Studies of gravity variations at Mars have revealed dense, large-scale structures hidden beneath the sediment layers of a lost ocean. The analysis, which combines models and data from multiple missions, also shows that active processes in the Martian mantle may be giving a boost to the largest volcano in the solar system, Olympus Mons.

Mars has many hidden structures, such as ice deposits, but the features discovered in the northern polar plains are a mystery because they are covered with a thick and smooth sediment layer believed to be deposited on the ancient seabed.

“These dense structures could be volcanic in origin or could be compacted material due to ancient impacts. There are around 20 features of varying sizes that we have identified dotted around the area surrounding the north polar cap—one of which resembles the shape of a dog.”