The Lunascan Project

 A small, relatively young crater, 2.4 km in diameter, 600 m deep. It is surrounded by bright ejecta
material. Under high-illumination it appears through the telescope as a bright spot. In the
literature, since the second half of the nineteenth century, numerous mysterious changes and
disappearances of Linne have been recorded. These are classic examples of observing errors which
occur when lunar details are close to the limit of resolution of a telescope. (Source: Atlas of the
Moon, Antonin Rukl,  1990)

The resurgence of serious interest in the moon that followed World War II focused attention on the problem of "outgassing," the release of gas from the lunar interior. One of the strangest cases involves the crater Linne'.

Linne' was discovered by Riccioli during the seventeenth century; Lohrman, Madler, and Schmidt all observed it as a crater. Schmidt drew it as a crater in eight of eleven drawings he made between 1840 and 1843. In 1866, however, he made the spectacular announcement that the crater no longer remained and that only the bright mound was observable. During 1867 numerous observers could find only the mound. Late in that year Schmidt announced that he could discern a mountain in the center of the mound. During 1868 Knott, Buckingham, and Key observed a shallow depression at the center. Later a craterlet was detected by Secchi, who estimated its diameter at barely half a mile. Still later Huggins measured its diameter as two miles. Since that time Linne' has been observed, when near the terminator, more or less easily through large telescopes.

In a rather recent program hundreds of pairs of photographs have been made of the lunar surface. The two plates of each pair were exposed with a minimum time interval between them. One of the two plates used only violet and blue light, and the other used infrared light almost exclusively.

                         Plate 10-15                                                        Plate 10-16
Plate 10-15. Mt. Wilson & Palomar Observatories, 1958 July 7d 10h 32m UT, phase 20.11 days, colongitude 161-degrees. Right hand part of Mare Serentitatis and the crater pit on Linne. Plate 10-16. Made one minute later using Kodak II-0 plate sensative to blue-green and shorter wavelenghts. (Source: Pictorial Guide to the Moon, Dinsmore Alter,  1973)
The Linne' mound has been photographed on some of these dual plates. An infrared exposure is reproduced as Plate 10-15, and its blue-violet mate as Plate 10-16. Both show the surface of the right part of Mare Serenitatis nicely, but the infrared plate is distinctly the better. Because the moon has no appreciable atmosphere, this difference in over-all clarity must be due to the terrestrial atmosphere; the passage of the light through the lower air of the Earth absorbs and scatters the shorter violet waves more than it does the longer red ones.

The craterlet on the Linne mound shows sharply in the infrared (Plate 10-15), but is invisible in the blue-violet (Plate 10-16). The difference in Linne's appearance is greater than one would expect from examination of the rest of the plate, although it is not enough to make it absolutely certain that it is not merely an accidental effect from our own atmosphere. If the effect is in fact not due to the terrestrial atmosphere, it must be from something on the moon itself. The reasonable assumption in such a case is the existence of a slight haze between the craterlet and the observer. Such a haze might be either dust or gas, or both. The hypothesis of a dust cloud can no longer be considered favorably, but a very low density of gas leaking from the craterlet would be almost completely ionized by sunlight. The ionization makes it luminous and also increases its scattering effect on light passing through it, especially on the shorter wavelengths.

Indeed, a gas only one-billionth as dense as earth's sea-level atmosphere could produce a noticeable effect. It appears rather probable, but not certain, that there is such a residual outgassing from Linne'. The fact that such able nineteenth-century astronomers as Schmidt, Secchi, and Huggins believed that they had observed changes makes the evidence for such a conclusion much more convincing than if the current observations pertained merely to some previously unobserved craterlet. If the crater disappeared as described, there was much more of such out- gassing for several months in 1866 than has occurred since.

When photographed by Lunar Orbiter (Plate 10-16A), no obscuration of the craterpit of Linne' appeared. Residual outgassing might be a rare event, however, Hyginus rill and crater (Plate 15-1-B) have also seemed obscured on occasion, as have other features on the moon. All of these obscurations, as well as red flashes and patches, reported by many observers, are known as "transient lunar phenomena." Recently, some evidence has come in of such events in Alphonsus.


Plate 10-16A. Lunar Orbiter IV photograph of Linne crater from an altitude of only 2705 km, the best view of the crater ever made. Here it appears like a typical small crater, although observers have reported changes in its pit and shape and obscuration of its pit.  (Source: Pictorial Guide to the Moon, Dinsmore Alter,  1973)

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