Astro News
- Recent Updates of Astro News
- Active Mercury(07/09)
- Hubble Status Report: Directly Observes a Planet Orbiting Another Star(01/09)
- A Non-trivial Answer to a Trivial Astronomical Question-The Origin Of Absolute Magnitude(07/08)
- Assault by a Black Hole(04/08)
- New Lakes Discovered on Titan(01/08)
- ¡§Deviant Behaviour¡¨ in the Solar System(10/07)
- Cosmic Ripples - Cosmic Microwave Background - CMB(07/07)
- Interplanetary Superhighway(04/07)
- Is Pluto a Planet?(01/07)
- Breathing Moonrocks(10/06)
- My Thoughts on the Theory of Relativity, Quantum Mechanics, Superstring Theory and Dark Matter(07/06)
- Space-time Vortex(04/06)
- Radio Astronomy(01/06)
- Neutrino Astronomy(10/05)
- The Active Earth(07/05)
- What is Dark Energy?(04/05)
- The Mysterious Black Holes(01/05)
- Intermediate-Mass Black Holes And Quasisoft X-Ray Sources(10/04)
- Time Travel: From a Scientific Approach(07/04)
- What is Astrobiology?(04/04)
- Black Hole: From Fantasy To Reality (II)(01/04)
- Black Hole: From Fantasy To Reality (I)(10/03)
- From The Oldest Light In The Universe To The Fate Of The Universe(7/03)
- The Cosmic HERO(4/03)
- Quaoar - the Tenth Member of the Solar System?(1/03)
- The First Chinese Telescope in Space(10/02)
- Diamonds and Other Stardust(7/02)
- Supermassive Black Hole in Andromeda Galaxy(4/02)
- Detection of Solar Neutrinos(1/02)
- Simultaneous Multiple Wavwlength Observation(10/01)
- Celestial Distance(7/01)
- Solar-Terrestrial Relations(7/00)
- Fundamental Particles in Astronomy(4/00)
- The Solar Maximum in 2000(1/00)
- Hubble Constant(10/99)
- New Findings on Cosmology(7/99)
- Strange Stars(4/99)
- How Strong Stellar Magnetic Field Can Be?(1/99)

Important notices

A NASA spacecraft gliding over the surface of Mercury has revealed that the planet¡¦s atmosphere, magnetosphere, and its geological past display greater levels of activity than scientists first suspected. The probe also discovered a large impact basin named ¡§Rembrandt¡¨ measuring about 700 kilometres (430 miles) in diameter.

These new findings and more are reported in four papers published in the May 1 issue of Science magazine. The data come from the Mercury Surface, Space Environment, Geochemistry, and Ranging spacecraft-- MESSENGER for short. On October 6, 2008, MESSENGER flew by Mercury for the second time, capturing more than 1,200 high-resolution and colour images of the planet.


Figure 1: The Rembrandt impact basin discovered by MESSENGER during its second flyby
of Mercury in October 2008

(Credit: NASA/Johns Hopkins University Applied Physics Laboratory/
Smithsonian Institution/Carnegie Institution of Washington)

¡§This second Mercury flyby provided a number of new findings,¡¨ said Sean Solomon, the probe's principal investigator from the Carnegie Institution of Washington. ¡§One of the biggest surprises was how strongly [Mercury's magnetosphere] had changed from what we saw during the first flyby in January 2008.¡¨

The magnetosphere is a region of space around Mercury enveloped by the planet's magnetic field. Gusty solar wind buffeting the global bubble of magnetism can potentially trigger magnetic storms and other space weather-related phenomena.

¡§During the first flyby, MESSENGER measured relatively calm dipole-like magnetic fields close to the planet. Scientists didn't detect any dynamic features other than some Kelvin-Helmholtz waves,¡¨ said James Slavin of NASA's Goddard Space Flight Centre. Slavin is a mission co-investigator and lead author of one of the papers.

¡§But the second flyby was a totally different situation,¡¨ he says. MESSENGER observed a highly dynamic magnetosphere with ¡§magnetic reconnection¡¨ events taking place at a rate 10 times greater than what is observed at Earth during its most active intervals. ¡§The high rate of solar wind energy input was evident in the great amplitude of the plasma waves and the large magnetic structures measured by the spacecraft's magnetometer throughout the encounter.¡¨


Figure 2: An artist's concept of Mercury's surprisingly active magnetosphere. (click photo to enlarge)

Credits: Image produced by NASA/Goddard Space Flight Center/
Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
(Image reproduced courtesy of Science/AAAS)

Another exciting result is the discovery of a previously unknown large impact basin. The Rembrandt basin is more than 700 kilometres in diameter and, if formed on the east coast of the United States, would span the distance between Washington, D.C., and Boston. 

Rembrandt formed about 3.9 billion years ago, near the end of the period of heavy bombardment of the inner Solar System, suggests MESSENGER Participating Scientist Thomas Watters, lead author of another of the papers. Rembrandt is significant, not only because it is big, but also because it is giving researchers a peek beneath the surface of Mercury that other basins have not.

¡§This is the first time we've seen terrain exposed on the floor of an impact basin on Mercury that is preserved from when it formed,¡¨ explains Watters. ¡§Landforms such as those revealed on the floor of Rembrandt are usually completely buried by volcanic flows.¡¨

¡§After mapping the surface, we see that approximately 40 percent is covered by smooth plains,¡¨ said Brett Denevi of Arizona State University in Tempe, a team member and lead author of a paper. ¡§Many of these smooth plains are interpreted to be of volcanic origin, and they are globally distributed. Much of Mercury's crust may have formed through repeated volcanic eruptions in a manner more similar to the crust of Mars than to that of the moon.¡¨

Another finding of the flyby is the first detection of magnesium in Mercury's exosphere. The exosphere is an ultrathin atmosphere where the molecules are so far apart, and they are more likely to collide with the surface than with each other. Material in the exosphere comes mainly from the surface of Mercury itself, knocked aloft by solar radiation, solar wind bombardment and meteoroid vaporisation: 


Figure 3: In this interpretive map of Mercury's surface, shades of yellow denote smooth plains of
mainly volcanic origin. This type of terrain covers approximately 40% of the planet.
The white (empty) slice is the portion of Mercury not yet photographed. (click photo to enlarge)

(Credit: NASA/Johns Hopkins University Applied Physics Laboratory/
Arizona State University/Carnegie Institution of Washington)

The probe's Mercury Atmospheric and Surface Composition Spectrometer detected the magnesium. Finding magnesium was not surprising to scientists, but the abundance was unexpected. The instrument also measured other exospheric constituents including calcium and sodium. Researchers believe that big day-to-day changes in Mercury's thin atmosphere may be caused by the variable shielding of Mercury's active magnetosphere.

¡§This is an example of the kind of individual discoveries that the science team will piece together to give us a new picture of how the planet formed and evolved,¡¨ said William McClintock of the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder. McClintock is a co-investigator and lead author of one of the four papers.


Figure 4: Murcury's Surface-Bounded Exosphere (click photo to enlarge)

¡§The third Mercury flyby [coming up on September 29] is our final dress rehearsal for the main performance of our mission, the insertion of the probe into orbit around Mercury in March 2011,¡¨ said Solomon. ¡§The orbital phase will be like staging two flybys per day and will provide continuous collection of information about the planet and its environment for one year.¡¨

¡§Mercury has been coy in revealing its secrets slowly so far, but in less than two years the innermost planet will
become a close friend.¡¨



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