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Discovering Mercury: The 2006 Transit

Written By: Susanne Douglas, Ph.D.

What are Transits? A transit is observed when a planet passes across the disk of its parent star. When transits occur, the planet passing in front of the star may cause an apparent reduction of the star's brightness as seen from Earth. This is, in fact, one of the ways in which planets have been found orbiting stars in distant planetary systems, but from Earth, only very large, Jupiter-sized planets can be found in this way. The Kepler Mission is a space-borne telescope that will detect planets indirectly, using the "transit" method.

The Kepler telescope will allow, for the first time, the detection of Earth-sized worlds around a distant star. Three transits of a star, all with a consistent period, brightness change and duration, will provide strong proof of an Earth-sized planet's presence. The measured orbit of the planet and the known properties of the parent star are used to determine if each planet discovered is in the habitable zone; that is, at the distance from its star where liquid water could exist on the surface of the planet.

Mercury transit of the sun May 7, 2003.
Mercury transit of the sun May 7, 2003.

Mercury's Transit of the Sun

On November 8th, 2006 Mercury will transit the Sun. From Earth, the only planets that can be viewed transiting the Sun are Mercury and Venus, since these are the only planets that have orbits within that of Earth's. Starting at 19:12:04 UT (Universal Time) and ending at 00:10:08 UT, the entire transit will last about 5 hours. This is the time from which the disk of Mercury first contacts the Sun to the time of last contact. Images of the 2003 Mercury transit can be seen at the SOHO (Solar and Heliospheric Observatory) website.

The transit will not be visible in its entirety from all parts of the world. Only part of the transit will be visible from the Americas, eastern Asia, the Pacific Ocean and Australia. The entire transit will be observable from western North America, eastern Pacific, New Zealand, Southeastern Australia, and Antarctica. Further details on viewing tips, exact transit times and the timing of past and future Mercury transits can be found at the Sun-Earth Program at Goddard Space Flight Center.

The Planet Mercury

Mercury depicted from Mariner 10 data.
Mercury depicted from Mariner 10 data.

Mercury is the innermost planet of our solar system. It is the least well studied but what is known about it suggests it is unique compared to the other inner planets, which include: Mercury, Venus, Earth, and Mars. The inner planets share a similar rocky composition, in which the solid material is layered into an inner metal-rich core, a silicate crust, and a mantle composed of molten silicate rock, which surrounds the core. Earth is the largest inner planet and the only one that has substantial liquid water on its surface. Earth, Mars, and Venus all have significant atmospheres, while that of Mercury is so thin that the molecules and elements of which it consists are so widely scattered they rarely interact. These arise from bombardment of Mercury's solid surface by the solar wind, a stream of charged particles emanating from the Sun.

Mercury is also the densest of the inner planets because the ratio of its heavy metal core to its much lighter silicate crust is much higher than for the other inner planets. Geophysicists estimate that the core makes up 70 to 80 % of Mercury's weight (compared to about 32 % for Earth's core). There are a number of theories that have been put forth in order to explain this. These theories depend on a distinct understanding of how our solar system formed and evolved and this is still not precisely known. In general, it is believed that the planets and Sun condensed out of a cloud of hot gases (the nebular cloud) and that the differential composition of the planets resulted from their varying distance from the Sun. Planet density and number of orbiting satellites generally decrease with distance from the Sun. The high density of Mercury may have arisen from:

1. The lighter particles, which would form the silicate crust were preferentially dragged away by the nebular cloud to eventually become part of the sun. This would leave behind a collection of particles in which the composition of the silicates is unchanged but their proportion in relation to the heavier metal-rich particles is decreased.

2. The early planet formed but tremendous heat from the young Sun vaporized part of the outer rock layer, leaving a metal-rich cinder covered by silicates poor in easily-vaporized elements like sodium and potassium.

3. Giant impacts, soon after Mercury formed, stripped off the primordial crust and upper mantle. In this case the surface would be depleted in elements that would have been concentrated in the primordial (initially-developed) crust (Al, Si, O).

Why Study Mercury?

