Post by Dr. Jonathan Clarke
Soon after landing at Meridiani Planum the Opportunity rover imaged some curious wind erosion features. These were the haematite concretions commonly known as “blueberries” standing out from the substrate on stalks up to a cm or so in length. Good examples were seen at Eagle Crater, others were imaged at Fram Crater (Image 1). In places, the concretion has protected the underlying substrate from erosion. Sediments hosting the hematitic concretions have been eroded, leaving some concretions perched on small stalks. Several rocks at the Spirit landing site also show pedestals or fingers projecting away from rock surfaces.
Similar features occur on Earth, and have been called “dedos” (Spanish for finger). They have been reported from the Sahara, Mojave, and Namib deserts and are typically formed in hard, fine-grained crystalline rocks, such as marble.
The Perry Sand Hills are a group of active, transverse, source bordering dunes near Wentworth, New South Wales, Australia, adjacent to the confluence of the Darling and Murray Rivers. Small yardangs of older, very weakly indurated sand are found in some swales. Classic dedos reminiscent of those on Meridiani, and also up to one centimetre in length occur on the upwind side of the yardangs (Image 2). Unlike those at Meridiani there is no distinct head to these features and the tips are composed of only slightly more cemented material than the unconsolidated sand which disintegrates back to loose sand with firm finger pressure. Their age is unknown, but it would seem questionable that they would survive a heavy fall of rain. Thus it is possible that they formed over a period of a few months.
It is planned to visit the Perry Sand Hills during a field trip prior to the International Association of Geomorphology Conference in Melbourne in 2009, along with other south eastern Australian landforms of interest to planetary scientists.
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