6. Mercury: a battered world
Radar probes of Mercury
The halting spin of old age
In 1965, Mercury's true rotational period was determined with radio signals that rebounded from the planet. The world's largest radio telescope, located in Arecibo, Puerto Rico, was used to transmit mega-watts of pulsed radio power at Mercury, and to receive the faint echo. This technique is known as radio detection and ranging, abbreviated radar, and it is used to locate and guide airplanes near airports.
Each pulse was finely tuned, with a narrow range of wavelengths. Upon hitting the planet, its rotation de-tuned the pulse, slightly spreading the range of wavelengths. One side of the globe was rotating away from the Earth, while the other side was rotating toward our planet. These motions produced slight changes in the wavelength of the echo and from these changes, the speed of the surface and the rotational period were calculated, using the well-known expression for the Doppler effect.
Long, hot afternoons on Mercury
The spin-orbit coupling has a curious effect that may have misled astronomers who reported the wrong rotation period. Because three times the true rotation period is equal to twice the orbital period, any surface markings on Mercury would have returned to the same side after two orbital revolutions. Thus, astronomers could have been fooled, because looking at Mercury after two of its orbital periods they would see the same markings on the sunlit side and would find no disagreement with the 88-day period that they expected. Many of the conflicting observations were apparently ignored or missed. This is a striking example of curious observational circumstances and theoretical expectations that misled nearly everyone.
The days are certainly long on Mercury, longer than the planet's year. At any given point on Mercury, the daylight interval between sunrise to sunset lasts 88 Earth days, and the night lasts 88 Earth days more. This means that at one location on the surface, successive sunrises occur every 176 Earth days. So, the full day on Mercury is twice the length of its year.
Possible water ice at the poles of Mercury
Despite the heat on the sunlit side of Mercury, radar astronomers have found evidence for ice at both the north and south poles. Both the intensity and orientation, or polarization, of the bright radar echoes suggest the presence of water ice. Their radar characteristics are similar to those seen on icy surfaces elsewhere in the solar system, such as the ice deposits on the Galilean satellites and the south polar ice cap on Mars.
Moreover, the radar-bright features have been matched with specific polar craters, located on Mariner 10 images, and these craters should contain permanently shadowed interiors. On the other hand, the shaded polar craters could contain other volatile substances, such as sulfur, which could produce strong radar echoes but have a higher melting point than water ice. So, we may not definitely know if there is water ice at the top and bottom of Mercury until inquisitive robot spacecraft land there and make the appropriate tests. In the meantime, we turn to the startling results of Mariner 10.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University