UChicago: For Mars, rapid formation stunted growth

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For Mars, rapid formation stunted growth
University of Chicago | National Science Foundation | 2011 May 25

[i]Credit: NASA/JPL[/i]

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[i]Credit: Lunar and Planetary Institute[/i]

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Mars developed in as little as two to four million years after the birth of the solar system, far more quickly than Earth, according to a new study published in the May 26 issue of the journal Nature. The red planet’s rapid formation helps explain why it is so small, say the study’s co-authors, Nicolas Dauphas at the University of Chicago and Ali Pourmand at the University of Miami (Fla).

Mars probably is not a terrestrial planet like Earth, which grew to its full size over 50 to 100 million years via collisions with other small bodies in the solar system, said Dauphas, an associate professor in geophysical sciences. Mars instead is a planetary embryo that never developed into a full-fledged planet.

“Earth was made of embryos like Mars, but Mars is a stranded planetary embryo that never collided with other embryos to make an Earthlike planet,” Dauphas said. The new work provides supporting evidence for this idea, which was first proposed 20 years ago on the basis of planetary growth simulations.

The new evidence likely will change the way planetary scientists view Mars, observed Pourmand, assistant professor in marine geology and geophysics at the UM Rosenstiel School of Marine & Atmospheric Science. “We thought that there were no embryos in the solar system to study, but when we study Mars, we are studying embryos that eventually made planets like Earth.”

There had been large uncertainties in the formation history of Mars because of the unknown composition of its mantle, the rock layer that underlies the crust. “Now we can shrink those uncertainties to the point where we can do interesting science,” Dauphas said.

Dauphas and Pourmand were able to refine the age of Mars by using the radioactive decay of hafnium to tungsten in meteorites from Mars as a way of estimating their age. Hafnium 182 decays into tungsten 182 in a half-life of nine million years. This relatively rapid decay process means that almost all hafnium 182 will disappear in 50 million years, providing a way to assemble a fine-scale chronology of early events in the solar system.

“To apply that system you need two gradients,” Pourmand explained. “You need the hafnium-tungsten ratio of the mantle of Mars, and you need the tungsten isotopic composition of the mantle of Mars.” The latter was well known from analyses of martian meteorites, but not the former.

Previous estimates of the formation of Mars ranged as high as 15 million years because the chemical composition of the martian mantle was largely unknown. Scientists still wrestle with large uncertainties in the composition of Earth’s mantle because of composition-altering processes such as melting.

“We have the same problem for Mars,” Dauphas said. Analyses of martian meteorites provide clues as to the mantle composition of Mars, but their compositions also have changed.

Solving some lingering unknowns regarding the composition of chondrites, a common type of meteorite, provided the data they needed. As essentially unaltered debris left over from the birth of the solar system, chondrites serve as a Rosetta stone for deducing planetary chemical composition.

Cosmochemists have intensively studied chondrites, but still poorly understand the abundances of two categories of elements that they contained, including uranium, thorium, lutetium and hafnium.

Dauphas and Pourmand thus analyzed the abundances of these elements in more than 30 chondrites, and compared those to the compositions of another 20 martian meteorites. Scientists have previously identified some meteorites as having come from Mars based on their composition; such rocks likely were knocked into space when a separate meteorite hit Mars.

“Once you solve the composition of chondrites, you can address many other questions,” Dauphas said, including a refinement of the age of the Milky Way galaxy, which he published in 2005.

Hafnium and thorium both are refractory or non-volatile elements, meaning that their compositions remain relatively constant in meteorites. They also are lithophile elements, those that would have stayed in the mantle when the core of Mars formed. Thus, if scientists could measure the hafnium-thorium ratio in the martian mantle, they would have the ratio for the whole planet, which they need to reconstruct its formation history.

Mars-meteorite connection

The relationships between hafnium, thorium, and tungsten dictated that the hafnium-thorium ratio in the mantle of Mars must be similar to the same ratio in chondrites. To derive the martian mantle’s hafnium-tungsten ratio, they divided the thorium-tungsten ratio of the martian meteorites by the thorium-hafnium ratio of the chondrites.

“Why do you do that? Because thorium and tungsten have very similar chemical behavior,” Dauphas said.

Once Dauphas and Pourmand had determined this ratio, they were able to calculate how long it took Mars to develop into a planet. A computer simulation based on these data showed that Mars must have reached half its present size only two million years after the formation of the solar system.

A quickly forming Mars would help explain the puzzling similarities in the xenon content of its atmosphere and that of Earth.

“Maybe it’s just a coincidence, but maybe the solution is that part of the atmosphere of Earth was inherited from an earlier generation of embryos that had their own atmospheres, maybe a Marslike atmosphere,” Dauphas said.

