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.
Full Version: Viking Landing Site Selection
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
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
Viking had approximately the same engineering requiremets as all American landers since.... low altitude <to give the EDL working altitude before the spacecraft impacts> and a "safe" landing ellipse. In addition, Viking was specifically searching for life.... requiring some potential for liquid water <low altitude, relatively warm, humid>.
The orbiter (a souped-up Mariner 9 in many ways) carried only 3 instruments: Candidate landing site mapping camera, infrared thermal mapper (surface temp, roughness, rock abundance), and MAWD: Mars Atmosphere Water Detector, which was going to TRY to look for unusually humid areas <it didn't have the resolution or signal-to-noise ratio to really do so, but established the basics of the global atmosphere water cycle>. The orbiter carried no science instruments independent of the tasks supporting finding a landing site.
Earthbased radar was extremely crude in the early 70's. Data consisted of measuring the strength and width of the sub-radar specular reflection and estimating radar reflectance and meter-plus-scale roughness along strips at a fixed latitude as the planet rotated. It could, however, give a rough assessment of a regional terrain where that terrain was dominated by one geology. It could NOT return data outside the sub-Earth latitude range, and only return data when Mars was near Earth and at a given latitude <Martian season> and longitude <rotational phase between Mars' 24 2/3 hour day and Earth's 24 hour day.>
Preliminary selection of a Viking 1 Chryse/Ares Valles landing site was bases on Mariner 9 global geologic mapping with the support of a handful <5 or 6?> scattered frames in the nominal landing area. Mapping as soon as they could phase the orbit over the landing site showed a scabland terrain with severe sub-quarter-kilometer roughness. The polite term was "Uhoh". They kept mapping to the west across Chryse looking for smoother terrain with good radar properties and found it at the VL-1 site.
Lander 2 was sent to "northern plains", at least in part due to avaiable orbit parameters for that launch/arrival date range. Mariner photo coverage was poor at that latitude and radar data was not available. I can't recall if they orbit-inserted into a non-24-hour orbit at first to be able to sample different longitudes with different candidate landing sites or what. They ended up doing an extended landing site search as well. I think it boiled down to "low elevation safe-looking plains in the summer that are different from the Viking 1 site", but I'd have to review old papers and articles on the site-selection process. They thought they were going to a flat and smooth site with scattered low-relief dunes. Boy, were they surprised!.
The orbiter (a souped-up Mariner 9 in many ways) carried only 3 instruments: Candidate landing site mapping camera, infrared thermal mapper (surface temp, roughness, rock abundance), and MAWD: Mars Atmosphere Water Detector, which was going to TRY to look for unusually humid areas <it didn't have the resolution or signal-to-noise ratio to really do so, but established the basics of the global atmosphere water cycle>. The orbiter carried no science instruments independent of the tasks supporting finding a landing site.
Earthbased radar was extremely crude in the early 70's. Data consisted of measuring the strength and width of the sub-radar specular reflection and estimating radar reflectance and meter-plus-scale roughness along strips at a fixed latitude as the planet rotated. It could, however, give a rough assessment of a regional terrain where that terrain was dominated by one geology. It could NOT return data outside the sub-Earth latitude range, and only return data when Mars was near Earth and at a given latitude <Martian season> and longitude <rotational phase between Mars' 24 2/3 hour day and Earth's 24 hour day.>
Preliminary selection of a Viking 1 Chryse/Ares Valles landing site was bases on Mariner 9 global geologic mapping with the support of a handful <5 or 6?> scattered frames in the nominal landing area. Mapping as soon as they could phase the orbit over the landing site showed a scabland terrain with severe sub-quarter-kilometer roughness. The polite term was "Uhoh". They kept mapping to the west across Chryse looking for smoother terrain with good radar properties and found it at the VL-1 site.
Lander 2 was sent to "northern plains", at least in part due to avaiable orbit parameters for that launch/arrival date range. Mariner photo coverage was poor at that latitude and radar data was not available. I can't recall if they orbit-inserted into a non-24-hour orbit at first to be able to sample different longitudes with different candidate landing sites or what. They ended up doing an extended landing site search as well. I think it boiled down to "low elevation safe-looking plains in the summer that are different from the Viking 1 site", but I'd have to review old papers and articles on the site-selection process. They thought they were going to a flat and smooth site with scattered low-relief dunes. Boy, were they surprised!.
