Association of phyllosilicates and the inverted channel in Miyamoto crater, Mars



[1] The western floor of the Miyamoto crater in Sinus Meridiani on Mars exhibits both geomorphic and spectral evidence for aqueous history. It contains a sinuous and narrow ridge that is interpreted to be an inverted channel and is suggestive of past fluvial activity. Phyllosilicates occur in materials that are proximal to the paleochannel, but are not detected on top the ridge. The simultaneous use of the spectroscopic data, high-resolution images, and a digital elevation model show that Fe/Mg-smectites are exposed by erosion. They are associated with polygonally-fractured bedrock which occurs on the slopes of both sides of the sinuous ridge. The observations provide direct evidence of the presence of water and possibly of multiple aqueous events throughout the area.

1. Introduction

[2] Miyamoto crater is a 160-km-diameter impact crater of Noachian age of Sinus Meridiani, located southwest of Meridiani Planum (Figure 1a). The regional geologic history of the Meridiani Planum region has been extensively studied [Hynek et al., 2002; Arvidson et al., 2003; Newsom et al., 2003; Hynek and Phillips, 2008]. It is interpreted to have begun with the formation of the ancient cratered crust and a subsequent major fluvial episode carved an extensive valley network, originating from an area located south of the Schiaparelli impact basin [Newsom et al., 2003]. Because of the presence of phyllosilicates in the crater floor [Wiseman et al., 2008] Miyamoto crater was proposed as a candidate landing site for the upcoming Mars Science Laboratory (MSL) rover [Vasavada and MSL Science Team, 2007; H. E. Newsom et al., Southwest Meridiani Planum (Runcorn crater), paper presented at the Second MSL Landing Site Workshop, Mars Science Laboratory, Pasadena, Calif., 23–25 October 2007] and, only recently, it has been excluded, not being part of the final four perspective landing sites [Golombek et al., 2009]. For reference, the proposed location of the landing site ellipse (20 × 25 km) is shown in Figure 1a.

Figure 1.

(a) THEMIS IR day-time photomosaic at 256 pixel/degree of the southwestern area of Miyamoto crater. White hourglass shape represents the CRISM footprint of observation FRT0000C71F. Gray ellipse is the location of the candidate MSL landing site. The inset shows a colorized MOLA (Mars Orbiter Laser Altimetry) map of Mars and the white dot indicates the location of Miyamoto crater. Images credit: NASA/JPL/Arizona State University. (b) CRISM IR observation FRT0000C71F in false RGB color where red = 2.3 μm, green = 1.5 μm, and blue = 1.1 μm. Credit: NASA/JPL/John Hopkins University.

[3] One of the most striking morphological features of the crater floor is a sinuous ridge (Figure 1) that descends from near the rim toward the center of the crater [Newsom et al., 2009a]. The feature occurs north to the region of the crater floor enriched in phyllosilicates [Wiseman et al., 2008]. Here we present the first analysis of a high spatial resolution (17.6 m/pixel) observation by CRISM (Compact Reconnaissance Imaging Spectrometer for Mars [Murchie et al., 2007]) centered on this feature (3.0S, 352.1E). After a brief summary of the geological context we describe our analysis process. The results from spectroscopic measurements are associated with images from HiRISE (High Resolution Imaging Science Experiment [McEwen et al., 2007]) and the digital elevation model of the region obtained by HRSC (High Resolution Stereo Camera [Jaumann et al., 2007]). Then we discuss our findings and draw some conclusions.

2. Geology

[4] The north and north-eastern half of Miyamoto crater is buried by a plains-forming unit composed of light-toned layered sedimentary rock overlain by a thin regolith of basaltic sand, granules, and nanophase iron oxides [Arvidson et al., 2006] with outcrops of “dirty” sulfate-rich evaporites [Grotzinger et al., 2005; Morris et al., 2006]. The extent of the plains unit corresponds with occurrences of crystalline gray hematite [Christensen et al., 2000, 2001] which hosts the landing site of the MER Opportunity rover.

[5] In the western portion of the Miyamoto crater floor extensive deposits of Fe/Mg-smectites have been identified and interpreted as exhumed materials pre-dating the formation of the sulfate- and hematite-bearing plains unit [Wiseman et al., 2008], suggesting a general transition from neutral pH to a more acidic pH through time [Bibring et al., 2006; Wiseman et al., 2008].

