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).