Asteroid Ryugu Likely Link in Planetary Formation

Formation scenario for Ryugu. More than one year ago, the Japanese Hayabusa2 orbiter deployed the German lander, MASCOT, which investigated the approximately one-kilometre-diameter asteroid Ryugu. Scientists are now imagining the history of its formation 4.5 billion years ago. First, flakes and grains of dust formed in the disc of dust and gas rotating around the Sun (1), before porous planetesimals agglomerated due to the accretion of these loose flakes (2). Recent investigations suggest that Ryugu’s parent body hardly condensed and was also highly porous. This may have resulted in the formation of a firmer core, but scientists also believe that a gradual increase in density towards the centre of the parent body is conceivable (3). Impacts and collisions with other asteroids (4) led to a fragmentation of the parent body; the large boulders on Ryugu probably originated here. Part of the debris was then the source material for the accretion of Ryugu (5), with porous blocks and loose material, and also some more compact blocks of higher density from the original core, some of which remain on the surface. Ryugu‘s present diamondlike shape (6) occurred over time due to its rotation. (Credit: Okada et al. Nature 2020)
  • Infrared images show that Ryugu is almost entirely made up.
  • The asteroid was formed largely from fragments of a parent body that was shattered by impacts of highly porous material.
  • DLR scientists participate in the publication in the scientific journal Nature.

COLOGNE, Germany (DLR PR) — The Solar System formed approximately 4.5 billion years ago. Numerous fragments that bear witness to this early era orbit the Sun as asteroids. Around three-quarters of these are carbon-rich C-type asteroids, such as 162173 Ryugu, which was the target of the Japanese Hayabusa2 mission in 2018 and 2019. The spacecraft is currently on its return flight to Earth.

Numerous scientists, including planetary researchers from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), intensively studied this cosmic ‘rubble pile’, which is almost one kilometre in diameter and can come close to Earth. Infrared images acquired by Hayabusa2 have now been published in the scientific journal Nature. They show that the asteroid consists almost entirely of highly porous material.

Temperature measurements on Ryugu’s surface. Close-up of a rock examined by DLR’s MARA radiometer directly on the surface of Ryugu. The yellow arrow shows the direction of illumination, the dotted line separates the observed rock from the background. The red area shows the part of the rock where the surface temperature was measured by the MARA radiometer, the dotted line shows a ledge in the rock. The scale in the centre of the image shows the dimensions at this distance from the camera. The image was acquired by the DLR MASCAM camera on board MASCOT. (Credit: MASCOT/DLR/JAXA)

Ryugu was formed largely from fragments of a parent body that was shattered by impacts. The high porosity and the associated low mechanical strength of the rock fragments that make up Ryugu ensure that such bodies break apart into numerous fragments upon entering Earth’s atmosphere. For this reason, carbon-rich meteorites are very rarely found on Earth and the atmosphere tends to offer greater protection against them.

Thermal behaviour reveals density

The Allende Meteorite is a carbonaceous chondrite, a carbon-rich class of rocky meteorites. It was named after the Pueblito de Allende in Mexico, in whose surroundings numerous pieces of an asteroid weighing several tonnes were found. It broke up in the atmosphere on 8 February 1969. Typical are the spherical ‘chondrules’. The silicate spheres are considered to be the 4.5 billion-year-old building blocks of the planets. The Hayabusa2 scientists also assume that the material on Ryugu corresponds chemically approximately to that of the chondritic meteorites. Hayabusa2 has taken samples of Ryugu that are on their way to Earth. Their analysis in the laboratory is eagerly awaited. (Credit: Shiny Things)

This investigation of the global properties of Ryugu confirms and complements the findings of the landing environment on Ryugu obtained by the German-French ‘Mobile Asteroid Surface SCOuT’ (MASCOT) lander during the Hayabusa2 mission.

“Fragile, highly porous asteroids like Ryugu are probably the link in the evolution of cosmic dust into massive celestial bodies,” says Matthias Grott from the DLR Institute of Planetary Research, who is one of the authors of the current Nature publication. “This closes a gap in our understanding of planetary formation, as we have hardly ever been able to detect such material in meteorites found on Earth.”

