September 27, 2000
From: M. Golombek and Tim Parker
Re: Preliminary Definition of Mars Exploration Rover Landing Site Engineering Requirements and Initial Identification of Potential Landing Sites
This memo describes the preliminary definition of engineering requirements on Mars Exploration Rover (MER) landing sites, maps these requirements into remote sensing criteria, and uses these criteria to identify potential landing sites. To first order, many of the engineering requirements are the same as for the Mars Pathfinder mission [Golombek et al., 1997], because the landing system is the same. The preliminary engineering requirements stated below have been adopted by the MER Project.
Analysis of the Entry, Descent and Landing (EDL) system and atmospheric profiles indicates that the MER spacecraft are capable at landing below -1.3 km MOLA defined elevation, with respect to the MOLA defined geoid [Smith and Zuber, 1998; Smith et al, 1999]. This requirement stems mostly from the need for an adequate atmospheric density column for the parachute to bring the spacecraft to the correct terminal velocity. Because the landing system has no means to reduce horizontal velocity, low-altitude winds and wind shear together are major concerns and must be below about 20 m/s.
Preliminary analyses of power generation/usage and thermal cycling of the rovers restricts the landing sites to be near the subsolar latitude at arrival to last the required 90 sols. This translates to 15S to 5N for MER-A and 5S to 15N for MER-B, which arrives at Mars about 52 sols after MER-A. Operations considerations require the two landing sites to be separated by a minimum of 37° (solid angle) on the surface.
Surface slopes are an obvious concern for the landing system. Large slopes can spoof the radar altimeter and/or cause premature or late firing of the solid rockets and airbag inflation. Small slopes over large distances can lead to significant additional horizontal velocity and prolonged bouncing by the lander within the inflated airbags. Reconstruction of Mars Pathfinder landing indicates the lander traversed a horizontal distance of about 2 km in 10-20 large bounces across the surface [Golombek et al., 1999a], even though it landed at 3 am local time when winds should have been calm. Slopes can also affect the stability of the lander, rover deployment and trafficability, and power generation. As a result, surface slopes should generally be less than 15°. One relation between measured radar RMS slopes and slope suggests surfaces with < 6° RMS slopes have about 5% of their surfaces with slopes >15° [e.g., Golombek et al, 1997]. Images must appear hazard free (i.e., relatively smooth and flat without obvious hazards, such as fresh craters).
The airbags of the Mars Pathfinder landing system were qualified to protect the lander from damage when landing on 0.5 m high rocks in any orientation [Golombek et al., 1997]. As for Pathfinder, this required a landing site with less than 1% of the surface covered by rocks greater than 0.5 m high. Model rock size-frequency distributions based on Viking, Mars Pathfinder and rocky locations on the Earth [Golombek and Rapp, 1997; Golombek et al., 1999b], generally suggest this requirement can be satisfied at locations with total rock coverage of < 20% as derived from thermal infrared measurements [Christensen, 1986b].
The surface must be radar reflective for the descent radar altimeter to work properly, so radar reflectivity must be greater than 0.05. The surface must be load bearing for the rover and too much dust would coat rocks and could reduce surface lifetime by covering the solar panels. Extremely high albedo and low thermal inertia regions should therefore be avoided [Christensen and Moore, 1992]. Areas with fine component thermal inertia of less than 3-4 x 10 -3 cal cm -2 s -1/2 K-1 or cgs units (equivalent to 125-165 J m -2 s -1/2 k-1 or SI units) should therefore be avoided [Christensen, 1982, 1986a; Mellon et al, 2000].
We plotted the MOLA elevations within the 30° latitude band from 15N to 15S. Because the southern hemisphere of Mars is dominantly heavily cratered highlands, little area is actually below -1.3 km in elevation for MER-A (between 5N and 15S). The largest area below this elevation is in southern Elysium and Amazonis Planitiae. Unfortunately, most of this area (150W to 200W) is dominated by extremely low thermal inertia, with fine component thermal inertias below 3 x 10-3 cgs units and so is excluded. For the latitude band of MER-B (5S to 15N), more area is above -1.3 km elevation. Nevertheless, most of the area between 135W and 190W is excluded on thermal inertia grounds. Areas available to seek landing sites are thus reduced to southern Isidis and Elysium Planitiae in the eastern hemisphere and western Arabia Terra, Terra Meridiani, Xanthe Terra, Chryse Planitia, and the bottom of Valles Marineris in the western hemisphere.
Because of the arrival geometry and the prograde entry into the atmosphere, landing ellipse size and orientation change significantly with latitude and time of arrival. Analysis of the expected flight path angle at atmospheric entry and dispersions produced by the atmosphere yield landing ellipses of 80 km by 30 km for MER-A at 15S up to 360 km by 30 km for MER-B at 15N. In addition, the orientation of the ellipse rotates from 80° at 15S for MER-A to 109° at 15N for MER-B for the opening of the launch window (for MER-A the ellipse orientations rotate about 3° counterclockwise by the end of the launch period; for MER-B the ellipse orientations rotate about 14° counterclockwise by the end of the launch period). Using the preliminary landing site dispersion data from P. Knocke and P. Desai, we used the following ellipses for each 2.5° in latitude for each lander, applicable for the opening of the launch periods. In reality, the ellipse size probably varies smoothly with latitude, although the ellipses used are within about 20 km of the correct length and within a degree of the correct orientation (sufficient for this preliminary identification). Note all ellipse widths are 30 km.
Table 1.
