Near-Earth asteroid Ryugu was born in the outer solar system 4 billion years ago

In note here the reference paper cited in the article.

Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples, https://www.science.org/doi/10.1126/science.abn8671, 22-Sep-2022. "Abstract Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed seventeen Ryugu samples measuring 1-8 mm. CO2-bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu’s parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and Ca, Al-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed by aqueous alteration reactions at low temperature, high pH, and water/rock ratios < 1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate Ryugu’s parent body formed ~ 2 million years after the beginning of Solar System formation."

The PDF report is 109 pages, much to digest :) An earlier report on Ryugu indicated its overall surface age was young, perhaps 17 Myr. Ref - Touching the asteroid Ryugu revealed secrets of its surface and changing orbit, Touching the asteroid Ryugu revealed secrets of its surface and changing orbit (phys.org)

"...What I found most exciting was that, from the analysis of the size and colors of craters on Ryugu, the Hayabusa2 team concluded that at some point the asteroid must have been closer to the Sun that it is now. That would explain the amount of reddening of the surface. Using two different models for calculating the age of craters, the team estimated that this solar heating induced reddening must have happened either eight million years ago or as recently as 300,000 years ago—a mere blink of an eye, cosmologically speaking. These crater statistics, based on images collected by Hayabusa2, even show that the age of the overall asteroid surface itself is likely no more than around 17 million years, much younger than the time when the main-belt parent asteroids of Ryugu are thought to have broken apart, which happened hundreds of millions to over a billion years ago…"

today's Space.com report provided an age 4 Gyr and possible breakup time 1 Gyr. There is some juggling going on to work the various ages and observations found. The 109-page report provides some good details on the postulated parent body and impactor body need to fit the pieces together. Here are some geeky details I found.

See 109-page PDF report. My note, from the paper, “Thermal evolution modeling To explore the formation time of the Ryugu’s parent body, we modeled the thermal evolution of a 100 km-sized body. We assumed that Ryugu’s parent was a spherical body of a mixture of water ice and rock. Since the spectral properties of Ryugu are similar to the Euralia (or Polana) family (3, 4), we assumed that the rocky component of the Ryugu’s parent body was 50 km in radius (85). Since the W/R ratio is ranging from 0.2 to 0.9 based on the thermal equilibrium calculation (see Fig. 7A), we took four values for the W/R ratio, 0.3, 0.6, and 0.9 in our simulations for the thermal evolution. Thus, the estimated radius of the initial ice-rock mixture body ranges from 60 km for W/R = 0.3 to 70 km for W/R = 0.9, which contains the rocky component corresponding to 50 km in radius. The finding of CO2 in fluid inclusions (see the main text) may suggest that water-ice and CO2 ice coexisted in Ryugu’s parent body. Thus, we adopted the initial temperature as –203 ºC (70 K), which corresponds to the temperature that CO2 can condense (150). We considered two exothermic and one endothermic reaction. The primary heat source is the decay heat of short-lived radionuclides, 26Al (half-life of 0.72 Myr). Taking the canonical value of 26Al/27Al from CAIs (5.25 × 10^–5, (151)), the formation time of Ryugu’s parent body after the CAIs represents the initial amount of the heat source. The earlier formed body has more 26Al than the later one, which results in reaching high temperatures.”...“The formation time of the Ryugu’s parent body depends on the initial water ice to rock mass ratio (W/R, see Figure S30). A water-rich body requires additional heat to melt the water-ice and also takes a longer time to melt all the ice. Since the water-rich body has less heat source than the same sized water-poor body, it should be formed earlier to reach a certain temperature at a certain time. Thus, the water-rich Ryugu’s parent body should have formed at 1.8 Myr (W/R = 0.9 in Figure S30) to reach 40 ºC at ~5 Myr. Its formation occurred earlier than other water-poor cases (W/R = 0.6 and 0.3 in Figure S30).” The paper presents good detail concerning impactor modeling used to explain Ryugu today. “Impact modeling…A uniform temperature for the entire bodies of the impactor and the target was assumed as the initial condition. The initial temperature was set to 220 K, which corresponds to the equilibrium temperature at the orbit that roughly corresponds to the current Eulalia (2.49 au) and Polana (2.42 au) ones…”

My note, the original parent body for Ryugu and the impactor body used in the modeling, neither are observed today in the solar system apparently. There are 29 references to *parent body* in the paper and 15 to *impactor*. The paper presents good detail concerning impactor modeling used to explain Ryugu today.

“Impact modeling…A uniform temperature for the entire bodies of the impactor and the target was assumed as the initial condition. The initial temperature was set to 220 K, which corresponds to the equilibrium temperature at the orbit that roughly corresponds to the current Eulalia (2.49 au) and Polana (2.42 au) ones…”

More detail is provided on how long the parent body took to form for Ryugu using the modeling, some 1.4 Myr to 2.8 Myr. "Fig. S30. Time to reach 40 ºC as a function of the formation age of Ryugu’s parent body. We measured these times at the center of the body. Each line represents the result from a different W/R and corresponding radius (r); black solid line for W/R = 0.9 with 70 km, green dashed-dotted line for W/R = 0.6 with 65 km, and red dotted line for W/R = 0.3 with 60 km, respectively. The shaded region depicts the formation time range of carbonate minerals, 4.5–6.0 Myr, which formed at the temperature of ~ 40 ºC (29). As the late formed body has less 26Al, it takes more time to have a temperature of 40 ºC. Later formation times do not reach that temperature, because the ice melting consumes the heat."

So, it looks like a postulated parent body somewhere out near 2.5 AU or perhaps farther out with an impactor leaving behind Ryugu perhaps 1 Gyr that migrates to its present location with a young surface age now. I am still attempting to digest all of this :)
 
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