Applying a new way in planetary formation modeling, where celestial bodies develop from tiny objects named ‘pebbles,’ researchers reveal why Mars is so much tinier than Earth. This same process also reveals the rapid creation of the gas giants Jupiter and Saturn, as announced earlier this year.
“This simulation mirrors the makeup of the inner solar system, with Earth, Venus, and a tinier Mars,” told Harold F. Levison, an Institute researcher at the SwRI Planetary Science Directorate. He is the primary author of a new paper that was announced in the Proceedings of the National Academy of Sciences(PNAS) Early Edition.
Mars contains only 10 percent of the total mass of the Earth.
The reality that Mars contains only 10 percent of the total mass of the Earth has been a mystery for solar system theorists and had puzzled them for quite some time now. In the standard model of planet creation, similarly sized objects gather and absorb through a process called accretion; rocks integrated other rocks, making mountains; then mountains combine to create city-size bodies, and further down the line planets. While common growth models make good analogs to Earth and Venus, they predict that “The Red Planet” should be identical, or even bigger in size than our planet. Also, these models also miscalculate the total mass of the asteroid belt.
“Figuring out why Mars is tinier than anticipated has been a huge problem that has foiled our modeling attempts for several decades,” told Levison. “Right now, we have an answer to that dilemma that originates directly from the planet formation process itself.”
New measurements by Levison and his colleagues Katherine Kretke, Kevin Walsh and Bill Bottke, all of SwRI’s Planetary Science Directorate analyze the development and evolution of a system of celestial bodies. They indicate that the formation of the inner solar system that we see today is really the natural outcome of a new mode of planetary creation known as Viscously Stirred Pebble Accretion (VSPA). With VSPA, dust quickly and effortlessly grows to “pebbles” – objects just a few inches ( 1 inch = 2,54 centimeters ) in diameter – part of which gravitationally collapse to create asteroid-sized objects. Under the right circumstances, these primordial asteroids can intensively absorb the remaining pebbles, as aerodynamic drag pulls pebbles into orbit, where they fall down and combine with the planetary body. This allows some asteroids to grow planet-sized over relatively short periods of time.
Still, those new techniques discover that not all of the primordial asteroids are evenly well-positioned to gather pebbles and grow.
For instance, an object the size of Ceres ( about 600 miles, or around 965 kilometers across ), which is the biggest asteroid in the asteroid belt, would have grown very rapidly near the current location of the Earth. But it would not have been able to become larger effectively near the current location of The Red Planet, or beyond, because aerodynamic drag is too low for pebble absorption to take place.
“This model has big implications for determining the past of the asteroid belt,” stated Bottke. Past models have foreseen that the belt basically contained a few of Earth-masses’ worth of material, meaning that planets started to develop there. The current model predicts that the asteroid belt never had much mass in bodies like Ceres.
“This gives the planetary science community a verifiable prediction between this model and previous models ones that can be analyzed using data from meteorites, remote sensing, and spacecraft missions,” told Bottke.
Pebbles can make the cores of the giant planets.
This work is an addition to the recent study announced in Nature by Levison, Kretke, and Martin Duncan (Queen’s University), which shows that pebbles can make the cores of the giant planets and reveals the makeup of the outer solar system. Combined, the two studies offer the means to create the entire solar system from a single, unifying process.
“As far as I know, this is the first model to recreate the structure of the planets that orbit the Sun-Venus, Earth, a small Mars, a low-mass asteroid belt, two gas giants, two ice giants (Uranus and Neptune), and a pristine Kuiper Belt,” stated Levison. The study, “Growing the Terrestrial Planets from the Gradual Accumulation of Sub-meter Sized Objects,” is published online by PNAS. Authors H.F. Levison, K.A. Kretke, K. Walsh, and W. Bottke are all of Southwest Research Institute’s Space Science and Engineering Division. This research was supported by the NASA Solar System Exploration Research Virtual Institute (SSERVI) through institute grant number NNA14AB03A.