Sorting indicates former floods
This grading, with the thickest chunks at the bottom, is characteristic of material that was transported by water and then settled. Large rocks sink to the bottom first, then as the water calms down, smaller rocks, gravel, sand and mud follow. The upper, such layer is about 20 meters thick, the one below about 50 meters. According to the working group, the radar data indicate that in the lowest layer at a depth of 80 meters there could be rock debris up to several meters in size.
Li’s team indirectly determined the age of the suspected flood deposits. Based on the number and size of the craters in the region, it concludes that around 1.6 billion years ago a layer around 40 meters thick covered the region. This fits well with the top of the two sorted layers, which would have been deposited when Mars already had a relatively cool and dry climate. The lower, thicker layer could be around 3.2 billion years old, based on similar estimates from large craters. Beneath both lies the even older Vastitas Borealis Formation, which is being suggested as a possible residue of a once permanent North Ocean.
The findings of the ground radar thus suggest a possible chronology of the water in Utopia Planitia. Once upon a time, a gigantic ocean would have covered the lowlands of the northern Martian hemisphere. As Mars became colder and drier, the sea disappeared. But twice more, massive floods that swept away mud and debris temporarily filled the area with water. Finally, an icy, dry plain remained, in which only traces tell those who know about the humid past.