I only just noticed this thread. I spent this spring going through the minutes of the Landing Site Staff, a group headed by Hal Masursky to do the site certification for Viking (not the original site selection based mainly on Mariner 9 data). The minutes were provided to me by Norm Crabill, who wrote them for the group. Now they are in the hands of Matt Golombek before going to a permanent home at LPI in Houston.
For Viking 1, the original site (A-1) was at the mouth of Ares and Tiu Valles, the same location used by Pathfinder 20 years later. But as noted above it turned out to be erosional, not depositional as expected. A frantic search, pushing the limits of team endurance and computing power (batch processing ordered in advance and slower than desired!), examined many sites. Radar was a help but was ambiguous and not fully trusted. The backup site, Cydonia, was too rough. They kept pushing west and northwest of A-1 until they got a smooth site, which looked more like the moon than anything else - craters and wrinkle ridges. Masursky said - we've landed in similar places lots of times. It's beyond any obvious signs of fluvial erosion.
For Viking 2, with an overworked team and limited time before conjunction, the story was different. They took lots of images of B-1 and B-2 candidate sites but both were too rough and poorly understood. They actually chose B-3 (Utopia) in the blind, knowing almost nothing about it, because the other sites were no good. The images looked promising despite low quality, suggesting a mantle which didn't exist!
It's amazing how much data we have these days in comparison.
Phil
For Viking 1, the original site (A-1) was at the mouth of Ares and Tiu Valles, the same location used by Pathfinder 20 years later. But as noted above it turned out to be erosional, not depositional as expected. A frantic search, pushing the limits of team endurance and computing power (batch processing ordered in advance and slower than desired!), examined many sites. Radar was a help but was ambiguous and not fully trusted. The backup site, Cydonia, was too rough. They kept pushing west and northwest of A-1 until they got a smooth site, which looked more like the moon than anything else - craters and wrinkle ridges. Masursky said - we've landed in similar places lots of times. It's beyond any obvious signs of fluvial erosion.
For Viking 2, with an overworked team and limited time before conjunction, the story was different. They took lots of images of B-1 and B-2 candidate sites but both were too rough and poorly understood. They actually chose B-3 (Utopia) in the blind, knowing almost nothing about it, because the other sites were no good. The images looked promising despite low quality, suggesting a mantle which didn't exist!
It's amazing how much data we have these days in comparison.
Phil
It is also important to remember that before the MGS TES hematite detection, Meridiani's main selling point wasn't known. Add to that the fact that a lot of other areas looked just as safe in low resolution images.
Very true, Ted. Some kind of association with water was desirable - they looked at water sources like valley heads, or chaos, water sinks like depositional basins, and areas with fogs or ground ice (to the extent the data suggested it). There was absolutely nothing to suggest Meridiani was like that. Also I think its elevation would have been too high - though their knowledge of topography was very poor compared with ours. Gerry Soffen said he was amazed they got two landers down safely. I expect most people were.
Phil
Phil
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Phil[/quote]
Totally agree Phil.
I confirm that the VL sites were chosen to avoid ALL rocks and the Viking Lander mission was (almost) entirely devoted to biology : i.e. sifting for life in the sands of Mars. When the MPF project arose in 1991, they decided to look just for ROCKS and searched for a "grab-bag" site where all types of rocks could be found. Thus, the VL "A1" site was chosen back, because it was one of the best cartographed place on Mars because of the search for a site for Vl1 (remember : at this time NO MGS data available, just VO images). This is why, ironically, MPF landed in Ares Vallis, quasi at the same place that was originally chosen for VL1. This is why we used to say at JPL that the Viking mission was ended two decades later by Mars Pathfinder...
Phil[/quote]
Totally agree Phil.I confirm that the VL sites were chosen to avoid ALL rocks and the Viking Lander mission was (almost) entirely devoted to biology : i.e. sifting for life in the sands of Mars. When the MPF project arose in 1991, they decided to look just for ROCKS and searched for a "grab-bag" site where all types of rocks could be found. Thus, the VL "A1" site was chosen back, because it was one of the best cartographed place on Mars because of the search for a site for Vl1 (remember : at this time NO MGS data available, just VO images). This is why, ironically, MPF landed in Ares Vallis, quasi at the same place that was originally chosen for VL1. This is why we used to say at JPL that the Viking mission was ended two decades later by Mars Pathfinder...
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