A planet as small and as hot as Mercury has no possibility of retaining a significant atmosphere, if it ever had one. To be sure, Mercury's surface pressure is less than one-trillionth that of Earth. Nevertheless, the traces of atmospheric components that have been detected have provided clues about interesting planetary processes. Mariner 10 found small amounts of atomic helium and even smaller amounts of atomic hydrogen near Mercury's surface. These atoms are mostly derived from the solar wind—the flow of charged particles from the Sun that expands outward through the solar sys-tem—and remain near Mercury's surface for very short times, perhaps only hours, before escaping the planet. Mariner also detected atomic oxygen, which, along with sodium, potassium, and calcium, discovered subsequently in telescopic observations, is probably derived from Mercury's surface soils or impacting meteoroids and ejected into the atmosphere either by the impacts or by bombardment of solar wind particles. The atmospheric gases tend to accumulate on Mercury's nightside but are dissipated by the brilliant morning sunlight.
Many atoms in Mercury's surface rocks and in its tenuous atmosphere become ionized when struck by energetic particles in the solar wind and in Mercury's magnetosphere. Unlike Mariner 10, the Messenger spacecraft has instruments that can measure ions. During Messenger's first flyby of Mercury in 2008, many ions were identified, including those of oxygen, sodium, magnesium, potassium, calcium, and sulfur. In addition, another instrument mapped Mercury's long cometlike tail, which is prominently visible in the spectral emission lines of sodium.
Although the measured abundances of sodium and potassium are extremely low—from hundreds to a few tens of thousands of atoms per cubic centimetre near the surface—telescopic spectral instruments are very sensitive to these two elements, and astronomers can watch thicker patches of these gases move across Mercury's disk and through its neighbourhood in space. Presumably many other gases that are harder to detect are present in similar minuscule quantities. Where these gases come from and go was primarily of theoretical, rather than practical, importance until the early 1990s. At that time Earth-based radar made the remarkable discovery of patches of highly radar-reflective materials at the poles, apparently only in permanently shadowed regions of deep, near-polar craters. Scientists believe that the reflecting material might be water ice.
The idea that the planet nearest the Sun might harbour significant deposits of water ice originally seemed bizarre. Yet, Mercury must have accumulated water over its history—for example, from impacting comets. Water ice on Mercury's broiling surface will immediately turn to vapour (sublime), and the individual water molecules will hop, in random directions, along ballistic trajectories. The odds are very poor that a water molecule will strike another atom in Mercury's atmosphere, although there is some chance that it will be dissociated by the bright sunlight. Calculations suggest that after many hops, perhaps 1 out of 10 water molecules eventually lands in a deep polar depression. Because Mercury's rotational axis is essentially perpendicular to the plane of its orbit, sunlight is always nearly horizontal at the poles. Under such conditions the bottoms of deep depressions would remain in permanent shadow and provide cold traps that could hold water molecules for millions or billions of years. Gradually a polar ice deposit would build up. The susceptibility of the ice to subliming away slowly—e.g., from the slight warmth of sunlight reflected from distant mountains or crater rims—could be reduced if it gradually became cloaked by an insulating debris layer, or regolith, made of dust and rock fragments ejected from distant impacts. Radar data suggest that the reflecting layer indeed is covered with as much as 0.5 metre (1.6 feet) of such debris.
It is far from certain that the volatile material near Mercury's poles is water ice. Additional radar studies found small patches of high reflectivity at latitudes as low as 71°, where water ice would be far less likely to form and survive. Moreover, the same reasoning about the possibility of water ice near Mercury's poles also has been applied to the Moon, where the accumulation process should have been even more robust. In its lunar orbital mission in 1998-99, the Lunar Prospector spacecraft found evidence for, at most, minimal water ice near the lunar poles. Perhaps another easily evaporated substance, but one less volatile than water, has been "cold-trapped" on Mercury. One candidate, atomic sulfur, is fairly abundant in the cosmos and, for other reasons, may be especially abundant on and within Mercury.
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