[X] My Assistant

[nprev]

Hindsight being 20/20, I have been and continue to be more puzzled by the selection of Chryse & Utopia Planitia as the landing sites for V1 & V2 respectively. Hard to understand why Meridiani was not picked, for example, and now the Phoenix site shows very benign terrain as well (save for the winter weather, of course.)

Admittedly we did not have high-res (by today's standards) orbital imagery of Mars then, and IIRC a lot of the selection was driven by Earth-bound radar estimation of terrain roughness. Was the main driver the expected science return from the sites rather then enhanced probability of successful landing? Was the radar data of insufficient resolution so that its interpretation was problematic? (Thinking Meridiani here; presumably the polar regions would be pretty hard to get decent returns from due to the incidence angle.)

Whenever I look at Big Joe in Chryse or that big boulder not far at all from V2, I get a cold chill. We were VERY lucky on both counts.

[dvandorn]

Well, Nick -- my admittedly imperfect memory tells me the following on the Viking site selection rationales:

For VL1, they wanted a landing site where the landforms showed that water had once flowed. Where you had water, you might have remanent hydrological cycles, the thinking went, and thus a higher possibility of finding life. The Chryse landing sites (both the original site that was disqualified due to overt roughness and the actual site) are located on the debris apron from one of the ancient catastrophic floods, and so are sites where water had, quite obviously, once flowed over Mars' surface.

Another reason for landing in a catastrophic flood plain was that rocks and soils from a variety of places on Mars would have been deposited in the lee of the flood. Boulders the size of Big Joe (and even larger) were likely transported to the VL1 site from many hundreds of kilometers away. It gave you the opportunity to look at a wide variety of Martian rocks, all in one place. At the, in hindsight, rather high cost of sacrificing any possibility of seeing the context in which these rocks were formed.

Note that the Pathfinder site, in Ares Vallis, is also located on a catastrophic flood plain, one that (if memory serves) simply diverges into a side lobe from the same flood event that so strongly altered Chryse. I imagine the reasons for its selection were pretty much the same as those for VL1.

For VL2, my memory is that they wanted to see what the northern plains looked like and were made of, to contrast against the geology seen on the flood plains. If those northern plains really were the dessicated ancient floors of a great Martian ocean, then you once again have a better shot at finding remanent water and life.

Also, the actual expectation for the fine-scale structure of the Utopia site was that the surface would be covered by relatively low-profile sand dunes, rather similar to what was eventually found at Meridiani. For the life detection experiments, digging into soil under a dune base was considered a better-than-average bet for finding moisture and microbes.

The actual character of the VL2 site was quite a surprise to the Viking team, in fact. It's the rockiest surface any successful lander has ever set down in, and had the team known how seriously rocky it was, they would have ruled it unsafe. Indeed, VL2 was lucky to have only landed a single footpad onto a rock, and that one a relatively small rock.

The rocks at the VL2 site are far more uniform in nature than those at the VL1 site. The VL2 site seems covered with the broken fragments of a once-contiguous lava field, the rocks most all look like chunks of basalt. Whether the site is actually sitting in the ejecta blanket of the crater Mie, or the thermal cycling that far north broke up the surface into the well-sorted, dense boulder population we see, hasn't been completely settled yet, I don't think.

What was curious and somewhat disappointing, of course, was how similar the soils at the VL1 and VL2 sites seemed to be. No matter how ubiquitous the global dust distribution, you would expect regional variation in the soil elements derived from local rocks and deposits. No real variations were found, within the ability of the Viking sensors to detect.

In summary, I'd say that the VL1 site was more similar to what was expected than the VL2 site, but that neither truly exhibited the characteristics for which they were selected.

-the other Doug