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Why Mars is so small

A theory proposed by Brazilian researchers explains the size of the Red Planet

Lightweight: named after the Roman god of war, Mars, contrary to expectations, has only 10% of the Earth's mass

NASALightweight: named after the Roman god of war, Mars, contrary to expectations, has only 10% of the Earth’s massNASA

If the ancient Romans had known the actual size of the planet Mars, maybe they would not have named it after their god of war. Mars would be more of a dwarf warrior than a giant if its body were in accurate proportion to the dimensions of the planet of the same name. Mars is the second smallest planet in the solar system, with one-tenth the mass of Earth. And the reason for its size is one of the principal open questions for astronomers and geophysicists studying the formation of planets. Specialists in celestial mechanics at São Paulo State University (Unesp), however, believe they have finally found a satisfactory answer to the question.

Computer simulations of the formation of the solar system have already explained the position and physical properties of many of the planets and other celestial bodies that revolve around the Sun. Mars, however, is still among the bodies whose origin is a mystery. According to these simulations, the mass of the red planet should be as great as that of Earth or Venus, which are similar to each other. Some researchers have proposed theories to explain the disparity. The main one, known as the Grand Tack scenario, assumes that a series of unlikely events during the movement of the planets in the early solar system, about four billion years ago, created favorable conditions for the formation of a small planet Mars. “The beauty of our work is to explain Mars in a much simpler and more likely way,” says astronomer Othon Winter of the School of Engineering, Unesp Guaratinguetá, who is part of the team that suggested a new model for the formation of Mars in February 2014 in the Astrophysical Journal.

048-053_Marte_221-1The astronomer André Izidoro, who completed his doctorate at Unesp in 2013 under the guidance of Winter, had the idea to test whether the reduced size of Mars could be a consequence of a lack of “construction material” in the region of Mars in the early solar system. Under this new scenario, four billion years ago there would have been a large gap without raw material in the protoplanetary disk region—composed of thousands of bodies similar to today’s moons and asteroids that originated the rocky planets through collisions—near Mars’ current orbit. Currently undertaking postdoctoral studies at the Nice Observatory of the University of Nice, France, Izidoro built this model based on recent theories that gaps like the one assumed may have arisen naturally in the planetary disk.

Computer simulations based on this new scenario suggest that Mars had begun to form either near the current location of Earth, or closer to where the asteroid belt is today, between the orbits of Mars and Jupiter. Both in the former and the latter case, Mars could have migrated very quickly to the region with the greatest lack of planetary building material and remained there at a distance one-and-half times greater than that between Earth and the Sun, according to computer simulations performed by Izidoro and Winter in partnership with Nader Haghighipour, of the University of Hawaii at Manoa, and Masayoshi Tsuchida, of the Institute of Biosciences, Letters and Exact Sciences, Unesp São José do Rio Preto.

Original disk
Astronomers believe they already know the history of the origins of the solar system, although there are still details to be filled in. The Sun, like many other stars, was created from the gases and dust of the interstellar medium that condensed into a cloud 4.6 billion years ago. The majority of the material collapsed, forming the sun, while the rest remained in the form of a disk rotating around the new star. In this disk, the dust grains agglomerated over millions of years to form rocky bodies similar to asteroids, called planetesimals, with diameters of about 100 kilometers.

048-053_Marte_221-2Most planetesimals continued to collide with each other to form planetary embryos: bodies like planets, with masses between that of the Moon (one hundredth of the Earth’s) and that of Mars. Some early embryos grew enough for their gravitational pull to start sucking gases from the protoplanetary disk, forming the current gas giant planets: Jupiter, Saturn, Uranus and Neptune.

This first phase of solar system formation lasted at most 10 million years and ended when all the gases in the disk dissipated or were captured by the gas giants or the Sun. The solar system was still very different from the present: the gas giants orbited closer to the Sun, immersed in a sea of planetesimals and planetary embryos. Collisions and gravitational shaking during the following 500 million years eventually moved the gas giants to their present positions, pushing the smaller bodies into specific bands further from the Sun, forming the Kuiper belt, where Pluto is located, and beyond that the Oort cloud, which is the source of many comets.

The astronomers Hal Levison, Alessandro Morbidelli, Kleomentis Tsiganis and the Brazilian Rodney Gomes, currently at the National Observatory, presented this model of the initial formation of the solar system in 2005, in a series of articles published in the journal Nature. This theory became known as the Nice model, as it was developed while the authors worked together at the Nice Observatory.

While the gas giants were forming, the collisions of planetesimals and planetary embryos gathered in the region between the Sun and Jupiter began to give rise to the current rocky planets—Mercury, Venus, Earth and Mars—beyond the asteroid belt between Mars and Jupiter. It took 50 million to 150 million years for Mercury, Venus and Earth to reach their current forms, while Mars was formed very quickly, in less than 10 million years. Izidoro dedicated his doctoral research to simulating this final period of planet formation. “Our simulations, like most carried out by other researchers, tended to fail to produce Mars,” Izidoro explains. “They generated two to three planets similar to Earth and Venus, but never something like Mars.”

Sunset on Mars, captured by the robot Spirit: the Red Planet formed over a period of 2 million years, 25 times faster than Earth

MARS EXPLORATION ROVER MISSION / TEXAS A&M / CORNELL / JPL / NASASunset on Mars, captured by the robot Spirit: the Red Planet formed over a period of 2 million years, 25 times faster than EarthMARS EXPLORATION ROVER MISSION / TEXAS A&M / CORNELL / JPL / NASA

At the same time that Izidoro started his PhD, the international astronomical community began to understand the main problem with the simulations. They assumed that the amount of planetesimals and planetary embryos varied evenly throughout the protoplanetary disk. In response, several studies began to show that a smaller planet could arise in the current vicinity of Mars if the distribution of material varied more abruptly, with a narrow band containing more material near Earth’s current orbit, and a band with less material in the region where the Red Planet is now located.

