. Unlike the planets which orbit the Sun beyond the Earth, Mercury and Venus can never be seen high in the sky in the dead of night. Because they are close to the Sun and inside Earth's orbit, these planets are always seen soon after sunset, or shortly before sunrise, and they show phases much as the Moon does. The elongation of Mercury and Venus, or their angular distances from the Sun as viewed from the Earth, never exceeds 28 degrees and 47 degrees, respectively.
. The world's largest radio telescope is nestled into the hills near Arecibo, Puerto Rico. Its metal reflecting surface has a spherical shape with a diameter of 305 meters. The reflected radio signals are focused to detectors suspended on the triangular structure hanging from the three towers. The facility can also transmit powerful radio pulses, sending them off the metal surface into space. Such pulses, sent and received from this giant telescope, first measured Mercury's rotation period in 1965. Until then it had been wrongly thought that Mercury kept one side permanently facing the Sun, with a rotation period equal to its orbital period.
. A radio signal spreads out as a spherical wave, and only a small fraction is intercepted by Mercury. As the wave sweeps by the planet, it is reflected in spherical wavelets whose wavelengths are Doppler-shifted by the rotational motion of Mercury's surface. The waves from the receding side are red-shifted towards longer wavelengths and those from the approaching side are blue-shifted to shorter wavelengths. The total amount of wavelength change, from red to blue, reveals the speed of rotation, and the rotation period can be obtained by dividing the planet's circumference by this speed.
. Because of its spin-orbit coupling, Mercury rotates once every 58.6 Earth days, and orbits the Sun in 88.0 Earth days. After two orbits the planet has rotated three times, but from Mercury's surface the Sun appears to have moved only once around the planet. So sunrise is repeated at a given point on the planet's surface (black dot) every two orbits, and a solar day on Mercury lasts two of the planet's years and 176 Earth days. The labels on this figure refer to Earth days.
. During its three encounters with Mercury in 1974-75, the Mariner 10 spacecraft obtained thousands of images of the same sunlit hemisphere of the planet. Individual frames acquired by the approaching spacecraft have been joined together in this mosaic. The tiny, brightly rayed crater (just below center top) was the first recognizable feature on the planet's surface and was named in memory of astronomer Gerard Kuiper (1905-1973), a Mariner 10 team member. From this perspective, Mercury looks similar to the Moon, with a heavily cratered surface, but close-up inspection of individual surface features reveals some fundamental differences. (Courtesy of JPL and NASA.)
. After passing on the dark side of the planet, the departing spacecraft looked back at the sunlit hemisphere of Mercury and photographed these images that have been assembled into a mosaic. It shows large tracts of smooth plains, that may be due to extensive volcanism or the splashed-out debris from large impacts. Partially visible along the day-night terminator (left) is half of the Caloris basin (above center toward the top), a gigantic multi-ringed impact scar. (Courtesy of JPL and NASA.)
. In this Mariner 10 frame, south is down and the south pole is located on the right-hand edge of the large crater that has only its rim sticking up into the light. Radar reflection data suggest that water ice might be present in the cold, dark, permanently shadowed floors of craters at the poles of Mercury. (Courtesy of JPL and NASA.)
. The highlands of Mercury exhibit abundant craters, just as the lunar highlands do; but unlike the Moon, intercrater plains occur between large craters. Intercrater plains and heavily cratered terrain are typical of much of Mercury outside the area affected by the formation of the Caloris basin and photographed by Mariner 10 (Courtesy of JPL and NASA.)
. The smooth-plain terrain of Mercury is a possible example of a secondary crust of volcanic origin; but unlike the dark lunar maria the smooth terrain on Mercury is light in color. It was created after the heavy bombardment that excavated the older craters on the planet, but before the younger craters that are superposed on the smooth plains. The largest young crater shown here is about 100 thousand meters in diameter; such large craters have central peaks, flat floors, terraced walls, radial ejecta deposits and surrounding fields of secondary craters. The smooth plains shown here, in a Mariner 10 image, have well-developed ridges. (Courtesy of JPL and NASA.)
. This false-color composite was formed from re-calibrated Mariner 10 images at different wavelengths to highlight differences in surface minerals on Mercury. The color units in this and similar images are consistent with wide-spread plains formed by volcanism on the planet. The yellow splash at the lower right marks the location of a 60-km-wide ray Kuiper crater believed to have excavated unusual material from below the surface about 1 billion years ago. Reddish areas contain fewer opaque minerals and may represent primitive crustal material; the dark and blue regions are consistent with enhanced titanium content. The composite also suggests that the planet has undergone differentiation with heavy minerals sinking inside and lighter ones rising to the surface. The Kuiper crater is named after the astronomer Gerard Kuiper (1905-1973), a Mariner 10 team member. (Courtesy of JPL and NASA.)
. Mercury's surface is distinguished from the Moon and from the other terrestrial planets by having enormous cliffs, or scarps, that cut across its surface. The dark scarp cutting vertically across the center of this image, taken from Mariner 10, is about 350 thousand meters long. The cliff transects two craters 35 thousand and 55 thousand meters in diameter, proving that it was created after these craters formed. These long cliffs, which are as much as 4 thousand meters high, were probably crated when the planet cooled and contracted. (Courtesy of JPL and NASA.)
. Mercury is an anomaly in this comparison of the size and mean mass density of the Moon and the terrestrial planets. Mercury is just slightly bigger than the Moon and a little smaller than Mars, but its mean mass density is comparable to that of the larger terrestrial planets, Earth and Venus. As shown in the internal cross sections, Mercury's unexpectedly large mass density is due to a large dense core extending up to 75 percent of its radius. The Moon's core occupies just 20 percent of its radius. The Earth's inner and outer core take up 55 percent of its radius, while that of Mars is smaller.
. The magnetic field of Mercury is a miniature version of Earth's magnetic field, complete with bow shock, magnetosphere and magnetotail. Mercury's magnetic axis is closely aligned with its rotation axis, and its polarity is the same as the Earth's, with magnetic north corresponding to geographic north. This magnetic field is probably generated within the planet's large iron core, which takes up three-quarters of the planet's radius, but the exact mechanism for creating the field remains a mystery. The electrified solar wind compresses the magnetic field into a bow shock on the sunlit side and draws it out into a tail on the opposite side.
. Instead of always tracing out the same ellipse, the orbit of Mercury pivots around the focus occupied by the Sun. The point of closes approach to the Sun, the perihelion, is slowly rotating ahead of the point predicted by Newton's theory of gravitation. This was at first explained by the gravitational tug of an unknown planet called Vulcan, but we now know that Vulcan does not exist. Mercury's anomalous motion was eventually explained by Einstein's new theory of gravity in which the Sun's curvature of space makes the planet move in a slowly revolving ellipse.
. A massive object creates a curved indentation upon the flat Euclidean space that describes a world which is without matter. Notice that the amount of space curvature is greatest in the regions near the object, while further away the effect is lessened.
. Summary diagram