So fine, we'll do anniversaries. 40 years ago today the press on the NASA beat were shown the image that launched a thousand theories.
The image was actually acquired the previous day, 25 July 1976, from a range of 1873 kilometers (1162 miles).note 3 note 4 The Viking 1 orbiter was still in its site certification orbit, 1513 x 33,000 km. The image was designated 035A72, meaning the 72nd image taken on the 35th orbit.The resolution of the image was 48 m/px at the center. A second Viking image (070A13) was acquired on 30 July, 35 orbits later, with slighly improved resolution (44.7 m/px.)note 5. Viking Orbiters acquired sixteen more images of "Owen Mesa" but at much poorer resolutionsnote 6.
It was not to be until 5 April 1998 that the next image was acquired, by Mars Global Surveyor, and it was a massive disappointment. Even contrast-enhanced, it looked like this:
When Malin Space Science specialists used every enhancement trick they knew, they came up with this:
The resolution of that image is 4.3 m/px -- almost exactly ten times better than the best Viking shot. The problem was that haze and dust covered the area.note 7. The above image was notoriously dubbed "a catbox" by Art Bell during discussion with Richard Hoagland on Coast to Coast AM. I heard that show and I'll never forget Hoagland making an utter fool of himself by yelling "Somebody stole six gray levels!"
However, a better enhancement was carried out by Tim Parker of JPLnote 8, and that became the Astronomy Picture OTD on 7 April.
MGS went off and did real science for three years, but on 8 April 2001 another opportunity to shoot Owen Mesa arose, this time in clear conditions and from directly overhead. Hooray! With a lower orbit, the resolution improved to 1.56 m/px.
A full resolution version (5.3 MB) can be viewed here.
In April 2002, Mars Odyssey returned images from its polar oebit in both infra-red and visible light. The IR image had a resolution of 100 m/px, but the visible-light image was much better, 19 m/px.
Next up was the ESA's Mars Express. Initially frustrated by the same appalling weather as had bedeviled MGS, on 22 July 2006, during orbit #3253, the High Resolution Stereo Camera acquired enough color images at a resolution of 13.7 m/px for a 3-D compilation to be made.note 9. An animation was even released.
Until men land on Mars and explore, we're unlikely to see a better image than this one, acquired on 5 April 2007 by the HiRISE telescope on Mars Reconnaissance Orbiter at a range of 300 km.note 10. Its resolution is 25 cm/px -- the size of a small dinner plate. Click to see the real thing.
credit: MSSS/JPL/Univ. Ariz.
So, to sum up, we've gone from a resolution of 44.7 m/px to 0.25 m/px in these steps:
Viking 1976: 44.7
Mars Global Surveyor 1998: 4.3 but fogged
Mars Global Surveyor 2001: 1.56
Mars Odyssey 2002: 19
Mars Express 2006: 13.7 color, stereo
Mars Reconnaissance Orbiter 2007: 0.25
If this were a purposefully created artifact, it stands to reason that as resolution improved we would see and appreciate more and more detail of the workmanship. In fact, the reverse has happened -- we get more and more convinced that Owen Mesa is... well, just a mesa.
 Speaking of anniversaries, July 21 was the first anniversary of Hoagland's show. My personal prediction that it would sink without trace before Thanksgiving was way off.
 Kerry's blog, 15 July
 Viking Press Release P-17384
 Data page on 035A72
 Data page on 070A13
 Malin Space Science Systems page, 1995. I call the feature "Owen Mesa" in honor of Tobias Owen who first noticed it
 MSSS data page
 JPL release 6 April 1998
 ESA release
 HiRISE data page