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Supercomputers Comet, Bridges Provide Single Model of Both Inner and Outer Solar System

Never-before generated model lends insight into formation of multiple planets

Published December 9, 2021

By Kimberly Mann Bruch, SDSC Communications 

Supercomputer-enabled models generated at the San Diego Supercomputer Center (SDSC) at UC San Diego and the Pittsburgh Supercomputing Center (PSC) revealed new insights into the solar system’s formation. While attempting to better understand the relationship between Earth and Mars, postdoctoral fellow Matt Clement (Carnegie Institution of Washington’s Earth and Planets Laboratory) and his colleagues used allocations from the National Science Foundation’s Extreme Science and Engineering Discovery Environment (XSEDE) on Comet at SDSC and Bridges at PSC to illustrate the formation of both the inner and outer solar system in a single model. Their work was recently published in the journal Icarus.

The international team included Clement, Nate Kaib (University of Oklahoma), Sean Raymond (University of Bordeaux) and John Chambers (Carnegie Earth and Planets Laboratory). Clement said that the team used numerical models to study the formation of the solar system’s inner, terrestrial planets: Mercury, Venus, Earth and Mars. 

“We know the giant planets (Jupiter, Saturn, Uranus and Neptune) must have formed far quicker than the terrestrial planets because observations of other forming solar systems in the galaxy indicate that the main ingredient for the gas giant planets, free gas, has been around for only a million years or so,” Clement said. “Because isotopic dating of rocks from the Earth and moon give much later dates, the inner terrestrial planets must have formed in the presence of the fully grown giant planets and we think this process occurred over a time span of around 100 million years, with the planets themselves slowly accreting and coalescing from a sea of small, asteroid-like objects.” 

This would have been a very violent time for the young Earth, as it would have been continuously smashing into other proto-planets and small objects en route to achieving its present size and orbit, according to Clement. 

As for the outer solar system planets, Clement said that many peculiar aspects – such as swarms of asteroids that orbit along with Jupiter and Neptune and irregular moons such as Triton – are explained by the giant planets passing through a tumultuous epoch of instability at some point after their formation. During this instability, Clement explained that the planets’ orbits evolved rapidly and substantially with their orbits becoming more elliptical and diverging from one another. 

“It is suspected that planets similar to Uranus and Neptune once existed in between Saturn and Uranus, but they were ejected during this ‘giant planet instability’, which has become known as the Nice Model – as in Nice, France, where it was developed by scientists in the early 2000s,” Clement said. “The Nice Model is arguably the consensus model for the formation of the outer solar system, however, it does not reconcile the orbits and masses of inner terrestrial worlds, and contemporary models of the inner solar system’s formation fail to replicate the low mass of Mars, and its rapid geologically inferred formation time with respect to the Earth.”  

According to Clement, when the instability happens while the inner planets are still growing, the instability also explains why the inner solar system looks the way it does. “Thus, our work explains both the inner and outer solar system in a single model,” said Clement.

More about Mars

While the primary objective of Clement and the team was to better understand the relationship between Earth and Mars, their study also revealed a potential resolution to another solar system mystery: Mercury’s diminutive size and isolated orbit. That is, while Mercury is physically close to Venus, gravitationally speaking it is quite isolated as it makes nearly three revolutions around the sun for every one Venus cycle. The team’s study found that, in certain simulations, interactions between the giant planets and the forming terrestrial material liberate a proto-planet from the Mars-region. That is, one of those objects that might have turned Mars into a bigger planet if the instability hadn’t stunted its growth, has been implanted in the inner solar system on a Mercury-like orbit. 

“If this genesis scenario is correct, it would have huge implications for our understanding of the terrestrial planets’ compositions, as we would expect Mercury to be made of much the same material as Mars,” Clement explained. “While Mercury’s bulk composition is fairly unconstrained, forthcoming missions such as BepiColombo to the innermost planet should help improve our understanding of the planet’s makeup and inform our models – we will continue to study this scenario on the new supercomputers Expanse at SDSC and Bridges-2 at PSC.

The Early Instability Scenario is an evolutionary model for the solar system that aims to explain several peculiar aspects of the inner solar system. While the x axis shows the semi-major axis of the planets (distance from the sun), the y axis shows the orbital eccentricity (the degree to which the orbit is elliptical or non-circular). The size of each dot is proportional to the mass of the object in the simulation, and the color is related to the amount of water and other volatile elements the object contains. Credit: Matt Clement

Why It’s Important

Understanding why Earth, Venus and Mars are so different not only gives us insights into our own origins and place in the universe, but also helps lay the groundwork for understanding habitability around other stars in the galaxy. While the Earth is a lush, hospitable place for life, our planet’s next-door neighbors are not so friendly for humans. That is, while Venus is about the same size as Earth, its atmosphere and surface are much different – with the former being a product of the runaway greenhouse effect. Venus also lacks a moon and a magnetic field – both of which partially contributed to the Earth’s climate regulation and shielding from excessive bombardment by meteoroids.  

