Why are Small Planets so Diverse?
When
Where
Dr. Erik Asphaug
Professor
Arizona State University
Venus and Earth contain 93% of the inner solar system mass, and are geochemically similar at least in bulk. Formation models predict this similarity. But Mars, the Moon and Mercury are profoundly unique: the Moon is missing its metallic core and is devoid of volatiles, Mercury is missing its silicate mantle but is replete with volatiles, and Mars seems ‘average’ or ‘chondritic’. Models have been constructed for the Moon and Mercury around the idea of one or more giant impacts, but the scenarios have problems. For one thing, the canonical giant impact model for the Moon is ‘gentle’, impacting at near escape velocity, while the prevailing giant impact model for Mercury is as energetic as can be, several times v-escape. More critically, if proto-Mercury was shredded catastrophically, at the required energy, why does the finished planet retain the volatiles and semivolatiles detected by MESSENGER and groundbased observations, yet not the Moon? And where does Mercury’s silicate mantle disappear? Other planetary populations also exhibit striking diversity: the KBOs (bulk densities ranging from 0.6 to 3-4 g/cc), the middle-sized moons of Saturn (rocky worlds alongside pure-ice worlds), and main belt asteroids (metallic dwarfs like Psyche and silicate-dominated worlds like Vesta). These oddities can be explained as signatures of the distinct mess that is pairwise accretion. I will survey this diversity, consider a ‘multiple hit and run’ solution for Mercury, venture a ‘hit and run return’ solution for the Earth and Moon, and as time permits, attempt to explain small planetary diversity more generally by analogy with the Anna Karenina Principle, that “happy families are alike; every unhappy family is unhappy in its own way.”