Mercury is the smallest planet in our Solar System and the closest to the Sun. It’s a rocky world with a cratered surface that to the untrained eye somewhat resembles our own Moon. It’s orbital parameters mean that it experiences extreme temperature variations, ranging from -180°C at night to 430°C during the day. It completes an orbit around the Sun every 88 Earth days and rotates slowly on its axis and, despite its proximity to the Sun, still has ice in permanently shadowed craters near its poles. We know a good amount about Mercury but one aspect that has eluded us is the story of its formation.
Mercury (Credit : NASA/Johns Hopkins University)
Understanding Mercury’s formation is crucial to explaining its unusual structure. Researchers have proposed that the planet’s unique geology might have resulted from a massive collision that stripped away much of its original rocky mantle. While earlier studies focused on impacts between objects of vastly different sizes, more recent computer simulations suggest that collisions between similarly sized bodies accounts for one-third of all collisions in the early Solar System. Such an event could have been a key moment in Mercury’s geological history.
In a study recently published by a team led by Patrick Franco from the National Observatory in Brazil, they used computer simulations to explore how collisions between similar sized objects could create a planet like Mercury. The team used a specific proto-Mercury object with a mass of about 0.13 Earth masses and a 30% iron composition, in their simulations which collided with target objects of varying masses and iron content. By carefully controlling impact velocities (2.8 to 3.8 times the mutual escape velocity) and angles, they explored scenarios that were typical of the early Solar System but focussing on finding configurations that could produce a planet matching Mercury’s current mass and composition.
It’s not just Mercury that has suffered a collision. A similar collision is thought to have led to the formation of the Moon although the conditions were vastly different. Artist impression of the formation of the Moon.
The researchers ran three computer simulations; in the first set, using standard impact parameters, they failed to create a Mercury-like planet, the second run involved reducing the impact angle to create a more destructive collisions and the results were promising. The third run however was most successful and so they refined their approach by adjusting various impact parameters. This final simulation produced a remnant planet that matched Mercury’s current mass within 5% and had an iron core fraction of 0.65-0.75 (in comparison to the known current value of 0.7)
The study demonstrated that carefully controlled computer simulations of planetary collisions can create a planet remarkably similar to Mercury. By making adjustments to impact angles, speeds, and impactor masses, researchers have revealed a very likely scenario for the planet’s origins. Their findings suggest that collisions between two similarly sized protoplanets in the early Solar System were likely responsible for Mercury’s unique characteristics.
Source : Forming Mercury by a grazing giant collision involving similar mass bodies