Central Elysium Planitia

Planetary Science Institute, Tucson, AZ

3

IDES, Université Paris Sud, CNRS, Orsay, France

4

LDTP, Observatoire Midi-Pyrénées, Université Paul Sabatier, CNRS, Toulouse, France

2.4.1 Introduction

The comparison of neutron signals measured by the Neutron Spectrometer to

geological maps [Scott and Tanaka, 1986; Greeley and Guest, 1987] highlights a

particular correlation in the region of Central Elysium Planitia. By Central Elysium

Planitia we refer to the flat and smooth area south east of Elysium Mons, also known as

the Cerberus Plains. The most recent volcanic unit within Central Elysium Planitia

indeed corresponds to a local maximum of neutron emission in all three ranges of

energy. This correlation makes this region an especially interesting object regarding

neutron interpretation. Such a maximum constrains the chemical composition and H

content of the material in which the signal is produced.

As the gamma subsystem onboard Mars Odyssey also constrains the

composition, this study aims to inter-compare the compositional constraints provided by

these two instruments. The first step is the identification of the geological unit from

where the anomaly originates. Particular attention will be paid to two lines of evidence:

the geologic knowledge of this region built from other experiments and the difference of

spatial structures seen by the GRS GS and GRS NS instruments. This analysis will lead

us to deepen our understanding of the actual structure of the region through integration

of the information collected by these two instruments.

2.4.2 Central Elysium Planitia

Early work [Plescia, 1990 and references herein] suggested that the Cerberus

Plains have a volcanic origin and a large number of volcanic features have been

documented subsequently. In addition to these features, a large variety of geological

structures indicate a complex history, also involving tectonic and fluvial processes

[Tanaka, 1986]. Most studies focus on the analysis of volcanic episodes involved in the

formation of the different geologic units of the area [i.e.: Plescia, 1990; Lanagan et al.,

2001a] or on the recent water flows and their interaction with the lavas [i.e.: Burr et al.,

2002; Berman and Hartmann, 2002; Head et al., 2003; Plescia, 2003]. The low viscosity

of these lavas partially explains the exceptional surface smoothness of this region. This

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smoothness results also from the young ages, some of them below a few millions of

years, as determined by crater counts [Hartmann et al., 2000; Burr et al., 2002; Vaucher

et al., 2008, in revision].

Alternative scenarios of formation have been proposed. Murray et al. [2005] and

Page [2008] defend the hypothesis of a frozen sea. However this theory would imply a

high concentration of hydrogen at shallow depth, which is not consistent with the

observations of the neutron spectrometer. This latter data reveal a very dry surface

(among the driest regions of the planet). And as will be seen in the discussion, the

compositional information retrieved from the GRS states in favor of basaltic origins, in

agreement with Jaeger et al. [2007, 2008] conclusions.

The topography of Elysium Planitia as revealed by the MOLA experiment

(Figure 2.50) shows that the volcanic unit is very flat and very smooth at three different

scales: 0.6 km, 2.4 km and 19.2 km [Kreslavsky and Head, 2000]. At a smaller scale

(decimeter scale), the region is shown to be very rough by earth-based 12.6 cm Arecibo

radar data [Harmon et al., 1992]. These observations are consistent with a lava flow

texture, while the smoothness at large scale suggests the widespread occurrence of low

viscosity lavas. The possible emplacement styles of the flows have been extensively

discussed [i.e.: Keszthelyi et al., 2000; Plescia, 2003] and include both flood lavas and

plains style volcanism.

The map derived from MGS-TES data [Putzig et al., 2005] reveals that Elysium

Planitia generally has a thermal inertia below 100 J m

s

K

(Figure 2.51). Such a

low inertia implies an at least partial dust or sand covering. This mantling probably

results from the deposition of surficial material over the fresh lavas by atmospheric

processes [Mellon et al., 2000, Putzig et al., 2005].

Figure 2.50: Topography measured by MOLA. Cylindrical projection centered on the

coordinates: latitude 15° and longitude 170° (note: parallels and meridians appear

curved because the projection is not centered on the equator). Superimposed are

contours of our region of interest.

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Figure 2.51: Thermal inertia map in J m-2 K-1 s-½ derived from the MGS-TES dataset

[Putzig et al., 2005] (same projection as in Figure 2.50). Superimposed are contours of

our region of interest.

Elysium Planitia has a uniformly bright albedo (among the highest of the planet,

except for the bare ice at high latitudes), likely a result of this mantling. The analysis of

a sample of HiRISE images of the region provides evidence of this mantling. At

HiRISE resolution (Figure 2.52), we see that this albedo is also generally high for small

objects. Figure 2.52 displays samples of HiRISE images taken at three geographic

locations (see the green squares in Figure 2.53). The first image shows platy flows that

are part of Central Elysium Planitia. Two types of terrains can be identified: a

homogeneous smooth surface at the top and hummocky terrains formed by rocks

contributing to meter scale roughness usual in lava flows at the bottom. The second

image shows sand dunes formed on a smooth terrain at the north edge of the Central

Elysium Planitia. The third image is a region of contact between Elysium Planitia and

the Highlands marked by debris aprons.

The apparent smoothness of the different observed terrains and the relatively

high thermal inertia of the entire region suggest the presence of a general mantling of

superficial material. In the upper part of the first image of Figure 2.53, we observe that

rocks poorly crop out whereas they are clearly visible in the lower part. This can be

explained by three theories: the two geological units consist in lava flows of originally

different surface roughness, or the age of the units is different, leading to a dust

mantling of variable thickness, or the flows were initially emplaced at different

elevations, which led the lower regions to trap a larger amount of dust. The presence of

nearby dunes in the second image also indicates that a global mantling hypothesis is

reasonable. It also indicates that at least part of the surficial material is sand rather than

dust. These different images tend to prove the presence of a layering of surficial

material of variable thickness.

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Figure 2.52: Several MRO HiRISE images extracts (see light green squares in Figure

2.53 for positioning). (a) Elysium Planitia platy flows (image TRA_000854_1855

centered on coordinates latitude 5.5° north and longitude 152.2° east). (b) Many small

Zunil secondary craters (image PSP_001342_1910 centered on latitude 10.7° north and

longitude 158.5° east). (c) Cerberus Plains and uplands contact (image

TRA_000840_1810 centered on latitude 1° north and longitude 174.6° east).

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Figure 2.53: Geomorphic map of Central Elysium Planitia [Lanagan, 2004]. Surfaces in

red have been interpreted as lava flows emplaced after the incision of Marte Valles.

Geological maps of the region were achieved by Scott and Tanaka [1986] and

Greeley and Guest [1987]. More detailed studies were performed by Lanagan et al.

[2001a, 2001b], Lanagan and McEwen [2003], Lanagan [2004] and Vaucher et al.

[2008, in revision]. The geological map from Lanagan’s dissertation (Figure 2.53) is

used for comparison with the neutron signals. There is a strong resemblance between

the boundary of the young lava flow unit (referred to as post Marte Valles lavas, in red

in Figure 2.53) and the region where the maximum neutron currents are emitted (the

region is the same for all three ranges of neutron energy).