Mercury as a Puzzle

            The innermost planet in the Solar System, Mercury, is unique in many ways.  It is surprisingly dense, reflecting a massive metallic core that has 60% of the mass of the planet (versus less than 31% for Earth, Venus, and Mars).  It has a highly eccentric orbit, approaching as close as 0.3236 AU to the Sun and retreating as far as 0.4506 AU on each orbit, covering the entire range in half an orbital period, 44 Earth days.  The intensity of sunlight striking Mercury’s orbit drops off with the square of the distance from the Sun, so it varies by nearly a factor of two. 

            Even odder, the rotation of Mercury is locked onto its orbital period.  Most examples of spin-orbit resonances in the Solar System are 1:1 (one rotation per orbit), which means that the smaller body always keeps the same side toward the larger body.  Examples include Earth’s Moon and the large Galilean satellites of Jupiter.  The most extreme example of a strong lock between rotation and orbit is Pluto and its satellite Charon:  they are locked into a 1:1:1 spin-spin-orbit state, in which both Charon and Pluto always keep exactly the face toward each other.  Mercury is noteworthy in that it has a 3:2 spin-orbit resonance: Mercury rotates exactly 3 times every 2 Mercury years, or 1 ½ times per year.  Thus in consecutive closest approaches to the Sun (perihelion), opposite sides of the planet get baked.  This situation is understandable if Mercury is elongated along one of its equatorial axes: the planet then points the ends of this long axis (the so-called tidal bulges) at the Sun alternatingly at each perihelion passage.

            Mercury shows no trace having ever had a significant atmosphere or oceans.  The faint wisp of gases surrounding Mercury today is in part due to solar wind gases from the Sun temporarily captured by Mercury’s gravity, and in part to atoms, such as sodium, baked out of the surface by the extreme heat and the impact of high-speed solar wind ions.  Gases released from Mercury’s interior, such as argon-40 from the radioactive decay of potassium-40 in the crust and mantle, can easily be ionized by ultraviolet radiation from the Sun, entrapped in the magnetic field of the solar wind, and swept away.

            How did Mercury get to be like this?  The spin-orbit resonance is essentially unavoidable for a planet that orbits so close to a star.  The absence of atmosphere and oceans is also unavoidable for a planet with such weak gravity in so hostile an environment.  But the high density of Mercury is a continuing puzzle.  The smaller bodies that accreted to form Mercury may have collided so violently that brittle silicates were preferentially crushed to dust, while tough grains of metal survived.  Dissipation of the finest dust would then leave behind the ingredients of a metal-rich, dense planet.  A second possibility is that the material in the zone where Mercury was to accrete was so strongly heated by the early superluminous Sun that the most volatile minerals were evaporated and lost from Mercury’s formation zone, leaving dense, involatile solids behind to form the planet.  A third scenario is that Mercury formed normally with a composition similar to that of the other terrestrial planets, but that post-accretion impacts of comets and asteroids eroded away the crust and much of the mantle, blasting them off at high speeds and leaving behind a part of the lower mantle and the well-protected dense metallic core.  Each of these three scenarios predicts different surface compositions for present-day Mercury.  The goal of the MESSENGER spacecraft, which is due to enter orbit around Mercury in January, is to test these hypotheses by analyzing the crust by means of gamma-ray spectroscopy and probing the interior of the planet by means of measuring its gravitational and magnetic fields and the interaction of Mercury’s core with the magnetic field of the solar wind.

            If all goes well, the answers to these puzzles will soon be in our hands.

Mercury: MESSENGER Spacecraft in Future Headlines

Paradoxically, Mercury is the hardest planet to land on (or in) in the Solar System.  Only a single mission from Earth has flown by Mercury, the Mariner 10 spacecraft launched by NASA in 1973.  Mariner 10 used a flyby of Venus  to bend its orbital path inward to Mercury and to shorten its orbital period, eventually placing the spacecraft in an orbit around the Sun that repeatedly passed by Mercury at close range.  That orbit had a period of 176 days, exactly two Mercury years, assuring that it would fly by Mercury at close range that often.  Because Mercury is locked into a resonance with the Sun, the planet rotates exactly 3 times every 2 Mercury years—meaning that the exact same face of Mercury is presented to the Sun (and illuminated for imaging purposes) on every flyby date.  Mariner 10 carried out three brief high-speed flybys of Mercury (and one of Venus) during its operational lifetime.

Placing a spacecraft in orbit around Mercury is made very difficult by the high speed of a fly-by spacecraft.  After falling in the Sun’s gravitational field in from Earth’s orbit to Mercury’s orbit, the spacecraft is traveling so fast that it would require impractically large masses of rocket propellant to slow it down enough to be captured.  Landing is even harder, since it requires additional propellant to resist the planet’s gravitational field and decelerate to zero velocity upon arrival at the planetary surface. Instead, other deceleration methods need to be undertaken.

A second spacecraft mission, MESSENGER, uses planetary swing-bys to slow the probe. MESSENGER will soon arrive at Mercury after a 6.6 year journey and no less than six planetary flybys.  After its launch in August 2004, there were close encounters with Earth (August 2005), Venus (October 2006 and June 2007), and Mercury (January and October 2008 and September 2009), putting MESSENGER into an orbit around the Sun that will arrive at Mercury with a low relative speed on March 18, 2011.  MESSENGER will then fire its rocket engine and drop into orbit around Mercury.  The orbit will be eccentric (to permit both large-scale mapping and close-up imagery) and highly inclined (to permit coverage of the entire surface as Mercury rotates beneath the spacecraft’s orbit). 

Even with this clever trajectory design, most of MESSENGER’s launched mass (55%) had to be rocket propellant. By the end of the mission it will have been used for mid-course trajectory tweaks, to slow it down, and to bring about orbital insertion, and to maintain the desired orbit. By far the most costly of these maneuvers in terms of fuel consumption is orbital insertion. To keep posted on this exciting mission consult the website maintained by Johns Hopkins University’s Applied Physics Laboratory, the managers of the mission: http://messenger.jhuapl.edu/spacecraft/index.html.  
 If you are interested in the instruments carried to Mercury to explore its atmosphere, magnetic and gravitational fields, radiation environment, and surface composition and structure, go the The Mission and click on Instruments on this same site.