A collapsed volcanic feature on the Moon

The caldera of Hyginus.

Seen here is a caldera, a rare type of lunar crater. The caldera of Hyginus and the accompanying rilles (channels) on both sides are actually collapsed volcanic features.

Hyginus crater (caldera) at the center, and the rille Rima Hyginus, as captured by NASA's Lunar Orbiter 3. Credit: NASA / James Stuby

The craters that we are used to seeing on the Moon are formed by asteroid/cometary impacts. But a caldera forms when magma from the lunar interior rises above and tries to escape through the lunar crust but fails. The introduction of magma beneath the crust softens the structural foundation on which the crust is resting. This results in a surface collapse, leaving behind a hollow pot like crater—a caldera. See GIF below for visualization.

An analogue experiment showing the formation of a collapsed volcanic feature called caldera. The box filled with flour hides the balloon attached to a tube, which begins to inflate the air. Once it stops inflating, the air begins to escape due to gravity and starts to drop its elevated flour, which illustrates the process of filling and emptying of magma, and the subsequent drop making the caldera. Credit: Chmee2

A paper explaining the origin of Hyginus caldera also says the attached linear rilles formed as collapse features. End-to-end, the collapsed feature runs for about 220 kilometers. The linear rille is similar to the previously covered Rima Ariadaeus.

Hyginus caldera was a candidate landing site for the cancelled Apollo 19 mission. Future lunar missions to Hyginus will reveal important information about the dynamics of such collapse features and may give insight into elusive Ina.

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