30. September 2021
New insights into the formation of Earth-like planets

Heavy bom­bard­ment ex­pe­ri­enced by the plan­ets in the ear­ly So­lar Sys­tem

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Space
Asteroid Vesta
As­ter­oid Ves­ta
Image 1/5, Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Asteroid Vesta

Dis­cov­ered in 1807 and 525 kilo­me­tres across, the ce­les­tial body Ves­ta is the sec­ond largest ob­ject in the as­ter­oid belt af­ter the dwarf plan­et Ceres and is thus the largest as­ter­oid in the So­lar Sys­tem. It com­pris­es ap­prox­i­mate­ly eight per­cent of the to­tal mass of the Main As­ter­oid Belt. Ves­ta's in­te­ri­or is 'dif­fer­en­ti­at­ed' – af­ter the as­ter­oid's for­ma­tion, the com­po­nents that were ini­tial­ly ho­mo­ge­neous­ly mixed then seg­re­gat­ed and sep­a­rat­ed. The heat re­leased by the de­cay of ra­dioac­tive el­e­ments led to the for­ma­tion of melts. The com­po­nents dom­i­nat­ed by heavy el­e­ments such as iron sank to the cen­tre of the body, while the melts rich in lighter com­po­nents such as sil­i­cate rocks rose to the sur­face. This cre­at­ed an iron core, a sil­i­cate man­tle and an iron- and mag­ne­sium-rich crust of basaltic rocks.
Model of the material circulation in Vesta’s interior
Mod­el of the ma­te­ri­al cir­cu­la­tion in Ves­ta’s in­te­ri­or
Image 2/5, Credit: W. Neumann (2019)

Model of the material circulation in Vesta’s interior

Dur­ing Ves­ta's ear­ly evo­lu­tion­ary phase, con­vec­tive up­heavals took place in­side the as­ter­oid. The di­men­sion­less tem­per­a­ture shown in the en­ve­lope in­creas­es with depth. In this ‘stag­nant-lid’ sit­u­a­tion, the rock is rigid and im­mo­bile due to its very low tem­per­a­ture. Con­vec­tive cir­cu­la­tion takes place un­der the ‘lid’ of Ves­ta’s so­lid­i­fied crust – warmer rock mass­es rise and cool­er rock mass­es sink. The di­men­sion­less con­vec­tion ve­loc­i­ty in­di­cates ‘plumes’, bub­bles of ris­ing, warmer and less dense ma­te­ri­al, and sink­ing, cold­er and denser ma­te­ri­al.
Meteorites from Vesta in polarised light
Me­te­orites from Ves­ta in po­larised light
Image 3/5, Credit: NASA/Universität Tennesse

Meteorites from Vesta in polarised light

This im­age shows three thin sec­tions of HED me­te­orites (from left: howardite, eu­crite and dio­gen­ite) in po­larised light. The Dawn mis­sion has con­firmed their ori­gin as the gi­ant as­ter­oid Ves­ta. Com­bin­ing nu­mer­i­cal mod­elling, ob­ser­va­tions of the as­ter­oid Ves­ta and a com­par­i­son with the chem­i­cal-min­er­alog­i­cal com­po­si­tion of Ves­ta me­te­orites led to the re­al­i­sa­tion that Ves­ta was ex­posed to an ex­ten­sive se­ries of im­pacts from large rocky bod­ies much ear­li­er than pre­vi­ous­ly as­sumed.
Howardite meteorite from the asteroid Vesta
Howardite me­te­orite from the as­ter­oid Ves­ta
Image 4/5, Credit: Hap McSween (Universität Tennessee) und Andrew Beck/Tim McCoy (Smithsonian Institution)

Howardite meteorite from the asteroid Vesta

The HED me­te­orites (howardite, eu­crite and dio­gen­ite) are a group of me­te­orites that very prob­a­bly all orig­i­nate from the as­ter­oid Ves­ta. In this im­age, the Bununu howardite found in 1942.
The Dawn mission at the asteroid Vesta
The Dawn mis­sion at the as­ter­oid Ves­ta
Image 5/5, Credit: NASA/JPL-Caltech

The Dawn mission at the asteroid Vesta

Dawn was a NASA Dis­cov­ery Pro­gram mis­sion launched in 2007 on a re­search mis­sion to the Main As­ter­oid Belt be­tween Mars and Jupiter. The mis­sion's first tar­get was the as­ter­oid Ves­ta, which was ob­served from or­bits of vary­ing al­ti­tudes be­tween Ju­ly 2011 and Septem­ber 2012. Ves­ta was al­most com­plete­ly mapped us­ing the four in­stru­ments on board Dawn. The min­er­alog­i­cal com­po­si­tion of the as­ter­oid was ex­plored, and its grav­i­ta­tion­al field was pre­cise­ly de­ter­mined. The Dawn space­craft then flew on to its sec­ond tar­get, the dwarf plan­et Ceres.
  • Asteroid Vesta was subjected to violent bombardment by other bodies many millions of years earlier than previously thought
  • This also affected Earth and the other planets in the inner Solar System 4.5 billion years ago.
  • The extra material mixed with the hot interior of the asteroid and is found in meteorites.
  • Focus: Space, Solar System exploration, asteroids, meteorites

At approximately 500 kilometres in size, Vesta is the largest known asteroid in the Solar System. Like its numerous companions in the Main Asteroid Belt, it is made of the 'primordial matter' of the Solar System. A new study published in Nature Astronomy concludes that Vesta was exposed to an extensive impact series of large rocky bodies much earlier than previously assumed. This suggests that the entire inner Solar System, and thus the rocky planets, was affected by such an early 'bombardment'. As such, this observation also provides important insights into the early development of our Earth. This is the conclusion reached by an international research team with the participation of geoscientists from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), the University of Heidelberg, the Free University of Berlin and the Berlin Natural History Museum.