[6] The western floor of the Miyamoto crater contains morphological features that are suggestive of past fluvial activity and the presence of water. Along with other mesa-like features to the south, a raised curvilinear feature has been interpreted as a sinuous, narrow inverted paleochannel deposit ∼25 km in length and 30–50 m height [Newsom et al., 2009a]. In Terrestrial desert settings, paleochannels form initially as deposits of fluvial sediments within stream channels that often contain a significant amount of gravel. These stream sediments become cemented rapidly upon deposition and the relatively large grain size (usually from sand to gravel) provides increased resistance to erosion [Holm, 1960]. Subsequent erosion may eventually exhume these channels by removing the surrounding unconsolidated material and leaving the indurated channel deposits in relief [Newsom et al., 2009a]. Below we follow the notation from Newsom et al. [2009a] when applicable.

3. Analysis and Results

[7] CRISM observation FRT0000C71F (Figure 1) is centered on the sinuous ridge and offers a unique opportunity to further investigate the mineralogical characteristics of this feature and its surroundings. CRISM measures reflected energy from the surface that provides the ability to recognize primary and secondary minerals [Murchie et al., 2007]. CRISM data have been converted to apparent I/F (i.e., the ratio of reflected to incident sunlight), then divided by the cosine of the incidence angle to correct for the illumination geometry. The gas atmospheric contribution has been removed using a volcano-scan technique correction proposed by McGuire et al. [2009], which is an improvement to previous atmospheric corrections [Erard and Calvin, 1997; Murchie et al., 2007]. This choice retains the rapid analyses capabilities as the previous volcano-scan techniques and provides for more-thorough analyses of surface hydration.

[8] In the considered CRISM observation a material dominated by a Fe/Mg-smectite occurs, previously described by Wiseman et al. [2008] based on CRISM data south of the putative inverted channel. The most prominent feature of the spectrum is a band at about 1.92 μm typical of hydrated minerals. A weaker hydroxyl band at 1.41 μm and a series of bands at 2.31 μm, 2.39 μm, consistent with Fe/Mg-smectites [Clark et al., 1990], are also present. The phyllosilicate deposits have been mapped based on the presence of the 2.3 μm spectral absorption feature using the D2300 parameter index defined by Pelkey et al. [2007]. Values of D2300 are shown as green to white shade in Figure 2b and represent the potential locations of phyllosilicates.

Figure 2.

(a) 1.3 μm IR albedo [Pelkey et al., 2007] of CRISM observation FRT0000C71F. (b) D2300 parameter in shades from green to white representing different relative amounts of phyllosilicates. Figures 2a and 2b are wrapped on a digital elevation model by HRSC at 75 m/pixel with 20× vertical exaggeration. HiRISE details from observation PSP_009985_1770 (red channel) of (c) a portion of the putative paleochannel and (d) a region showing multiple geological units and a proposed stratigraphy. The approximate location of the two HiRISE images is shown in Figure 2a. Polygonally-fractured terrains are associated with phyllosilicates (KF). Capping units (PRd and PRu) have been spectroscopically compared showing no significant differences. Stratigraphic units follow the notation from Newsom et al. [2009a]: unit PR is divided here into two distinct units (PRd and PRu) and unit Bp does not extend in the studied region. CRISM credit: NASA/JPL/John Hopkins University, HiRISE credit: NASA/JPL/University of Arizona, HRSC credit: ESA/DLR.

[9] Both the CRISM observation and locations of the Fe/Mg-smectite, as defined by D2300, shown in Figures 2a and 2b, respectively, have been wrapped on digital terrain model from Mex/HRSC [Jaumann et al., 2007]. Phyllosilicates occur on the slopes of the sinuous ridge and in areas adjacent to those slopes. In particular, they seem to be absent on top of the ridge while they are located on both its sides. We have selected an area on top of the ridge (unit PRu, Figure 2c) and have compared its spectral signature to a region of equivalent area (∼100 pixels) and D2300 parameter value (unit PRd, Figure 2d). The spectral signature of the cap layer of the putative inverted channel is similar to that of the surrounding material without the phyllosilicate signature, independent of altitude (Figure 3, middle).

Figure 3.

(top) Average I/F corresponding to high values of the D2300 parameter (label KF). (middle) A continuum removed spectrum and its ratio (label KF ratio) to a region without the phyllosilicate signature (unit PRd) and two continuum removed average spectra (offset for clarity) from the cap units (units PRd and PRu from Figures 2c and 2d, respectively). The spectral regions where the strongest residual of the atmospheric correction occur are indicated in gray. The function used for removing the continuum of the D2300 spectrum is shown in Figure 3 (top) as a thick dashed line. (bottom) Continuum removed laboratory spectra of selected minerals from the CRISM spectral library (offset for clarity).

