The red planet’s size is at the heart of a rift of ideas among researchers modeling the solar system’s formation.

The presence of water isn’t the only Mars mystery scientists are keen to probe. Another centers around a seemingly trivial characteristic of the Red Planet: its size. Classic models of solar system formation predict that the girth of rocky worlds should grow with their distance from the sun. Venus and Earth, for example, should exceed Mercury, which they do. Mars should at least match Earth, which it doesn’t. The diameter of the Red Planet is little more than half that of Earth (1, 2).

Mars’ small stature remains a mystery. Image courtesy of NASA/JPL-Caltech.

“We still don’t understand why Mars is so small,” says astronomer Anders Johansen, who builds computational models of planet formation at Lund University in Sweden. “It’s a really compelling question that drives a lot of new theories.”

Understanding the planet’s diminutive size is key to knowing how the entire solar system formed. And that’s where the trouble begins. Despite decades of debate, data, and computer simulations, until recently most models predicted that Mars should be many times its observed size. Getting Mars to form at the right place with the right size is a crucial part of any coherent explanation of the solar system’s evolution from a disk of gas and dust to its current configuration. In trying to do so, researchers have been forced to devise models that are more detailed—and even wilder—than any seen before.

Nebulous Notions The solar system began as an immense cloud of dust and gas called a nebula. The idea of a nebular origin dates back nearly 300 years. In 1734, Swedish scientist and mystic Emanuel Swedenborg—who also claimed that Martian spirits had communed with him—posited that the planets and sun had once been part of the same big space cloud (3). The modern version of the hypothesis says that the nebula began contracting under its own gravity and started spinning faster and faster. The bulk of the matter collapsed to form the Sun, and what was left formed the planets. By the mid-20th century, the nebular hypothesis morphed into a more sophisticated idea that dust grains in the cloud stuck together to form small rocks, which grew into planetesimals, a kilometer or more in diameter. Those in the inner solar system coalesced to form rocky planetary embryos. (Those that didn't form embryos became small asteroids.) Supersized cores formed in the outer solar system, where they swept up vast amounts of surrounding gas and became giants. It is a highly plausible scenario, except that the details keep tripping up scientists. If you simply begin with the protoplanetary disk—a flattened ring of dust and gas swirling around the newly formed Sun, stretching outward from inside the current orbit of Mercury—and let the disk evolve according to the laws of physics, then “Mars comes out to be larger and more massive than it is,” says Nader Haghighipour at the University of Hawai’i’s Institute for Astronomy in Honolulu. Many models that tried to solve the Mars problem have managed to winnow the mass down, but still predicted a Mars at least half as massive as Earth (Mars actually has only one-tenth the mass of Earth). In addition to addressing the size of Mars, any model has to account for the so-called meter barrier, which confounds astronomers because it suggests planetary embryos shouldn’t even form in the first place. In models of the turbulent conditions of the protoplanetary disk, grains of dust grow into rocks that are too large to continue sticking together electrostatically and too small to attract each other gravitationally. In addition, those rocks move so fast that they’re more likely to collide and break into pieces than they are to coalesce, making it impossible to form planetary embryos. This is called the “meter barrier” because it’s most pronounced for meter-sized bodies, which have the highest predicted speed. There’s another problem connected to the size of Mars and the meter barrier. Many models don’t work unless the cores that eventually formed the gas giants grew as big as 10 Earths in the outer system, even as planetary embryos were forming nearer the sun. A robust model for Mars must account for all of these conditions. In addition, it has to accommodate the accepted idea that Mars finished forming tens of millions of years earlier than Earth, evidence for which came from a 2011 study of radioactive elements in meteorites from Mars. Based on that study, astronomers concluded that Mars is likely a stunted embryo that stopped growing after it ran out of material (4). Earth, it seems, didn’t have that problem. The notion that Mars’ size is a result of arrested development raises questions about the composition of the protoplanetary disk in the inner region where the rocky planets formed. In particular, what were the conditions that made it possible for an embryonic Mars to end up in a part of the disk where it couldn't keep accumulating mass? This is where the theories diverge. The Red Planet’s diameter is about half that of the Earth’s; its volume is about 15% of Earth’s. Image courtesy of Dave Jarvis (davidjarvis.ca/dave/gallery).

