Planetary Provenance – Mercury

I will be speaking at the Sunday, September 7 meeting of the Humanist Society of Greater Phoenix about “Images of Mars and Interplanetary Science.” This series of posts in preparation for the event will explore current spacecraft activities in planetary science.

After the astronomy revolt that left the solar system with eight planets, four dwarf planets, and a huge variety of other celestial objects, Mercury became the smallest planet.  Despite its newfound stature, Mercury remains an important destination for better understanding the inner and rocky solar system.  It also remains little explored, a state that has only recently been addressed by a new spacecraft with a comprehensive suite of instruments.

NSSDC Photo Gallery: Mercury - Mercury Mosaic #1

Credit: NSSDC Photo Gallery: Mercury – “Mercury Mosaic #1” taken by Mariner 10

Mercury can be difficult to see from the Earth despite being one of the brightest objects in the night sky.  The planet orbits close to the Sun, which also makes it a difficult destination to reach for spacecraft.  In fact, until recently only one spacecraft had visited Mercury.  In 1974 and 1975, Mariner 10 explored Mercury during three flybys.  Due to the geometry of the encounters, Mariner 10 was only able to image approximiately 45% of the planet’s surface.  The rest of the planet has been mapped at low resolution using Earth-based radar.

33 years later, MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) has finally provided the first close up images of surface features unexplored by Mariner 10.  The first flyby of the MESSENGER mission occurred on January 14, 2008.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington - Mercury Shows Its True Colors

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington – “Mercury Shows Its True Colors” taken by MESSENGER

MESSENGER returns to Mercury for another pass on October 6, 2008 and again on September 29, 2009. However, on March 18, 2011 the mission gets especially interesting; the spacecraft will fire its thrusters to slip into orbit around Mercury. Although nearly all of the planet will have been mapped by this time, the closer proximity will allow a year-long study of the environment in which Mercury orbits the Sun and its tenuous atmosphere, as well as inspection of its geology and composition. Mercury hosts particularly interesting landmarks like the enormous Caloris impact basin and polar craters that may be shadowed enough to trap water in the form of ice despite the planet’s proximity so close to the Sun.

Mercury apparently has a massive iron core as a result of formation in the inner solar system and possibly due to a traumatic collision with another body that skinned the developing planet soon after differentiation. MESSENGER has spotted good evidence for volcanism to explain Mercury’s smooth lava plains, though it is likely that such activity occurred early in the planet’s history. Extensive systems of faults, ridges and cliffs as well as a spider-like formation of fractures within Caloris Basin show that Mercury has had a complex geological history.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington - The Spider - Radial Troughs within Caloris

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington – “‘The Spider’ – Radial Troughs within Caloris” taken by MESSENGER

This history, among other data, may provide further information related to the formation of Mercury, the rocky inner planets, and the Solar System as a whole. Our Solar System is striking for the rocky composition of its inner planets and the gaseous and icy nature of its outer planets. Mercury is on the extreme end of this spectrum of planetary composition dictated by nearness to the developing Sun. Just how “dry” is Mercury? Why is the planet so dense? These questions and more can finally be addressed by the renewed exploration of the innermost planet. The next few years promise a wealth of discovery for scientists who have long clamored for a return visit to Mercury and finally saw their dreams realized after the turn of the century, and should help to keep feeding the public’s interest in planetary science and exploration.

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