A giant impact: Solving the mystery of how Mars' moons formed

July 4, 2016
Chronology of events that may have created Phobos and Deimos. Mars is struck by a protoplanet one-third its size (1). A debris disk forms within a few hours. The elementary building blocks of Phobos and Deimos (grains smaller than a micrometer) condense directly from gas in the outer part of the disk (2). The debris disk soon produces a moon near Mars that moves further away and propagates its two areas of dynamical influence like ripples (3), which over the course of a few thousand years causes the accretion of more dispersed debris into two small moons, Phobos and Deimos (4). Under the effect of the tidal pull of Mars, the large moon falls back to the planet within approximately five million years (5), while smaller Phobos and Deimos take up their current positions in the ensuing billions of years (6). Credit: Antony Trinh / Royal Observatory of Belgium

Where did the two natural satellites of Mars, Phobos and Deimos, come from? For a long time, their shape suggested that they were asteroids captured by Mars. However, the shape and course of their orbits contradict this hypothesis. Two independent and complementary studies provide an answer to this question. One of these studies, to be published in The Astrophysical Journal and predominantly conducted by researchers from the CNRS and Aix-Marseille Universite, rules out the capture of asteroids, and shows that the only scenario compatible with the surface properties of Phobos and Deimos is that of a giant collision. In the second study, a team of French, Belgian, and Japanese researchers used cutting-edge digital simulations to show how these satellites were able to form from the debris of a gigantic collision between Mars and a protoplanet one-third its size. This research, which is the result of collaboration between researchers from Université Paris Diderot and Royal Observatory of Belgium, in collaboration with the CNRS, Université de Rennes 1 and the Japanese Institute ELSI, is published on July 4, 2016 in the journal Nature Geoscience.

The origin of the two Martian moons, Phobos and Deimos, remained a mystery. Due to their small size and irregular shape, they strongly resembled asteroids, but no one understood how Mars could have " captured " them and made them into satellites with almost circular and equatorial orbits. According to a competing theory, toward the end of its formation Mars suffered a giant collision with a protoplanet: but why did the debris from such an impact create two small satellites instead of one enormous moon, like the Earth's? A third possibility is that Phobos and Deimos formed at the same time as Mars, which would entail that they have the same composition as their planet, although their low density seems to contradict this hypothesis. Two independent studies have now solved the puzzle: the Martian moons must have arised from a giant collision.

In one of these studies, a team of Belgian, French, and Japanese researchers offers, for the first time, a complete and coherent scenario for the formation of Phobos and Deimos, which would have been created following a collision between Mars and a primordial body one-third its size, 100 to 800 million years after the beginning of the planet's formation. According to researchers, the debris from this collision formed a very wide disk around Mars, made up of a dense inner part composed of matter in fusion, and a very thin outer part primarily of gas. In the inner part of this disk formed a moon one thousand times the size of Phobos, which has since disappeared. The gravitational interactions created in the outer disk by this massive moon apparently acted as a catalyst for the gathering of debris to form other smaller, more distant moons. After a few thousand years, Mars was surrounded by a group of approximately ten small moons and one enormous moon. A few million years later, once the debris disk had dissipated, the tidal effects of Mars brought most of these satellites back down onto the planet, including the very large moon. Only the two most distant small moons, Phobos and Deimos, remained (see the graphic at the end of the press release).

Due to the diversity of physical phenomena involved, no digital simulation is able to modelize the entire process. Pascal Rosenblatt and Sébastien Charnoz's team thus had to combine three successive cutting-edge simulations in order to provide an account of the physics behind the giant collision, the dynamics of the debris resulting from the impact and its accretion to form satellites, and the long-term evolution of these satellites.

In a second study, researchers from the Laboratoire d'astrophysique de Marseille (CNRS/Aix-Marseille Université) ruled out the possibility of a capture on the grounds of statistical arguments based on the compositional diversity of the asteroid belt. They moreover show that the light signature emitted by Phobos and Deimos is incompatible with that of the primordial matter that formed Mars (meteorites such as ordinary chondrite, enstatite chondrite and/or angrite). They therefore support the collision scenario. From this light signature they deduced that the satellites are made of fine-grained dust (smaller than a micrometer).

Yet the very small size of grains on the surface of Phobos and Deimos cannot, according to the researchers, be solely explained as the consequence of erosion from bombardment by interplanetary dust. This means that the satellites were from the beginning made up of very fine grains, which can only form by gas condensation in the outer area of the (and not from the magma present in the inner part). Both studies are in agreement on this point. Moreover, the formation of Martian moons from these very fine grains could also be responsible for a high internal porosity, which would explain their surprisingly low density.

Artist's rendering of the giant collision that may have produced Phobos and Deimos along with the Borealis basin. The colliding body would have been approximately one-third the size of Mars. At the time, Mars was young, and perhaps had a thicker atmosphere and liquid water on its surface. Credit: Université Paris Diderot / Labex UnivEarthS

The theory of the giant collision, which is corroborated by these two independent studies, could explain why the northern hemisphere of Mars has a lower altitude than the southern hemisphere: the Borealis basin is most probably the remains of a giant collision, such as the one that gave birth to Phobos and Deimos. It also helps explain why Mars has two satellites instead of a single one like our Moon, which was also created by a . This research suggests that the systems that were created depended on the planet's rotational velocity, because at the time Earth was rotating very quickly (in less than four hours), whereas Mars turned six times more slowly.

New observations will soon make it possible to know more about the age and composition of Martian moons. Japan's (JAXA) has decided to launch a mission in 2022, named Mars Moons Exploration (MMX), which will bring back samples from Phobos to Earth in 2027. Their analysis could confirm or invalidate this scenario. The European Space Agency (ESA) has planned a similar mission in 2024 in association with the Russian space agency (Roscosmos).

Explore further: How many moons does Mercury have? (Update)

More information: 1. Reconciling the orbital and physical properties of the martian moons, Thomas Ronnet, Pierre Vernazza, Olivier Mousis, Bastien Brugger, Pierre Beck, Bertrand Devouard, Olivier Witasse, Fabrice Cipriani. The Astrophysical Journal, in press.

2. Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons, Pascal Rosenblatt, Sébastien Charnoz, Kevin M. Dunseath, Mariko Terao-Dunseath, Antony Trinh, Ryuki Hyodo, Hidenori Genda & Stéven Toupin. Nature Geoscience, 4 July 2016, nature.com/articles/doi:10.1038/ngeo2742

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cantdrive85
1.6 / 5 (28) Jul 04, 2016
EDM gives a much, much simpler explanation than this guess that can't even be modeled due to all the different magic that needs to occur to make it happen.
nkalanaga
4.4 / 5 (20) Jul 04, 2016
"100 to 800 million years ago".

http://www.stuff....happened

"...Nina Lanza, a researcher at Los Alamos National Laboratory in New Mexico. In a study published this week in the journal Geophysical Research Letters, she and her colleagues argue that the discovery of manganese oxide (which forms in wet, oxygen-rich conditions) on the Martian surface suggests that the planet was once much more Earth-like.

Now, of course, Mars is a frozen, barren wasteland, covered in dull red rock that's been bombarded and twisted into strange formations. Another study of the geology of Mars (this one published on Thursday in the journal Science) suggests that a particular kind of formation - a ripple in the sand that forms only in Mars' current thin, arid atmosphere - can be found in fossilised form going back 3.7 billion years."

800 MYrs would be about the right time to lose the atmosphere.
Shootist
2 / 5 (23) Jul 04, 2016
I present to you something quite germain

http://onlinelibr...343/full
BoatRocker
4.3 / 5 (17) Jul 04, 2016
" In the inner part of this disk formed a moon one thousand times the size of Phobos, which has since disappeared. The gravitational interactions created in the outer disk by this massive star apparently acted as a catalyst for the gathering of debris to form other smaller, more distant moons."

A moon, an order of magnitude over Phobos is a massive star gathering debris... wait. What?
nkalanaga
3.6 / 5 (20) Jul 04, 2016
Shootist: A very interesting paper. It doesn't sound like they're describing the same event, but their conclusions seem reasonable, and certainly Mars has been hit by quite a few asteroids in its history. It's going to take a LOT of study, and drilling, to even begin to understand Mars' geologic history. We've found multiple craters on Earth that apparently formed from either binary asteroids, or ones the broke up on approach, so it makes sense the same thing happened on Mars.

We've been exploring Earth for millennia, mining it for centuries, and drilling for decades, and we're just starting to understand our own planet.

BoatRocker: "Star" is probably a translation error, given that none of the authors seem to be native English speakers. After all, "asteroid" means "resembles a star".
LuisDineman
1.5 / 5 (15) Jul 04, 2016
I think both Phobos and Deimos, as well as our own Moon, formed following a slightly different scenario, the thermodynamics of which could be visualized following this video
youtu.be/cp5gdUHFGIQ?t=272
While accreting, Mars was absorbing not only meteorite matter, but also heat energy released by their impacts. At some point its crust got too hot and started vaporizing. This could've been a continuous ejection of vaporized lava sea, a series of small explosions (volcano eruptions), or a one giant explosion, delivering a massive amounts of gas in its atmosphere and/or beyond it. Eventually this ejected gas would cool and condense into bigger dust like particles, or even bigger rocks. Some of them would fall fall back on Mars (LHB?). Some would break free and end up in other parts of the solar system. The remaining would find itself on stable orbits, first accreting into a layers and/or rings (like the ones Saturn has) and finally satellites.
LuisDineman
1.5 / 5 (15) Jul 04, 2016
Another scenario: After initial accretion, the planets would rapidely cool on the outside, while still superhot inside. This increased thermal gradient would be counteracted by increased dynamic activity, like giant mantle plums accelerating into and breaking thin crust, forming supervolcanos, ejaculating it into space.
blazmotronic
1.5 / 5 (16) Jul 04, 2016
contradict this hypothesis..excuse me..change your theory
to fit the new observations..this is called science...bunch of hacks!
Whydening Gyre
4.3 / 5 (18) Jul 04, 2016
EDM gives a much, much simpler explanation than this guess that can't even be modeled due to all the different magic that needs to occur to make it happen.

Model it.
BartV
1.6 / 5 (19) Jul 04, 2016
Please, please, give one decent computer simulation result of the above hypothesis. How could you possibly have a magical debris disk around a planet after such a collision? Please show me the math! I don't believe rotational velocities and gravity works that way. What gets sent up from a planet that is not past the escape velcoity will come right back to the planet.

Whydening Gyre
4.4 / 5 (14) Jul 04, 2016
Please, please, give one decent computer simulation result of the above hypothesis. How could you possibly have a magical debris disk around a planet after such a collision? Please show me the math! I don't believe rotational velocities and gravity works that way. What gets sent up from a planet that is not past the escape velcoity will come right back to the planet.

Read the source article from above. Here, I'll make it easy for you -
nature.com/articles/doi:10.1038/ngeo2742
Would you know the math if you saw it? Many objects in orbit around earth did not achieve escape velocity - or they would have "escaped" (ie- The Moon) They don't drop out of a building like rock.
And lastly, rotational velocities and gravity work differently on different bodies, dependent on mass and rotation, to name just 2 of a number of variables...
nkalanaga
4.5 / 5 (17) Jul 04, 2016
BartV: For solitary objects you're correct, they either escape, or return to the surface. But in a massive debris disk, the objects can collide with each other, and any gas or vapor will produce friction, both of which can alter orbits. Much of the debris will still escape or reimpact, but there will also be a substantial fraction that achieves at least temporarily stable orbits. That portion can then accrete into larger objects, in nearly circular orbits.

This has been well modeled in studies of our own Moon's formation, which is now widely believed to be the result of a similar impact, as the authors mentioned.
RealScience
4.5 / 5 (17) Jul 04, 2016
Please, please, give one decent computer simulation result of the above hypothesis. How could you possibly have a magical debris disk around a planet after such a collision? Please show me the math! I don't believe rotational velocities and gravity works that way. What gets sent up from a planet that is not past the escape velcoity will come right back to the planet.


@BartV - the article references just such a simulation:
a team of French, Belgian, and Japanese researchers used cutting-edge digital simulations to show how these satellites were able to form from the debris of a gigantic collision between Mars and a protoplanet one-third its size. This research... is published on July 4, 2016 in the journal Nature Geoscience.

Ah - I see that nkalanaga pointed the difference between single objects and clouds of debris.
Shootist
2 / 5 (16) Jul 05, 2016
nkalanaga, given the odds of 6 or 7 great impacts all occurring within a couple of degrees of a great circle, I have to conclude, unless shown compelling data otherwise, that those major basins formed as described in the paper. Also there is at least one additional basin that was unknown at the time of the paper (2005), a 2700 km Circular Thin Area/Quasi-circular depression from MOLA data in Amazonia (which is larger than Hellas!) and matching gravity anomalies discovered in the seasonal and static gravity field data gathered from MGS, Mars Odyssey and MRO.

What I found germain -- the putative debris disk described in this article could have provided the impactors described in the paper I cited, as the moon that crossed Mars' Roche limit and broke up. Remove the hypothetical moon and replace with debris disk.
jonesdave
4.3 / 5 (17) Jul 05, 2016
EDM gives a much, much simpler explanation than this guess that can't even be modeled due to all the different magic that needs to occur to make it happen.

Model it.


Why would they? Then it would no longer be simple! Somebody with actual qualifications would have to do a lot of work to figure out that it's wrong.
Otto_Szucks
1.5 / 5 (16) Jul 05, 2016
And how do you know it's wrong if you've never even seen the "explanations"? Obviously, they're keeping it under wraps until the right opportunity arises. There would be no point in explaining their Model to a layman - or a non-scientist - or a troll.
Whydening Gyre
4.4 / 5 (14) Jul 05, 2016
And how do you know it's wrong if you've never even seen the "explanations"?

And none forthcoming.
Obviously, they're keeping it under wraps until the right opportunity arises.

(snicker) THAT sounded like sarcasm...:-)
There would be no point in explaining their Model to a layman - or a non-scientist - or a troll.

How bout an artist?
There isn't even a claim to HAVE a model...
Exactly why I requested CD to produce one. For scientists, if not for me...
antialias_physorg
4.3 / 5 (17) Jul 05, 2016
A moon, an order of magnitude over Phobos is a massive star gathering debris

Yeah...I guess the translator for this article wasn't a native speaker:
the Martian moons must have arised from a giant collision.
...
no digital simulation is able to modelize the entire process

Arised? Modelize? Sheesh.

Enthusiastic Fool
4.5 / 5 (12) Jul 06, 2016
With low density of the moons and the massive volcanism on Mars is it possible that these are just the few remains of a period of eruptions where the all the ejecta did not quite hit escape velocity? Maybe Im being anthropocentric with thinking that Earth is locally unique for its giant impact moon. Another factor is what do current models of the solar system at the proposed time of this impact have to say about a 1/3 Martian mass planetoid floating about the inner solar system?
Whydening Gyre
4.6 / 5 (11) Jul 06, 2016
With low density of the moons and the massive volcanism on Mars is it possible that these are just the few remains of a period of eruptions where the all the ejecta did not quite hit escape velocity?

EF,
Seems kinda what LuisDineman was going for.
Maybe Im being anthropocentric with thinking that Earth is locally unique for its giant impact moon.

Just have to be special, donchya..:-)
Another factor is what do current models of the solar system at the proposed time of this impact have to say about a 1/3 Martian mass planetoid floating about the inner solar system?

Pretty sure that would be an unconsidered (at this point) element to previous models...
nkalanaga
4.6 / 5 (14) Jul 06, 2016
Shootist: I agree that the paper you linked to probably is accurate, and all of those basins formed from a single object. The reason I don't think it is directly related to this article is the timing. This article says the impact occurred 100 to 800 million years after Mars formed, while yours is dated 1 to 2 billion after formation.

However, you may very well be right that the "very large moon" formed in this article's impact was the impactor in your paper. This one says it reimpacted "a few million years" after formation, and given all of the other uncertainties, 200 million after the latest date here would match the earliest date in your paper. And, the moons formed here would almost certainly be in the equatorial plane, as it seems to be natural for rings and disks to form there.
DonCarloFantasia
Jul 06, 2016
This comment has been removed by a moderator.
Shootist
2.2 / 5 (10) Jul 06, 2016
nakalnaga

No. Mars coalesced at ~4.55Gy. The Terminal Lunar cataclysm/Late Heavy Bombardment which formed the Great Basins of Mars occurred 4.1 Gy - 3.9 Gy, certainly well within the time frame specified.

The author is speaking of both the cratering process and the formation of Tharsis. Quote: "The major impact cratering, tectonic and volcanic processes have occurred during the first 1–2 Gyr of the planet's history".

The LHB occurred well within the first billion years of Martian history.

The LHB may have continued until as late as 3.8Gy with two bursts of activity at 3.5Gy and 2.5Gy (17 asteroid impact ejecta horizons older than 2.5 billion years have been found in Africa/Australia). But the crater retention ages of Mars and the Moon correlate well.
BiteMe
Jul 25, 2016
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