No, this is NOT a doorway for Martians. Although the internet broke out Thursday after a photo of NASA’s Curiosity rover appeared to show an “alien door,” experts are pretty sure it’s just a natural feature of the Martian landscape.
“This is a very curious picture,” British geologist Neil Hodgkins, who has studied the geology of Mars, told Live Science. “But in short: it seems to me natural erosion.”
Curiosity took the photo with its Mast camera (“Mastcam” for short) on May 7, and it was released by NASA later in the week.
Several colorized images were created from the original black-and-white photo, including a panorama created by stitching together several Curiosity photos, as seen on Gigapan.comthe website of a panoramic photography company.
Related: Seeing things on Mars: a history of Mars illusions
Several clues make it clear that the subject of the image isn’t an actual door: For starters, it’s less than 1 meter high, planetary geologist Nicholas Mangold of the University of Nantes in France told Live Science in an email.
Or maybe this shows that the Martians were small, he joked.
Other ironic suggestions from the internet were the idea that it is the space grave of Jesus; a crib for ET; or a save point for a video game, the Vice website reported†
But the real answer is it’s none of those things. Instead, what looks like a door is in fact a shallow opening in the rock almost certainly caused by natural forces, the experts say.
So if the door isn’t a door, what is it?
Hodgkins, a vice president at the British geoscience company seekerthinks the “door” is caused by erosion.
Rock layers called strata can be seen on the rock, dipping on the left and higher on the right. “These are silt beds, with harder sand beds that stand out,” he told Live Science in an email.
“They may have been deposited under sedimentary conditions 4 billion years ago, possibly in a river (I’d have to see more of the outcrop to be sure) or a wind-blown dune.”
Related: Is there water on Mars?
Martian winds have eroded the layers since they were exposed to the surface, and the images even show traces of them in the “door,” he said.
The image also shows several natural vertical fractures, including those caused by the way rocks on Mars weather; and the small cave or “door” appears to have formed where the vertical fractures intersect the layers, he said.
It appears that “a large boulder has fallen under its weight” to create the door-shaped cave, he said. †Gravity isn’t as strong on Mars, but it’s strong enough to do this.”
The culprit is the rock that lies on the surface just to the right of the “door”, which appears to have a smooth vertical edge – possibly because it fell relatively recently and has not been exposed to the Martian winds for a long time: it is “all very natural, and similar to outcrops you can see in many dry places on Soil,” he said.
Mangold, who studies geological data from the rovers Curiosity and Perseverance, agrees that Mars’ “door” was created naturally by the structure of the rock.
“These are splits in two directions, creating an ‘open box’ with the appearance of a door — nothing artificial,” Mangold said.
Internet speculation has raised the possibility that the small door-shaped cave may have been caused by a seismic “Marsquake” – two of the largest Marsquakes ever recordedhappened late in 2021, for example.
But Mangold is cool about the idea: “The whole mountain is seriously fractured, no major Marsquakes are needed,” he said. Instead, the fractures may have formed before the rock was exposed, due to the hydraulic pressure of water in the cracks; or they could be the result of thermal stress caused by the seasonal variations in temperature on the planet’s surface.
“It is indeed a very beautiful, broken rock,” said geologist Angelo Pio Rossi of Jacobs University in Bremen, Germany. Rossi has created panoramas of the outcrop based on successive photos from the Curiosity rover, and he, too, thinks the door-shaped cave was created by the visible fractures in the rock.
Part of his work is to find analogs on Earth for geological structures seen on Mars, and there are numerous similar structures here on our own planet, he said.
And Marsquakes probably had little to do with it: “Any block isolated by fractures can eventually fall down, even if the slope is gentle,” Rossi told Live Science in an email. “The faults themselves are not created directly by Marsquakes, but simply by deformation through geological time,” he said.
Hodgkins adds that the image illustrates how useful photos of the Mars rovers can be: “This is a really good image … it just shows what good geology we can do with the images coming back from Curiosity and Perseverance.”
Originally published on Live Science.