Curiosity has been climbing Mount Sharp, the five-kilometer mound in the middle of Gale Crater, since 2014. In the spring of 2022 the rover was working its way across the Greenheugh Pediment, a sloping shelf of sandstone that formed from wind-blown dunes long after the lake sediments beneath it were laid down. On May 7, 2022, the mission’s sol 3,466, the Mast Camera took a routine set of frames of a rock mound the team had nicknamed East Cliffs. One of them would become the most shared Mars image of the year.
What happened
On May 11 a user posted a crop of the frame to the r/interestingasfuck forum on Reddit with the caption that it showed a formation resembling a portal and, nearby, a wall that looked artificial. The crop was black and white, grainy and enlarged many times, and in it a dark rectangle with straight sides and a flat lintel sat in a cliff face like the entrance to a tomb. Within a day the picture was on newspaper front pages and television bulletins from Chile to New Zealand under headlines about an alien doorway. Commenters argued over who had cut it, what lay behind it and why NASA was not driving in.
The size was the first casualty of the crop. Nothing in the frame gave a scale, and the eye assumed a door built for people. In fact the opening is tiny. A JPL spokesperson told Snopes the same day that the frame was a very zoomed-in shot of a small crevice, around 30 centimeters wide and 45 across, and that it lay where several linear fractures running through the outcrop happened to intersect. NASA’s own caption, released later that month with a wider mosaic as image PIA25288, gave the opening as about 30 centimeters tall and 40 wide, similar in size to a dog door, and called it a naturally occurring open fracture of a kind common in bedrock on Earth and Mars.
The investigation
Ashwin Vasavada, the project scientist for the Mars Science Laboratory mission, explained the geology to Gizmodo on May 13. The rover had been crossing terrain that began as sand dunes, cemented over time into sandstone and then buried and unburied as the surface shifted, which cracked it. The fractures in that area run mostly vertical. The doorway, he said, was simply the space between two of them: either a pair of vertical cracks from which the middle block had fallen out, or a single crack whose two sides had moved slightly apart. He put the height at about a foot. A full-resolution mosaic assembled by an amateur from the public raw images shows the same mound riddled with similar holes and shed blocks, with loose rock lying on the slope below.
The InSight lander, far to the north in Elysium Planitia, had recorded a magnitude 5 marsquake on May 4, the largest ever measured on another planet, and NASA announced it on May 9. Reddit’s more sober contributors noticed the timing and suggested the doorway was a fresh shear fracture, a break caused by strain in the rock that the quake might have helped along. It was a more reasonable idea than aliens, and some outlets ran it as the explanation, but NASA never connected the two events. Without an earlier picture of that face of East Cliffs there is no way to say whether the gap opened in May 2022 or ten thousand years ago, and the surrounding outcrop is full of older breaks of the same kind.
The explanations
Snopes rated the Reddit claim Miscaptioned, and PolitiFact rated a Facebook version False, quoting JPL’s Andrew Good that the team had seen nothing that was not easily explained. The mission’s own Curiosity account answered the public on May 18 with the word scientists use for the phenomenon: pareidolia, the mind’s habit of reading faces, animals and architecture into random shapes. Mars has supplied a long series of them, from the Cydonia face photographed by Viking 1 in 1976 to the spoon, the squirrel and the cannonball found in rover frames, and each has dissolved when a better image arrived. The doorway’s straight edges are the ordinary result of a rock that breaks along flat bedding planes and flat joints at right angles to them.
Where it stands
Curiosity drove on. The East Cliffs opening was never a candidate for a closer look, and the rover has since climbed well above the pediment. The frame remains a favorite of social media accounts that circulate it without the scale bar, and it periodically resurfaces as new. The answer has not changed: it is a crack in a sandstone outcrop about the size of a cat flap, photographed from several meters away and blown up until it looked like a door.