Mercury鈥檚 pumpkin pie recipe:
鈥� One ready-made impact crater crust
鈥� One large helping of gooey volcanic filling
鈥� A pinch of global cooling
The planet Mercury used these ingredients to bake several strange crater pies, complete with wrinkled crusts and cracked fillings. Spotted by a NASA probe, the craters are unlike anything seen on other rocky worlds, adding to the diversity of geologic processes known to occur in the solar system.
The pies were found in Mercury鈥檚 northern highlands, which were flooded repeatedly with volcanic eruptions early in the planet鈥檚 history. These floods buried underlying impact craters, but as the lava cooled, the crater rims became visible as the material above them wrinkled and split, forming the edges of the pie crust.
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The interiors of the craters are criss-crossed with cracks called graben, which form when rock is stretched horizontally. These cracks reminded scientists of the fissures that can form as a pumpkin pie cools.
鈥淚 haven鈥檛 really done the tectonics of the pie,鈥� admits of the Smithsonian Institution in Washington, DC. 鈥淏ut there are a number of similarities.鈥�
Still shrinking
Since becoming the first satellite to orbit Mercury last March, NASA鈥檚 Messenger spacecraft has been beaming back a plethora of surprises, from huge magnetic whirlpools to bright 鈥榟ollows鈥�, pits that are unique to the tiny world.
Other recent data from Messenger shows that Mercury has a large, partially liquid core, implying that the planet is still cooling and shrinking. Most of the surface is covered with scarps and ridges that formed when blocks of material squeezed together.
Images from Messenger now reveal the pie-like craters throughout the northern volcanic plains, which cover about 6 per cent of the planet鈥檚 surface.
鈥淲e鈥檝e never seen anything like that on Mercury, and we鈥檝e never really seen anything like that on any of the other terrestrial planets,鈥� Watters says. 鈥淲hat is happening here? Something definitely different is going on on Mercury that hasn鈥檛 happened on the moon or Mars.鈥�
Deep dish
Watters and colleagues modelled the formation of the craters and concluded that the cracks could have formed only in these pies, where lava arrived in quick bursts and then pooled deep enough to act just like pie filling.
鈥淚t鈥檚 the same process,鈥� Watters says. 鈥淚t鈥檚 cooling and contracting, and you鈥檙e getting some resistance to that contraction because the pie filling is in contact with the edges of the pie crust, and that crust is resisting its contraction.鈥� By contrast, Mars and the moon鈥檚 volcanic flooding happened in thin gushes over the course of hundreds of millions of years, limiting their baking skills.
鈥淚 think it鈥檚 another indication of just how unusual Mercury is among the terrestrial planets,鈥� Watters says. 鈥淭he tectonics of Mercury may be as unique in our solar system as plate tectonics is on Earth.鈥�
Journal reference: , DOI: 10.1130/G33725.1
