In contrast, the sparsely cratered lowlands formed near the end of the Late Heavy Bombardment, about 3.8 billion years ago. Mariner 10 data suggested that the lowlands formed either from volcanic activity or from the molten material splashed onto the surface after large impacts. Although the spacecraft found no obvious smoking gun for volcanism — such as lava flows, volcanic domes, or volcanic cones — it did uncover strong circumstantial evidence.
Mariner 10’s successor, NASA’s MESSENGER spacecraft, provided the proof. During its initial flyby in January 2008, the probe revealed a fractured region of ridges and furrows within the huge Caloris Basin. MESSENGER would go on to fly past Mercury twice more, in October 2008 and September 2009, and then orbit the inner world for four years starting in March 2011. While in orbit, the spacecraft discovered at least nine overlapping volcanic vents, each up to 5 miles (8 km) across and a billion years old, near Caloris’ southwestern rim. Elsewhere on Mercury, MESSENGER uncovered residue from more than 50 ancient pyroclastic flows — violent outbursts of hot rock and gas — tracing back to low-profile shield volcanoes, mainly within impact craters.
Caloris itself is an impressive relic from Mercury’s tumultuous early days. The Sun illuminated only half the basin during Mariner 10’s visits, so it was left to MESSENGER to fully reveal its structure. Caloris spans 960 miles (1,550 km), placing it among the largest impact features in the solar system, and it is ringed by a forbidding chain of mountains that rises 1.2 miles (2 km) above the surroundings. Beyond its walls, ejecta radiate in meandering ridges and grooves for more than 600 miles (1,000 km). The impact that created Caloris was so globally cataclysmic that strong seismic waves pulsed through Mercury’s interior and fragmented the landscape on the planet’s opposite side, leaving a region of jumbled rocks, hills, and furrows that some scientists have dubbed “weird terrain.”
Despite Caloris’ huge dimensions, Mercury itself is quite small — just 3,032 miles (4,879 km) in diameter. The planet’s small size and high temperature led mid-20th-century astronomers to suspect it could not retain an atmosphere. But Mercury is full of surprises. Mariner 10 discovered a thin layer of loosely bound atoms, known as an exosphere, albeit with a surface pressure trillions of times less than that at sea level on Earth. It contains hydrogen and helium atoms captured from the solar wind — the stream of charged particles emanating from the Sun — together with oxygen atoms liberated from the surface by micrometeoroid impacts. Spectroscopic observations also revealed sodium, potassium, calcium, magnesium, and silicon. Caloris and the weird terrain appear to be key sources of sodium and potassium, indicating that impacts can release gases from below the surface.