The present engineering constraints imposed on the Mars Pathfinder mission leave only three broad regions available for site selection: Amazonis, Chryse, and Isidis Planitia. Because of the knowledge gained by the Viking 1 mission, Chryse Planitia would make an ideal primary landing site. Geological characteristics of Isidis Planitia are discussed with a view to making it a back-up landing site

Craddock, Robert A.

English

NASA Technical Reports Server

L P I  Technical Report 94-W 2 1 
the hazards may be in the other areas. The extensive photographic 
coverage of Chryse Planitia by the Viking Orbiters and Earth-based 
radar observations has provided 100-m-resolution topography in the 
vicinity of the Viking 1 lander [2]. Analysis of these data and lander 
photographs indicate that chryse Planitia may be unique in that 
features >50 km away from the lander (such as the rimsof Lexington 
and Yorktown Craters) are visible over the horizon [3]. This type of 
information could potentially aid in roving-vehicle navigation. How- 
ever, the most important use of the Viking orbiter data will be in 
simply determining the location of the Mars Pathfinder spacecraft 
on the surface. These same data were useful in determining the 
location of the Viking 1 lander to within -50 m [4]. 
A landing site should ideally include access to as many different 
geologic units as possible. In addition to the materials debouched 
into the Chryse Basin by the large martian channel complex [5], the 
Hesperian-age ridged plains covering much of the region [6] repre- 
sent the single most important geologic unit needed for agedating 
materials on Mars. Composing -3% of the total Mars surface area 
(71, the ridged plains are fairly widespread in comparison to other 
geologic units, and, more importantly, are the Hesperian epoch 
referent [8]. Because the Hesperian epoch represents the interval 
of time immediately following the period of heavy bombardment 
(-3.8 Ga [9]), an absolute age determined from a ridged plain 
sample would allow estimates of the postheavy bombardment im- 
pact flux on Mars to be calibrated. It may then be possible to 
determine the absolute ages of every younger geologic unit on Mars 
based on crater statistics. Potential Hesperian ridged plains out- 
crops identified in Viking 1 lander images may represent the only 
known bedrock exposures on Mars. Mars Pathfinder rover analyses 
of these materials could provide data to support the hypothesis that 
these are bedrock materials, which could be crucial for future 
sample return missions. In addition, materials washed down from 
the highlands may be present in Chryse Planitia. Although the 
absolute ages of these materials almost certainly correspond to the 
period of heavy bombardment, analysis of their composition could 
provide some insight into the early geologic history of Mars. Also, 
the distribution of the materials in Chryse Planitia as determined by 
a long rover traverse may be indicative of the channel formation 
mechanism. For example, catastrophic flooding would lead to a 
Bouma sequence deposit in the Chryse basin [IO]; in liquefaction, 
an accretionary lobe in the debouching area results in larger par- 
ticles dropping out first with smaller particles being transported 
greater distances [ I  11. The Mars Pathfinder Meteorology Package 
(MET) would almost certainly augment the data obtained from the 
Viking Meteorology Experiment. The Viking Meteorology Experi- 
ment was capable of providing information at only one height, which 
is insufficient for determining the boundary layer profile in Chryse 
Planitia. However, because the MET will provide information from 
multiple heights, profiles from the Viking data may be derived. 
Because of the likelihood of running water debouching into 
Chryse Planitia in the past, the Viking I landing site was considered 
an ideal place to look for complex organic molecules [ 121. Although 
the Viking biological experiments did not identify the presence of 
organic life [ 131, controversy still exists as to the meaning of the 
Labeled-Release Experiment [14]. A landing in Chryse Planitia 
would make it possible to investigate the composition of the same 
soil samples investigated by the Viking 1 lander. Rocks seen in 
lander images could also be analyzed, answering questions concern- 
ing their compositional and erosional properties. Depending on the 
exact Mars Pathfinder landing site and the accuracy of rover navi- 
gation, it may be possible to examine the Viking 1 lander 1 itself! 
In situ erosional analysis of Lander 1 could allow the current martian 
weathering rate and eolian deposition to be determined. Such infor- 
mation could also serve as a valuable aid in developing future 
