Click for full image.
The photo to the right, released today by the science team of Lunar Reconnaissance Orbiter (LRO), takes a overhead view of the unusual crater dubbed Ryder (named after lunar scientist Graham Ryder).
The crater is located on the Moon’s far side, on the edge of the South Pole-Aitken Basin, the Moon’s largest and possibly oldest impact basin. What makes Ryder Crater intriguing is its strange shape, as well as its interior north-south interior ridge.
This crater was featured previously in 2012 in a spectacular oblique image looking east across the crater. Then, the scientists theorized its strange shape was caused by two factors, first that the impact was oblique, and second that it occurred on a steep slope.
Today’s release adds another factor that might explain the interior ridge. The context map below makes that explanation obvious.
Click for full image.
Ryder crater is in the center of the map, on the rim of the much larger crater to its west. As the scientists note at the link,
The steep ridge upon which Ryder formed is the degraded rim of an older crater that is ~70 km [about 44 miles] across, with a vertical drop of over 3000 meters (approximately 10,000 feet) from its rim to its floor. Even this older crater may have been affected by the uneven terrain on which it formed – its eastern rim appears to have formed on a topographic high perhaps related to the [South Pole-Aitken] basin, leaving it well above its western rim. Thus the impactor that formed Ryder crater struck the very highest point of the older crater’s rim, and in this case it is not hard to imagine that the final shape of Ryder crater was strongly affected by its lucky (or unlucky?) encounter with the extra steep slope. The high point of Ryder’s rim is approximately 1500 meters (5000 feet) higher than the low point of its rim!
Impacts tend to increase the density of the materials below, including the rim. When the Ryder impact occurred it was hitting the larger crater’s already squashed rim. The ridge thus appears to be the more resistant dense meterial of that rim, squashed further by the Ryder impact but dense enough already to survive the impact.
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