Mars north polar ice far cleaner than earlier estimates, spectral reanalysis finds
A reanalysis of orbital spectra and Phoenix surface data puts dust in exposed north polar water ice below three per cent by mass, against earlier figures of up to 25 per cent.
Revised modelling of the Martian north pole has cut the estimated dust content of exposed water ice to below three per cent by mass — a maximum of roughly 2.2 per cent in the new analysis — against values as high as 25 per cent in earlier work. The sharpest revision concerns Korolev crater, where one patch there could hold about 22 per cent dust on the older reading; the new model gives about 0.4 per cent for the upper layer and 0.02 per cent for the coarser ice beneath it. The study, by Pari Mohan of the University of Edinburgh and Aditya Khuller, appears in npj Space Exploration.
The gap comes not from fresh observations but from how surface properties are retrieved from spectra. Ice brightness depends simultaneously on ice-grain size, the size and concentration of dust particles and the arrangement of layers. The recalculation assumes dust radii of about 1.6 micrometres, consistent with orbital and surface observations; finer particles influence visible-light reflectance more strongly, so fewer are needed to reproduce the observed brightness. The authors report that the new values agree better with radar measurements of the northern ice sheet. Summer sublimation of the seasonal carbon-dioxide cap exposes a permanent ice layer; beneath it lies a layered sequence almost 1.5 kilometres thick holding the record of recent climate change. Mohan and Khuller propose that relatively clean strata carrying 0.05–0.5 per cent dust alternate with dirtier 'marker' layers where dust reaches tens of per cent. They attribute seasonal brightening to a thin, fine-grained frost of one to two per cent dust that sublimates as the pole warms, revealing coarser, cleaner ice — a transition beginning earlier at Korolev because of latitude and local microclimate.
The analysis draws on Mars Phoenix, which sampled ice near the north pole in 2008, together with orbital observations. Khuller had earlier identified a problem with the prevailing method for interpreting the physical properties of Martian ice: it had been devised for lunar soil and, tested against terrestrial data, appeared wrong. The recalculation adapts an approach developed by Steve Warren, professor emeritus of Earth and Space Sciences at the University of Washington. In Khuller's account, dust darkens the ice and changes how much sunlight is reflected back to space, warming it and hastening sublimation. Outside the polar regions, most Martian ice lies buried under dust.
Layered ice and dust may carry wider implications. Khuller has previously argued that dark layers could trap sunlight and generate meltwater pockets within the ice, enriched by nutrients from the dust — conditions that could suit bacteria and primitive life, by analogy with shallow dust-laden meltwater on Earth that concentrates life in summer. He asks why one planet has life and the other does not, given the similar ice-and-dust layers. The next step is extending the revised method to other regions of Mars.