martian spacecraft materials. Alternatively, it may be possible to 
navigate from the lander to the crater caused by the jettisoned 
Viking aeroshell. Ejecta from this fresh crater would represent 
Chryse stratigraphy to a depth of -I m. providing additional in- 
formation on the nature of the surface materials observed by the 
Viking 1 lander. Although crater ejecta is frequently suggested as 
material that should be sampled by a spacecraft during a traverse, 
such stops are rarely justified. Crater ejecta, especially the outer 
ejecta blanket, typically has the same composition as the surround- 
ing rock. It is the traverse up to the crater rim crest where material 
at depth is gradually exposed, the deepest material being exposed 
directly at the rim crest, typically from a depth equivalent to one- 
tenth the crater diameter. However, a simple examination of crater 
ejecta from a larger-diameter crater could potentially provide some 
valuable information. “Rampart” [ 151 or “fluidized ejecta” craters 
[I61 have been suggested as forming from the incorporation of 
volatile material from depth [ 15) or from the interaction of the ejecta 
curtain with a thin atmosphere during emplacement [17]. The de- 
rived volatile content andlor sediment distribution from a rampart 
crater (e.& Yorktown, 7.9 km diameter, -45 km northwest of the 
Viking 1 landing site) could provide clues as to which formation 
mechanism is the most viable. 
[ 1 1  Moore H. J. et al. (1987) USGS Prof. Paper 
1389, 222. (21 USGS Misc. Inv. Ser. Map I-1059 (1977) Denver. 
(31 Craddock R. A. and Zimbelman J. R. (1989) L P S X X ,  193-194. 
[4] Moms E. C. and Jones K. L. (1980) Icarus, 44, 217-222. 
[5] Craddock R. A. et al. (1993)LPSXXIV, 335-336. [6] Scott D. H. 
and Tanaka K. L. (1986) USGS Mix .  Inv. Ser. Map 1-1802A, 
Denver. [7] Watters T. R. (1988) MA’TV-LPI Workshop: Early 
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W. K. et al. (1981) in Basaltic Volcanism, New York. [IO] Komar 
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Mars. 226228. 
N95= 161 84 
. d -  3- . , -g I -  
RATIONALE FOR ISIDIS PLANITIA AS A BACK-UP 
LANDING SITE FOR THE MARS PATHFINDER MISSION. 
R. A. Craddock. Center for Earth and Planetary Studies, National 
Air and Space Museum, Smithsonian Institution, Washington DC 
20560, USA. 
As discussed previously [ 11. the present engineering constraints 
imposed on the Mars Pathfmder mission leave only three broad 
regions available for site selection: Amazonis, Chryse, and Isidis 
Planitiae. Because of the knowledge gained by the Viking 1 mission, 
Chryse Planitia would make an ideal primary landing site. The 
https://ntrs.nasa.gov/search.jsp?R=19950009769 2020-06-16T08:53:23+00:00Z
22 Mars Pathfinder Landing Site Worhhop 
principal objectives of this mission should be to determine the 
composition and distribution of surface materials. Analysis of rocks 
in Chryse Planitia would build upon results obtained by the Vi- 
king 1 lander and answers questions concerning composition and 
origin of these materials. Of particular interest would be determin- 
ing whether proposed bedrock materials are indeed in situ materials 
and the degree of weathering these materials may have undergone. 
Because these materials may represent Hesperian-aged ridged plains, 
they could potentially be the key to Understanding the absolute ages 
of the martian epochs. Results of the Mars Pathfinder could deter- 
mine whether the bedrock materials are indeed Hesperian ridged 
plains materials, which could influence the priorities of future 
sample return missions. 
Isidis Planitia also contains material that is Hesperian in age [2]. 
These materials, however, are from the Late Hesperian epoch and 
mark the end of this period. Nonetheless, identifying exposed bed- 
rock materials from this unit would also be important as radiometric 
age dates obtained by future missions could determine which model 
for the absolute ages of the martian periods is correct (Fig. I). 
Analysis of the temperature contrast measured by the Viking Infra- 
red Thermal Mapper [3] suggests that the spatial distribution of 
rocks in Isidis Planitia may be as high as 20%, similar to that 
observed at both Viking landing sites. Central and northeastern 
Isidis Planitia appear to be much smoother, containing <IO% rocks. 
These data suggest that Mars Pathfinder could expect to find similar 
or perhaps more favorable conditions than observed at the Viking 1 
landing site. In addition, extensive Viking orbiter data with resolu- 
ticm 150 &pixel exists for most of Isidis Planitia. Without an 
additional orbiter imaging system, these data will be critical for 
