10 mission, astronomers really did not know how the smooth plains on Mercury were formed.

Some scientists thought they were created by materials ejected out of large meteorite impacts bombarding the planet from outer space. Others thought the smooth plains were formed from inside the planet by lava erupting from giant volcanoes.

Astronomers also did not know much about the magnetic field of Mercury. They knew it had an active magnetic field, like on Earth, but these astronomers did not know its source. The core of Mercury was previously identified as consisting of a large iron-rich inner core and a molten outer core. However, it was unknown if that core produced the magnetic field on Mercury.

In addition, some of surface details on Mercury indicated that the planet had been slightly larger in its past than what it is now. Such contraction of the planet was a mystery to scientists, too.

Before MESSENGER, only one other spacecraft has gotten close to the planet--the robotic Mariner 10 spacecraft, which mapped about 45% of the planet between 1974 and 1975. It fly by the planet on March 29, 1974, September 21, 1974, and March 16, 1975.

Launched on November 3, 1973, the Mariner 10 mission was to measure Mercury’s and Venus’ atmospheres, surfaces, and other characteristics. Most of the other observations of Mercury were made with ground-based telescopes and other instruments.

The scientists had theories about these and other unknowns of the planet, but no facts to collaborate those hunches.

What did the scientists learn from the first flyby of Mercury by MESSENGER? Please read on.

When the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft traveled past Mercury on its mission to explore the characteristics and environment of the planet from its orbit above it, it discovered some of the answers to these questions.

After exploring about 20% of the planet that had not been observed earlier by Mariner 10, the MESSENGER spacecraft found that the smooth Mercurial plains was formed by flowing lava from active volcanoes from the planet’s past.

It also discovered that the origin of its very active magnetic field is from the planet’s core.

The researchers found evidence of volcanic vents along the margins of the Caloris basin, one of the solar system's youngest impact basins. They also found that Caloris has a much more complicated geologic history than previously believed.

James Head (Brown University), lead author of one of the Science articles stated, "By combining Mariner 10 and MESSENGER data, the science team was able to reconstruct a comprehensive geologic history of the entire Caloris basin interior. The basin was formed from an impact by an asteroid or comet during a period of heavy bombardment in the first billion years of Solar System history. As with the lunar maria, a period of volcanic activity followed, producing lava flows that filled the basin interior. This volcanism is responsible for the comparatively light, red material of the interior plains intermingled with [newer] impact crater deposits." [NASA: “New Discoveries at Mercury”]

Please also go to the NASA “New Discoveries at Mercury” website to see dramatic pictures of Mercury taken by MESSENGER. Pictures are also available on websites listed at the end of this article.

The magnetic field on Mercury was discovered by the Mariner 10 spacecraft. However, Mercury continued to puzzle scientists because its iron core should have cooled sufficiently over its past and stopped producing magnetism.

Nevertheless, MESSENGER found that Mercury does have a global magnetic field and that its source is an active dynamo in its outer core. It is powered by the cooling of the core and is very complex and dynamic.

The magnetic field on Mercury is only the second one found on a planet in our solar system. The other one being here on Earth.

Brian Anderson (Johns Hopkins University Applied Physics Laboratory), a lead author in one of the Science articles states, "The fact that it is dipolar [with north and south magnetic poles], and that we did not find the signature shorter-wavelength anomalies that would signify patches of magnetized crust, supports the view that we’re seeing a modern dynamo. We are eager for the October flyby [of MESSENGER] and the year in orbit to see if this is the case elsewhere on the planet and confirm that the field comes from the core." [NASA]

The core at the center of Mercury is very large. In fact it is at least twice as large as any other core on any other planet in the solar system. Its core makes up over half of its mass.

The cooling of the core on Mercury has produced some interesting features on the surface. Please read on.

As this core cooled over Mercury’s long history, it caused the planet to shrink (contract). This has led to surface features called lobate scarps (or cliff-like “wrinkles”).

In fact, MESSENGER found many of these contractional faults--many more than what was found with Mariner 10.

Sean Solomon (Carnegie Institution of Washington), the principal investigator of the mission, states, "The dominant tectonic landforms on Mercury are lobate scarps, huge cliffs that mark the tops of crustal faults that formed during the contraction of the surrounding area. They tell us how important the cooling core has been to the evolution of the surface. After the end of the period of heavy bombardment, cooling of the planet's core not only fuels the magnetic dynamo, but also led to contraction of the entire planet. And the data from the flyby indicate that the total contraction is a least one third greater than we previously thought." [NASA]

Impact craters were also observed by MESSENGER. The first measurements of the altitudes on Mercury show that some of the craters on the planet are about half the depth of craters on the Moon. Such images help astronomers to determine the ages of surface features and to resolve the steps involved in shaping the planet’s surface. Meteorite impacts and the solar wind, because the Sun is so close to the planet, have caused a great deal of weathering on the planet.

The results from the January 14, 2008 fly-by of Mercury by MESSENGER is found in a series of eleven papers published in a special section of Science magazine on July 4, 2008. Many of them discuss the surface of Mercury with respect to its reflectance and color variation, chemistry, and altitude. Many of the instruments aboard MESSENGER targeted different wavelengths of radiation in order to collect a complete picture of Mercury.

If you have a subscription to the magazine, the eleven articles are found at Science MESSENGER.

The study of Mercury, like the study of the other planets of our solar system helps us understand our planet Earth in better and more complete ways. We learn more about how Earth evolved and developed by learning how these planets also evolved and developed—sometimes similar to our Earth but at other times very differently.

Solomon adds, "It's remarkable that this rich lode of data came from two days of imaging, just 30 minutes of sampling the planet's magnetosphere and exosphere, and less than ten minutes carrying out altimetry and collecting other data near the time of its closest approach. MESSENGER's flyby was a huge success." [NASA]

The MESSENGER spacecraft will fly past Mercury on two future occasions (again in October 2008 and September 2009), before it finally is inserted into orbit about the planet in 2011.

For more information on the NASA mission MESSENGER to the planet Mercury, go to:

NASA: “MESSENGER Settles Old Debates and Makes New Discoveries at Mercury

Johns Hopkins University Applied Physics Lab: “Mercury’s Newly Named Features

NASA Solar System Exploration: “Mission to Mercury

Johns Hopkins University Applied Physics Lab: “Mercury Flyby 1

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