May 24 2009
Science magazine (May 1, 2009) celebrates up-to-the-minute views of planet Mercury’s magnetosphere, exosphere, and surface from the second flyby in October, 2008; the third flyby is scheduled for 9/29/09 and orbital insertion of Messenger will occur next year on March 18.
Although few people have ever actually seen it in the sky because of its closeness to the Sun (0.4 AU), Mercury is an extraordinary scientific puzzle with almost unlimited potential for human utilization. As we approach the transformative 2015 Maslow Window and pursue human expansion toward the Moon and Mars, global interest in the spectacular large-scale potential for Mercury will emerge.
The mysteries began decades ago with recognition of Mercury’s extremely high density (5.43) for such a small planet (just over 1/3 of Earth’s radius); it had to be the Iron World! If all this iron formed a core, there would be only a thin layer of rocks (~ 600 km) for Mercury’s mantle and crust, resulting in rapid freeze-over for a molten core. In the 1970s, Mariner 10 scientists decided to include a magnetometer on the spacecraft — just to be sure — and were rewarded with a surprise discovery: a weak, but definitely Earth-like magnetic field surrounding Mercury.
Today, the Top Two Mercury Mysteries are: 1) how can such a small planet have an Earth-like magnetic field? and 2) how can a planet have such a thin silicate mantle; i.e., such a low silicate-to-iron ratio?
In Mercury (Vilas et al., 1988) Gerry Schubert — National Academy of Sciences member and my research boss while a graduate student at UCLA — showed that for a relatively stiff, insulating mantle, Mercury could retain a partly molten core with a few % of sulfur. More recent studies support this idea and include Earth-like dynamo processes in the core; see the MESSENGER team of Kabin et al. (2008).
At the Lunar and Planetary Laboratory, we determined (Cordell and Strom, 1977) from Mariner 10 images (with about 45% surface coverage) that the planet had cooled and actually shrunk early in its history, as has been confirmed by MESSENGER’s survey of the whole planet. Despite this contraction, Denevi et al. (2009) indicate that widespread, effusive (lunar mare-like) volcanism has been an important process on Mercury. Earth-based observations have indicated a generally basaltic composition and MESSENGER color data suggest low-iron pyroxenes or olivines.
There is little evidence of an “ancient feldspar-rich crust such as that of the lunar highlands.” Although not addressed in Denevi et al. (2009), this is consistent with theories for Mercury’s high density that involve removing much of the planet’s outer mantle with a large impact. Also interesting is the main inference from MESSENGER that “much of Mercury’s crust formed as a result of the eruptions of magmas of varying composition over an extended duration of geologic time,” (italics mine). This is consistent with a continuously cooling core that has remained warm enough over the planet’s lifetime to produce the currently observed field.
Despite the difficulties of just getting there, a rich speculative literature describing Mercury’s future already exists. Mercury offers 21st Century humans 3 wonderful things: 1) more solar energy than you can imagine, 2) a moderate planetary gravity well with 1/3 Earth’s surface gravity (similar to Mars), and 3) a primordial, airless surface. I suppose that given enough time and determination, we could produce April showers and rose gardens on Mercury — but it would be a cosmic crime to do so. In Terraforming, Martyn Fogg agrees that “some lesser planets may be of more use to a Solar System civilization generally if they are left airless so that the products of mineral extraction can be easily launched into space.”
Think of the entire surface of Mercury as a solar cell farm where the Sun is almost 7x brighter than at Earth. (Except for the polar craters, where there may even be Moon-like deposits of ice, the rest of Mercury gets a little toasty for settlements, to say nothing of the amazingly long solar days — 176 Earth days). So the big question is: what are you going to do with all that energy?
If you want to build things, the entire ferromagnesian Mercurian crust is available. It will liberate useful elements like iron, silicon, titanium, and oxygen if you apply enough solar energy in the right ways. Mercury’s airless surface and moderate gravity promise relatively easy access to space for electromagnetic launchers (mass drivers). It’s a dream come true!
One suggestion is to build solar sails on Mercury, shoot them into space, and then use the abundant sunlight as free propellant (Gillett, 1996). If you construct sails that are configured to use laser beams instead of sunlight, a Mercury-based laser could zip you back to Earth or Mars. Larger versions could send you toward the stars. (See Robert Forward).
For those who don’t want to leave the Solar System just yet, why not expand your sociological horizons and build a space colony with a few thousand of your closest friends using materials from Mercury and energy from the Sun? Initially these colonies might be in Mercury orbit, but eventually they’d circle the Sun independently. Over time, if you produce enough Mercury/Sun space colonies, they’ll remind us of something very special: a Dyson Sphere
All it takes is the energy and materials of Mercury and almost anything is possible. However, as we approach the 2015 – 2025 Maslow Window and stress over a 2020 return to the Moon, it appears that even Mars bases are doubtful, much less the large-scale utilization of Mercury. Unless a significant human beachhead in deep space — i.e., near the Moon or beyond — is established before 2025, as part of an international multi-decade Human Solar System Initiative, it appears Mercury spectaculars will be relegated to the late 21st Century Maslow Window…
…It starts near 2071.