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Boulder halos are circular arrangements of clasts present at Martian middle to high latitudes. Boulder halos are thought to result from impacts into a boulder-poor surficial unit that is rich in ground ice and/or sediments and that is underlain by a competent substrate. In this model, boulders are excavated by impacts and remain at the surface as the crater degrades. To determine the distribution of boulder halos and to evaluate mechanisms for their formation, we searched for boulder halos over 4,188 High Resolution Imaging Science Experiment images located between similar to 50-80 degrees north and 50-80 degrees south latitude. We evaluate geological and climatological parameters at halo sites. Boulder halos are about three times more common in the northern hemisphere than in the southern hemisphere (19% versus 6% of images) and have size-frequency distributions suggesting recent Amazonian formation (tens to hundreds of millions of years). In the north, boulder halo sites are characterized by abundant shallow subsurface ice and high thermal inertia. Spatial patterns of halo distribution indicate that excavation of boulders from beneath nonboulder-bearing substrates is necessary for the formation of boulder halos, but that alone is not sufficient. Rather, surface processes either promote boulder halo preservation in the north or destroy boulder halos in the south. Notably, boulder halos predate the most recent period of near-surface ice emplacement on Mars and persist at the surface atop mobile regolith. The lifetime of observed boulders at the Martian surface is greater than the lifetime of the craters that excavated them. Finally, larger minimum boulder halo sizes in the north indicate thicker icy soil layers on average throughout climate variations driven by spin/orbit changes during the last tens to hundreds of millions of years.

Plain Language Summary Boulder halos are enigmatic rings of boulders scattered across the Martian polar latitudes. Boulder halos have been thought to result from meteorite impacts into icy soils that sit atop rocky geological units. To test this model, we mapped boulder halos over 4,188 High Resolution Imaging Science Experiment images located between 50 and 80 degrees north and south latitude. We also examined geological and climate conditions at each site with and without boulder halos to determine if ground conditions or climate processes determine where halos are or are not present. Boulder halos are about three times more common in the northern hemisphere than in the southern, and, based on their abundance, appear to have formed over the tens to hundreds of millions of years. We show that while excavation of boulders from beneath ice and sediment layers is necessary to form boulder halos, that alone is not sufficient. Rather, surface processes like soil churning by ice sublimation or burial of wind-blown sediments either promote boulder halo preservation in the north or destroy boulder halos in the south. Intriguingly, boulder halo rocks seem to outlast the impact craters that originally excavated them.