determining the location of the lander once on the surface. Similar 
data were useful in determining the location of the Viking I lander 
to within -50 m [4]. 
MARS 
A 
1 1 
2 
Fig. 1. Geologic time for the Earth can be broken into the Archean (Ar), 
Proterozoic (R),andPhanerozoic(m)periods.Thedashedlinerepresenlsthe 
amount of time life is known to have existed on this planet. Geologic time for 
Mars is divided into the Noachian (N), Hesperian (H), and Amazonian (A) 
periods.’lheabsoluteagesareestimatedfrom twodifferent modelsofcratering 
rates.AsampleofHesperianmaterialcoulddeterminewhethermodel1[17]or 
model 2 [ 181 is comt.  Because geologic evidence suggests that liquid water 
existcdonIhcmamansurface~gboththcNoachianandHesperian.absolute 
agesbasedonmodel2implythatmicrobiallifemayhavealsoevolvedonMars. 
(Reproduced from [ 191.) 
The material contained in the interior of Isidis Planitia is fre- 
quently interpreted to be an eolian deposit [2,5,6] based on the 
morphology and relative ages of these units. Examination of high- 
resolution Viking orbiter images suggest that these materials also 
exhibit a systematic pattern of terrains from the interior outward to 
the basin rim [6]. These investigators suggested that Isidis Planitia 
had been the location of a thick, volatile-rich debris layer that was 
subsequently removed. Crater statistics and the buried morphology 
of craters contained within the annulus of material surrounding the 
basin also suggest some type of resurfacing event. However. it has 
also been proposed that water may have stood in Isidis Planitia [71. 
perhaps as part of an Ocean covering the northern hemisphere of 
Mars (81. Such a hypothesis may also explain the stratigraphy. 
Examination of the grain size and distribution of surface materials 
may yield clues as to the viability of either of these hypotheses. 
Particularly useful may be the analysis of material excavated by the 
crater resulting from the jettisoned Mars Pathfinder aeroshell, if it 
could be located. A long rover traverse (i.e., several kilometers and 
out of the view of the lander), however, may ultimately be required 
to examine such unit differences on the surface. 
A variety of mechanisms has been proposed to explain the 
enigmatic mounds and arcuate ridges in the interior of Isidis Planitia. 
including volcanic cinder cones [9-1 I], pingoes [ 121, and glacial 
features [6,12,13]. Because of their close spacing (tens of meters), 
it is very likely that Mars Pathfinder would land in the vicinity of one 
of these features. Surface images and compositional analyses of 
surface material would provide valuable clues as to their origin. 
Such information is important for understanding the geologic his- 
tory of Mars and the climatic transition that planet may have expe- 
rienced from the late Hesperian into the Amazonian. 
As in Chryse Planitia, the Isidis Basin contains both lunarlike 
and rampart craters. “Rampart” [ 14) or “fluidized ejecta” craters 
[IS] have been suggested as forming from the incorporation of 
volatile material from depth [ I41 or from the interaction of the ejecta 
curtain with a thin atmosphere during emplacement [ 161. The de- 
rived volatile content and/or sediment distribution from a rampart 
crater near the Mars Pathfinder landing site could provide clues as 
to which formation mechanism is the most viable. 
[ 11 Craddock R. A., this volume. [2] Greeley R. 
and Guest J. E. (1987) USGS Mix .  Inv. Ser. Map 1-18028, Denver. 
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and Jones K. L. (1980) Icarus, 44, 217-222. [SI Meyer J. D. and 
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scale 1:5,OOO,OOO, Denver. [6] Grizzaffi P. and Schultz P. H. (1989) 
Icarus, 77,358-38 1. [7] Scott D. H. et al. (1992)Proc. LPS, Vol. 22, 
53-62. [SI Parker T. J. et al. ( 1993yGR, 98,1106 1-1 1078. [9] Moore 
H. J. and Hodges C. A. (1980) NASA TM-82385, 266-268. 
[IO] Plescia J. B. (1980) NASA TM-82385,263-265. [ I  I] Frey H. 
and Jarosewich M. (1982) JGR, 87, 9867-9879. [ 121 Rossbacher 
L. A. and Judson S .  (1981) Icarus, 45,39-59. [ 13) Lucchitta B. K. 
( I  98 1) Icarus. 45, 264-303. [ 141 Cam M. H. et a]. ( 1977) JGR, 82, 
4055-4066. [IS] Mouginis-Mark P. J. (1979)JGR, 84,801 1-8022. 
[ 161 Schultz P. H. and Gault D. E. (1981) Third Inlernarional 
Colloquium on Mars, 226-228. [17] Neukum G. and Wise D. U. 
(1976)Science, 194,1381-1387. [18]Hmmann W. K.etal. (1981) 
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Third Inrl. Con& Eng. Consnuc. Oper. Space, 1488-1499. 
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Rationale for Isidis Planitia as a back-up landing site for the Mars Pathfinder mission

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