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A Natural Land Bridge on The Moon

On the morning of July 29, 1953, John J. O'Neill, science editor of the New York Herald Tribune turned his telescope, a 4-inch refractor, towards the moon and began studying the western rim of Mare Crisium, a vast oval-shaped plain more than five hundred kilometers in diameter with a flat floor and a ring of wrinkle ridges around its boundaries. The low sun hit the region’s lofty peaks and exaggerated their heights and at the same time created tiny islands of light in a black sea of shadow. But O'Neill’s attention was drawn towards another sliver of light, that appeared to breach through a tiny gap between the tips of two lunar headlands. After studying the feature for some time, O’Neill was convinced it was a gigantic natural bridge, arching over a gap like one of the stony marvels in the American Southwest. Except it was bigger than anything Earth had ever seen. O'Neill estimated the land bridge to be about 19 kilometer in length. For comparison, the longest natural arch on earth is just over 300 feet, or less than 100 meters.

lunar land bridge

Drawing made by H.P. Wilkins of O'Neill’s Lunar Bridge (left) and 60 years later NASA’s Lunar Orbiter imaged the same area. Photo credit: LPOD

News of this sensational discovery soon reached other astronomers, and conformations, both from amateur observers as well as professionals, began to trickle in. The most prominent among these was Hugh Percy Wilkins, director of the British Astronomical Association's (BAA) Lunar Section, who used a 15-inch Newtonian reflector to verify the arch's existence. Wilkins claimed to have seen not only the arch with sunlight streaming beneath it, but also the bridge's shadow on the surrounding lunar terrain. In a radio interview to BBC later that year, Wilkins blurted out: “…there’s no mistake at all. It’s been confirmed by other observers. It looks artificial…it looks almost like an engineering job…it casts a shadow under a low sun angle and you can see the sunlight streaming in beneath it.”

The director of a prestigious institution of astronomy remarking that the bridge was artificial caused many eyebrows to be raised. It was a thoughtless remark casually thrown, which was exaggerated by the media to such disproportion that Wilkin’s reputation within the BAA was damaged beyond repair. Later Wilkins observed ruefully: “My experience has been that it does not matter how carefully you explain things to most reporters, they always get things wrong.”

But the damage was already done. The story became fodder for UFO buffs and conspiracy theorist. In the 1955 book The Flying Saucer Conspiracy, Major Donald E. Keyhoe described O'Neill's discovery as a “huge structure appeared sharply in outline, an unbelievable engineering marvel apparently erected in weeks, perhaps in days.” Keyhoe added that there was so much more to the story that “not even O'Neill dared to tell” the truth.

In the end, O'Neill’s bridge, as it was called, turned out to be a case of mistaken identify, an optical illusion played by light and shadow. But the question lingered. Can such a natural bridge exist on the moon—bridges that take million of years of wind and water erosion to form, of which there is no appreciable amount on our earth’s satellite? The answer was delivered by NASA’s Lunar Reconnaissance Orbiter in 2010.

lunar land bridge

The LRO spotted not one but two natural arches on the lunar surface, and this time there was no ambiguity in observation. If you look at the image (above) closely, you can see a little crescent of light that came from under the bridge.

This bridge is about 20 meters (66 feet) long, and 7 meters (23 feet) wide on top and perhaps 9 meters (30 feet) on the bottom side. The land bridge is located on the rim of the King crater, which is about 72 kilometers across. The rim itself is about 15 kilometers wide.

lunar land bridge

Left shows the bridge when the Sun is 42 degrees above the horizon and the right is the same area when the Sun is 80 degrees above the horizon (near noon).

According to NASA’s interpretation of the satellite's photograph, when the asteroid (that created the crater) slammed into the lunar surface, the heat generated by the violent impact melted the lunar rocks and the ejecta collected on the crater’s rim in several tens of meters thick. As the molten ejecta cooled it formed a hardened crust, but the magma underneath the cooled exterior was still liquid. At some point before the whole ejecta hardened, the magma found a way out, perhaps by a breach in the crust, and it drained out to leave a void, like a lava tube. The crusted roof then partially collapsed to form a bridge. At least that’s how NASA’s scientists see it. We don’t know for sure, not until we get some more close-up shots of the feature, possibly from a rover or something. At the time the news was reported, Dr. Mark Robinson, the Principal Investigator for the imaging system on the Lunar Reconnaissance Orbiter, wrote that the LROC team was processing stereo images into topographic maps to aid scientists in determining exactly what took place on this fascinating region.

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