The short formation history of Mars further raises the possibility that aluminum 26, which is known from meteorites, turned the planet into a magma ocean early in its history. Aluminum 26 has a half-life of 700,000 years, so it would have disappeared too quickly to contribute to the internal heat of Earth.

If Mars formed in two million years, however, significant quantities of aluminum 26 would remain. “When this aluminum 26 decays, it releases heat and can completely melt the planet,” Pourmand said.

Hf–W–Th evidence for rapid growth of Mars and its status as a planetary embryo - N Dauphas, A Pourmand

• Nature 473 489 (26 May 2011) DOI: [url=htttp://dx.doi.org/10.1038/nature10077]10.1038/nature10077[/url]

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Why Mars Is a Planetary Runt
Science NOW | Richard A. Kerr | 2011 May 25

Something stunted the growth of Mars in the earliest years of the solar system, planetary scientists report today in Nature. And other researchers are offering two new ways that a nascent Mars could have been starved of the building blocks it needed to grow into a full-size rocky planet. Both ways involve shuffling young planets to and fro as they grew.

With a mass 11% that of Earth, Mars is definitely undersized. But planetary dynamicists have struggled to explain why, as in their computer models of the swirling gas, dust, and rubble of the early solar nebula, Mars tends to grow to the size of Earth or Venus. In the models, kilometer-size planetesimals agglomerate into moon- to Mars-sized “embryos” out where Mars is today. These embryos, in turn, keep on agglomerating until an Earth-size Mars has formed. Obviously, the models have been missing something.

Unlike Mars in the models, the real Mars simply stopped growing once it reached the embryo stage, according to planetary geochemists Nicolas Dauphas of the University of Chicago in Illinois and Ali Pourmand, who is now at the University of Miami in Florida. The two came to that conclusion after they made a new estimate of how long it took the planet to form.

Many researchers had gauged how long Mars took to form using the steady decay of radioactive hafnium-182 to tungsten-182, but the answers were all over the place. The problem had been that the hafnium-tungsten dating technique depends not only on measuring the relevant isotopes in meteorites long ago blasted off Mars but also on knowing the relative proportion of hafnium and tungsten in the deep martian mantle. But martian meteorites are bits of martian crust, a rock derived from the mantle by melting, which alters the ratio of hafnium to tungsten. Researchers’ estimates of the extent of alteration and, thus, the time it took to make Mars varied wildly.

To shrink the uncertainty, Dauphas and Pourmand went looking in a variety of non-martian meteorites for a stand-in for the hafnium-tungsten ratio that would not be altered. They found it in the ratio of hafnium-176 to hafnium-177, which are identical chemically. When they redid the dating calculation using published isotopic ratios and their more precise elemental ratio, they found that it took just 2 million to 4 million years to form Mars, not the tens of millions of years Earth must have taken to agglomerate. Mars therefore must be an embryo that for some reason stopped merging with other embryos and failed to become a full-size rocky planet.

So why did Mars grow no further? Two studies presented at last March’s Lunar and Planetary Science Conference in Houston, Texas, suggest that the best way to arrest Mars’s development was to starve it of building material. In simulations run by planetary dynamicists David Minton and Harold Levison of Southwest Research Institute (SwRI) in Boulder, Colorado, Mars grows rapidly by colliding with kilometer-sized planetesimals close to the sun. But at the same time, Mars is nudged outward by trillions of close encounters with planetesimals that give infinitesimal gravitational kicks to the growing Mars without hitting it. When the embryo reaches about where Mars now orbits, its growth by planetesimal accretion slows, but the other embryos have been left behind, closer to the sun. Starved of its only building materials, Mars remains embryo-size while Earth and Venus loiter closer to the sun and continue to grow.

Or Jupiter could have starved Mars, according to planetary dynamicist Kevin Walsh, who is also at SwRI in Boulder, and his colleagues. They modeled an early solar system in which the gas that briefly surrounded the sun dragged Jupiter toward the sun, as appears to have happened to gas giants in most known exoplanetary systems. An inward-migrating Jupiter would have flung planetesimals out of the way to clear a gap. The first embryo to wander into the gap would become Mars after Saturn gravitationally latched on to Jupiter and both moved outward again.

“It seems crazy,” Minton says, but “there are all these ways of moving planets around early in the solar system’s history when there was a lot going on.” Exactly how the planetary shuffling played out must await much more modeling.

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Small Mass of Mars Could Be Due to Planetary Orbital Migration
Planetary Science Institute | 2011 June 05

A long-ago inward migration by Jupiter during the formation of our Solar System could explain why Mars is small in relation to Earth and Venus, according to a paper published in Nature.

