Mars' wild years

The early Solar System was a chaotic place, with evidence indicating that Mars was likely struck by planetesimals, small protoplanets up to 1,200 miles in diameter, early in its history.

An important open issue in planetary science is to determine how Mars formed and to what extent its early evolution was affected by collisions. This question is difficult to answer given that billions of years of history have steadily erased evidence of early impact events. Luckily, some of this evolution is recorded in martian meteorites. Of approximately 61,000 meteorites found on Earth, about 200 or so are thought to be of martian origin, ejected from the Red Planet by more recent collisions. These meteorites exhibit large variations in iron-loving elements such as tungsten and platinum, which have a moderate to high affinity for iron. These elements tend to migrate from a planet’s mantle and into its central iron core during formation. Evidence of these elements in the martian mantle as sampled by meteorites is important because they indicate that Mars was bombarded by planetesimals sometime after its primary core formation ended.

To investigate how projectile materials were delivered to early Mars, we performed smoothed-particle hydrodynamics impact simulations (Figure 1). Based on our model, early collisions produce a heterogeneous, marble-cake-like martian mantle. These results suggest that the prevailing view of Mars formation may be biased by the limited number of meteorites available for study.

Figure 1. An animation of a high-resolution, smoothed-particle simulation of a large, differentiated projectile hitting early Mars after its core and mantle had formed. The projectile’s core and mantle particles are indicated by brown and green spheres respectively, showing local concentrations of the projectile materials assimilated into the martian mantle (click here to enlarge in a new page) (SwRI/Marchi).

In addition, studying isotopes of particular elements produced locally in the mantle via radioactive decay processes helps scientists understand when planet formation was complete. Based on the ratio of tungsten isotopes in martian meteorites, it has been argued that Mars grew rapidly within about 2–4 million years after the Solar System started to form. However, large, early collisions could have altered the tungsten isotopic balance, which could support a Mars formation timescale of up to 20 million years, as shown by our model.

The martian meteorites that landed on Earth probably originated from just a few localities around the planet. Our research shows that the martian mantle could have received varying additions of projectile materials, leading to variable concentrations of iron-loving elements. The next generation of Mars missions, including plans to return samples to Earth, will provide new information to better understand the variability of iron-loving elements in martian rocks and the early evolution of the Red Planet.

It is possible that these early, large collisions helped in establishing liquid water at the surface of Mars, with consequences for its early habitability (Figure 2). The bottom line is that to fully understand Mars, we need to understand the role the earliest and most energetic collisions played in its evolution and composition.

Figure 2. An artistic rendering of how early Mars may have looked, showing signs of liquid water, large-scale volcanic activity, and cratered terrain due to heavy bombardment from planetary projectiles (click here to enlarge in a new page) (SwRI/Marchi).

This research was published in the February 12th 2020 issue of Science Advances (here).