From the right angle, construction workers at the National September 11 Memorial and Museum at New York's ground zero can make out a sad-looking face that's been dubbed the "Angel of 9/11," seemingly emerging from the twisted steel of a girder that was hit by the first hijacked plane. The facelike feature is the result of natural corrosion plus a trick of the eye — but that shouldn't make the angel any less inspiring.
The angel's dark eyes, nose and open mouth stem from a combination of factors, starting with the way the steel was twisted when American Airlines Flight 11 slammed into it on the morning of Sept. 11, 2001.
After the attack, salts, oxides and moisture on the steel's surface affected how the girder corroded, said P. Chris Pistorius, a materials scientist at Carnegie Mellon University who is co-director of the Center for Iron and Steelmaking Research. Air circulation around the ruined steel played a role as well.
"Atmospheric corrosion is very sensitive to microclimates," he told NBC News. "It's actually difficult to get even corrosion of such a surface. It's more likely to get a pattern than to get uniform corrosion."
If layers of steel are lying on top of each other, as appears to have been the case with the 9/11 girder, moisture can "wick in different areas and leave all kinds of different patterns," said Thomas Eagar, a professor of materials engineering at the Massachusetts Institute of Technology.
Pareidolia at work
The pattern of twisted, corroded steel looks like a face in large part because of a perceptual phenomenon known as pareidolia, a term derived from the Greek words for "mistaken image." It's the same phenomenon that's behind the Face on Mars, New Hampshire's Old Man of the Mountain, the Virgin Mary cheese sandwich and the "cloud angel" that was spotted over Florida when Pope Francis was chosen.
Our brains are well-suited to finding patterns even in seemingly random patches of light and dark — and that's how splotches of rust gave rise to the Angel of 9/11.
The perspective and lighting angles have to be just right, but some of the workers at the 9/11 Memorial and Museum have marveled over the sight. "You can see the face clear as day, as if it’s looking down over a sacred spot," one visitor who toured the exhibit in advance of its public opening was quoted as saying in The Sun, a British tabloid.
"People often find meaning in tragedy," a spokesperson for the memorial museum told NBC News. "For us, this piece of steel is historically important to include in the museum, as it helps to tell the story of 9/11."
After looking at a photo showing the angel, Pawan Sinha, a neuroscientist at MIT who specializes in visual recognition, agreed that the face is a "compelling example" of pareidolia at work. The effect reminded him of Auguste Rodin's famous Gates of Hell — a sculpture at the Kunsthaus in Zürich that depict faces of the damned emerging from the background.
"Even without being cued to the right location, I could instantly see the face," Sinha told NBC News. "If one thinks of this as sculpture, it's almost as if the sculptor has put in this three-dimensional structure."
Some artists, such as sculptor Richard Serra, take advantage of the way that metal naturally weathers when it's exposed to the elements. But Pistorius said that as long as the girder is being kept under climate-controlled conditions, the corrosive process should be held in abeyance — ensuring that the angel will endure. You can see for yourself when the museum opens to the public next year.
More about remembering 9/11:
- PhotoBlog: Museum balances grief with remembrance
- How a supertower rose above New York's 9/11 nightmare
- Americans pause to remember day 'like none other'
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.