Shackleton is an impact crater that lies at the lunar south pole. The peaks along the crater's rim are exposed to almost continual sunlight, while the interior is perpetually in shadow. The low-temperature interior of this crater functions as a cold trap that may capture and freeze volatiles such as water shed during comet impacts on the Moon. Measurements by the Lunar Prospector spacecraft showed higher than normal amounts of hydrogen within the crater, which may indicate the presence of water ice. The crater is named after Antarctic explorer Ernest Shackleton.
Geography The Moon's south pole passes through Shackleton's crater rim. The crater is conical in shape, measuring 21 km (13 miles) in diameter and 4.2 km (2.6 miles) deep. Its interior is marked by several smaller craters hundreds of meters in diameter, and its floor is hilly. The rim is slightly raised about the surrounding surface and it has an outer rampart that has been only lightly impacted. No significant craters intersect the rim, and it is sloped about 1.5° toward the direction 50–90° from the Earth. From the Earth, it is viewed edge-on in a region of rough, cratered terrain. It is located near the rim of the ancient South Pole–Aitken basin on the edge of a mountain. Because the orbit of the Moon is tilted only 1.5° from the ecliptic, the interior of this crater lies in perpetual darkness. Estimates of the area in permanent shadow were obtained from Earth-based radar studies. Peaks along the rim of the crater are almost continually illuminated by sunlight, spending about 80–90% of each lunar orbit exposed to the Sun. Continuously illuminated mountains have been termed peaks of eternal light and have been predicted to exist since the early nineteenth century. The shadowed portion of the crater was imaged with the Terrain Camera of the Japanese SELENE spacecraft using the illumination of sunlight reflected off the rim. The interior of the crater consists of a symmetrical 30° slope that leads down to a 6.6 km (4.1 miles) diameter floor. The handful of craters along the interior span no more than a few hundred meters. The bottom is covered by an uneven mound-like feature that is 300 to 400 m (980–1,310 ft) thick. The central peak is about 200 m (660 ft) in height.
Naming This crater is named after Ernest Shackleton, an Anglo-Irish explorer of Antarctica from 1901 until his death in 1922. The name was officially adopted by the International Astronomical Union in 1994. Nearby craters of note include Shoemaker, Haworth, de Gerlache, Sverdrup, Slater, and Faustini. Somewhat farther away, on the eastern hemisphere of the lunar near side, are the larger craters Amundsen and Scott, named after two other early explorers of the Antarctic continent.
Geology Shackleton is a well-preserved simple crater in a region of ancient, rugged pre-Nectarian terrain associated with the South Pole–Aitken basin rim. The South Pole–Aitken basin is estimated to have formed 3.9 to 4.3 billion years ago, excavating large portions of the ancient lunar crust and potentially some of the upper mantle. Shackleton is expected to lie in a region made of ancient noritic crust. Its formative impact event exposed ancient material, including suspected deposits of purest anorthosite (>98% plagioclase). Due to its structural freshness, Shackleton was originally thought to have formed about 1.1 to 3.3 billion years ago in the Moon's Eratosthenian period. Later analysis using data from the Lunar Orbiter Laser Altimeter places Shackleton's formation 3.69 billion years ago, during the Moon's Imbrian period. It is several hundreds of millions of years younger than some neighboring craters. Simulations show that it was likely formed by a chondritic impactor around 1.5 km (0.93 mi) in size, striking the surface with a vertical velocity of 15 km/s (9.3 mi/s). After impact, at least 20 cubic kilometres (4.8 mi3) of impact melt gathered at the crater's floor. Its uneven floor may be due to large blocks of rock, central peaks or rings, or from landslides.
Water ice Shackleton has been in the proximity of the south lunar pole for at least the last two billion years. The continuous shadows in the south polar craters cause the floors of these formations to maintain a temperature that never exceeds about 100 K (−173 °C; −280 °F). For Shackleton, the average temperature was determined to be about 90 K (−183 °C; −298 °F), reaching 88 K at the crater floor. Under these conditions, the estimated rate of loss from any ice in the interior would be 10−26 to 10−27 m/s. Any water vapor that arrives here following a cometary impact on the Moon would lie permanently frozen on or below the surface. However, the surface albedo of the crater floor matches the lunar far-side, suggesting that there is no exposed surface ice.
Exploration
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