Analysis of Candidate Landing Sites for Future Mars Rover Missions
Executive Summary
Robotic exploration of the surface of Mars is central to NASA’s long-held goal of determining the past habitability, and the past or present existence of life, on the planet. The process of identifying candidate landing sites for a mission is critical to the overall success of these missions. The aim of this project is to identify those areas on Mars that are likely to provide the best evidence of past habitability, while simultaneously being accessible to robotic exploration. Candidate sites for future missions will be assessed via a global vector-based, ‘fuzzy’, suitability analysis. The suitability analysis will include both engineering and science criteria. Engineering criteria, such as latitude, elevation, slope, and surface properties, dictate where a rover can safely be landed and operated; while science criteria, such as geologic context, mineralogy, and geomorphology, define the areas most likely to provide evidence of past habitability, or even life itself. Data relating to the engineering criteria largely exist as raster datasets already (e.g., elevation), or can be derived from such datasets (e.g., slope). Data relating to the science criteria largely exist as feature or xy data, and will be used to generate new raster datasets (e.g., distance rasters). The result of the initial suitability analysis will be a global discrete raster illustrating the best candidate locations for future mission landing sites. This raster will be further analyzed to identify suitably large areas of contiguous high scoring cells, and a comparison will be made with sites considered as finalists during the process to identify the landing site for the current Mars Science Laboratory mission and upcoming Mars 2020 mission. If time allows, additional detailed maps of the most suitable sites will be constructed.
1. Introduction
The discovery of extraterrestrial life would be one of, if not the most, significant scientific discoveries in history. For many decades, Mars was considered the Solar System body most likely to contain evidence for life, or the evidence of past life. This belief is the primary reason that NASA (the American ‘National Aeronautics and Space Administration’), and more recently the European Space Agency (ESA), have spent billions of dollars studying the “red planet”. This is achieved primarily in two different ways. First, through remote sensing capabilities on spacecraft in orbit around the planet (current, or recently active, examples include the Mars Global Surveyor (MGS), Mars Odyssey, and Mars Reconnaissance Orbiter (MRO)). The remote sensing tools on these spacecraft provide critical context for the second mode of planetary exploration – the delivery of landers, and more recently rovers, to the surface of the planet itself, beginning with the Viking missions in 1975 and 1976. Such missions are enormously complex and expensive undertakings (the total life cycle cost of the current MSL mission is estimated at US$2.5 billion), so deciding where the mission will land is a critical component of the whole program. This project seeks to undertake a planet-wide suitability analysis to identify candidate landing sites for future robotic rover missions to Mars.
2. The Research Question
The primary scientific objective of recent Mars rover missions (Spirit and Opportunity, the two Mars Exploration Rover (MER) mission rovers; and the currently active Mars Science Laboratory (MSL) rover Curiosity) has been to determine the past habitability of Mars, and the potential that evidence of past life may be preserved on the planet. The upcoming Mars 2020 mission (scheduled for launch in July 2020), and its rover Perseverance, will take Mars exploration to the next level by actively searching for direct indicators of past life (i.e., ‘biomarkers’). Clearly, an understanding of the geology of the proposed landing site is a critical aspect of the selection process – candidate landing sites are carefully examined and scrutinized in an effort to select the site that will yield the most new science, and that is most likely to meet the mission objectives. However, this is only half of the equation that must be considered during the landing site selection process. The chosen site must also be as safe as possible for an attempted landing, and subsequently be as safe as possible for robotic investigation for the duration of the primary mission (at least one Mars year – 669 Mars days (‘sols’) or 687 Earth days – for MSL and Mars 2020).
Thus, the selection of a landing site is a trade-off between a locality that will maximize new knowledge gained during the mission (science criteria) and the physical constraints of operating a rover there (engineering criteria). The goal of this research is to identify those areas on Mars that are likely to provide the best evidence of past habitability, while simultaneously being accessible to robotic exploration. I intend to construct this analysis as a model in ModelBuilder, such that the effects of varying certain criteria can be more easily investigated.