The formation of the inner planets followed similar paths, but, due to conditions specific to each planet, these bodies diverged and differed over time to the forms we see today. Each planet was subjected to unique conditions which influenced its ultimate geological path so that by studying them, we can get glimpses into what Earth may have been like under different circumstances. For example, if Earth were closer to the Sun, it may have been hot, like Venus, and unable to hold liquid water or harbor life. If it were as small and far from the Sun as Mars, its liquid core may have frozen, halting tectonic activity and eliminating its magnetic field.

Mercury allows us a glimpse of what Earth may have been like without its relatively light and mobile crust. It also will give us an idea of what Earth's mantle layer is like and how it would behave if it were the outermost layer of the planet. Mercury is part of a continuum of possibilities in planetary composition and physical characteristics as is Earth. By studying our nearby neighbors, we can better understand the Earth.

By comparative studies of the rocky planets, we can also gain a better understanding of how a parent star influences the formation and characteristics of its planets. This, together with studies of the outer planets will allow us to better understand the evolution of our solar system. Then, when we find other planetary systems, we may be able to determine more detailed characterisitcs of the planets around distant stars through our ability to relate mineralogical, atmospheric, and physical characterisitcs of planets to the physics and chemistry of their parent star.

Missions to Mercury

Mariner 10

So far, our knowledge of Mercury's surface composition is based on results from the Mariner 10 mission, the only mission which has so far studied this planet. This mission yielded images of about 45% of Mercury's surface but reworking of the data using the technology available today has led to some insights on Mercury's geological history and present characteristics. Detailed descriptions of how Mariner's data was re-worked and the interpretations that resulted can be found here. Mercury's surface has lava flows, fault scarps (cliffs) that are hundreds of km long and several km high, and many craters as much as 1300 km across. The Mariner mission also revealed that, surprisingly, Mercury has a global magnetic field that is about 1/100 the strength of Earth's. This is unusual for such a small planet and implies that there is circulation of liquid metal at the core which can produce the convection-driven movement necessary to generate a magnetic field.

MESSENGER spacecraft
MESSENGER spacecraft


The NASA Mercury Surface, Space Environment, Geochemistry and Ranging mission was launched 8/3/04 to arrive in Mercury orbit by March, 2011 after 2 Venus flybys and 2 Mercury flybys. Mercury is the least studied planet in our solar system and by studying it, the mission aims to shed light on Earth's evolution. The Messenger spacecraft will map the entire planet in color, image most of the areas unseen by Mariner 10, and measure the composition of the planet's surface, atmosphere, and magnetosphere.

Messenger will carry instruments designed to measure many aspects of Mercury's chemistry and physical properties. X-ray, gamma ray, and VIS-IR spectrometers will determine the mineralogy and elemental composition of rock units. Nearly the entire planet's surface will be imaged in stereo so that heights can be determined and topographical maps and 3-D visualizations can be constructed. A laser altimeter will measure the topography of surface features more precisely in the Northern Hemisphere. Comparing the topography with the planet's gravity field, measured by tracking the spacecraft, will allow determinations of local variations in the thickness of Mercury's crust.


A joint venture between ESA and JAXA, the Japanese aerospace exploration agency, this mission will consist of two orbiters. The science payload was selected in 2004 and the mission is scheduled for launch in 2013. Mercury orbit will be achieved in 2019-2020 for one full Earth year of observations with the possibility of an extension. The mission was named after Professor Giuseppe (Bepi) Colombo (1920-1984) from the University of Padua, Italy. He was an engineer/mathematician who explained the reason for Mercury's orbital vs. rotational period (3 rotations for every two trips around the Sun. He also determined how to use gravity assist from Venus to get the Mariner 10 spacecraft into its orbit around Mercury in 1974-1975.

This mission will carry an array of instruments on two orbiters which will determine the composition of Mercury's atmosphere and surface, measure surface topography, study Mercury's magnetic field and also test Einstein's Theory of Relativity.

Last Updated: 14 February 2011

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Last Updated: 14 Feb 2011