162173 Ryugu
162173 Ryugu is a near-Earth object and also a potentially hazardous asteroid of the Apollo group. It measures approximately in diameter and is a dark object of the rare spectral type Cb, with qualities of both a C-type asteroid and a B-type asteroid. In June 2018, the Japanese spacecraft Hayabusa2 arrived at the asteroid. After making measurements and taking samples, Hayabusa2 left Ryugu for Earth in November 2019 and returned the sample capsule to Earth on 5 December 2020. The samples showed the presence of organic compounds, such as uracil and vitamin B3.
Discovery and name
Ryugu was discovered on 10 May 1999 by astronomers with the Lincoln Near-Earth Asteroid Research at the Lincoln Lab's ETS near Socorro, New Mexico, in the United States. It was given the provisional designation. The asteroid was officially named "Ryugu" by the Minor Planet Center on 28 September 2015. The name refers to Ryūgū-jō, a magical underwater palace in a Japanese folktale. In the story, the fisherman Urashima Tarō travels to the palace on the back of a turtle, and when he returns, he carries with him a mysterious box, much like Hayabusa2 returning with samples.Geological history
Ryugu formed as part of an asteroid family, belonging either to Eulalia or Polana. Those asteroid families are most likely fragments of past asteroid collisions. The large number of boulders on the surface supports a catastrophic disruption of the parent body. The parent body of Ryugu probably experienced dehydration due to internal heating and must have formed in an environment without a strong magnetic field. After this catastrophic disruption, part of the surface was reshaped again by the high speed rotation of the asteroid forming the equatorial ridge, through internal failure and/or mass wasting. The geologically distinct western area is probably the result of asymmetrical internal failure. It is hoped that surface samples will help to reveal more of the geological history of the asteroid.Ryugu is hypothesized to be an extinct comet.
Characteristics
Orbit
Ryugu orbits the Sun at a distance of 0.96–1.41 AU once every 16 months. Its orbit has an eccentricity of 0.19 and an inclination of 6° with respect to the ecliptic. It has a minimum orbital intersection distance with Earth of, equivalent to 0.23 lunar distances.Physical
Early analysis in 2012 by Thomas G. Müller et al. used data from a number of observatories, and suggested that the asteroid was "almost spherical", a fact that hinders precise conclusions, with retrograde rotation, an effective diameter of 0.85–0.88 km and a geometric albedo of 0.044 to 0.050. They estimated that the grain sizes of its surface materials are between 1 and 10 mm.Initial images taken by the Hayabusa2 spacecraft on approach at a distance of were released on 14 June 2018. They revealed a diamond-shaped body in diameter and confirmed its retrograde motion. Between 17 and 18 June 2018, Hayabusa2 went from from Ryugu and captured a series of additional images from the closer approach. Astronomer Brian May created stereoscopic images from data collected a few days later. After a few months of exploration, JAXA scientists concluded that Ryugu is actually a rubble pile with about 50% of its volume being empty space.
The acceleration due to gravity at the equator has been evaluated at about 0.11 mm/s2, rising to 0.15 mm/s2 at the poles. The mass of Ryugu is estimated at 450 million tonnes. The asteroid has a volume of 0.377 ± 0.005 km3 and a bulk density of 1.19 ± 0.03 g/cm3 based on the shape model.
Shape
Ryugu has a round shape with an equatorial ridge, called Ryujin Dorsum. Ryugu is a spinning top-shape asteroid similar to Bennu. The ridge was shaped by strong centrifugal forces during a phase of high-speed rotation, through landslides and/or internal failure. The western side, also called the western bulge, has a distinct shape. It has a smooth surface with a sharp equatorial ridge. When modeling a high-speed rotation of present-day Ryugu, subsurface material appears structurally intact and relaxed in the western bulge, while other regions are more sensitive to structural failure. This indicates past structural failure in the western bulge. The western bulge is bordered by the Tokoyo and Horai Fossae.Surface
Observations from Hayabusa2 showed that the surface of Ryugu is very young and has an age of years based on the data collected from the artificial crater that was created with an explosive by Hayabusa2.The surface of Ryugu is porous and contains no or very little dust. The measurements with the radiometer on board of MASCOT, which is called MARA, showed a low thermal conductivity of the boulders. This was an in situ measurement of the high porosity of the boulder material. This result showed that most meteorites originating from C-type asteroids are too fragile to survive the entry into Earth's atmosphere. The images from the camera of MASCOT, which is called MASCam, showed that surface of Ryugu contains two different almost black types of rock with little internal cohesion, but no dust was detected. One type of rocky material on the surface is brighter with a smooth surface and sharp edges. The other type of rock is dark with a cauliflower-like, crumbly surface. The dark type of rock has a dark matrix with small, bright, spectrally different inclusions. The inclusions appear similar to CI chondrites. An unanticipated side effect from the Hayabusa2 thrusters revealed a coating of dark, fine-grained red material.