Researchers have long sought to explain the small mass of Mars, which has remained an outstanding problem in terrestrial planet formation, said David P. O’Brien, a Research Scientist at the Planetary Science Institute and co-author of A low mass for Mars from Jupiter’s early gas-driven migration that appears in Nature.

“This work not only solves a difficult problem in Solar System formation,” O’Brien said, “it shows that the solution lies in the giant planets of our Solar System undergoing significant early migration, which was generally thought to only have occurred in extrasolar planetary systems.”

Simulations of the formation process of the four inner planets in the Solar System – Mercury, Venus, Earth and Mars – generally produced a version of Mars far more massive than the real planet.

“We tried a large variety of simulation parameters to solve this problem, but nothing seemed to work,” O’Brien said.

A 2009 paper by Brad Hansen from UCLA offered a new clue: Hansen showed that if the initial distribution of solid material in the solar system was assumed to have an outer boundary at 1 Astronomical Unit (1 AU being the current distance from the sun to Earth), a smaller Mars could form.

The presence of a sharp outer boundary at 1 AU required in Hansen's work was hard to explain, given the existence of the asteroid belt between 2 and 4 AU, the giant planets between 5 and 30 AU and the Kuiper Belt beyond that.

However, it has been shown in numerical simulations over the past decade that Jupiter and Saturn could migrate in the early Solar System when gas was still present, and in some cases could move inwards and then back outwards to roughly their current locations.

“Rapidly the pieces of the story came together,” said Kevin J. Walsh, lead author of the paper who began work on the project at the Observatoire de la Cote d'Azur in Nice, France and is now at the Southwest Research Institute in Bounder, CO. “If Jupiter had moved inwards from its birth place down to 1.5 AU from the sun and then had turned around because of the formation of Saturn, eventually migrating outwards towards its current location, it would have truncated the distribution of solids in the inner Solar System at about 1 AU, as required to explain the small mass of Mars.”

Jupiter now orbits the sun at 5.2 AU.

“The problem was to understand whether the inward and outward migration of Jupiter through the 2-4 AU region could be compatible with the existence of the asteroid belt today,” Walsh said. “So we started to do a huge number of simulations.”

“The asteroid belt, which was a priori our main problem, turned out to be the main strength of our model,” said O’Brien.

“The result was fantastic,” Walsh said. “The simulations showed that the migration of Jupiter was consistent with the existence of the asteroid belt, but it also explained properties of the belt never understood before.”

The passage of Jupiter depleted then re-populated the asteroid belt region, with inner-belt bodies originating between 1 and 3 AU and outer belt bodies originating in a very distinct region between and beyond the giant planets, naturally producing the significant compositional differences existing today across the belt.

The model was called the “Grand Tack Scenario” with Jupiter’s motion similar to a sailboat tacking around a buoy.

New solar system formation models indicate that Jupiter's foray robbed Mars of mass
Southwest Research Institute (SwRI) | 2011 June 05

Planetary scientists have long wondered why Mars is only about half the size and one-tenth the mass of Earth. As next-door neighbors in the inner solar system, probably formed about the same time, why isn't Mars more like Earth and Venus in size and mass? A paper published in the journal Nature this week provides the first cohesive explanation and, by doing so, reveals an unexpected twist in the early lives of Jupiter and Saturn as well.

Dr. Kevin Walsh, a research scientist at Southwest Research Institute (SwRI), led an international team performing simulations of the early solar system, demonstrating how an infant Jupiter may have migrated to within 1.5 astronomical units (AU, the distance from the Sun to the Earth) of the Sun, stripping a lot of material from the region and essentially starving Mars of formation materials.

"If Jupiter had moved inwards from its birthplace down to 1.5 AU from the Sun, and then turned around when Saturn formed as other models suggest, eventually migrating outwards towards its current location, it would have truncated the distribution of solids in the inner solar system at about 1 AU and explained the small mass of Mars," says Walsh. "The problem was whether the inward and outward migration of Jupiter through the 2 to 4 AU region could be compatible with the existence of the asteroid belt today, in this same region. So, we started to do a huge number of simulations.

"The result was fantastic," says Walsh. "Our simulations not only showed that the migration of Jupiter was consistent with the existence of the asteroid belt, but also explained properties of the belt never understood before."

The asteroid belt is populated with two very different types of rubble, very dry bodies as well as water-rich orbs similar to comets. Walsh and collaborators showed that the passage of Jupiter depleted and then re-populated the asteroid belt region with inner-belt bodies originating between 1 and 3 AU as well as outer-belt bodies originating between and beyond the giant planets, producing the significant compositional differences existing today across the belt.

