The redness of the sand comes from centuries of iron oxidation – the older the dunes, the redder they grow (http://www.jenmansafaris.com/african-countries/namibia/attractions/sossusvlei.html). These dunes here are among the world’s oldest, allegedly dating some 5 million years back. Iron oxidation, by the way, is what gives Mars its distinctive color. But if everything in the Solar System was formed around the same time – a not-too-distant 4.57 billion years ago – shouldn’t Earth be red as well? According Space.com, because of Earth’s higher mass (and hence stronger gravitational pull), much of its iron constituents sank to the core during the formative years (http://www.space.com/16999-mars-red-planet.html). A notable exception here would be Mercury, Solar System’s smallest planet yet with the most iron-dense core. As of this writing, the jury is still out on why that is the case. If only we can live forever just to watch the mysteries of our universe unfold.