On 18 January 2000, after the explosion of a large fireball over Canada, hundreds of small meteorites fell onto Earth and numerous fragments were found on the ice of the frozen lake that gave it its name. The Taglish Lake meteorites are very rare stone meteorites from the class of what are referred to CI-Chondrites. The “C” stands for the chemical element carbon, and the “I” for the similarity with the Ivuna meteorite from Tanzania.. (Credit: Michael Holly, Creative Services, University of Alberta)

In autumn 2018, the scientists working with first author Tatsuaki Okada of the Japanese space agency JAXA analysed the asteroid’s surface temperature in several series of measurements performed with the Thermal Infrared Imager (TIR) on board Hayabusa2. These measurements were made in the 8 to 12 micrometre wavelength range during day and night cycles. In the process, they discovered that, with very few exceptions, the surface heats up very quickly when exposed to sunlight.

“The rapid warming after sunrise, from approximately minus 43 degrees Celsius to plus 27 degrees Celsius suggests that the constituent pieces of the asteroid have both low density and high porosity,” explains Grott.

MASCOT radiometer MARA. The MARA radiometer is used to measure the surface temperature on Ryugu in high resolution as well as the temperature differences during a full day/night cycle on the asteroid. [Credit: DLR (CC-BY 3.0)]

About one percent of the boulders on the surface were colder and more similar to the meteorites found on Earth.

“These could be more massive fragments from the interior of an original parent body, or they may have come from other sources and fallen onto Ryugu,” adds Jörn Helbert from the DLR Institute of Planetary Research, who is also an author of the current Nature publication.

From planetesimals to planets

Asteroid lander MASCOT on board the Hayabusa2 space probe. The Japanese Hayabusa2 space probe has completed a 3200-million-kilometre long journey carrying the German-French lander MASCOT (Mobile Asteroid Surface Scout). [Credit: DLR (CC-BY 3.0)]

The fragile porous structure of C-type asteroids might be similar to that of planetesimals, which formed in the primordial solar nebula and accreted during numerous collisions to form planets. Most of the collapsing mass of the pre-solar cloud of gas and dust accumulated in the young Sun. When a critical mass was reached, the heat-generating process of nuclear fusion began in its core.

The remaining dust, ice and gas accumulated in a rotating accretion disk around the newly formed star. Through the effects of gravity, the first planetary embryos or planetesimals were formed in this disc approximately 4.5 billion years ago.

The planets and their moons formed from these planetesimals after a comparatively short period of perhaps only 10 million years. Many minor bodies – asteroids and comets – remained. These were unable to agglomerate to form additional planets due to gravitational disturbances, particularly those caused by Jupiter – by far the largest and most massive planet.

However, the processes that took place during the early history of the Solar System are not yet fully understood. Many theories are based on models and have not yet been confirmed by observations, partly because traces from these early times are rare.

Between June 2018 and October 2019, the approximately one-kilometre-diameter asteroid 162173 Ryugu was the target of the Japanese Hayabusa2 spacecraft, which was launched in 2014. The ‘Peregrine Falcon’ examined Ryugu from different distances with several cameras, spectrometers and a laser altimeter. Small landing modules also directly explored the surface. The German Aerospace Center (DLR) provided the MASCOT landing module, which was equipped with four experiments from Germany and France. Ryugu is a Near Earth Object (NEO); the asteroid’s orbit comes close to that of Earth, but there is no danger of collision. The observations of Ryugu also serve to characterise such carbon-rich C-type asteroids, the most common group of minor bodies. Hayabusa2 is currently on its way back to Earth, carrying samples from two sites on Ryugu in a sealed capsule, which are due to be recovered in Australia in late 2020. (Credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST, Kobe University, Auburn University)

“Research on the subject is therefore primarily dependent on extraterrestrial matter, which reaches Earth from the depths of the Solar System in the form of meteorites,” explains Helbert.

It contains components from the time when the Sun and planets were formed.

