Ejecta

• 1. Introduction

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  Ejecta are particles thrown out of an area, and their absence once helped settle a debate about a feature on Mars called Eden Patera. In the 2010s, scientists began to suspect that Eden Patera was a collapsed volcanic caldera and not an impact crater. One of their reasons was a gap: the debris that an impact should have scattered around it was missing. That raises a curious puzzle. How can material that is not there count as evidence? To answer it, the documentary follows ejecta across very different settings. It asks what forces fling matter outward, what that matter is made of, and what it reveals about the place it left. The word applies to volcanoes on Earth, to craters on distant moons, to dying stars, and even to human experiments.

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• 2. Trapped Pressure

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  Inside an explosive eruption, large amounts of gas are dissolved in lava that is extremely viscous. As this lava froths toward the surface, the trapped pressure drives the material out rapidly. Sometimes the vent itself becomes the problem. Lava that solidifies inside a volcano's vent can form a lava plug, or a volcanic neck, which seals the opening. Heat and pressure then build to an extreme, with no way to escape. Eventually the blockage can no longer hold, and it breaks. The result is a more violent eruption that throws material out of the volcano. Ejecta from a volcanic vent or crater may travel through the air or through water. It then falls back onto the ground surface or onto the ocean floor. What exactly comes down depends on where each fragment began.

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• 3. Three Kinds of Debris

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  Volcanologists sort ejecta by origin. Juvenile particles are the freshest: fragmented magma and free crystals. Cognate or accessory particles are older volcanic rocks that belong to the same volcano. Accidental particles come from the rocks lying beneath the volcano, torn loose and carried upward by the blast. Together, these pyroclastic materials are known as tephra. Size varies widely. At the fine end sits ash. Next come lapilli, whose name refers to little stones. At the heavy end are volcanic bombs, which are large enough that the source gives their cutoff as a size above 2.5 inches. The same idea of thrown-out debris also applies far from any volcano, where the cause is a collision.

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• 4. Impact Blankets

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  In planetary geology, the term ejecta covers the debris thrown out when an impact crater forms. When an object that is massive enough strikes another with enough force, a shock wave spreads out from the point of impact. The object breaks apart and excavates the ground and rock. At the same moment, it sprays material outward. This debris lands outward from the crater's rim as loose material or as a blanket that thins toward its outer edges. If enough accumulates, it forms an ejecta blanket, full of dust and debris from the original impact. Researchers read these blankets closely. Their composition reveals the geology of the surface that was hit and of the projectile that hit it. Their distribution and shape hint at the impact angle and how the debris settled. The size of the crater together with its blanket can indicate how large and intense the impacting object was. On Earth, analyzing these blankets can even identify the source location of the impact. Scientists also classify ejecta features by distance from the crater, appearance, and the terrain beneath.

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• 5. Rays and Rings

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  Look at the Moon or Mercury and you see radial ejecta patterns, where debris spreads outward from a crater in rays or streaks. These rays are often more prominent on solid surfaces like those two worlds. Icy surfaces tell a different story. On the moons of Jupiter and Saturn, ejecta often form concentric patterns, with multiple circular layers around the crater. Such rings are commonly seen there and are taken as a sign of subsurface volatiles, like water or other ices. A third common feature is the secondary crater, another mark left where thrown material lands. Impacts are only one cause of outward-flung matter, though. Some of the largest releases of material happen in the sky.

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• 6. Beyond the Ground

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  In astrophysics and heliophysics, ejecta means material expelled in a stellar explosion. A supernova is one example. A coronal mass ejection, or CME, is another. The scale is vastly larger than a volcanic vent, yet the word still describes matter blasted outward. Humans have created ejecta of their own. People launch material into space with a range of launch systems. Some events, particularly nuclear ones, produce artificial ejecta as well. The best-known case is the Pascal-B test, which might have ejected an object into space at a speed exceeding Earth's escape velocity.

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