The image below is the ‘Blue Marble’ photo of Earth. Purported to be taken from space, this was believed to be what Earth looked like from an astronaut’s point of view. Turns out that isn’t quite accurate.
It’s Earth alright. But it’s actually about 10,000 photos digitally stitched together to be more like what NASA thinks Earth looks like. The image’s creator, Robert Simmon, has just unveiled some little-known bits of information about the image and a personal anecdote about how he felt when he saw the Blue Marble featured on the start-up iPhone image.
Unlike Apollo 17′s photograph of the whole Earth, NASA’s Blue Marble is not a real photograph. It’s a composite made of many other images. A synthetic, but accurate and deliciously crispy representation of our home planet made by Robert Simmon:
In 2002 my colleague Reto Stöckli (now back in Switzerland) was working on a global map of the Earth that was going to blow away all previous examples. A new NASA satellite (Terra) was gathering the first color pictures of the entire Earth, and we wanted to showcase the imagery. Reto put together about 10,000 satellite scenes (each file over 300 MB) collected over 100 days, stripped out the clouds, and created a 43,200-pixel by 21,600-pixel map of the Earth in (this was the hard part, everything I did afterwards was just adding chrome).
To make the Earth look realistic, or at least how I imagined the Earth would look, I needed to do some work. First of all, the satellite images weren’t usable over deep water (it collects data, but there’s no automated process to detect clouds and correct for the atmosphere), so I needed to add some color into the water. NASA measures chlorophyll in the ocean (a way of monitoring phytoplankton), so I grabbed a month’s worth of that data, colored it blue and green (I looked at individual satellite images to get a sense of what hues to use), and used that map for the ocean. I also had to add a stand-in for sea ice, since it’s impossible to measure chlorophyll beneath a few meters of snow and ice. At least that was simple–I just replaced missing data near the poles with white. In addition to the sea ice, I brightened and reduced the saturation of Antarctica, which was pasted into the original from a different dataset. The combined ocean color and ice look like this:
One of the best surprises of my life was turning on my brand-new iPhone-before it had even been activated-glancing down at the screen, and seeing an image I had made. Apple chose the NASA Blue Marble for the default welcome screen and wallpaper, and I had no idea beforehand.
Simmon created the image using a 43,200-pixel by 21,600-pixel map of the Earth stitched together by Reto Stöckli. Stöckli used about ten thousand 300-megabyte satellite scenes captured by the Terra satellite over a period of 100 days. (Back then, Terra was NASA’s latest Earth-monitoring satellite.) Stöckli took out all the clouds and left that huge image clean. Then Simmon came in and added some details: “To make the Earth look realistic, or at least how I imagined the Earth would look, I needed to do some work.”
He used NASA’s data on chlorophyll levels in the ocean—which is “a way of monitoring phytoplankton”—and textured Earth’s water according to it. At the same time, he added the ice to the base texture, resulting in the image above.
Then he created a “map of clouds stitched together from 200 satellite scenes” as well as a topographic map to add elevation to our planet’s land masses. He entered all these textures into Electric Image, a classic Mac-based 3D software that was popular at the time, and the magic started to take form. (Ultra-nerd bonus: Electric Image was used by Industrial Light & Magic’s John Knoll to create the space battle scenes in the original Star Wars trilogy remake too.)
There he mapped them onto a sphere, rendering separate images for everything: “land and ocean, specular highlight, clouds, a couple day/night masks, and atmospheric haze”.
A Few Notes
All the source files are archived on the original Blue Marble page. They’re free to use and modify, but you can’t use them to imply you’re associated with NASA.
NASA subsequently made a new and improved version of the base maps, the Blue Marble Next Generation. It’s not only twice the resolution (86,400 pixels by 43,200 pixels), but there’s a separate image for each month, so you can see the changing seasons. Reto didn’t make new clouds because it’s a really long and painful process of stitching images together by hand that’s never going to be perfect.
In the existing cloud map some people have noticed a few repeating features that appear photoshopped. They are. There are gaps between orbits near the equator, and there’s no way to fill them with real data. The specular highlight off Baja and the thickness and fuzziness of the atmosphere. There’s a weird streak in the clouds near Greenland that’s entirely due to an error on my part, and Simmons has no idea why the shading on the east coast of Greenland is incorrect.
Yes, it’s centered on North America: Simmons says he has spent the vast majority of his life there and is biased. He did, however, make a version centered on South Asia at the same time, as well as a rotating Earth centered on the Equator. He subsequently has created a few more versions, including the Pacific Ocean.
