Report

Radar Sounding of the Medusae Fossae Formation Mars: Equatorial Ice or Dry, Low-Density Deposits?

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Science  16 Nov 2007:
Vol. 318, Issue 5853, pp. 1125-1128
DOI: 10.1126/science.1148112

Abstract

The equatorial Medusae Fossae Formation (MFF) is enigmatic and perhaps among the youngest geologic deposits on Mars. They are thought to be composed of volcanic ash, eolian sediments, or an ice-rich material analogous to polar layered deposits. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument aboard the Mars Express Spacecraft has detected nadir echoes offset in time-delay from the surface return in orbits over MFF material. These echoes are interpreted to be from the subsurface interface between the MFF material and the underlying terrain. The delay time between the MFF surface and subsurface echoes is consistent with massive deposits emplaced on generally planar lowlands materials with a real dielectric constant of ∼2.9 ± 0.4. The real dielectric constant and the estimated dielectric losses are consistent with a substantial component of water ice. However, an anomalously low-density, ice-poor material cannot be ruled out. If ice-rich, the MFF must have a higher percentage of dust and sand than polar layered deposits. The volume of water in an ice-rich MFF deposit would be comparable to that of the south polar layered deposits.

Units of the Medusae Fossae Formation (MFF) occur discontinuously at equatorial latitudes along the boundary of the hemispheric dichotomy from Amazonis to Elysium Planitiae (∼130°E to 240°E) (1, 2). The MFF may be among the youngest surficial deposits on Mars, unconformably overlying ancient Noachian heavily cratered highlands and young Amazonian lowlands (18). However, pedestal craters on the outer edge of the MFF deposits have been cited as evidence of an older age (9). The local topographic relief of the MFF units varies greatly, reaching a maximum of more than 3.5 km (57). The morphology of the MFF units is complex and variable. Over large horizontal scales (tens of kilometers), the undulating hills of the MFF are relatively smooth (Fig. 1). At smaller scales, many of the MFF units are marked by systems of parallel ridges and grooves interpreted as yardangs (1014) (Fig. 2). Remnant yardangs and outliers some distance from the thicker units suggest that MFF deposits once covered a larger area of the northern lowlands (3, 6) (Fig. 2A). Layering is observed in the MFF deposits that varies in scale from coarse, indurated layers that cap weaker, more friable material to thin, pervasive layering (3, 4, 6, 1517).

Fig. 1.

The Medusae Fossae Formation in Elysium and Amazonis Planitiae along the dichotomy boundary. The locations of MARSIS orbit tracks 2896, 4011, 3868, 3824, 4117, and 3996 are indicated by back lines (from left to right, respectively) overlaid on Mars Orbiter Laser Altimeter (MOLA) color-coded shaded relief. The locations of Thermal Emission Imaging System (THEMIS) images shown in Fig. 2 are indicated by the small white rectangles.

Fig. 2.

High-resolution THEMIS images of the MFF material in Elysium Planitia. (A) THEMIS visible image (frame #V05275021) shows an outlier of MFF material that is being stripped, partially exposing the underlying lowlands plains. (B) THEMIS visible image (frame #V13163010) shows numerous yardangs and a valley stripped of MFF material. The locations of images (A) and (B) are to the left and right, respectively, of orbit track 2896 shown in Fig. 1.

A variety of origins have been proposed for the MFF deposits. These include ignimbrite or volcanic ash deposits from now-buried vents (1, 2, 6, 11), eolian deposits from materials weathered early in martian history (1, 18), and deposits analogous to polar layered and circumpolar deposits formed either as a consequence of polar wandering (9) or during periods of high obliquity (7) [see supporting online material (SOM) text]. Units of the MFF are associated with the “Stealth” region on Mars (SOM text), so named because no echo is detected in 3.5- and 12.6-cm Earth-based radar data (1921).

We report here on observations of the MFF deposits by the MARSIS radar sounder (22) (SOM text). Subsurface echoes are detected that correspond to the basal interface between the MFF material and the underlying plains material. We also characterize the thickness and electrical properties of the MFF deposits as a guide to their bulk porosity and/or ice fraction.

