JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. E10, PAGES 23,571–23,593, OCTOBER 25, 2001 Mars Global Surveyor observations of Martian fretted terrain Michael H. Carr U.S. Geological Survey, Menlo Park, California Abstract. The Martian fretted terrain between latitudes 30Њ and 50ЊN and between 315Њ and 360ЊW has been reexamined in light of new Mars Orbiter Camera (MOC) and Mars Orbiter Laser Altimeter (MOLA) data from Mars Global Surveyor. Much of the terrain in the 30Њ–50Њ latitude belt in both hemispheres has a characteristic stippled or pitted texture at MOC (1.5 m) scale. The texture appears to result from partial removal of a formerly smooth, thin deposit as a result of sublimation and deflation. A complex history of deposition and exhumation is indicated by remnants of a former, thicker cover of layered deposits. In some hollows and on some slopes, particularly those facing the pole, are smooth textured deposits outlined by an outward facing escarpment. Throughout the study area are numerous escarpments with debris flows at their base. The escarpments typically have slopes in the 20Њ–30Њ range. At the base of the escarpment is commonly a deposit with striae oriented at right angles to the escarpment. Outside this deposit is the main debris apron with a surface that typically slopes 2Њ–3Њ and complex surface textures suggestive of compression, sublimation, and deflation. The presence of undeformed impact craters indicates that the debris flows are no longer forming. Fretted valleys contain lineated fill and are poorly graded. They likely form from fluvial valleys that were initially like those elsewhere on the planet but were subsequently widened and filled by the same mass-wasting processes that formed the debris aprons. Slope reversals indicate that downvalley flow of the lineated fill is minor. The ubiquitous presence of breaks in slope formed by mass wasting and the complex surface textures that result from mass wasting, deflation, and sublimation decreases the recognizability of the shorelines formerly proposed for this area. 1. Introduction been attributed to a wide variety of processes [e.g., Sharp, 1973; Lucchitta, 1984; Carr, 1995]. Fourth, the terrain lies at the The term “fretted terrain” was first used by Sharp [1973, p. latitudes where debris aprons [Squyres, 1979] and terrain soft- 4073], in reference to terrain “characterized by smooth, flat, ening [Squyres and Carr, 1986] are most common. Finally, lowland areas separated from a cratered upland by abrupt within the fretted area are several discontinuous channels that escarpments of complex planimetric configuration.” He was could plausibly be interpreted as having been caused by mas- Њ referring mainly to the plains-upland boundary between 30 N sive, channelized, subsurface flow. Thus a wide variety of phe- Њ Њ Њ and 50 N and from 280 W westward to 360 W, but the term nomena can be observed in a small area. has also been applied to the plains-upland boundary near the Њ Њ This reassessment of the characteristics of the fretted terrain equator between 180 and 240 W[Parker et al., 1989]. Our is based mainly on the Mars Global Surveyor Mars Orbiter main concern here is with the northern fretted terrain in which Camera (MOC) and Mars Orbiter Laser Altimeter (MOLA) broad flat-floored, steep-walled valleys reach from the north- data. The main emphasis is on the western part of the fretted ern plains deep into the upland, and the uplands transition terrain, that between 30Њ and 47.5ЊN and from 315ЊW westward northward into numerous mesas separated by plains. The me- to 0ЊW (Figure 1). It is the area covered by the U.S. Geological sas become smaller and more widely spaced to the north and Survey (USGS) 1:2,000,000 charts MC-5 SW and MC-5 SC. At ultimately merge with the knobby terrain of the northern the time of writing, roughly 200 MOC narrow-angle frames plains. The transitional region is several hundred kilometers were available of the area. These were mostly taken from the wide, across which surface elevations fall ϳ2km[Smith et al., mapping orbit with resolution of 1.5 m/pixel. Mapping frames 1999]. are identified by three symbols that indicate the mapping phase The terrain is of interest for a variety of reasons. First, it lies followed by five digits to indicate the sequence in that phase. astride the plains-upland boundary, one of the most funda- Thus frame M02-00123 is frame 123 of mapping phase 2. MOC mental and least understood features of the planet. Second, frames taken before the mapping phase are identified by var- many of the stratigraphic contacts, terraces, and other linear ious designators such as SP1, for the first science-phasing orbit features of the fretted terrain have been interpreted as shore- [Albee