The most famous scenario to explain this unusual distribution of material is called the Grand Tack. According to this scenario, proposed in 2011 in Nature, at the end of the first phase of solar system formation, when the gas giants had already arisen, gravitational forces—between the remainder of the gases that still permeated the protoplanetary disk and the gas giants—caused Jupiter and Saturn to move toward the Sun. On this voyage, the gas giants escaped their original orbits by about four astronomical units—one astronomical unit is the distance between Earth and the Sun—and migrated to the region where Mars is located now, about 1.5 astronomical units from the Sun. Then, the complex interactions of gravitational forces acting on the gases and on the gas giants reversed the migration of the planets, with Jupiter and Saturn returning to their more distant orbits. Simulations showed that the abrupt movement of these two planets would have scattered the bodies in the protoplanetary disk, creating an uneven distribution of material that could explain Mars. This scenario was named the Grand Tack by one of its authors, Alessandro Morbidelli of the Nice Observatory, in allusion to the tacking maneuver sailboats use to change their course in relation to the direction of the wind.

The big gap
Although the Grand Tack scenario is possible, Izidoro notes that the model only works when a very precise combination of physical properties is assumed for the protoplanetary disk and the gas giants. “It is very unlikely that Jupiter’s reversal of motion occurred exactly in Mar’s present orbit,” he explains. “If the properties of the disk and the planets are just a bit different, simulations of the model form a solar system completely different from ours.”

Seeking an alternative to the Grand Tack, Izidoro decided to explore an idea proposed in 2008 by astronomer Liping Jin, at the University of Jilin, China. Jin and his colleagues proposed that the distribution of rocky bodies in the protoplanetary disk could have contained a large density gap near the orbit of Mars. But the origin of this gap could be older than the Grand Tack scenario assumes. It could have been created by the properties of the gases and dust in the infancy of the protoplanetary disk, before the formation of the gas giants. At the same time, the effects of solar radiation and cosmic rays, combined with the fact that the gases in the planetary disk rotated faster closer to the Sun, could have created a density gap—an orbital band with less gas and dust that, millions of years later, could have resulted in a band with fewer planetesimals and planetary embryos, precisely in the present orbit of Mars.

Inspired by this possibility, Izidoro and his colleagues conducted computer simulations that assumed the presence of a disk with almost a thousand planetesimals and around 150 planetary embryos between the Sun and Jupiter, with a gap in density near the current orbit of Mars. The team carried out 84 simulations using the computer cluster in the laboratory of the Orbital Dynamics and Planetology Group at Unesp Guaratinguetá. Each simulation assumed different initial conditions, varying parameters such as the orbits of Jupiter and Saturn, and the width, position and degree of the density gap.

The result of each one- to three-month-long simulation is a sort of high-speed film portraying one billion years of interplanetary collisions and acrobatics. The result of a single simulation is like a science fiction film, telling an alternative history of the solar system, but faithful to the laws of physics. By comparing the results of many different simulations, however, researchers can get an idea of what is most likely to have happened in the solar system’s past.

The simulations in which a planet with the size and current position of Mars formed and began orbiting the Sun stably were those that assumed a gap in the protoplanetary disk density in a band between 1.5 and 2.5 astronomical units from the Sun, with 50% to 75% less material than the disk average. The simulations also made clear that, contrary to what was thought, Mars does not begin to form in the low-density region. In half of the successful simulations, Mars appears near where Earth and Venus formed, while in the rest of the simulations it arises further from the Sun, on the other side of the gap. The gravitational forces between the Sun, the gas giants and the planets being formed, however, eventually launch Mars into the gap, where its growth is interrupted. “The gap has so little material that there are almost no collisions in the region,” explains Winter. “Not even a small planet could form there.”

In addition to Mars, the simulations also form planets very much like Earth and Venus, and a belt of asteroids with orbits similar to actual asteroids. The simulations could not, however, form an analog of Mercury. In fact, Mercury has been relatively ignored by most models so far. “But some researchers are already adapting our model to address it,” says Izidoro. “Now, Mercury is the hot topic.”

The time that planets similar to Earth and Mars take to form in the simulations is also consistent with the formation times estimated by geochemists by comparing the proportion of radioactive chemical elements in terrestrial rocks and Martian meteorites. Mars is believed to have stopped growing, prematurely, just two million years after starting to form. Earth’s growth phase, however, is estimated to have taken 50 million years.

Winter stresses that the study has applications beyond the formation of Mars and the solar system. “A wide variety of extrasolar planetary systems have been discovered that are very different than our solar system and still unexplained,” says the astronomer. “The models for their origin still assume a protoplanetary disk of uniform density with no gaps.”

“The local deficit of planetesimals and embryos that they assume, though extreme, is expected,” says Brazilian astronomer Wladimir Lyra of the NASA Jet Propulsion Laboratory. In 2008, Lyra and his colleagues performed simulations to study the effect of the turbulent motion of the material in the protoplanetary disk on the formation of planetesimals. “The non-homogeneous distribution of gases and rocks that results from our models coincides fairly well with the distribution that Izidoro and his colleagues need in their model.”

Orbital dynamics of minor bodies (nº 2011/08171-3); Grant mechanism Thematic Project; Principal investigator Othon Cabo Winter  (School of Engineering/Unesp Guaratinguetá); Investment R$ 560,886.80 (FAPESP).

Scientific article
IZIDORO, A. et al. Terrestrial planet formation In a protoplanetary disk with a local mass depletion: a successful scenario for the formation of MarsThe Astrophysical Journal. v. 782: 31. 10 fev. 2014.