MARSIS data obtained between March 2006 and April 2007 cover all the units of the MFF (Fig. 1). Radargrams, or time-delay renderings of the sounding data along the spacecraft ground track, show subsurface echoes, offset in time-delay from the surface return, where the tracks cross the MFF (Fig. 3). The subsurface echoes generally parallel the surface return except near the margins where, in some cases, the subsurface and surface echoes converge (Fig. 3). The observed time-delay in the radargrams is consistent with the expected depth to the interface between the MFF deposits and the underlying terrain.

Fig. 3.

Radargrams showing MARSIS data for orbit 2896 (A), 4011 (B), 3868 (C), 3824 (D), 4117 (E), and 3996 (F). Echoes are plotted in time-delay versus position along the orbit. The subsurface echoes are offset in time-delay from the surface echo and are interpreted to be nadir reflections from the interface between the MFF deposits and the lowland plains material. The peak surface return is corrected to agree with the MOLA topography along the orbit track. The radargrams are resampled to a uniform along-track length of ∼1000 km. All the orbits are ascending except for orbit 2896.

The westernmost MFF deposits form low-relief, undulating hills (Fig. 1) and overlie relatively young (Late Amazonian-aged) lowlands volcanic plains associated with Cerberus Fossae (2, 8) (Fig. 2A). The inferred elevation of the subsurface interface corresponds closely with the floor of a valley separating two hills where MFF material has been almost completely stripped away, nearly exposing the Cerberus plains (Fig. 2B and Fig. 3A). The MFF material that forms Lucus Planum is deposited on older (Hesperian-aged), lowlands knobby terrain (2, 8) (Fig. 1). The interface beneath the eastern flank of this unit is flat and largely continuous (Fig. 3B). The MFF material exposed in the pronounced valley of Medusae Fossae itself extends from the northern lowlands into the ancient heavily cratered (Noachian-aged) southern highlands, locally burying the dichotomy boundary and the cratered highlands (2, 8) (Fig. 1). A generally flat, continuous subsurface interface that extends for several hundred kilometers is separated in time-delay from a shallower, discontinuous interface associated with a layer internal to the MFF deposits (Fig. 3C). The subsurface echo from the eastern flank of Eumenides Dorsum is more spread out in time-delay but appears to delineate the north-downward slope of the buried dichotomy boundary (Fig. 3D). MFF material overlying Amazonian-period volcanic plains (1, 8) forms the prominent ridges of Amazonis Mensa and Gordii Dorsum (Fig. 1). There are two parallel subsurface echoes from the valley between the ridges (Fig. 3E) that correspond to the base of the MFF material and an internal dielectric horizon. The discontinuous subsurface echoes associated with the northern tip of Gordii Dorsum correlate in time-delay with the basal echo from the valley floor (Fig. 3E, far right in radargram). The easternmost MFF deposits overlie the dichotomy boundary and Amazonian volcanic plains of Olympus Mons and the Tharsis Montes (1) and narrow northwestward into a ridge (Fig. 1). A discontinuous subsurface reflection from beneath the western part of the ridge suggests a flat basal interface (Fig. 3F).

Previous analyses suggested that some MFF units are draped over preexisting topographic rises in the lowlands (13, 23). The subsurface interfaces revealed by MARSIS suggest that MFF materials are deposited on generally planar materials in the northern lowlands and the downward slope of the dichotomy boundary (Fig. 3). MARSIS data support estimates of the total volume of MFF material calculated using apparent base-level elevations in the lowlands. These estimates range from 1.4 × 106 km3 (4) to 1.9 × 106 km3 (6).