[10] Using the HiRISE images of the area we suggest an overall stratigraphy for the CRISM scene. HiRISE images of the region, as reported in Figures 2c and 2d, show that phyllosilicate deposits are located in a unit containing polygonal fractures (unit KF). From the HiRISE observations, a relatively smooth, thin (∼1–3 m) unit (PRd) overlies KF and is limited to low elevation areas of the scene. The PRd material is consistent, from a textural point of view, with that observed further south [cf. Wiseman et al., 2008, Figure 4]. An additional capping unit (PRu) sharing similar spectral properties as PRd occurs on top of the sinuous ridge and in several scattered outcrops in the southern part of the region (e.g., Figure 2d). However, based on the elevation of the PRu terrain relative to the other units, between 20 to 100 meters, and textural differences, we identify it as a distinct capping unit which postdates the formation of KF and PRd. Aeolian deposits (Ao) are observed widespread in the region and partially cover the three units described above. A summary of the stratigraphy occurring in the scene is provided in Figure 2d.

4. Discussion

[11] We considered two possible scenarios to account for the phyllosilicates presence in the area around the sinuous ridge; 1) aeolian deposition from the deposits to the south in relatively recent times; and 2) deposition or formation of phyllosilicates in the local environment related to water activity.

[12] Aeolian deposits are widespread in the region raising the possibility that the phyllosilicates were transported from the southern region where phyllosilicate-bearing deposits have been reported [Wiseman et al., 2008], and have been emplaced in the current topography. The spectral signature of the aeolian material indicates that it is more similar to the capping units, PRd and PRu, instead of the KF, where phyllosilicates occur. This is consistent with the previous interpretation by Newsom et al. [2009a] that the ridge is covered by dust. The simultaneous presence of smectites on both sides of the sinuous ridge, their association with steep walls, and the absence of smectites in the low elevation areas immediately to the south of the putative channel and on top of the ridges, all argue against the deposition of phyllosilicates due to aeolian processes. In addition, the observed spectral signature of the aeolian material is not consistent with phyllosilicates. This leads us to reject an aeolian origin for the phyllosilicate deposits.

[13] The phyllosilicates may have been emplaced in the local environment due to water activity. The phyllosilicates could have resulted from alteration of materials that were later deposited forming the bedrock, or were formed by aqueous in situ alteration of the bedrock. Subsequently the hydrological system could have emplaced a more coherent cap, which has armored the channel(s) from erosion, leading to the sinuous ridge formation. The presence of capping units on top of the phyllosilicate-bearing material suggests that the phyllosilicates have also been exhumed by erosion and their origin pre-dates the inverted channel. Thus the observations are consistent with deposition or formation of phyllosilicates in the local environment related to water activity. However, the available data preclude us from selecting between in situ formation of the phyllosilicate deposits and their deposition due to fluvial activity. In either case there is evidence for at least two episodes of aqueous activity: an earlier one which emplaced or formed the phyllosilicate deposits followed by a second event which led to the presence of the overlying capping layers that are the armoring materials that assisted in the formation of the putative inverted channel.

5. Conclusions

[14] We have analyzed a CRISM observation centered on a sinuous ridge in Miyamoto crater, previously interpreted as an inverted paleochannel deposit [Newsom et al., 2009a]. The putative inverted channel is located in an area enriched in Fe/Mg-smectite which is associated with the slopes of the ridge and areas adjacent to those slopes. Geomorphological data suggests that the phyllosilicates are consistent with ancient deposits exhumed by erosion, as previously suggested [Wiseman et al., 2008; Marzo et al., 2009; Newsom et al., 2009a, 2009b]. The identification of phyllosilicates and the geological observations build confidence for the presence of water in the past on the floor of Miyamoto crater. Thus, the site remains a potentially interesting location for future in situ study.


[15] This research was supported by an appointment to the NASA Postdoctoral Program at the Ames Research Center, administered by Oak Ridge Associated Universities through a contract with NASA. TLR acknowledges NASA Mars Reconnaissance Orbiter Program for supporting his participation as a CRISM science team member. We are grateful to the CRISM, HiRISE, HRSC, MOLA, and THEMIS teams for making data available for this project. We wish to thank Alberto G. Fairén and an anonymous reviewer for helping us to improve the manuscript and Mark R. Salvatore for useful discussions.