The collaborators call their simulation the "Grand Tack Scenario," from the abrupt change in the motion of Jupiter at 1.5 AU, like that of a sailboat tacking around a buoy. The migration of the gas giants is also supported by observations of many extra-solar planets found in widely varying ranges from their parent stars, implying migrations of planets elsewhere in universe.

Jupiter's Youthful Travels Redefined Solar System
NASA Goddard Space Flight Center | 2011 Jun 06

Jupiter, long settled in its position as the fifth planet from our sun, was a rolling stone in its youth. Over the eons, the giant planet roamed toward the center of the solar system and back out again, at one point moving in about as close as Mars is now. The planet's travels profoundly influenced the solar system, changing the nature of the asteroid belt and making Mars smaller than it should have been. These details are based on a new model of the early solar system developed by an international team that includes NASA's Goddard Space Flight Center in Greenbelt, Md. The work is being reported in a Nature paper posted on June 5, 2011.

"We refer to Jupiter's path as the Grand Tack, because the big theme in this work is Jupiter migrating toward the sun and then stopping, turning around, and migrating back outward," says the paper's first author, Kevin Walsh of the Southwest Research Institute in Boulder, Colo. "This change in direction is like the course that a sailboat takes when it tacks around a buoy."

According to the new model, Jupiter formed in a region of space about three-and-a-half times as far from the sun as Earth is (3.5 astronomical units). Because a huge amount of gas still swirled around the sun back then, the giant planet got caught in the currents of flowing gas and started to get pulled toward the sun. Jupiter spiraled slowly inward until it settled at a distance of about 1.5 astronomical units—about where Mars is now. (Mars was not there yet.)

"We theorize that Jupiter stopped migrating toward the sun because of Saturn," says Avi Mandell, a planetary scientist at NASA Goddard and a co-author on the paper. The other co-authors are Alessandro Morbidelli at the Observatoire de la Cote d'Azur in Nice, France; Sean Raymond at the Observatoire de Bordeaux in France; and David O'Brien at the Planetary Science Institute in Tucson, Ariz.

Like Jupiter, Saturn got drawn toward the sun shortly after it formed, and the model holds that once the two massive planets came close enough to each other, their fates became permanently linked. Gradually, all the gas in between the two planets got expelled, bringing their sun-bound death spiral to a halt and eventually reversing the direction of their motion. The two planets journeyed outward together until Jupiter reached its current position at 5.2 astronomical units and Saturn came to rest at about 7 astronomical units. (Later, other forces pushed Saturn out to 9.5 astronomical units, where it is today.)

The effects of these movements, which took hundreds of thousands to millions of years, were extraordinary.

Jupiter's Do-Si-Do

"Jupiter migrating in and then all the way back out again can solve the long-standing mystery of why the asteroid belt is made up of both dry, rocky objects and icy objects," Mandell says.

Astronomers think that the asteroid belt exists because Jupiter's gravity prevented the rocky material there from coming together to form a planet; instead, the zone remained a loose collection of objects. Some scientists previously considered the possibility that Jupiter could have moved close to the sun at some point, but this presented a major problem: Jupiter was expected to scatter the material in the asteroid belt so much that the belt would no longer exist.

"For a long time, that idea limited what we imagined Jupiter could have done," Walsh notes.

Rather than having Jupiter destroy the asteroid belt as it moved toward the sun, the Grand Tack model has Jupiter perturbing the objects and pushing the whole zone farther out. "Jupiter's migration process was slow," explains Mandell, "so when it neared the asteroid belt, it was not a violent collision but more of a do-si-do, with Jupiter deflecting the objects and essentially switching places with the asteroid belt."

In the same way, as Jupiter moved away from the sun, the planet nudged the asteroid belt back inward and into its familiar location between the modern orbits of Mars and Jupiter. And because Jupiter traveled much farther out than it had been before, it reached the region of space where icy objects are found. The massive planet deflected some of these icy objects toward the sun and into the asteroid belt.

"The end result is that the asteroid belt has rocky objects from the inner solar system and icy objects from the outer solar system," says Walsh. "Our model puts the right material in the right places, for what we see in the asteroid belt today."

Poor Little Mars

The time that Jupiter spent in the inner solar system had another major effect: its presence made Mars smaller than it otherwise would have been. "Why Mars is so small has been the unsolvable problem in the formation of our solar system," says Mandell. "It was the team's initial motivation for developing a new model of the formation of the solar system."

Because Mars formed farther out than Venus and Earth, it had more raw materials to draw on and should be larger than Venus and Earth. Instead, it's smaller. "For planetary scientists, this never made sense," Mandell adds.

