Emily LakdawallaSep 30, 2015

The solar system at 1 kilometer per pixel: Can you identify these worlds? The answers

On Friday I posted an image containing 18 samples of terrain, all shown at the same scale. The squares are about 500 kilometers on a side, and when fully enlarged, the images have a resolution of 1 kilometer per pixel. There are 18 solid-surfaced worlds in our solar system that are large enough to include here and for which we have imagery of sufficient resolution: Mercury, Venus (though only in radar), Earth, the Moon, Mars, Ceres*, Io, Europa, Ganymede, Callisto, Tethys, Dione, Rhea, Titan (though only in infrared), Iapetus, Triton, Pluto, and Charon*. I starred Ceres and Charon because we're a bit limited for our images of those at the moment; the images I included in the composite actually only cover about 430 kilometers square.

Were you able to figure out which square was which? Below the image I'll include a labeled version with the answers.

The solar system at 1 kilometer per pixel
The solar system at 1 kilometer per pixel

Ready? Here you go. As a bonus, I've included information on the instruments and missions that captured each image.

The solar system at 1 kilometer per pixel (labeled)
The solar system at 1 kilometer per pixel (labeled)

Some of the worlds' identities can be deduced from features that occur nowhere else: Europa with its bands, Ganymede with its grooved terrain, the smooth lunar maria, the funny dark splotches in Mars' flat-floored craters. You won't find rivers dammed to make reservoirs anywhere but Earth. (This particular piece of Earth is entirely contained within Spain, in case you were wondering.) People who are experienced observers of spacecraft imagery might recognize the fuzziness of Cassini's Titan images, and the peculiar patterns of light and dark in Magellan radar images of Venus. I would really like to know whether I would be able to tell Jupiter's moon Io and planet Venus apart if we were able to shoot them with the same camera.

After that, it gets a bit harder. I might possibly have recognized Ithaca Chasma on Tethys, but I don't think I would have been able to tell Dione and Rhea apart. And I think I would have been thrown by Ceres. Look how flat-floored its craters are! They look as much like Ganymede's or Mars' as they do like Tethys' or Rhea's.

The choice of 1 kilometer per pixel and 500-kilometer squares was not made at random. It was a deliberate choice based on my knowledge of what imagery is available and the sizes of solar system worlds, including the very new additions of photos from New Horizons at Pluto and Charon. The vast majority of cameras that have been sent to other worlds have detectors that are square and have 800 or 1024 pixels on a side. I needed to pick a square smaller than that, so that I could have some flexibility in resizing images to match their scales. There's also a limitation imposed by the solar system, a break in the size distribution below a diameter of about 1000 kilometers. The next-largest object in the solar system after Ceres that we have visited with a spacecraft is Vesta, which is barely more than half the size of Ceres. You can comfortably fit a 500-kilometer square with room to spare on a 1000-kilometer circle, so the choice of scale was basically made for me. In the future I may well do another of these comparisons looking at a much smaller scale, but I'll be limited by the availability of high-resolution images for many of the worlds shown above; it will take some work to identify the resolution that will allow me to include the highest possible number of places.

To locate these images, I went to a variety of sources. For Mercury, I began with the global mosaic from the MESSENGER website, which is available for download at a variety of resolutions. For Venus, I turned to some of the Magellan images that I had already processed for The Planetary Society. For Earth, I went to NASA's Visible Earth image gallery and filtered it by MODIS. For the Moon, I knew I could find the right resolution of imagery from the Lunar Reconnaissance Orbiter's Wide-angle Camera global lunar mosaics. For Mars, 1 kilometer per pixel is a kind of tricky in-between resolution that we haven't seen much of since early days, but I remembered that Mars Express had recently released an unusually wide-angle view of Mars that I could grab a crop from. For Ceres, there's no formally released data yet, so the best source was JPL's Planetary Photojournal for press-released Dawn Ceres images. For the moons of Jupiter, Saturn, and Neptune, I turned to the trusty Outer Planets Unified Search tool hosted by the Rings Node of the Planetary Data System. They have a relatively new feature that dramatically eases the search among Cassini images: if you click on "Cassini Surface Geometry" at the left, and click on "Cassini Target Name" and then select a target, you can specify a minimum and maximum pixel resolution to refine your search. And for Pluto and Charon, I went to my own index of publicly released New Horizons images.

How did you do at guessing which world was which?

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