Австралийн Кимберлигийн цөлд илэрсэн эртний солирын тогооны нууцыг Landsat 9 хиймэл дагуул тайлав

On Sept. 20, Landsat 9 passed over the Kimberley region of Western Australia and captured a strange sight in the desert: something resembling a giant spider, its long, crooked legs spread across the orange and tan landscape. At the middle sits a pale, broken-looking mass. The legs are sandstone ridges, and the body is the fractured core of an ancient impact crater.

Known as Spider Crater, the oval formation stretches roughly 13 by 11 kilometers (8 by 7 miles), although the spider-shaped portion at its center spans only about 4 kilometers (2.5 miles). The impact itself is thought to have occurred somewhere between 900 million and 600 million years ago, judging from the rocks in the surrounding region.

What survives is the crater’s central uplift, a section of the floor that sprang upward in the moments after the collision. Around it, tougher sandstone beds resisted erosion as the surrounding material gradually disappeared.

But there is something odd about the damage left behind. The most severely shocked rocks are not at the center.

The Spider Was Noticed Around 1960, and Some Geologists Blamed a Volcano

Geologist J. E. Harms recognized the unusual formation around 1960, according to a 2013 conference abstract by Thomas Kenkmann and Michael Poelchau of the University of Freiburg. Getting to Spider Crater is no simple matter, even in the sparsely populated Kimberley. The nearest infrastructure is the Gibb River Road and the Mount Barnett Roadhouse, and access is restricted because the formation sits on a pastoral lease within a declared Aboriginal homeland.

For a time, a volcanic explanation remained on the table: an underground explosion that had somehow left no lava at the surface.

Then geologists began finding shatter cones. These distinctive cone-shaped fractures, marked by radiating, fan-like grooves, form when an exceptionally powerful shock wave tears through rock. They are a telltale sign of an impact. “Shatter cones are ubiquitously present in the central dome,” Kenkmann and Poelchau wrote.

By 1980, Harms and his colleagues had published their case in Nature, pointing to the shatter cones, the raised central structure and sheets of rock shoved outward around it. The Impact Earth database, maintained at Western University, records shatter cones reaching roughly 50 cm (20 inches) in the central quartzite. Two other known impact structures, Goat Paddock and Wolf Creek, lie south of Spider.

At the very center is the Warton Sandstone, the oldest rock exposed at the site, dating back about 1.7 billion years. Shock damage extends through its quartz grains and into the mineral cement binding them, evidence that the sandstone had already hardened completely when the object struck.

The Strongest Shock Sits Away From the Crater’s Center

The impact origin may be established, but Spider’s peculiar geometry has proved harder to explain. For years, the prevailing interpretation was that an object came in obliquely from the north or northwest, accounting for rock slabs that tilt toward the north.

Researcher Alex Abels challenged that view, suggesting the terrain before the collision, together with a broad, gently inclined fold in the underlying rock, might explain the asymmetry without requiring the same impact trajectory.

In 2007, Kenkmann and Poelchau went into the central uplift to map its structure. They identified two overlapping stacks of rock slabs, arranged rather like roof tiles around the core. Individual beds leaned at angles ranging from 20 to 80 degrees.

To account for that arrangement, the researchers developed a simple model involving two forces working together: the crater floor collapsing inward and the impactor pushing sideways as it arrived at an angle. The result was a set of curved slabs much like those preserved at Spider.

A 2021 study in Meteoritics & Planetary Science tackled the same problem from an entirely different scale. Rather than mapping large slabs, the researchers studied 453 grains of zircon, a particularly durable mineral capable of preserving shock damage, collected from 12 samples of shatter-cone quartzite and breccia, the latter consisting of broken rock fragments.

The distribution was anything but uniform. Shocked zircons appeared unevenly around the crater’s center, a pattern the team described as the first mineral-based evidence of its kind for asymmetric shock associated with an oblique impact. Quartz examined from those same rocks, however, showed no comparable distribution.

The authors noted that the most intense observed shock deformation did not line up with the geographic center of the structure. NASA says analyses of this kind can help researchers work backward to determine the angle at which an impactor arrived.

Erosion Has Worn Spider Crater Down Below Its Original Floor

Precisely when the collision happened remains unresolved. No rocks from Spider have yielded a direct impact age, so the estimated date comes instead from the depth of regional erosion and the relative ages of surrounding geological layers. NASA describes the formation as “a creature of the late Precambrian.”

According to the Impact Earth database, geological evidence points to at least 1 kilometer of erosion across the region, and the ridges visible today may represent only the inner portion of what was once a much larger central uplift.

Without that enormous loss of rock, the spider might never have emerged. Softer material gradually crumbled and washed away, exposing the more resistant sandstone beds as long, raised ridges. From above, they look remarkably like legs. Landsat 9’s Operational Land Imager recorded the formation during a single pass, producing the image that served as the answer to NASA’s October satellite puzzler.

Spider is not the only ancient impact crater whose outline can still be picked out from orbit.

Among the geologists who visited and mapped the structure during fieldwork in 1984 and 1985 were Gene Shoemaker and his wife, Carolyn. Years later, on July 18, 1997, Gene Shoemaker was killed in a car crash near Alice Springs while the couple were studying Australian impact craters, and some of his ashes were subsequently carried to the Moon aboard NASA’s Lunar Prospector spacecraft.

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