Mercury’s Bombardments Reveal Earth’s Habitability

Mercury’s Bombardments Reveal Earth’s Habitability

Ancient battlefields do not readily offer evidence of what transpired in the heat of combat. Historians and scientists must construct scenarios from various lines of evidence, including obscure places, to learn what happened. In a similar way, sometimes scientists must search in the obscure places, such as the dark recesses of Mercury’s surface, for evidence of a different type of bombardment that ultimately exhibits evidence for Earth’s design.

Pictures of Mercury show a surface covered in craters, the vast majority of which, studies show, formed around the same time (the oldest craters date to 4.1 billion years ago). The sizes, distribution, and ages of the craters seem similar to the Moon’s craters. Taken together, these two cratered surfaces point to a period when an enormous number of asteroids and comets careened through the inner solar system and pummeled all the rocky planets (Mercury, Venus, Earth, and Mars, plus the Moon). The atmospheres of and geological activity on Venus, Earth, and Mars have erased crater signatures on those planets, but scientists can use the Moon and Mercury to study this era referred to as the Late Heavy Bombardment (LHB), which seems to carry design implications.

Due to difficulties in reaching Mercury by spacecraft, only two missions have visited the planet. In the 1970s, Mariner 10 entered orbit around the Sun in such a way that it was able to pass by the planet closely every few months. During the flybys, Mariner 10 provided close-up images for 45 percent of the surface (the same side each time), but the rest of Mercury remained in darkness. In March 2011, the MESSENGER spacecraft entered orbit around Mercury and has now mapped the entire surface (including the dark side) of the planet.

Scientists used MESSENGER data to demonstrate that no terrain on Mercury’s surface dates older than 4.0–4.1 billion years ago, the time corresponding to the start of the LHB. The impact of large bodies with Mercury also triggered massive volcanic activity that resurfaced the entire planet, removing any evidence from the first 400–500 million years of its history (the solar system planets formed 4.56 billion years ago).1

Until this study, the possibility of finding formations dating to periods earlier than the LHB remained open. Such a finding would have cast doubt on the general understanding of the LHB and the catastrophic effects it presumably wrought on Earth. These latest results remove that possibility and confirm scientists’ understanding of the timing and consequences of the LHB.

Statistically, once every 500,000 years, objects one kilometer in size hit Earth and release an amount of energy similar to the simultaneous detonation of the world’s entire nuclear arsenal. Without the LHB, impacts of this size would occur every 500 years, putting the planet in frequent peril. Much of the asteroid and comet debris left over from the formation of the solar system was cleared because of the LHB, thus reducing the number of impacts. Ultimately, by decreasing these catastrophic impacts, the LHB increased Earth’s capacity to support human life.

  1. Simone Marchi et al., “Global Resurfacing of Mercury 4.0–4.1 Billion Years Ago by Heavy Bombardment and Volcanism,” Nature 499 (July 4, 2013): 59–61.