Latitude
Ellipse Length
Ellipse Orientation
(°)
(major axis, km)
(orientation of
major axis, °)
MER-A
15S-12.5S
86
81
12.5S-10S
97
81
10S-7.5S
109
83
7.5S-5S
120
84
5S-2.5S
132
86
2.5S-0
140
87
0-2.5N
155
87
2.5N-5N
166
88
MER-B
5S-2.5S
132
103
2.5S-0
140
104
0-2.5N
155
104
2.5N-5N
166
105
5N-7.5N
195
106
7.5N-10N
240
107
10N-12.5N
286
107
12.5N-15N
330
109
Next using these landing ellipses, we tried to place them in all locations that are below -1.3 km in elevation, had acceptable fine component thermal inertia values, and were free of obvious hazards in the MDIMs (Mars Digital Image Mosaics). This is exactly the same procedure employed to initially identify potential landing sites for Mars Pathfinder. Only sites that appear smooth and flat in the MDIM without scarps, large hills, depressions or large fresh craters (>5 km) were acceptable.
To our surprise, we found 100 sites for MER-A and 85 sites for MER-B that met these criteria (30 sites overlap). Even though the area available to land north of the equator is at least twice as great as south of the equator, the smaller ellipse size towards the south compensates. For comparison, the landing ellipse for Mars Pathfinder (300 km by 100 km) and the 10° latitude band reduced available sites to just 10 [Golombek et al., 1997]. Virtually all ages and types of geologic mapped units are available for landing including ancient Noachian units, Hesperian channel and plains, and Amazonian volcanics, channel and smooth plains. The Hematite site studied for the Mars Surveyor '01 lander is assessable to both landers, with 4-5 sites for each (TM20B-TM23B; TM9A-TM12A) and 19 sites (VM35A-VM53A) can be placed within Valles Marineris (guaranteed spectacular views). We have not yet evaluated any of these potential sites in more detail, so it is unclear how many would survive the more careful review required to certify a landing site. If the Pathfinder site selection activity is used as a guide, at least half would be eliminated fairly quickly on the basis of inspection of existing MOC images. Nevertheless, this analysis shows a large number of landing sites are available for study for the MER landers. The 30 sites that overlap in area are: TM1A-TM11B, TM2A-TM11B, TM4A-TM12B, TM5A-TM13B, T6A-TM14B, TM7A-TM17B, TM8A-TM16B, TM9A-TM19B, TM10A-TM20B, TM11A-TM22B, TM12A-TM23B, TM13A-TM24B, TM14A-TM25B, XT18A-XT28B, XT20A-XT29B, XT21A-XT30B, XT33A-XT31B, XT34A-XT27B, EP 68A-EP52B, EP70A-EP56B, EP72A-EP57B, EP74A-EP61B, EP75A-EP63B, EP76A-EP64B, EP78A-EP71B, EP80A-EP70B, EP81A-EP72B, EP83A-EP79B, IP84A-IP96B, IP85A-IP98B.
Below is a list of all potential sites identified. This list is not meant to be exclusive or inclusive; it is merely to identify the scope of choices available. Each site is given a unique identifier consisting of 2 letters, followed by a number, followed by the letter A or B, for purposes of communication. The first two letters define the general region in which the landing site is located as follows: wA-western Arabia Terra, TM-Terra Meridiani, CP-Chryse Planitia, XT-Xanthe Terra, and VM-Valles Marineris (all in the western hemisphere), and EP-Elysium Planitia, IS-Isidis Planitia, and SM-Syrtis Major (all in the eastern hemisphere). The number is the unique site number for MER-A or MER-B, which is distinguished by the letter A for MER-A and B for MER-B. The location is for the center of the ellipse; corresponding ellipse sizes can be found in Table 1. The elevation is approximate for the center of the ellipse and is within 0.5 km for elevations -6 km to -2.5 km and within 0.1 km for elevations -2.5 km to -1.3 km. Geologic units from Scott and Tanaka [1986] and Greeley and Guest [1987] are: Npl1 - Noachian Plateau cratered unit; Npl2 - Noachian Plateau subdued cratered unit; Nplh - Noachian Plateau hilly unit; Npld - Noachian Plateau dissected unit; Hr - Hesperian Ridged plains; Hch - Hesperian Channel Material; Hvr - Hesperian Vastitas Borealis Formation, ridged; Hs - Hesperian Syrtis Major Formation (volcanics); Ael1 - Amazonian Elysium volcanics, Achu - Amazonian Channel material; Aml - Amazonian Medusae Fossae Formation; Aps - Amazonian Smooth Plains; Apk - Amazonian knobby material; Avf - Amazonian Valles Marineris floor material; S - Smooth plains material within craters.
Those at JPL (or with special access) can look at the maps on the MER library web site at: http://mars03-lib.jpl.nasa.gov/ (click on Work Area, then Project Science, then Science Team Meetings, then Athena Science Team Meeting Sept. 18-19, 2000, then Potential landing sites; viewgraphs under Golombek). The ellipses are drawn on an MDIM basemap with MOLA elevation in color, and with the geologic maps of Scott and Tanaka [1986] and Greeley and Guest [1987] in color. We will also have this material on an open web site for the community soon.
Table 2.
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Site ID: A = MER-A, B = MER-B Geological Units: Scott & Tanaka [1986], Greeley & Guest [1987] Latitude: ± 0.2° (°N, °S), relative to MOLA geoid Longitude: ± 0.2° (°W) |
References: