I’m sometimes asked why I write so seldom about Mars, a very interesting place indeed. The answer is that so many excellent sites are out there tracking events on the planet that I’m happy to keep my focus on the outer system and the starry gulf beyond. But now and then Mars news interrelates with broader stories about planet formation and what we might find in other solar systems. Such is the case with new work from Kevin Walsh (Southwest Research Institute) that looks at the migration of Jupiter during the formation of the Solar System.
At issue is the question of why Mars is so small, because if you run simulations of the planet formation process for the four inner planets of our system, you get a Mars that’s much heavier than the one we see. Tweaking the simulation parameters isn’t enough — it still doesn’t produce the smaller Mars. But a major migration scenario involving Jupiter can help to explain the situation. The trick is that it relies upon an initial distribution of solid material that had to have an outer boundary of about 1 AU, the current average distance of the Sun from the Earth.
That’s a well defined boundary, and it doesn’t fit well with either the asteroid belt (between 2 and 4 AU) or the current parameters of the outer system, including the Kuiper Belt. But numerical simulations have been able to show that both Saturn and Jupiter could have migrated at a time in the early system when gas was still present, moving not only inward but back out again. Such migrations could occur on timescales of as little as 100,000 years. Walsh explains the scenario:
“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.”
Hydrodynamic simulations reproduce this inward, then outer movement, with Jupiter migrating inward to about 1.5 AU and subsequently moving to its present position, resulting in a truncated planetesimal disk ending at 1 AU. The terrestrial planets would have then formed from this disk over the ensuing 30 to 50 million years, producing planets consistent with Earth and Mars.
Image: An international team of scientists led by Dr. Kevin Walsh of Southwest Research Institute is using complex modeling techniques to better understand the formation of our solar system. The “Grand Tack Scenario” demonstrates that the gas giant Jupiter may have briefly migrated into the inner solar system and influenced the formation of Mars (right), stripping away materials that resulted in its relatively small size in comparison to Venus (left) and the Earth. Credit: NASA.
Walsh began this work while at the Observatoire de la Cote d’Azur (Nice), trying to figure out whether a migrating Jupiter, which would have had to pass through the 2-4 AU region, could be reconciled with the presence of the asteroid belt today. Not only did the simulations show the migration was consistent with the asteroid belt, but they actually helped to explain some features of the belt that have not been understood before. The planetary migration repopulated the asteroid belt with inner-belt objects that originally formed between 1 and 3 AU and outer-belt bodies that originated in and beyond the gas giant planets. Thus we gain insight into why the asteroid belt contains both extremely dry objects as well as bodies that are water-rich.
David O’Brien (Planetary Science Institute), a co-author on the paper, says “The asteroid belt, which was a priori our main problem, turned out to be the main strength of our model.” Walsh calls this model of the peripatetic Jupiter the ‘Grand Tack Scenario,’ likening the motion of the giant planet to a sailboat tacking around a buoy as it makes its great pivot at 1.5 AU. We wind up with a smaller Mars whose mass suggests a story of ancient planetary wandering, and we see a model of gas giant migration that may also give us insight into the kind of migrations that must have occurred in exoplanetary systems, where the orbital distance of gas giants varies widely.
The paper is Walsh et al., “A Low Mass for Mars from Jupiter’s Early Gas-Driven Migration,” published online by Nature 5 June 2011 (abstract).
The paper also explains why we do not have a hot Jupiter. Jupiter would have become a hot Jupiter if Saturn had not been there to migrate it out again.
One word comes to mind for a solar system that comes to be from such a complicated dance of giant planets : RARE.
We’ll see if that’s really the case.
Maybe it is, maybe it isn’t. Certainly there seem to be a few examples of wide-orbiting Jupiter-Saturn pairs in the exoplanet catalogue (e.g. 47 UMa and some microlensing system with a horrendously unmemorable designation).
And given that a more massive Mars could have improved chances of habitability, maybe the rarity of a low mass Mars is not necessarily a bad thing in terms of the occurrence of life anyway.
I have long felt that the problem of asteroid belt composition is understated. It must have been very great in able to explain an anomaly in the history of science…
The exploded planet hypothesis for a time came to be seen as a respected possibility, despite the fact that energetic considerations showed the it should never have been a starter. This, in turn, suggests that Walsh et al. could have solved a great and understated mystery.
Mercury? What about poor little Mercury, with an iron core 3 sizes too big?
I agree, the Sol system is likely an uncommon pattern. I recall attending a Drake SETI talk in the early 90s, where he laid out his guesses about the values in his famous equation. He thought the fraction of single stars with planetary systems to be close to 1, which may be true.
(Multiple star systems are a more complex topic, and he thought close binaries like Alpha Centauri AB to be unfavorable for SETI)
He then thought the average number of rocky worlds with stable near circular orbits in the liquid water zone per planetary system to be close to 1 as well. A reasonable guess at the time, but my 2011 guess would be closer to 0.01.
However, uncommon does not mean rare. As there are over 10,000 stars within 100 ly, there can be many examples of even uncommon planetary system configuarations in the general neighborhood.
Looking at the large number of sun diving comets discovered by SOHO in the present era, I imagine that Mercury started out as a larger body and has been impact eroded over the age of the solar system. Thus the relatively large core.
I want to stress that a rare solar system doesn’t necessarily means scarcity of small rocky planets. Too little data to say that.
What seem to be rare are cold 5 AU Jupiters. This is not my idea but it comes from respected astronomer Greg Laughlin and particularly from this article in his blog :
The argument is simple : we had the precision and the time to detect cold Jupiters and we have only found a few. I’m not 100% sure but it is not just because people have not looked : even unassuming stars like Iota Horologi have been looked at and what was to be found, found quickly ( a 2.5 MJ at ~1 AU 11 years ago).
By the way, Laughlin’s article hints even more at how weird our systems is starting to look like.
If Jupiter did migrate inward to a distance of only 1.5 AUs from the sun:
(a) Why did the Jovian satellite survive intact? Would the sun have not ripped apart such system as as the distance from Jupiter to the Sun dramatically decreased? Perhaps the answer is that such satellite system formed after the proposed inward and outward migration.
(b) Why did Jupiter not destroy the embryonic Earth? Since Jupiter would have been only about 0.50 AUs from where the Earth was forming but the Sun was a full 1 AU from Earth, would Jupiter’s gravity not possibly have exhibited a disrupting effect? Of course, I am aware that the Sun is about 1,000 times more massive than Jupiter and would have still held on to Earth gravitationally.
(c) Why are there quite a few other extrasolar systems where (i) the mass ratio of the inner planet to the outer planet is approximately that of Jupiter to Saturn and yet (ii) the outer planet did not cause the inner planet to migrate back out (in other words, both planets are very close to the central star)? Examples are HD 134987, HD 155358, HD 200964, and HD 14810. Is it possible that it was not the outer planet triggering outward migration but instead photoevaporation from nearby giant stars in an enviornment similar to the Orion nebula which is an enviornment in which about 90% or so of stars seem to form. If this is the case, then earth-like planets may still be common because such photoevaporation would have primarily resulted in outward migration instead of inward migration.
Finally, it appears that only about 20% of stars have giant planets. Most stars seem to have Neptunes instead of Jupiters. In that case, would a migrating planet have disrupted the formation of Earth at all it it had passed to within 1.5 AU and then migrated out again? Some of the recent articles on dynamically interacting planets seem to be grossly pessimistic about the formation of Earths in that they are written only about systems in which there is one or more ginat planets interacting gravitationally so that one or more such planets is flung toward the central star. In a system whith Neptunes instead of Jupiters, such dynamic interactions would be far less frequent and a newly forming Earth would have a much better chance.
If this proposed mechanism is true, then this has important implications for stellar systems. Someone, I’ve forgotten who, in a paper on tidal perturbations of orbits pointed out that there is a minimum spacing of planets in a system equal to the maximum number of planets you can pack into a system with stable orbits. Any more planets and one gets ejected.
Jupiter having moved in and out through the orbit of Mars and the asteroid belt implies that every stable orbit in a system is occupied by something when a stellar system settles into its final configuration. If there is insufficient material to coalesce into a planet, then you get a planetismal belt. I call it the law of no gaps. So this maximum number may also be the minimum number*.
Once you get past what was the inner part of our solar nebula (which was presumably truncated just inside the orbit of Mercury) then something occupies every possible position all the way out to the Kuipier belt.
This means that if you look at a stellar system (with say the next generation of telescopes) and you can detect the larger planets and the planetismal belts then the remaining gaps must be occupied by planets. (Tau Ceti may be a good example.) The planet may be a very small (if Jupiter had for some reason been smaller so it didn’t stir up the asteroid belt so much, the belt may have coalesced into a small planet), but it will be there.
*This is not an absolute number. A given space in a stellar system may, for instance, hold say three larger planets in stable orbits or five small ones. If you find two large planets and no planetismal belt, then you know there will be a 3rd planet. If you find say three small ones and two gaps, then you know you have another two planets.
@Randy Kelley: the Hill sphere of Jupiter at 1.5 AU is still ~8 times wider than the orbit of Callisto. The outer irregular satellites on the other hand are probably captured and lost over the history of the solar system.
@Randy Kelly: why Jupiter did not destroy the embryonic Earth?
At the stage Jupiter migrated in to 1.5 au, the area around Earth’s orbit contained nothing but a mixture of protoplanets and planetismals. Earth would assemble itself some 10s of millions of years later.
Fogg and Nelson did a series of simulations of giant planets migrating in through an inner stellar system to form Hot Jupiters, and one of the effects they found is that the giant planet would bulldoze some of the protoplanets and planetismals inward through mean motion resonances, so Jupiter’s inward migration may have contributed to Earth’s and Venus’s size by pushing material from further out into their orbits.
In answer to the last part of your post, Fogg and Nelson also found that as the giant planet migrated inward it would scatter the protoplanets and planetismals out past it so that after it had passed through an area there was still about 50% of the material left behind to form planets.
In answer to question (c) planetary formation is obviously a chaotic process in which very small changes in input can produce large changes in outcome.
The whole idea of Jupiter sweeping up the protoplanetary dust disk (proplyd) material and in this way preventing the formation of another planet at the asteroid belt location and at the same time limiting the size of Mars, is not new by itself. Dole already wrote about it in his seminal work “Habitable Planets for Man” in 1964 (I think he spoke about the ‘forbidden zone’).
However what is new is the expansion of this idea with Jupiter migrating inward and subsequently outward again, in doing so widening the zone in which material was swept up and terrestrial planet formation prevented.
Also ref. to kurt9, Randy Kelly (last paragraph) and Dave Moore (like your theory):
My very initial impression is that the abundance and size of (giant) planets largely correllates with the thickness and density of the proplyd. And that very large giant planets have a greater tendency to migrate inward (and stay there), maybe as a result of drag plus lack of forces pulling/pushing them outward again.
Since proplyd mass (thickness, density) seems to correllate well with metallicity, this could imply that very high metallicity results in hot (giant) Jupiters and failed terrestrial planets. And inversely, lower metallicity would result in smaller subgiants (Neptunes) plus less inward migration, hence also enabling the formation of (more) terrestrial planets. Too low metallicity would result in a failed planetary system consisting of dust and planetesimals (asteroids).
This still does not satifactorily explain the common occurrence of hot Neptunes.
It would be fascinating to be able to correllate the Kepler results with metallicity data.
A bit further to my previous post, concluding: if the ‘ metallicity-proplyd-planet size&abundance’ correllation is correct, then this implies that there is an optimal metallicity range for terrestrial planets in stable orbits.
Annoying, of course, that Jupiter managed to eat up so much material from Mars. If Jupiter had just stayed away another 0.5 AU orso, Mars might have been more like Venus, holding on to a denser atmosphere and having a warmer climate, liquid surface water and possibly life.
Feb 14, 2013
By Kim DeRose
Top scientists debate whether life could survive on Mars
More than 50 of the world’s top Mars scientists gathered in Royce Hall last week to discuss whether life could survive on the red planet. Three dozen talks over two days covered topics ranging widely from the current liquid water activity on Mars to NASA’s planetary protection policies.
“The habitability of Mars is a pressing issue because we plan to send humans there in the next century,” said David Paige, a UCLA professor of Earth and space sciences and a co-organizer of the conference, held Feb. 4-5. “To do that in a responsible way, we should take into account that there could be an indigenous biosphere on Mars, and do our best to predict what would happen to any terrestrial organisms we might bring with us.”
A color-enhanced image of the inside rim of Newton Crater on Mars. The dark streaks, called Recurring Slope Lineae (RSL), may represent current subsurface liquid water activity. The image was taken by an instrument onboard the Mars Reconnaissance Orbiter. NASA/JPL/University of Arizona
Life is almost everywhere we look on Earth, but that’s not true for inhospitable Mars. Andrew Schuerger, an astrobiologist from the University of Florida and speaker at the conference, went so far as to list 17 separate environmental hazards that could freeze, irradiate or otherwise disrupt microbial life on the surface of Mars. Topping the list: powerful, microbe-frying, ultraviolet light from the sun; sub-zero temperatures; and a thin, oxygen-less atmosphere with pressure levels 100 times lower than those found on Earth.
Nonetheless, Schuerger and his colleagues have made it their mission to find the hardiest bacteria surviving in the harshest environments on Earth and to determine whether the tiny microorganisms could grow in Mars-like conditions. His search was rewarded with the discovery of several hypobarophiles, microorganisms that can grow in extremely low-pressure and low-temperature environments. While some of these single-celled survivors hail from the Canadian Arctic or the depths of Siberia, others live closer to home: Schuerger was particularly surprised to find hypobarophiles in a sample he took of human saliva.
Yet even the sturdiest of the hypobarophiles would shrivel without a stable source of liquid water, said Schuerger. While liquid water may be hard to come by on the Martian surface, there is plenty of evidence that water exists beneath the surface, according to Alfred McEwen, a planetary geologist from the University of Arizona.
First discovered from images taken by the Mars Reconnaissance Orbiter in 2011, Recurring Slope Lineae (RSL) are dark streaks that slowly creep down sun-facing crater rims and canyon walls during the Martian spring and summer and then fade in winter. The flow of liquid water, mixed with Martian salts several centimeters beneath the planet’s surface, may be responsible for tracing the finger-like patterns, though the source of the water is still unknown.
“The exact mechanism is not well understood,” said McEwen. “Given the seasonality and temperature dependence, we think a volatile must be involved, and briny water is the best candidate.”
While the formation of RSL may look remarkably like streaming water on Earth, McEwen is quick to put aside the notion, emphasizing that, on Mars, the trickle of briny water takes weeks to flow downhill and behaves more like “maple syrup slowly oozing down the slope.”
While liquid water on Mars may be subsurface and salty, one of the largest questions the conference dealt with was the role of perchlorate, a high-energy molecule toxic to most kinds of life.
Artist’s concept of the Phoenix lander on the surface of Mars. In 2008, scientists were surprised to find that the substance perchlorate was present in the Martian soil. Photo credit: NASA/JPL-Caltech
In 1976, twin Viking spacecraft landed on the surface of Mars and analyzed the composition of Martian soil for signs of past or present life. Viking found only a single organic molecule, which the science team dismissed as a remnant of cleaning products used on Earth prior to launch. However, in 2008, an experiment onboard the Phoenix lander surprised scientists when it indicated the presence of perchlorate.
“Perchlorate is a double-edged sword,” said Paige. “It is a reactive molecule that destroys organic molecules, yet we find a variety of organisms on Earth that, in fact, use it to survive.”
If perchlorate, a common component in pyrotechnics and rocket fuel, was present in those early Viking samples, it may have completely destroyed any interesting organics the experiment was meant to measure. A reanalysis of the Viking results published more than 30 years later revealed that the single organic molecule the experiment detected was not the result of Earth-based contaminants as originally suspected, but instead the predicted byproduct of a perchlorate reaction.
“This could be a very exciting explanation for the Viking results that showed no organic molecules except one that could easily be residue from the combustion of perchlorate and organics,” said Paige. “This opens up a whole new set of possibilities that just weren’t there before the perchlorate molecule was discovered on Mars.”
Strong among these possibilities is the fact that perchlorates can draw atmospheric water vapor into liquid form. Liquid water produced in such a way on Mars could potentially provide hydration for microbes capable of surviving in the presence of the reactive perchlorate molecules. Just as important, when perchlorate mixes with water on Mars, it forms salty brine that freezes at a much lower temperature than pure water, which extends the range of potentially habitable conditions on Mars.
Solving the puzzle of whether life could survive in the harsh and varied environments of Mars requires a community of scientists from across many disciplines, said Paige.
“Many different types of scientists are involved, from researchers who study orbital images to biologists who grow microorganisms in petri dishes, and everyone in between,” he said. “To get such a diverse community together was a lot of fun.”
This conference, sponsored by the UCLA Institute for Planets and Exoplanets (iPLEX), the NASA Astrobiology Institute and the UK Centre for Astrobiology, is the first iPLEX meeting to be open for virtual participation. Nearly 50 participants watched the conference online, asking speakers questions via webchat. Nine talks were given remotely by speakers located as far away as the United Kingdom, Hungary and Russia.
The iPLEX aims to advance research into planetary systems around the sun and other stars by facilitating interdepartmental collaboration. It is a joint venture bridging the interests of researchers in the departments of Earth and Space Sciences, Physics and Astronomy, and Atmospheric and Oceanic Sciences.
All conference talks were recorded and archived online; they can be streamed for free by the public from the conference website.