Edit - added more detail on the specific geological interests per the question revision
Selection Process
This letter to Dr Michael Meyer, lead scientist for the Mars Exploration Program, gives a synopsis of the assessments and findings at the February 2017 workshop. The workshop aimed to narrow the shortlist of landing site candidates from 8 to 3.
Five criteria were used to assess the sites (emphasis mine):
- Criterion 1:
The site is an astrobiologically relevant ancient environment and has geologic diversity that has the potential to yield fundamental scientific discoveries when it is
a) characterized for the processes that formed and modified the geologic record; and
b) subjected to astrobiologically relevant investigations (e.g., assessment of habitability and biosignature preservation potential). (scoring: 1=lowest potential, 5=highest potential)
- Criterion 2: A rigorously documented and returnable cache of rock and regolith samples assembled at this site has the potential to yield fundamental scientific discoveries if returned to Earth in the future. (scoring: 1=lowest potential, 5=highest potential)
- Criterion 3: There is high confidence in the assumptions, evidence, and any interpretive models that support the assessments for Criteria 1 and 2 for this site. (scoring: 1=lowest confidence, 5=highest confidence).
- Criterion 4: There is high confidence that the highest-science-value regions of interest at the site can be adequately investigated in pursuit of Criteria 1 and 2 within the prime mission. (scoring: 1=lowest confidence, 5=highest confidence).
- Criterion 5: The site has high potential for significant water resources that may be of use for future exploration - whether in the form of water-rich hydrated minerals, ice/ice regolith or subsurface ice. (scoring: 1=lowest potential, 5=highest potential)
The workshop assigned scores in each of the criteria for each of the landing site candidates, shown in this table:
From this it is clear that Jezero Crater is a prime candidate with NE Syrtis in second as summarised in the letter:
The summary plots reveal that two of the candidate sites (Jezero crater and NE Syrtis) were consistently assessed higher relative to criteria one and two and were as high or nearly as highly ranked as any other sites relative to criteria three and four.
Geological Interest of NE Syrtis
This article gives a brief summary of the geological interest of NE Syrtis. It explains that the site once was volcanically active; providing warm, mineral-rich surface water to potential microbial life.
JPL's Mars 2020 website has pages dedicated to each of the site selection workshops with detailed analyses of their geological interest and viability. There is far too much detail (and it's certainly far above my level of expertise) to summarise effectively here, but the essence is:
NE Syrtis is an ancient fluvial basin, giving it a rich geological historical record in the stratification of sediments as well as being an ancient active volcanic area. The North-East region was selected because more recent weathering has exposed various layers of the rock formations, allowing easy access to a wide range of geological samples in close proximity to each other. This would allow samples to be taken from many different time periods in Mars's early history. These exposed strata can be seen clearly in many of the images in the presentations:
The page for the 2017 workshop has several presentations focusing on NE Syrtis, one of which describes the site like this:
Northeast Syrtis Major mesas are spatially concentrated time
capsules that record the majority of geological processes active
on Mars during the Noachian and Hesperian and are special
because of the large carbonate deposit