“Our work demonstrates that these differences are partially a result of variation between the two planets’ formation routes – Venus formed much faster than the Earth, accumulated from many smaller objects and did not experience an ultimate merger with another large object like the final impact on Earth that formed the moon,” Clement explained. “Mars, on the other hand, is extremely similar to Earth in many ways, and is actually close enough to the sun for liquid water to have flowed on its surface; however, just like Venus, complex life did not emerge on Mars because of the planet’s diminutive size relative to Earth’s and its inability to retain a robust atmosphere.” 

The study conducted by Clement and his team shows that Jupiter and Saturn’s peculiar early evolution could be one reason for this lack of life on Mars. The research team’s study speculates that evacuation of material from the region around Mars and the modern asteroid belt as the newly formed giants evolved on to their modern orbits prevented additional terrestrial planets from forming where the asteroid belt is today and stunted Mars’ growth, which prevented it from fully maturing into an Earth or Venus-sized object capable of hosting life.

“This work unites our understanding of the inner and outer solar system’s formation and early evolution,” Clement said. “While previous explanations for inner solar system qualities like the mass of Mars, the lack of planets in the asteroid belt and the small mass of Mercury invoked additional mechanisms and processes beyond our conventional understanding of planet formation, our model shows that the event responsible for sculpting the outer solar system (the well-studied giant planet instability) also shaped the inner solar system.”

Because Mercury remains a solar system mystery, Clement and his team have also been working hard to try and understand why it is so peculiar. They recently published their Comet-enabled models regarding Mercury’s formation in the Astrophysical Journal. Clement said that this paper shows several working models that consistently produce Mercury-like planets and that the team has now focused on using XSEDE resources to test their viability within the early instability framework.

How Supercomputers Helped

The international team has been working on understanding how the giant planet instability affects the formation of the terrestrial planets for several years now. Their studies encompass computationally intensive problems, as they overlay two highly chaotic processes on top of one another: terrestrial planet formation and the giant planet instability. 

“Not only do we need enough simulations to achieve an adequate statistical picture of the range of possible outcomes in the terrestrial system, but we also need all of these realizations to be ones where the outer solar system turns out looking something remotely like the actual one,” Clement said. “Because the instability is so chaotic, it is impossible to repeat the same evolution from simulation to simulation without ‘telling’ the planets what to do, and thus restricting our analysis to only one plausible evolutionary path for the solar system.” 

“On the other hand, if we just let the giant planets evolve on their own, only a few percent of the outcomes resemble the solar system in broad strokes – this severely limited the scope and conclusions of our previous studies,” he continued. “But, in our current paper, we used XSEDE resources to develop a new pipeline that essentially restarts new simulations every time the giant planets veer too far off course. Producing the 360+ fully evolved solar systems for detailed analysis in this paper entailed upwards of two million CPU hours!”

Clement said the main reason the team turned to Comet was the massive computational requirements of our project – studying these two chaotic events statistically required enormous amounts of compute time. “Comet and XSEDE allowed us to sequester a large number of cores for months at a time to continuously run instability models and search for evolutions that matched the solar system.”

Clement learned about XSEDE through Floyd Fayton, the Campus Champion at the Carnegie Institution of Washington. When Fayton learned about Clement’s need for intense computational resources, he explained the XSEDE system and encouraged him to apply for a start-up allocation in 2019. “After benchmarking my code’s performance and building up my infrastructure, I was able to hit the ground running when I landed an XSEDE research allocation in 2020,” Clement said. “Having access to a free compute resource like XSEDE has been invaluable, particularly given that I am a post doc at an institution that does not possess an internal compute resource of comparable scale to Comet and I’d also like highlight our interactions with the XSEDE public outreach team – it has really helped get our research out there.”

The work on Comet and Bridges was allocated via XSEDE, which is supported by National Science Foundation (grant number ACI-1548562). Specifically, it used the Bridges system, which is supported by NSF award number ACI-1445606, at PSC; Comet is supported by NSF award number 1341698 at SDSC. Additional computing for this project was performed at the OU Supercomputing Center for Education and Research at the University of Oklahoma. The authors also acknowledge the Texas Advanced Computing Center at The University of Texas at Austin for providing resources that contributed to the results reported within this study. Additional computation for the work was supported by Carnegie Science’s Scientific Computing Committee for High-Performance Computing.