Pebble Power One path to a small Mars began with a 2012 study in which Johansen and his university colleague, Michiel Lambrechts, described how centimeter-sized pebbles, under the slowing influence of drag caused by turbulence in the gas, collapsed together quickly to form planetesimals up to 1,000 kilometers in diameter (5). These then had enough gravitational pull to accrete other nearby pebbles. That collapse happened so fast that it bypassed the meter barrier and also shows how cores as big as 10 Earths can come together in only 1,000 years. Hal Levison, at the Southwest Research Institute in Boulder, Colorado, says he was floored when he read the paper. “It’s rare when a truly new idea comes about that changes the direction of the field, and I think this approach to pebble accretion is one of those,” he says. “I think it’s going to change everything.” However, the early pebble accretion simulations had a problem: they produced hundreds of bodies the size of Earth. Levison and his colleagues, in a paper published last August in Nature, tweaked the recipe by slowing down the pebble-formation process (6). The theory could now explain how the gas giants (Jupiter, Saturn) and ice giants (Uranus and Neptune) formed quickly. In November, Levison et al. extended that approach to the terrestrial planets in a paper published in PNAS (7). They treated pebble accretion as a function of both distance from the sun and the relative speed between pebbles and embryos, and as a result their equations produced a deep-space cut-off. In the new scenario, planetary embryos accreted mass between the Sun and Earth’s present orbit, but beyond the orbit of Earth—in the neighborhood of Mars—accretion became inefficient because the pebbles were moving too fast to be captured by the embryos. Planetary embryos at that distance couldn’t grow bigger. Their simulations produced a small Mars and an acceptable asteroid belt. Levison was encouraged. “The same physics that solves the giant core formation problem also solves the small Mars problem,” he says. “We don’t need to invoke anything new. That’s what makes this compelling.” Johansen applauds Levison’s work. “All the components are coming together in one paper, and that’s major progress,” he says. “This is fantastic news for pebble accretion.” But not everyone agrees that pebble accretion tells the whole story. Alessandro Morbidelli, who has collaborated with Levison in the past, says Levison’s model is limited, noting that that many of the simulated runs still result in a large Mars. Levison’s simulations produce a Mars analog only about 30% of the time; classic models produce a Mars analog only about 4% of the time, which would suggest that pebble accretion models are a step in the right direction. But Morbidelli also says the asteroid belt produced by the model doesn’t contain enough mass and lacks the “excitation” of real asteroids, the tendency of asteroid orbits to be elliptical, rather than circular. He thinks the only way to get to a small Mars and an asteroid belt that matches observations is to have some outside force act on the disk. Like Jupiter.

Jovian Jaunts In 2011, Morbidelli and an international team of astronomers, led by Kevin Walsh at the Southwest The same physics that solves the giant core formation problem also solves the small Mars problem. —Hal Levison Research Institute, published a new simulation of the early solar system based on a wild ride (8). That theory, known as the Grand Tack, says Jupiter migrated toward the sun, through the asteroid belt, before migrating back out, dragging Saturn along with it. Their gravitational heft scattered the debris in the disk, even ejecting some of it out of the solar system. Such a migration effectively depletes the mass in the inner solar system, setting the stage for a small Mars and an excited asteroid belt full of rocks with elliptical orbits. “The idea of migration to the inner part of the disk does not shock me at all,” says Morbidelli. After all, giant Jupiters have been spotted orbiting close to their stars in extrasolar systems, suggesting that planets can roam from where they first formed. [Recently reported evidence of a ninth planet doesn’t undermine such models (9); indeed, Morbidelli and his colleagues have shown how such a distant world fits with the idea of migrating giants.] In the Grand Tack model, Mars started to form in a part of the disk that contained plenty of matter; but Jupiter’s gravity propelled it into a less-dense neighborhood. Johansen calls the Grand Tack a “leading theory,” although he thinks it’s a dramatic solution. “It’s also a controversial theory, and not everyone is convinced. A lot of people feel uncomfortable about having Jupiter pass through the asteroid belt.” Levison is among them. “I’ve never really liked the Grand Tack,” he says. The Grand Tack is “very fine-tuned. You need the migration of Jupiter to happen just right, and Saturn to show up at the right time. All the stuff needs to work out precisely right to give us a solar system that we see.” He says he prefers a less fussy approach. Haghighipour also harbors doubts. “As beautiful as it is, the Grand Tack has a lot of assumptions going into it,” he says. “Disk people will tell you that the possibility of two planets coming forward, grabbing each other, and going back is close to impossible.” But he’s not convinced about such pebble accretion models either, and thinks the answer lies in yet another idea.