3. Data Sources
The main sources of data for this project will be the Planetary Data System (PDS) archive of NASA mission data (PDS), and the Integrated Software for Imagers and Spectrometers (ISIS) portal hosted by the USGS Astrogeology Research Program (ISIS). Additionally, there are a variety of other datasets, often not in GIS formats, hosted by individual researchers on the web. One such example is the Integrated Database of Planetary Features hosted on WordPress and maintained by the NASA AMES Center (IDPF).
4. Project Workflow
The overall approach is to conduct a raster-based, or ‘fuzzy’, suitability analysis, with inputs representing both engineering and science criteria. I intend to conduct this suitability analysis at a global scale initially, at which point I will compare my analysis to known locations that have been considered as candidate landing sites for both the MSL and Mars 2020 missions.
The actual criteria that project scientists and NASA engineers use are described below. These are based on information presented on the NASA webpages relating both to the ongoing MSL mission (MSL homepage) and the upcoming Mars 2020 mission (Mars 2020 homepage). Alongside these criteria I have provide an evaluation of whether and how these will be accessible to me in a GIS context.
4.1 Engineering Criteria:
The engineering criteria specify the physical limitations of where the rover can be landed and where it can be operated. These criteria encompass all aspects of the mission, from initial entry into the Martian atmosphere to the driving of the rover once it (hopefully!) successfully lands. Many of them can be evaluated at a global planetary scale, although during the real landing site selection process, some are also investigated over much smaller areas once candidate sites have been evaluated. I will focus here, at least initially, on those criteria that can be evaluated globally.
i) Latitude
For a variety of reasons, including the dynamics of entering the Martian atmosphere, spacecraft communications, and the rover’s operational temperature range, the range of latitudes within which landing can safely occur are limited. The exact criteria vary from mission to mission based on a number of factors, however, I will create new rasters using the following criteria: i) landing sites between 30°N and 15°S are most preferable; ii) sites between 30-45°N and 15-30°S are less desirable; iii) sites polewards of 45°N and 30°S are not desirable.
ii) Elevation
The absolute elevation of the landing site is also a consideration. To a large extent this is related to the height of the atmospheric column that the spacecraft must traverse (the Martian atmosphere is very thin, so the lower the absolute elevation of the landing site the greater is the opportunity for air resistance to slow the spacecraft during descent towards the surface). On Earth, elevations are specified relative to mean sea level, while on Mars elevations are specified relative to the average planetary radius (frequently referred to as the ‘MOLA geoid’, from the instrument that produces the most accurate elevation data). The highest point on Mars is the summit of the volcano Olympus Mons (+21.3 km wrt. MOLA) and the lowest point is within the Hellas Basin (-8.2 km wrt. MOLA) Once again the precise elevation criteria vary for many reasons, but lower absolute elevations are preferable. I will reclassify a global MOLA-derived DEM along the following lines: i) elevations lower than -2 km are most preferable; ii) elevations between -2 and +2 km are less desirable; iii) elevations greater than +2 km are not desirable.
iii) Slope
Slope is perhaps the hardest parameter to classify, as a proper evaluation of landing sites requires slope, and thus elevation, data at a range of scales. The highest resolution global elevation data available is the MOLA DEM, with a resolution of 463 m per pixel. This will be converted to a slope raster and re-classified as follows: i) low slopes (most preferable) < 5°; ii) intermediate slopes 5 – 15°; high slopes (least preferable) 15 – 30°; slope threshold (not to be exceeded in the landing site) = 30°.
If time allows and candidate sites are identified, higher resolution DEMs and slope rasters may be obtained and analyzed.
iv) Surface Properties
The EDL (Entry, Descent, and Landing) system employed in both the MSL and Mars 2020 missions requires measurement of spacecraft altitude and velocity during descent via Doppler radar. Thus, the surface must be radar reflective, with an appropriate radar backscatter cross-section in the Ka band between -20 to +15 dB. It is currently uncertain whether these data are available at a global scale for Mars.
Another important consideration is whether the surface materials are suitably load bearing (i.e., relatively coherent rock rather than loose sediment and dust). Surfaces dominated by thick coverings of dust that the rover could bed down in are thought to be characterized by thermal inertia values less than 100 J m-2 s-0.5 K-1 (and possibly less than 150 J m-2 s-0.5 K-1) and albedo (i.e., reflectance) higher than 0.25.
v) Additional Criteria
There are a variety of additional criteria, most of which are assessed at a local scale for candidate landing sites, and as such will not be part of the initial analysis but may be used in follow-up analyses of candidate sites. First is the landing ellipse dimension, which specifies the best estimate of the uncertainty of landing the spacecraft at a given point. A conservative estimate of the size of this ellipse is 25 x 20 km (i.e., if a specific landing site is identified, the spacecraft should be able to touch down within an ellipse with major axis length of 25 km and minor (cross-track) axis length of 20 km, centered on that chosen site).
