APOD is such a favorite of mine that I use it as my home page. So criticizing it is a nip in that generous hand. Nevertheless, here is the text that goes with the above photo:
Credit: Viking Project, USGS, NASA
Explanation: The largest canyon in the Solar System cuts a wide swath across the face of Mars. Named Valles Marineris, the grand valley extends over 3,000 kilometers long, spans as much as 600 kilometers across, and delves as much as 8 kilometers deep. By comparison, the Earth's Grand Canyon in Arizona, USA is 800 kilometers long, 30 kilometers across, and 1.8 kilometers deep. The origin of the Valles Marineris remains unknown, although a leading hypothesis holds that it started as a crack billions of years ago as the planet cooled. Several geologic processes have been identified in the canyon. The above mosaic was created from over 100 images of Mars taken by Viking Orbiters in the 1970s.
I have always found this comparison infantile, especially when it is a guy (putatively) writing about Mars' canyon being bigger than our Canyon. Well, maybe juvenile. In any case, confusing and unenlightening, like comparing feces with snot.
(And wrong, too. HIS Grand Canyon is, in real units, 480 miles long, which is a stretch, and 18 across, although 12-15 is a more likely range, and mostly like 1.4 km deep. Was he using implants on the Canyon so we would not feel so inferior?)
And what is impressive about a canyon you cannot see? Not just because fewer people will get to visit Mars than get to run the Grand Canyon on their own. Look for instance at the ratio of depth to width (using apod numbers): On Mars, you have to look across 600 km for your 8 km down, or 1 down for 75 across; the Canyon is more like 1 down for 15 across. Now thats a gash. On Mars, well, look at it this way: If I put my head down on the curb out front (about 5"), and peer at the opposite curb 170" away, that is 1 down for ~37 across. TWICE as impressive as the Mars Big One, that barely visual bump.
Anyway, as everyone knows, of course canyons and mountains are bigger on Mars; the gravity is lower, so everything can swell up more.