Dark basaltic dunes represent the majority of Martian aeolian bed forms. However, on Earth there are only few places where basaltic dunes can be found, including New Zealand, Iceland, the western USA, Peru, and Hawaii [Edgett and Lancaster, 1993]. Is has been suggested that the Marian dunes sands are volcanic in origin because their mineralogical composition consists of pyroxene and olivine [e.g. Bandfield, 2002; Hoefen et al., 2003; Tirsch et al., 2009]. The dark dunes in Ka’u Desert on the Big Island of Hawai’i are located on the western flank of Kilauea volcano (Fig. 1). The dark sands are derived from volcanic ash and reworked pyroclastic material [e.g. Edgett and Lancaster, 1993; Power, 1948; Stearns, 1925]. Thus, the Hawaiian dark sand dunes are an adequate analogue to Martian dunes.
Samples of different dark dunes in Ka’u Desert were collected during a field trip in summer 2009. Several samples were taken from a large, dark vegetated parabolic dune (Fig. 2, sample 6), a falling dune (Fig. 3, sample 1), and a large dark climbing dune (Fig. 4, sample 2). Figure 5 presents a dark climbing dune on Mars for comparison with the climbing dune on Earth (Fig.4). The sand samples have a dark grayish color and are of fine- to coarse-grained sand sizes (Fig. 7 A, B, C). The sand samples were measured with an ASD field spectrometer [ASD, 2009] in a laboratory to gain insight into their mineralogical composition. The terrestrial spectra were compared with typical near-infrared spectra of Martian dark dune fields as measured by the OMEGA spectrometer onboard the MarsExpress orbiter [Bibring et al., 2004].
A comparison between spectra of Martian and terrestrial dark dunes and library spectra is shown in figure 8. The Martian OMEGA spectrum (black curve) reflects the basaltic composition of the dark dunes as it is typical for Martian dunes. The spectrum shows a deep broad absorption band at 1 μm (slightly shifted to shorter wavelengths due to the mineral mixture) and a broad shallower band around 2 μm. Both bands result from Fe2+ in the minerals and are indicative of a mixture of olivine and pyroxene. The terrestrial spectra (sample 1, 2, 6) strongly reflect the olivine content of the dark sands as indicated by the deep broad absorption band at 1 μm. A pyroxene absorption around 2 μm is less obvious in sample 1 and 6 and seems to be overlapped by other features, although it still exists. The increase of the spectra to higher wavelengths, results from the alteration of these silica materials. Sample 2 strongly exhibits a narrow 2.2 μm-band, probably related to bending metal-OH-bounds, and a further absorption at 1.9 μm, which is generated by molecular H2O in the minerals. Thus, a beginning aqueous alteration is obvious in the spectral shape of sample 2. The coarser grain size (1-3 mm) of this sample results in the decrease of the reflectance intensity and anincrease of the absorption band depths [e.g. Gaffey, 1987; Poulet et al., 2007].
The spectrum of Iddingsite is shown as a representative of a common alteration product of olivine on Earth, comprising phyllosilicates, iron oxides, and hydroxides [Cloutis, 2004; Orofino et al., 2006]. However, this library spectrum represents a sample, which is in its initial alteration phase, as indicated by the lack of the 1.4 μm band, and the weakness of the 1.9 μm water associated absorption band [Orofino et al., 2006]. However, it represents a good intermediate stage between unaltered and altered olivine-rich material.
The overall spectral shape of the terrestrial spectra reflects a basaltic composition of the sands similar to that of Martian dunes. The rock-forming minerals olivine and pyroxene form as the lava cools, and are commonly found in basaltic volcanic ash. The correlation in mineralogical composition of terrestrial and Martian dunes hints to a similar origin of the dark sands on
Mars and Earth. The sources of the Ka’u Desert dune sands are ashes erupted from the volcanoes in the vicinity and lava disintegration particles [e.g. Edgett and Lancaster, 1993; Power, 1948; Stearns, 1925]. A similar volcanic ash origin for Martian dunes has been suggested by [Tirsch et al., 2009], who found dark layers of fine-grained materials exposed in impact craters and a material transport to the dark intra-crater dune fields. Based on the mineralogical similarities and the morphological evidence, the sources of the dark material on Mars are probably layers of volcanic ash [Tirsch et al., 2009]. Since the terrestrial spectra show a beginning aqueous alteration of the dark sands these samples could be used to analyse alteration features of Martian dark dunes.
Last update: 09/11/2011 10:35