Craters
Ryugu has 77 craters on the surface. Ryugu shows variations of crater density that cannot be explained by randomness of cratering. There are more craters at lower latitudes and fewer at higher latitudes, and fewer craters in the western bulge than in the region around the meridian. This variation is seen as evidence of a complicated geologic history of Ryugu. The surface has one artificial crater, which was intentionally formed by the Small Carry-on Impactor, which was deployed by Hayabusa2. SCI fired a 2 kg copper mass onto the surface of Ryugu on 5 April 2019. The artificial crater showed a darker subsurface material. It created an Ejecta of 1 cm thickness and excavated material from up to 1 metre in depth.Boulders
Ryugu contains 4,400 boulders with a size larger than 5 metres. Ryugu has more large boulders per surface area than Itokawa or Bennu, about one boulder larger than 20 metres per 50 km2. The boulders resemble laboratory impact fragments. The high number of boulders is explained with a catastrophic disruption of Ryugu's larger parent body. The largest boulder, called Otohime, has a size of ~160 × 120 × 70 m and is too large to be explained as an ejected boulder from a crater.Sample analysis results
After the initial description, part of the sample was distributed to the Hayabusa2 Initial Analysis Team, consisting of six sub-teams, and two Phase-2 curation institutes at Okayama University and JAMSTEC Kochi Institute for Core Sample Research.In September 2022 the Hayabusa 2 initial Analysis Stone Team announced the results of their study, which includes:
- Ryugu samples contain grains that were formed at high temperatures above 1000 °C, which formed close to the Sun and were later transported towards the outer Solar System.
- The samples are soft enough to be cut with a knife and the samples preserve the magnetic field like a hard disk.
- A simulation of the formation was performed, which showed that the parent body of Ryugu accumulated 2 million years after the formation of the Solar System. It heated up to 50 °C over the next 3 million years, resulting in reactions of rocky material with water. In these reactions anhydrous silicates became hydrous silicates and iron became magnetite. The 100 km large parent body was then destroyed by a <10 km large impactor, with an impact speed of about 5 km/s. Ryugu then formed from material far from the impact.
Origin from the outer Solar System
Based on preserved magnetism in the samples researchers concluded that the parent body of Ryugu was probably formed in the darkness of nebular gas.
Volatiles
Water
The Hayabusa2 sample capsule was significantly upgraded from Hayabusa, to preserve water, light organics, gases, and other volatiles. This water was successfully sampled and preserved. Via a bulk sample, its water content was reported as 6.84 ±0.34 wt%.Independently, a research group with a far smaller allocation reported 4-7 percent water.
The lower-than-expected water signature seen by Hayabusa2 instruments was the result of space weathering, producing a dehydrated rind.
Liquid water and aqueous alteration
was discovered in one crystal. The water contained salts and organic matter. The liquid water was found inside a hexagonal iron sulfide crystal. The carbon dioxide was most likely CO2-ice inside the parent body. The water ice melted soon after the parent body formed and the CO2 dissolved into the water.Crystals "shaped like coral reefs" were found. These crystals probably formed in liquid water, which was once present in the interior of the parent body. The parent body had a dryer surface and a wetter interior. After the collision of the parent body with a smaller asteroid, the interior and surface material were mixed. Today Ryugu has both interior and parent body surface material on its surface.
An international team found particles in the samples that contained small amounts of material unaltered by water. The team found about 0.5 vol% of anhydrous silicates. The isotopic analysis of the magnesium-rich olivine and pyroxene in the sample suggests that two types of high-temperature objects accreted onto the surface of Ryugu: amoeboid olivine aggregates and magnesium-rich chondrules.