An additional criterion is the extent to which the primary targets of scientific investigation at any site fall within or out with this landing ellipse. If primary science targets are out with the landing ellipse, a significant portion of the mission duration could be occupied in driving from the eventual landing site to the points of scientific interest. Such ‘Go To’ sites are considered less desirable and should be of exceptionally compelling scientific interest.
4.2 Science (Geologic) Criteria:
Landing a rover safely on Mars is of little use if the area does not contain geology appropriate to answering the primary science questions of the mission. Although there exist a near limitless number of interesting scientific questions, the primary objective of recent Mars exploration has been the assessment of the planet’s past habitability. Primarily, this revolves around the question of whether liquid water ever existed on the surface of the planet for long enough that life may have been able to evolve. Thus, the geologic criteria are dominated by evidence for the past presence of liquid water on Mars, and sites that might preserve such evidence. This evidence can be investigated at a global scale in several ways.
i) Geology and Geochronology
Mars is believed to have been significantly warmer and wetter early in the planet’s history. Therefore, evidence of past habitability and/or life is more likely to be found in older rocks that date from such periods. A global Martian geologic map can be used to eliminate areas of younger rocks, such as the northern lowland plains and volcanic provinces such as Tharsis and Elysium.
ii) Mineralogy
The crust of Mars originally was volcanic in origin, and of a basaltic or basaltic andesite composition. Interaction of this material with, and/or deposition from, liquid water would create new, secondary minerals, such as phyllosilicates (i.e., hydrated clay minerals), carbonates, and sulfates. The spectral signature of such minerals has been mapped with the Thermal Emission Spectrometer (TES) aboard the Mars Global Surveyor (MGS) spacecraft.
iii) Geomorphology
Liquid water flowing across the surface will sculpt the landscape by eroding channels and shorelines and leaving characteristic deposits such as layered sedimentary rocks and delta structures. These cannot (yet) automatically be recognized but there are global vector datasets that can be useful. One example is a dataset of channel features on Mars. A distance raster based on this dataset will be used to indicate proximity to areas of past liquid water on the surface. Additional such datasets that exist either as ESRI shapefiles or simple xy coordinates in csv files include candidate delta deposits, open- and closed-basin lakes, and polygonal ridges, many interpreted as mineral-filled veins. Distance rasters will be created also for these features.
5. Expected Outcomes and Challenges
The primary expected outcome is a discrete classified raster, at the cell size of the global MOLA elevation input raster, representing the suitability analysis based on both engineering and geologic criteria. This will be masked by areas that are deemed not suitable – for example high latitudes and young volcanic provinces. I expect the main challenge at this stage will be choosing appropriate weighting for the different datasets and re-classifying rasters appropriately. This is where the use of ModelBuilder will help in automating the process so that it can be repeated with different weightings.
Assuming successful generation of a suitability raster there are several additional steps that could, or should, be taken. The first, and most obvious, is to identify regions of suitably large numbers of contiguous cells such that they might represent the area of the landing ellipse (25 x 20 km). One way to do this might be to set some arbitrary threshold score on the output raster of the suitability analysis, extract all cells exceeding this threshold, and convert contiguous areas into polygons. These polygons can then be analyzed in terms of area and total suitability score.
An additional step would be to compare such regions to the known locations of candidate landing sites for the MSL and Mars 2020 missions. The landing site selection process for the MSL mission considered 7 finalists. The landing site selection process for the Mars 2020 mission considered 8 finalists, of which 4 were part of the 7 MSL finalists, giving 11 unique candidate sites across the two missions. All of these sites have a wealth of additional data associated with them, such as high resolution imagery (from the HiRISE instrument on the MRO) and higher resolution spectral data (from the CRISM instrument on MRO), that could be used to make refined maps of candidate landing sites.
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