“In addition, we need missions such as Hayabusa2 to visit the minor bodies that formed during the early stages of the Solar System in order to confirm, supplement or – with appropriate observations – refute the models.”

A rock like many on Ryugu

Already in the summer of 2019, results from the MASCOT lander mission showed that its landing site on Ryugu was mainly populated by large, highly porous and fragile boulders. “The published results are a confirmation of the results from the studies by the DLR radiometer MARA on MASCOT,” said Matthias Grott, the Principal Investigator for MARA. “It has now been shown that the rock analysed by MARA is typical for the entire surface of the asteroid. This also confirms that fragments of the common C-type asteroids like Ryugu probably break up easily due to low internal strength when entering Earth’s atmosphere.”

On 3 October 2018, MASCOT landed on Ryugu in free fall at walking pace. Upon touchdown, it ‘bounced’ several metres further before the approximately 10-kilogram experiment package came to a halt. MASCOT moved on the surface with the help of a rotating swing arm. This made it possible to turn MASCOT on its ‘right’ side and even perform jumps on the asteroid’s surface due to Ryugu’s low gravitational attraction. In total, MASCOT performed experiments on Ryugu for approximately 17 hours.

Samples From Asteroid Ryugu on Their Way to Earth

In December 2020, the Japanese Hayabusa2 spacecraft will return to Earth and, before entering the Earth’s atmosphere (during which it will burn up), will release a sealed capsule containing samples from two different locations on asteroid Ryugu, which, slowed down by the Earth’s atmosphere and a parachute, will land in Australia. (Credit: DLR)

Hayabusa2 mapped the asteroid from orbit at high resolution, and later acquired samples of the primordial body from two landing sites. These are currently sealed in a transport capsule and are traveling to Earth with the spacecraft. The capsule is scheduled to land in Australia at the end of 2020.

So far, the researchers assume that Ryugu’s material is chemically similar to that of chondritic meteorites, which are also found on Earth. Chondrules are small, millimetre-sized spheres of rock, which formed in the primordial solar nebula 4.5 billion years ago and are considered to be the building blocks of planetary formation.

So far, however, scientists cannot rule out the possibility that they are made of carbon-rich material, such as that found on comet 67P/ Churyumov-Gerasimenko as part of ESA’s Rosetta mission with the DLR-operated Philae lander. Analyses of the samples from Ryugu, some of which will be carried out at DLR, are eagerly awaited.

“It is precisely for this task – and of course for future missions such as the Japanese ‘Martian Moons eXploration’ (MMX) mission, in which extraterrestrial samples will be brought to Earth – that we at DLR’s Institute of Planetary Research in Berlin began setting up the Sample Analysis Laboratory (SAL) last year,” says Helbert.

The MMX mission, in which DLR is participating, will fly to the Martian moons Phobos and Deimos in 2024 and bring samples from the asteroid-sized moons to Earth in 2029. A mobile German-French rover will also be part of the MMX mission.

About the Hayabusa2 mission and MASCOT

Hayabusa2 is a Japanese space agency (Japan Aerospace Exploration Agency; JAXA) mission to the near-Earth asteroid Ryugu. The German-French lander MASCOT on board Hayabusa2 was developed by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and built in close cooperation with the French space agency CNES (Centre National d’Études Spatiales).

DLR, the Institute d’Astrophysique Spatiale and the Technical University of Braunschweig have contributed the scientific experiments on board MASCOT. The MASCOT lander and its experiments were operated and controlled by DLR with support from CNES and in constant interaction with the Hayabusa2 team at JAXA.

The DLR Institute of Space Systems in Bremen was responsible for developing and testing the lander together with CNES. The DLR Institute of Composite Structures and Adaptive Systems in Braunschweig was responsible for the stable structure of the lander. The DLR Robotics and Mechatronics Center in Oberpfaffenhofen developed the swing arm that allowed MASCOT to ‘hop’ on the asteroid. The DLR Institute of Planetary Research in Berlin contributed the MASCAM camera and the MARA radiometer. The asteroid lander was monitored and operated from the MASCOT Control Center in the Microgravity User Support Center (MUSC) at the DLR site in Cologne.