Mapping the Planet of Fire and Ice
It is the closest planet to the sun. A roasting crisp of a world where daytime temperatures reach 800 degrees. And yet, stretched across the surface of Mercury are what look to be patches of ice.
It’s one of the biggest mysteries of this little planet--a bookend of the solar system and one of our nearest neighbors. While Mercury was visited briefly once, by Mariner 10 in the mid-1970s, less than half of the planet was seen by the spacecraft’s cameras.
Still, the data that streamed back from Mariner were more than enough to intrigue scientists. Mercury, the smallest planet after Pluto and the only inner planet besides Earth that has its own magnetic field, is the subject of a host of questions.
Why is it so dense, composed mostly of solid metal? Why is it so different from its rocky brethren, Venus, Earth and Mars? How did it form in the first place? And how could the hard-baked world sustain ice?
Many of those questions may soon be answered. NASA officials announced in June that in 2004 they will launch their second mission to Mercury. “Mercury has been neglected. But no more,” said Jeff Taylor, a planetary scientist at the University of Hawaii Institute of Geophysics and Planetology and an expert on Mercury.
The $256-million Messenger mission, managed by the Applied Physics Laboratory in Laurel, Md., is expected to reach Mercury in 2009. The orbiting spacecraft will map the entire planet, probe its thin atmosphere, test its internal makeup and closely image strange features on the planet’s surface that appear to be ancient volcanoes, ridges and enormous thrust faults that might have formed as the planet cooled and contracted.
“It’s one of our nearest planets, and we haven’t seen most of the surface,” said Sean Solomon, a planetary scientist at the Carnegie Institution of Washington who is the mission’s principal investigator. “We’re taking some obvious instruments to ask some very basic questions. It’s long overdue.”
Chief among questions is to determine how Mercury formed. Like other terrestrial planets, its heavier elements coalesced to form a metallic core. But Mercury has only a thin skin of rock on its surface--some 350 miles deep. Theories abound on why the veneer is so thin.
In the most violent scenario, the planet was bombarded by huge blasts of energy from the sun that “could have mostly boiled off” the rock, said Solomon. Alternatively, a giant impact--like the one that hit Earth and formed the moon billions of years ago--could have blasted off much of Mercury’s surface. Things may not have been so terrible for the little planet, though. It may be so strange, Taylor suggested, because it formed from material near the sun and not more distant reaches of the solar system. Mercury could also be a hodgepodge of material from around the solar system that came together quite randomly. “Is it a small, real planet?” asked Taylor. “Or is it an accident?”
Also still unclear is whether the vast plains on Mercury’s scarred surface are evidence of lava flows from ancient volcanoes or are merely examples of melting that occurred when asteroids and meteors hit the planet.
But perhaps the most intriguing question is about that apparent ice. In 1961, three Caltech scientists proposed that the moon might have ice on its poles. But no one thought that Mercury, where the surface temperature is hot enough to melt lead, could be similar.
Mariner 10 didn’t see anything that looked like ice, but it didn’t take images of the entire planet and carried no radar. Then, in 1991, a group including Martin Slade of the Jet Propulsion Laboratory in Pasadena started mapping the planet by sending radio signals from a 230-foot radio antenna in Goldstone, Calif., and receiving the echoes at the Very Large Array of 26 radio telescopes in Socorro, N.M. They found “radar bright features” that looked like the signals from ice on Mars and the icy moons of Jupiter.
“When we saw it, we said, ‘What could we have done wrong?’ ” recalled Slade. “That shouldn’t be there.”
The discovery, compared to “finding a snowball in hell,” shocked scientists, who to this day remain surprised at the radar findings. “The most common material that could explain this behavior is--ice! On the planet closest to the sun!” reads the usually restrained Messenger mission Web site ( sd-www.jhuapl.edu/MESSENGER/index.html ).
Over the years, Slade and colleague John Harmon have used the world’s largest radio telescope in Arecibo, Puerto Rico, as well as Goldstone, to get a better idea of what accounts for the signals. They have since determined the ice is not a widespread cap, as seen on Earth or Mars, but is much more spotty--”caplets” as Ashwin Vasavada, a Caltech planetary scientist, terms them.
The bright material appears only near Mercury’s poles--and only inside craters or other places that are in permanent shadow and never warmed. “The sun never, ever, ever shines there,” said Slade.
New radar data show that the largest features are round near the poles but crescent-shaped in regions that are not quite as shaded. The shapes fit the expected shadows cast by the sun in those regions. The radar images also suggest that the ice may be covered by a thin layer of dust.
Vasavada’s theoretical work has shown that the temperature in Mercury’s shady zones would fall below -262 degrees--cold enough to preserve ice for billions of years. If the features are water ice, the best guess is that the water was delivered to the planet’s surface by a comet. It wouldn’t take much water to explain what is seen on Mercury--about 7.5 million acre-feet, or the contents of the Salton Sea, Vasavada estimates.
“One big comet would have been enough,” he said.
But the bright features may not be water ice. Other explanations include ice made of carbon dioxide, or sulfur, or something “really exotic”--although those are less likely, Slade said. Mercury has been an elusive scientific target for two reasons. It is so close to the sun that it is difficult to observe from Earth-based telescopes. The Hubble Space Telescope has been prohibited from viewing Mercury because of the potential danger the bright sun poses to the telescope’s optics.
And getting a spacecraft into orbit around Mercury is no easy feat. “This is much more difficult than a Mars mission,” said Bobby G. Williams, who will lead Messenger’s JPL navigation team.
The spacecraft must slow down a great deal to enter Mercury’s orbit. When a spacecraft nears a planet with a thicker atmosphere, like Earth or Mars, it slows down. Mercury, with an extremely thin atmosphere, cannot provide this natural brake. But the spacecraft cannot carry enough fuel for rockets to burn for braking. Instead, a clever five-year, 3.88-billion-mile trajectory designed by JPL celestial scientist Chen-Wan Yen will be used to fly by Venus and Mercury twice to slow the spacecraft so it can enter Mercury’s orbit. Being close to Mercury raises its own problems. Heat from the sun can fry a spacecraft. Messenger will have its own sunshade--a high-tech parasol. There is also much heat radiating off the planet. To avoid a meltdown, the spacecraft has an elliptical orbit that will bring it close to the planet for only 20 minutes, then give it nearly 12 hours to cool off again after each pass.
“This is a harsh thermal environment,” said Solomon. “And the spacecraft will be at room temperature.”
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Mission to Mercury
The second unmanned expedition to Mercury is expected to launch in 2004 and begin orbiting the planet five years later.
Mercury: Closest planet to the sun
Diameter: 3,025 miles, about 40% larger than our moon
Average distance to sun: 36 million miles (Earth is about 93 million miles)
Length of one day from sunrise to sunrise: 176 Earth days
Warmest temperature: 800 degrees.
Coldest temperature: Minus 280 degrees.
Total distance of spacecraft journey: 3.88 billion miles Researched by LYNN MEERSMAN / Los Angeles Times
Sources: Harmon (NAIC/Arecibo), Slade (JPL/Caltech), Sky and Telescope; JPL; NASA; Johns Hopkins University; University of Michigan
