Boulders on asteroid Bennu shed new light on the space rock's history

rod

Oct 22, 2019
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This is an interesting asteroid report. I read some others. https://skyandtelescope.org/astronomy-news/osiris-rex-oldest-rocks/, https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/101955-bennu/in-depth/

Here are some observations from me on this asteroid. From the Sky & Telescope report I gathered this impression and content. My observation, a few millions years after solar system formed. 4.568E+9 - 3E+6 = 4.565E+9 years ago. 4.565E+9 years ago there was flowing water. 1.5E+9 years ago, a catastrophic collision. So we have 4.565E+9 - 1.5E+9 = 3.065E+9 years of orbital history from when the water was flowing to the catastrophic collision date.

From the NASA site report on Bennu, to explain Bennu we have formation 4.5E+9 - 10E+6 = 4.49E+9 years reported in the NASA site and whatever postulated events are needed in the model, including moving inwards from the main asteroid belt to just near 1.13 AU today, so a migration of 2 AU perhaps or more. In a period of 4.5E+9 years, Bennu could complete 3.75E+9 perihelion passages of the Sun using a period of 1.2 years. The explanation of this asteroid and features presents a very large extrapolation for its past based upon limited observations and its current orbit and position, likely does not extend back 4.5 billion years ago but only more recently in the solar system.
 
Mar 5, 2020
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The explanation of this asteroid and features presents a very large extrapolation for its past based upon limited observations and its current orbit and position, likely does not extend back 4.5 billion years ago but only more recently in the solar system.


The picture above shows that a large chunk is missing from the Martian moon Phobos.

Bennu was never a part of the main belt asteroids.

Clay is incredibly rare unless you are talking about an origin from a planetary body. There are plenty of types of igneous rocks that can turn into clay on Earth. Except those rocks usually do not form clays until they are close enough to the surface to contact surface moisture. A lot of terrestrial rocks (including igneous) would be considered “wet” by asteroid standards yet these processes do not occur.

The spitting rocks of Bennu are likely steam explosions from hydrated materials. The University of Arizona probably has data indicating this and neglected to mention it.

Bennu is a very young asteroid if the hydrated materials on its surface are still experiencing steam (or any kind of volatile) explosions. Mars has an abundance of hydrated rocks and has a relatively shallow gravity well. This would suggest that Mars or maybe Phobos was the origin for these hydrated materials.

The orbit shown below does suggest that Mars was the origin for the rubble that would become Bennu.

 
In a period of 4.5E+9 years, Bennu could complete 3.75E+9 perihelion passages of the Sun using a period of 1.2 years. The explanation of this asteroid and features presents a very large extrapolation for its past based upon limited observations and its current orbit and position, likely does not extend back 4.5 billion years ago but only more recently in the solar system.
From one of your own references:

Half-kilometer Bennu is likely a chip off that planetesimal block, formed from the debris in a catastrophic collision that happened some 0.8 to 1.5 billion years ago. It probably belongs to one of either the Eulalia or Polona asteroid families, both of which originated in collisions at about that time, says Daniella DellaGiustina (University of Arizona). Then, sometime between a few million and a few tens of millions of years ago, gravitational interactions with the giant planets and long-term interactions of solar radiation with spinning bodies shifted Bennu to its current near-Earth orbit.

That migration through the asteroid belt allowed Bennu to collect additional rubble from other sources, such as the six Vesta-like boulders reported in Nature Astronomy.
[ https://skyandtelescope.org/astronomy-news/osiris-rex-oldest-rocks/ ]


So no, the asteroid isn't all that old, but the material that it consists of has a history starting at the formation of the solar system that scientists are digging into. I wouldn't take the formation pathways too seriously as they will likely change as more data is gathered (as we know is the intention). But the datings are precise, and the broad water veins suggest a large parent body.
 
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Bennu was never a part of the main belt asteroids. ...

Bennu is a very young asteroid if the hydrated materials on its surface are still experiencing steam (or any kind of volatile) explosions. Mars has an abundance of hydrated rocks and has a relatively shallow gravity well. This would suggest that Mars or maybe Phobos was the origin for these hydrated materials.

The orbit shown below does suggest that Mars was the origin for the rubble that would become Bennu.
The found origin material suggest that the prior likely origin of the main asteroid belt is an even better posterior, and a Mars origin would not explain the Vesta material pick up. In any case the existence of a competitor to an idea means quantification - such as corresponding materials and origin ages [see my response to Rod for the putative parent body collision date] - must be used to settle the case.

Earth crossing asteroids have complex histories including many disturbances, making the resulting orbits rather uninformative of the origins.

Picking a crater among the many in the system - lots of bodies would have had flowing water after system formation - isn't more than eager pattern search. The Stickney crater could be < 0.5 Gyrs old, so would be much too young to explain a 0.8 - 1.5 Gyrs old asteroid.

In view of how crater-counting is unworkable for age-dating Stickney Crater we conclude an alternate age for Stickney Crater of 0.1–0.5 Ga that is constrained instead by the boulder evidence of Thomas et al. (2000), the boulder destruction rate analysis of Basilevsky et al. (2013, 2015), and the observed space weathering of Phobos regolith (Cipriani et al., 2011; Pieters et al., 2014).
[ https://www.sciencedirect.com/science/article/abs/pii/S0032063315300489 ]

Of course, such a dating is a bit precarious, but not as precarious as suggesting that Phobos is the main suspect for the Benny parent body.
 

rod

Oct 22, 2019
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In post #4 we read "But the datings are precise..." That is one interpretation. I did not see radiometric ages published for Bennu, CRE ages, yet the current orbit and position is not long lived. I do see reports like this though, "The current properties of small bodies provide important clues to their origin and history. However, how much small bodies were processed by past collisions and to what extent they retain a record of processes that took place during the formation and early evolution of the Solar System is still poorly understood...", Collisional heating and compaction of small bodies: Constraints for their origin and evolution, https://ui.adsabs.harvard.edu/abs/2020Icar..35013867J/abstract, November 2020
 
In post #4 we read "But the datings are precise..." That is one interpretation. I did not see radiometric ages published for Bennu,
From the article: "In the early days of the solar system, when it was a scant few million years old, heat from radioactive elements melted water ice, allowing it to interact with and change the rock of the parent body. The rapid decay of those elements means that the alteration had to happen early before the collision that destroyed Bennu's parent body. "

Your first comment argued that everything in the article represent "a very large extrapolation" and that we can't know about the "explanation" that far back, but as far as the parent body goes we know its age and its likely place (main asteroid belt) - Bennu itself is younger. The new hypothesis on its parent body would give a fairly conclusive age of Bennu itself since they have two asteroid families from the same body and with consistent dates. But that part would need more confirmation at a guess.
 

rod

Oct 22, 2019
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FYI, post #7 has issues. Here is an example, https://ui.adsabs.harvard.edu/abs/2004GeCoA..68.3779D/abstract, 39Ar- 40Ar evidence for early impact events on the LL parent body, Sep-2004. "These meteorites reveal a range of K-Ar ages from ≥3.66 to ≥4.50 Ga, and peak ages from ≥3.74 to ≥4.55 Ga. Significantly, three of the eight chondrites (LL4, 5, 6) have K-Ar ages of ∼4.27 Ga." There are other published reports on meteorite ages younger than the work of Clair Patterson in 1955/1956 that is used as the fixed age reference today for the solar system.

Bennu has no published radiometric age using various isotopes that I have found, no CRE ages published that I can find, and certainly is in its present location and orbit, not a long lived object in the ecliptic relative to Clair Patterson, fixed age for the solar system. In my opinion, the reporting here does feature a very large extrapolation to reconcile different ages found in the solar system.
 
Mar 5, 2020
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Any mechanism for Bennu spitting boulders (up to a meter in size) either by volatile release or even rock stress is a short term mechanism. The volatiles or rock stresses are a finite source of energy which are being rapidly consumed. If Bennu were billions of years old, it would now be entirely inactive.

If Mars (or Phobos) had been bombarded by interstellar asteroids like Earth about 13,000 years ago (the Younger Dryas), it would be consistent with the age of Bennu. A recent chance encounter with a cloud of interstellar asteroids is as good as any fiction that NASA or the Houston oil men might come up with.

I don’t see any evidence for two belt asteroids colliding. What I see is a pile of rubble that might indicate an asteroid impact into Martian sedimentary strata. The impact was deep enough to contribute distinct materials from different strata.

Geochronological techniques have the scientific potential to provide dates with high accuracy. But the technology of geochronology was developed for, and by, the oil companies. A bunch of young asteroids might lead people to conclude that they were created by interstellar asteroid impacts. The higher velocity of interstellar asteroids providing the energy for blasting material out of Mars’ gravity well, while producing a modest or magma filled impact crater.

Interstellar Asteroids and their relationship to oil. An explanation for the absence of Interstellar Asteroids in the geological record due to massive scientific corruption in America.
 

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