et al., 1998], followed by the five-digit number indicating lines of a former ocean [Parker et al., 1989, 1993], so the area can be viewed as a test bed for the ocean hypothesis. Third, the the sequence during that phase. The MOLA data used were wide, flat-floored, steep-walled, fretted valleys with their lin- those on the CD-ROMs PDS_MGSL_2001 through 2004 re- eated fill are very different from valleys elsewhere and have leased by the Planetary Data System. The main purpose of the paper is to describe what MGS has This paper is not subject to U.S. copyright. Published in 2001 by the recently revealed about the area. Much of the paper is descrip- American Geophysical Union. tive. The new data are puzzling, and much of what is in the Paper number 2000JE001316. imaging cannot be readily explained. Although a new hypoth- 23,571 23,572 CARR: MGS OBSERVATIONS OF MARTIAN FRETTED TERRAIN Figure 1. Index map of the study area showing figure locations. CARR: MGS OBSERVATIONS OF MARTIAN FRETTED TERRAIN 23,573 Figure 2. (a) Cratered uplands at 30.13ЊN, 324.4ЊW. A thin deposit with a stippled upper surface appears to be draped over the preexisting terrain. Illumination is from the left (SP2-51906). (b) Similar deposit on plains at 48.3ЊN, 343.9ЊW. Illumination is from the right (SP2-49907). esis for the formation of fretted channels is outlined, no new frames cannot be confidently located in the Viking frames used unifying theory is presented that explains everything seen in for context. the fretted terrain. The main intent here is to present some of The study area is similar to that discussed by Parker et al. the new data and discuss some of the relations and possible [1989]. Heavily cratered Noachian terrain is exposed in the implications in the hope of stimulating new ideas. southern half of the area. Although morphologically these The very high resolution and small size of the MOC frames cratered uplands resemble those elsewhere on the planet, they cause problems in presentation. The format size of the MOC are at a lower elevation than typical upland, being mostly at frames is typically so large that only a fraction of a frame can elevations of Ϫ0.5 to Ϫ2 km as compared with the more typical be displayed in a figure if the resolution is to be preserved. 0toϩ3km[Smith et al., 1999]. In addition, the crust under the Hundreds of figures would be required to adequately display cratered terrain is thinner than that under typical cratered the roughly 200 MOC frames in the area. Since typically a uplands and more like that under the northern plains [Zuber et journal can display only 10–20 figures, only a minute fraction al., 2000]. The plains upland boundary, as defined by the sur- of the information available can be displayed to support the face morphology, runs through roughly the middle of the area. arguments in the text. In addition, it is difficult to present an In the southern half of the area the uplands are mostly con- integrated picture because the frames normally cover only a tinuous, whereas in the northern half the uplands are broken small area (4 ϫ 12 km, for example) and are widely separated. up into numerous flat-topped mesas, each surrounded by a Only a minute fraction of the area of concern has actually been steep cliff at the base of which is a debris flow. Between the imaged by the MOC. A further complication is that the con- mesas and their surrounding debris flows are plains. Many, trast in resolution between the MOC frames and the previously although not all, of the rounded knobs, common on the plains, acquired Viking frames is so large that some of the MOC are probably upland remnants, like the mesas. In the northwest 23,574 CARR: MGS OBSERVATIONS OF MARTIAN FRETTED TERRAIN Figure 3. Deposits similar to those in Figure 2, except at southern midlatitudes. (a) SW of Gorgonum Chaos at 40.1ЊS, 171.8ЊW (M00-03091). (b) West of Hellas at 41.5ЊS, 257.3ЊW (FHA-00443). Illumination is from the top left in both images. corner of the area, typical thumbprint terrain [Guest et al., Vallis starts near the rim of the crater Cerulli at 32.5ЊN, 339ЊW 1977] occurs between the upland remnants. Clearly identifiable and winds northwards between 340Њ and 345ЊW, cutting across upland remnants disappear northward at ϳ50ЊN, beyond several craters. Near Cerulli it is 2 km across and 400 m deep, which are mostly mottled and knobby plains. but it widens and deepens downstream such that it is 30 km The plains-upland boundary runs through the middle of the wide and 2 km deep at 39ЊN. The other system of channels is area, but the concern here is not with the origin of the bound- informally referred to as the Ismeniae channels after the Is- ary but how the boundary has affected the geomorphological meniae Fossae.