Plain language summary
Linear chains of small craters are commonly observed on planetary bodies like Mars, Enceladus, or various asteroids. These craters, called pit craters, are not related to meteorite impacts. Based on comparison to similar craters observed on Earth and created in laboratory models, we think pit craters form because rock collapses into holes present beneath the surface. There are many suggestions for how these holes form, but all suggest their presence destabilises overlying rock, which drains into the hole and leaves a crater at the surface. We have not been able to test suggestions about how these holes form, because we cannot easily look beneath the surface of other planetary bodies. We use a technique called seismic reflection, which uses sound waves to create an ultrasound-like image of Earth’s subsurface. With this technique, we image the 3D shape of pit craters found offshore Australia. These data allow us for the first time to measure what pit craters look like in the subsurface and see whether they link to any structures that could have caused their formation. We show that some pit craters are connected to igneous dykes (solidified vertical sheets of molten rock) and others to faults (cracks in the rock).
Introduction
Quasi-circular topographic depressions are observed on the surface of Earth and many planetary bodies and asteroids (e.g., Figs 1A and B) (e.g., Abelson et al., 2003; Ferrill et al., 2004; Ferrill et al., 2011; Frumkin & Naor, 2019; Horstman & Melosh, 1989; Kling et al., 2021; Martin et al., 2017; Okubo & Martel, 1998; Sauro et al., 2020; Scott & Wilson, 2002; Whitten & Martin, 2019; Wyrick et al., 2004). These depressions, termed ‘pit craters’, have diameters of meters to thousands of meters and commonly arranged in linear chains (e.g., Kling et al., 2021; Whitten & Martin, 2019). The lack of raised rims and ejecta deposits around pit craters suggest they are not formed by meteorite impacts (e.g., Figs 1A and B) (e.g., Halliday, 1998; Poppe et al., 2015; Wyrick et al., 2004). Instead, pit craters are thought to reflect collapse of overlying rock and/or regolith into subsurface cavities or volumetrically depleted zones generated by (Fig. 1C) (see also Wyrick et al., 2004): (i) the dissolution of carbonate or salt (e.g., sinkholes; Abelson et al., 2003; Poppe et al., 2015; Spencer & Fanale, 1990); (ii) local porosity reduction of the host material following hydrothermal fluid flow or fault-related overpressure release (e.g., pockmarks; Velayatham et al., 2019; Velayatham et al., 2018); (iii) evacuation of lava tubes (see Sauro et al., 2020 and references therein); (iv) opening of tensile fractures (e.g., Ferrill et al., 2011; Smart et al., 2011; Tanaka & Golombek, 1989); (v) local dilation where faults are steeply dipping (e.g., Ferrill & Morris, 2003; Ferrill et al., 2011; Ketterman et al., 2015; Kettermann et al., 2019; Smart et al., 2011; Von Hagke et al., 2019); (vi) dyke intrusion (e.g., Mège & Masson, 1996; Okubo & Martel, 1998; Scott & Wilson, 2002; Wall et al., 2010); (vii) magma migration out of a reservoir (e.g., Mège et al., 2003; Poppe et al., 2015); and/or (viii) explosive volcanism (e.g., Hughes et al., 2018).