But if, as the Grand Tack model suggests, Jupiter spent some time parked in the inner solar system, it would have scattered some material available for making planets. Much of the material past about 1 astronomical unit would have been dispersed, leaving poor Mars out at 1.5 astronomical units with slim pickings. Earth and Venus, however, would have formed in the region richest in planet-making material.

"With the Grand Tack model, we actually set out to explain the formation of a small Mars, and in doing so, we had to account for the asteroid belt," says Walsh. "To our surprise, the model's explanation of the asteroid belt became one of the nicest results and helps us understand that region better than we did before."

Another bonus is that the new model puts Jupiter, Saturn, and the other giant planets in positions that fit very well with the "Nice model," a relatively new theory that explains the movements of these large planets later in the solar system's history.

The Grand Tack also makes our solar system very much like the other planetary systems that have been found so far. In many of those cases, enormous gas-giant planets called "hot Jupiters" sit extremely close to their host stars, much closer than Mercury is to the sun. For planetary scientists, this newfound likeness is comforting.

"Knowing that our own planets moved around a lot in the past makes our solar system much more like our neighbors than we previously thought," says Walsh. "We're not an outlier anymore."

Mass Arrest: Jupiter's Early Migration Could Explain Mars's Small Size
Scientific American | John Matson | 2011 Jun 06

The planets of our solar system follow nice, predictable orbits, but it was not always so. In the chaotic early days of the solar system, Jupiter and its fellow giant planets seem to have migrated from their birthplaces into the stable orbits that we observe today.

The migration of giant planets has been invoked to explain a number of features of planetary systems, such as the uneven spacing among the objects of the Asteroid Belt in our solar system. Migration would also explain the huge planets in other planetary systems known as "hot Jupiters" that orbit extremely close to their host stars, far closer than where they could have plausibly formed.

Now a new study, published online June 5 in Nature, demonstrates that a peculiar migration of Jupiter—first inward, then outward—could account for Mars's relatively small size.

Researchers have built a number of numerical simulations to try to trace how the planets formed in the tens of millions of years following the solar system's birth. But often, Mars has posed a problem. "As things would slowly build up, essentially what always would happen is you'd end up with planets of about the size of Earth and Venus where they should be," says lead study author Kevin Walsh, a planetary scientist at the Southwest Research Institute in Boulder, Colo. "But the object that ended up around the location of Mars was the size of Earth as well." In actuality, compared with Earth, Mars is only about half its diameter, and about one tenth its mass.

The problem would go away if there were simply less raw material available to Mars when the planet developed. To try to account for that paucity, Walsh and his colleagues designed a model in which Jupiter's motion sweeps out many of the planetesimals (small bodies) that collided to form the terrestrial planets. Today, Jupiter orbits the sun at about five times the Earth–sun distance, or five astronomical units (AU). But interactions with the gaseous disk that surrounded the young sun could well have drawn Jupiter inward in the first millions of years following the solar system's birth.

Walsh and his colleagues found that if Jupiter drew all the way in to about 1.5 AU, then retreated outward due to gravitational interactions among Saturn and the gaseous disk, its motion would scatter most of the planetesimals situated beyond Earth's orbit. So while Earth and Venus continued to accrete more protoplanetary material, Mars's growth further out was stunted. The entire inward and outward migration could have taken place within a few hundred thousand years; the planets of the inner solar system would then have finished their development over tens of millions of years.

Jupiter's migration would account for the divergent sizes of the terrestrial planets, and it also seems to gibe with the current state of the Asteroid Belt, which lies between the orbits of Mars and Jupiter. "The real big test for this was the Asteroid Belt, which is right there in the firing line where Jupiter goes back and forth," Walsh says. "We really didn't know if we'd get an asteroid belt that made any sense." But Jupiter's passage in and out appears to scatter the right amount of material to leave today's belt in place, and it could also account for the fact that the inner and outer parts of the Asteroid Belt have somewhat different populations.

In the simulation Walsh and his colleagues designed, one family of asteroids originates among the giant planets of the outer solar system, and another family originates much closer to the sun. The migratory motions of the giant planets mix those two asteroid families somewhat, but not completely, leaving two discernable populations within the belt. "We ended up with a really nice match with the Asteroid Belt," Walsh says. "The total mass worked out pretty nicely, and we were also able to reproduce this dichotomy in the Asteroid Belt."

A low mass for Mars from Jupiter’s early gas-driven migration - KJ Walsh et al

• Nature (online 05 Jun 2011) DOI: 10.1038/nature10201

http://asterisk.apod.com/viewtopic.php?f=31&t=21615