The article was originally published at The Conversation.
Professor of Planetary Geosciences at The Open University, David Rothery.
Enthusiasts lit up social media recently with images of a doorway into a hillside on Mars. Some wondered if it was evidence that the Red Planet was inhabited by aliens. The door was imaged by NASA's Curiosity rover on the slopes of Mount Sharp, the central massif within Gale crater, where it landed in 2012 Described on one website as a pharaonic tomb door, it is actually only about one foot high.
If you look at the individual frames below, you can see that the panoramic image mosaic of the hillside above is hard to spot. It looks like a doorway until you realize how small it is. The picture just about reveals a solid rock face at the back of the shadowed interior, if you boost the contrast in the dark parts of the image. It doesn't lead very far as a gateway into the hollow hills of Mars.
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There is a rocky gate to the ancient past on Mars.
Nobody with even a little geological experience is likely to mistake the feature for a door, and a geologist would immediately expect that they were looking at the eroded remains of hardened sandstone. In its gradual climb up through the layers of sedimentary rock, Mount Sharp was once covered by these.
When the weight of overlying rock layers is removed by erosion, the steep and fairly straight cracks running up the rock face are often referred to as "joints." There is a particularly obvious joint in the left of the image, but several others can be made out, including one that forms the smooth wall that lines up with the left side of the door itself. There is a joint on the right side of the feature.
There are photos of the face on Mars.
The hillside has been eroded back. The doorway is a place where the wind has been able to clean out the sand and dust from the rock face in an area that is close to the joints on either side. The base of an overlying bed of sandstone is the door lintel, and the sloping top of a bed of sandstone forms the gentle ramp that leads to the door.
It doesn't take much searching on the internet to find images taken by Mars rovers that show rock formations that look like other objects. Many of the rocks on Mars have been damaged by wind erosion for billions of years.
Apart from doors and hardware, there are also individual items. If seen from a different perspective, the others lose their distinctiveness.
Even when it isn't the familiar, there is a phenomenon called pareidolia. This shows what happens when you see faces in the random pattern of your wallpaper, or when you look out from the grain of wooden flooring. Jupiter looks angry in the image below because of the latter.
Seemingly mysterious objects don't always occur on Mars. The Chang'e 4 rover is still doing great things on the lunar far side more than three years after landing. It trundled towards it, and discovered it to be a boulder. I don't think anyone is saying that it was sculpted by aliens.
The Horsehead Nebula is one of the most famous examples of pareidolia. This is a huge cloud of gas and dust that is forming stellar systems. An image collected in the right part of the spectrum and with an appropriate exposure time shows a shape that most people would recognize as a horse's head. The shape will disappear if we shift the wavelength or look at it from a different direction.
Cat Rock, otherwise known as Sphinx Rock, is often seen by climbers on Great Gable, a mountain in Cumbria, U.K. The profile of the Sphinx's head can be seen on the side of the sitting cat. Everyone accepts the fact that this is a random occurrence and no one claims that it's proof that aliens have left clues in the landscape. People persist in making claims for rock formations on Mars.
Although you can generally believe your eyes, you should be cautious in believing your brain's interpretation.
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