Dr. Simone Marchi, a principal investigator at the Southwest Research Institute (SwRI) in Boulder, Colorado, recently expounded on the importance of impact histories. This is especially important in our ongoing efforts to detect habitable exoplanets like Earth. In a paper published in the journal Nature, titled “The shaping of terrestrial planets by late accretions,” Marchi and his team discuss how the evolution of terrestrial planets, including Earth, Mars, and Mercury, was significantly influenced by late large impacts.
Looking at the timing of Earth’s formation, we know that it had to be the last terrestrial planet to form. Marchi says that Earth only achieved about 99% of its final mass in a relatively short timespan of 60 to 100 million years. This rapid growth happened just after the first solids started to settle. This quick accumulation of mass indicates that late accretion events were very important. Early volcanoes didn’t just help sculpt our home planet—they helped forge its neighboring, rocky planets as well.
The Role of Late Accretion
Late accretion is a named eon in which the leftover material in our solar system continued to collide with the newly-formed planets. Marchi’s research shows that these impacts were decisive in shaping the geological and atmospheric characteristics of terrestrial planets. These enormous impacts on Mars have generated significant surface heterogeneity. These same impacts were responsible for creating the first major, lasting disparities between Earth’s hemispheres.
Further, the extreme metal-to-silicate mass ratio observed on Mercury is attributed to late accretionary impacts. This implies that impacts played a significant role in determining Mercury’s composition, making it the odd man out among rocky planets. Reconstructing these dynamics, Marchi said, will help scientists learn what conditions might be required for life on other planets.
Insights from Lunar Impacts and Dynamic Models
Scientists have a number of techniques to reconstruct the bombardment history of rocky planets. These techniques comprise lunar impact studies, further observations, and dynamical models. The moon is a time capsule of previous impacts, giving scientists valuable data to study and learn from. We can better understand the frequency and scale of these impacts by studying ancient craters and impact features on the lunar surface. Research like this is critical for understanding what Earth and other terrestrial planets went through.
Marchi and his colleagues use these techniques to drill down and figure out how late accretion sculpted the terrestrial planets into what we see today. Taken together, their research illuminates the relationship between tectonics, atmospheric composition, and the existence of liquid water on Earth and Venus. All of these factors appear to be influenced by major events.
Implications for Exoplanet Research
The findings from Marchi’s research have significant implications for the search for Earth’s twin among exoplanets. As a result, the scientific community is eager to find rocky planets. In particular they focus on ones with similar bulk properties, such as mass, radius, location within a habitable zone etc. As Marchi explains, collision history should be a key factor in this search, too.
By studying exoplanets that have had similar impacts to Earth, scientists can assess the likelihood that these planets are habitable in a much more informed way. Transient subduction events occur when one tectonic plate dives under another. In particular, these events could provide vital context as we search for geological processes that might nurture life.