MARSIS observations provide an opportunity to evaluate the electrical properties of the MFF where the material is deposited on lowlands plains that are exposed nearby (Fig. 2A). The observed time delays in the MFF deposits correspond to a bulk real dielectric constant ϵ′ of ∼2.9 ± 0.4 (SOM text and fig. S1), based on the projection of the surrounding plains beneath the material (Fig. 4). A variation in ϵ′ of 2.5 to 3.3 does not result in a large range in the radar-predicted thickness because h is a function of Embedded Image. The dielectric properties of a material are related to its density and composition. The real part of the dielectric constant is modulated strongly by density. The imaginary component of the dielectric constant ϵ″ and the loss tangent, tan δ = ϵ″/ϵ′, are strongly influenced by target composition. Radar losses due to attenuation in the deposits were estimated using the method outlined in Porcello et al. (24). At 4 MHz center frequency (Band 3), we obtain losses of ∼0.0048 ± 0.0024 dB/m (SOM text, fig. S2). For ϵ′ of 2.9 and a center frequency of 4 MHz, these losses correspond to a range in loss tangent of ∼0.002 to 0.006 (SOM text).

Fig. 4.

Radargram showing MARSIS data for orbit 2896 converted to depth using a dielectric constant ϵ′ = 2.9 for the MFF material. The dashed black line shows the projection of the lowland plains beneath the MFF deposits. There is good agreement between the basal reflector and the projection of the exposed surrounding plains (compare with Fig. 3A).

MARSIS studies of the PLD (22, 25) suggest a ϵ′ value of about 3, consistent with pure water ice, based on the agreement between the inferred depth of the basal interface and the projection of the surrounding surface. The loss tangent of the PLD is estimated to range from <0.001 to 0.005 (22, 25). Our analysis suggests a similar real dielectric constant (2.5 to 3.3) and a comparably low range of loss tangent (0.002 to 0.006) for the MFF materials. The loss tangents derived for the MFF deposits are below the range measured for terrestrial volcanic materials (26) but comparable to some low-titanium lunar materials (27). Thus, our first-order estimates of the dielectric losses span a range that includes some dry, unconsolidated geologic materials and mixtures of pure water ice and sediment. The real dielectric constant of the MFF and PLD deposits is also low relative to the behavior of compacted rock-derived materials, which are well fit by a function of the form ϵ′ = 1.96d, where d is the density in g/cm3 (26). A maximum ϵ′ of 3.3 corresponds to an average density of about ∼1.8 g/cm3, which is low for the expected self-compaction of 0.5 to 2.5 km of a dry geologic material.

There are two plausible interpretations of these observations. The first is that the MFF material is poorly consolidated and comprised of non-ice material with low dielectric loss. If the MFF material is an ice-poor ash or eolian deposit, it must have an unusually high porosity and low bulk density at depths up to 2.5 km to account for the estimated values of ϵ′. MFF deposits with a depth-averaged bulk density >1.9 g/cm3 will have an ϵ′ value outside the measured range.

The second possibility is that the MFF material is ice-rich, with a non-ice component of higher real dielectric constant and loss tangent (ice present as a minor component within a matrix of ϵ′ = 6 does not match the observed properties). The extensive fields of yardangs in the MFF deposits, landforms that occur in variably indurated to poorly consolidated material that is easily eroded by wind (13, 5, 6, 15), suggests that sublimation must have removed volatiles from the putative ice-rich deposits to leave meters of dust and sand. The accumulation of meters of sediments suggests that the non-ice component of an ice-rich MFF deposit may be larger than the maximum 10% estimated for the south polar layered deposits (SPLD) (25). This, in turn, suggests a higher modeled real dielectric constant than that of pure ice.

Although the real dielectric constant and dielectric losses may be consistent with an ice-rich material, the existing data do not exclude the possibility that the MFF deposits are an anomalously low density, ice-poor material. In either case, these deposits appear to have unique characteristics from other martian deposits studied to date by radar sounding. An ice-rich MFF raises the intriguing possibility of a large volume of water ice in the equatorial zone of Mars beneath a veneer of dust and sand. MARSIS observations suggest that the total volume of ice in the SPLD is ∼1.6 × 106 km3 (25). If the MFF deposits are ice-rich, estimates of their total volume suggest a volume of water comparable to that in the SPLD.

Supporting Online Material

www.sciencemag.org/cgi/content/full/1148112/DC1

SOM Text

Figs. S1 and S2

References

References and Notes

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