16. March 2020
From cosmic dust to the planets

As­ter­oid Ryugu like­ly link in plan­e­tary for­ma­tion

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Space
Close-up of Asteroid Ryugu
Close-up of As­ter­oid Ryugu
Image 1/9, 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

Close-up of Asteroid Ryugu

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.
Sur­face tem­per­a­ture of as­ter­oid Ryugu dur­ing the day
Video 2/9, Credit: JAXA, Hayabusa2 TIR team

Surface temperature of asteroid Ryugu during the day

Credit: JAXA, Hayabusa2 TIR team
Length: 00:00:10
The Japanese Hayabusa2 or­biter ob­served Ryugu in the ther­mal in­frared – from 8-12 mi­crome­tres – with its Ther­mal In­frared Im­ager (TIR). This an­i­ma­tion shows the tem­per­a­ture dis­tri­bu­tion on the day side over a pe­ri­od of ap­prox­i­mate­ly 1.5 as­ter­oid days. One day/night cy­cle on Ryugu lasts 7.63 hours. The false colours show the sur­face tem­per­a­ture in Kelvin, in ac­cor­dance with the scale on the right-hand side. Af­ter ‘sun­rise’ on the as­ter­oid, the as­ter­oid warms up rel­a­tive­ly quick­ly from 230 Kelvin (mi­nus 43 de­grees Cel­sius, dark blue colour fringes on the left) to 300 Kelvin (27 de­grees Cel­sius, or­ange) and cools down again quick­ly to 230 Kelvin af­ter ‘sun­set’ (blue fringes on the right). The fast warm­ing in­di­cates a low den­si­ty and al­so high poros­i­ty of the ma­te­ri­al with many voids in the as­ter­oid. The ex­treme­ly even tem­per­a­ture dis­tri­bu­tion on the day side in­di­cates a very ho­mo­ge­neous sur­face ma­te­ri­al.
Formation scenario for Ryugu
For­ma­tion sce­nario for Ryugu
Image 3/9, Credit: Okada et al. Nature 2020

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.
Temperature measurements on Ryugu's surface
Tem­per­a­ture mea­sure­ments on Ryugu's sur­face
Image 4/9, Credit: MASCOT/DLR/JAXA

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.
The 'Allende' Meteorite, a carbonaceous chondrite
The 'Al­lende' Me­te­orite, a car­bona­ceous chon­drite
Image 5/9, Credit: Shiny Things

The 'Allende' Meteorite, a carbonaceous chondrite

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.
December 2020 - Return to earth
De­cem­ber 2020 - Re­turn to earth
Image 6/9, Credit: DLR

December 2020 - Return 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.
MASCOT radiometer MARA
MAS­COT ra­diome­ter MARA
Image 7/9, Credit: DLR (CC-BY 3.0)

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.
Asteroid lander MASCOT on board the Hayabusa2 space probe
As­ter­oid lan­der MAS­COT on board the Hayabusa2 space probe
Image 8/9, Credit: DLR (CC-BY 3.0)

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).
Meteorite from Tagish Lake
Me­te­orite from Tag­ish Lake
Image 9/9, Credit: Michael Holly, Creative Services, University of Alberta

Meteorite from Tagish Lake

When eval­u­at­ing the MAS­Cam im­ages, the sci­en­tists iden­ti­fied two dif­fer­ent types of rock –one with sharp edges and smooth frac­tured sur­faces, and a sec­ond with sur­faces rem­i­nis­cent of a cauliflow­er. The im­ages of these some­what crum­pled sur­faces, ir­ra­di­at­ed with light-emit­ting diodes ac­quired dur­ing the night, the re­searchers dis­cov­ered bright min­er­al in­clu­sions in the al­most black rock ma­trix, rem­i­nis­cent of min­er­al in­clu­sions in me­te­orites from Lake Tag­ish (pic­ture). 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 ‘Tag­ish Lake Me­te­orites’ are very rare stone me­te­orites from the class of what are re­ferred to as CI-Chon­drites. The C stands for the chem­i­cal el­e­ment car­bon, and the ‘I’ for the sim­i­lar­i­ty with the Ivu­na me­te­orite from Tan­za­nia. They are among the most prim­i­tive and old­est com­po­nents of the So­lar Sys­tem, rem­nants of the first sol­id bod­ies formed in the pri­mor­dial so­lar neb­u­la. They are be­lieved to have evolved in­to the bod­ies of the So­lar Sys­tem.
  • Infrared images show that Ryugu is almost entirely made up of highly porous material.
  • 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.
  • Focus: Space, exploration

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. 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

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."

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. 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

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. "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

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.

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