![]() The image illustrates some of the relationships between color and geological features-in particular, how impact craters can serve as probes of the variation of crustal composition with depth. Mercury’s near-equatorial region, shown in enhanced color from MESSENGER’s second flyby. (Courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.)įigure 2. The blue material on the crater’s central peak is thought, on the basis of crater-formation models, to have been excavated from still greater depth. At left, Calvino crater, named for the 20th-century Italian writer, formed from an impact into the brown-appearing Rudaki plains the impact explosion excavated the bright orange material that makes up the crater rim. Material ejected from the crater appears blue, which represents a different composition. ![]() At right, the floor of the 121-km-diameter Titian crater, named for the Italian Renaissance painter, is coated with material brighter in orange than the surrounding area. The spectral reflectance displays a continuously positive, or “red,” slope from visible to near-IR wavelengths, consistent with the chemical and physical modification of surface material by micrometeoroid bombardment and sputtering by solar-wind ions-processes collectively termed “space weathering.” The general absence, in particular, of an absorption feature at wavelengths near 1 µm-nearly ubiquitous on the Moon and rocky asteroids-indicates that the silicate minerals at Mercury’s surface contain very little ferrous iron.įigure 2. But most reflectance spectra of Mercury’s surface obtained from ground-based telescopes and those from the MESSENGER flybys show no discernible absorption features. 4 Measurement of the surface reflectance at visible and near-IR wavelengths has been a productive tool for probing the surface composition of solar-system objects because a number of common minerals absorb radiation at characteristic wavelengths in that region of the solar spectrum. Information on the silicate composition of Mercury, however, is limited. Nonetheless, the permanently shadowed floors of Mercury’s high-latitude craters appear to be sufficiently cold to have trapped water ice and perhaps other frozen volatiles. The closest planet to the Sun, Mercury also experiences a variation in surface temperature of 600 ☌ over the course of a solar day, which because of Mercury’s slow spin rate equals two Mercury years. ![]() Mercury is the only inner planet other than Earth to possess an internal magnetic field and an Earthlike magnetosphere capable of standing off the solar wind. Mercury’s orbit is the most eccentric of the planets, and it is the only known solar-system object in a 3–2 spin–orbit resonance, in which three sidereal days are precisely equal to two periods of Mercury’s revolution about the Sun. But its uncompressed bulk density-the density in the absence of internal pressure-is the highest. Among the planets of our solar system, it is the smallest, at about 40% of Earth’s diameter and little more than 5% of its mass. Although a sibling of Earth, Venus, and Mars, Mercury is an unusual member of the inner planet family.
0 Comments
Leave a Reply. |