As part of the study published today, Wladimir Neumann from the DLR Institute of Planetary Research and the Institute of Geosciences at the University of Heidelberg carried out numerous calculations to model Vesta's thermal evolution. This made it possible to better narrow down the time period in which the early impacts occurred. "In order for the material of the impacting bodies to be mixed into the rocky mantle of young Vesta in a reasonably homogeneous way, the mantle must have been hot enough and circulate convectively, driven by internal heat," Neumann explains. "Our models have shown that this is only true for impacts within the short time span of around 4.56 to 4.50 billion years ago – almost immediately after the formation of the planets in the inner Solar System."

Exciting landscapes at Vesta's south pole
Exciting landscapes at Vesta's south pole
This view of the asteroid Vesta, calculated using a digital terrain model, shows an oblique view of the impact-ravaged south pole region. The image has a resolution of approximately 300 metres per pixel and the vertical scale is exaggerated to 1.5 times the horizontal scale. The mountain in the centre of the image rises approximately 20 kilometres from the Rheasilvia Basin.
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Rare meteorites provide important evidence

Until now, it was assumed that the main phase of this bombardment occurred several hundred million years later, around the time that some of the large impact craters were formed on the Moon. However, for Earth's moon, and probably also for the terrestrial planets, this study indicates that the main mass of this bombardment reached the planets very soon after their formation, as occurred with Vesta.

This finding is also based on analyses of meteorites in terrestrial collections whose parent body is almost certainly Vesta – the so-called HED meteorites. The acronym HED is derived from the first letters of a subgroup of rare stony meteorites, the howardites, eucrites and diogenites. This group shows similarities to magmatic rocks on Earth. Due to their chemical composition, they must have come from an already differentiated planetary body in which heavy, metallic elements accumulated in a core that was surrounded by a lighter rock mantle and an even lighter crust, and in which magmatic processes caused changes.

Planetary bodies continued to grow as a result of bombardment

Numerical simulations and investigations using data on Vesta collected by NASA's Dawn spacecraft in 2011 and 2012 reveal a new picture of the chronology of collision history in the early Solar System. The Earth-like planets in the early Solar System initially grew via the clumping together of tiny, adhering dust grains. Then, in their final stage, by impacts of increasingly larger rocky bodies. This is also true of the asteroid Vesta. During the growth process, Vesta increasingly heated up during the early phase of its development, resulting in the formation of a near-surface ocean of molten silicate rock and a liquid metallic core below.

Over time, other bodies struck Vesta's crust, causing material to be hurled into space and transported into the inner Solar System. Thus, rock debris from Vesta occasionally reached Earth as meteorites. Chemical analyses of these meteorites have shown that even after Vesta's core formed, further cosmic impacts changed the composition of the asteroid's crust and mantle. "However, this supply of material was much greater during the early phase than afterwards" explains Harry Becker from the Free University of Berlin (FU Berlin), one of the authors of the study. Vesta was struck by at least two very large bodies from the Main Asteroid Belt, as evidenced by two impact basins several hundred kilometres in size at the south pole, which were discovered using a camera developed by DLR and the Max Planck Society on board the Dawn spacecraft.

Traces of large collisions on Vesta
Traces of large collisions on Vesta
The surface of the asteroid Vesta has two vast, overlapping impact craters at the south pole, roughly 400 and 500 kilometres in diameter, named Rheasilvia and Veneneia. A central mountain 20 kilometres tall rises at the geometric centre of Rheasilvia. It was formed when the surface rebounded after the impact. In the process, a large quantity of material was flung away from Vesta and numerous new, smaller asteroids were formed – known as the Vestoids. Some fragments reached Earth as meteorites.
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Earth also had a magma ocean and a blazing hot atmosphere

Furthermore, the impacting bodies apparently did not originate in today's asteroid belt, as previously assumed. They originated in the inner Solar System, just like the terrestrial planets. "For Earth, this once again underlines the significance of an early hot phase with a magma ocean that was continuously replenished by large impacts. During this time, Earth's first atmosphere was red-hot for many millions of years. Only much later were oceans of water able to form as the hot water vapour cooled and rained down," explains Kai Wünnemann from the Natural History Museum and FU Berlin.

The research work at Heidelberg University was funded by the Klaus Tschira Foundation. The contributions from Berlin and Münster are part of the Collaborative Research Centre Transregio TRR 170 'Late Accretion onto Terrestrial Planets' which is funded by the German Research Foundation (DFG). The international study also involved scientists from the Macau University of Science and Technology (Macau), the Université de Nice Sophia-Antipolis (France), the University of California, Davis and the University of California San Diego (both USA), the University of Bayreuth, the Planetary Science Institute in Tucson (USA), and the Institute of Planetary Research of the German Aerospace Center (DLR).

Original publication:

M.-H. Zhu, A. Morbidelli, W. Neumann, Q.-Z. Yin, J.M.D. Day, D.C. Rubie, G.J. Archer, N. Artemieva, H. Becker, K. Wünnemann: Common feedstocks of late accretion for the terrestrial planets. Nature Astronomy (30 September 2021), https://doi.org/10.1038/s41550-021-01475-0

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  • Falk Dambowsky
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    Ger­man Aerospace Cen­ter (DLR)
    Me­dia Re­la­tions
    Com­mu­ni­ca­tions and Me­dia Re­la­tions
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  • Dr. rer. nat. Wladimir Neumann
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Plan­e­tary physics
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    12489 Berlin
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  • Prof. Dr. Kai Wünnemann
    Mu­se­um für Naturkunde
    Leib­niz In­sti­tute for Evo­lu­tion and Bio­di­ver­si­ty Sci­ence
    Telephone: +49 30 2093-8857
    Invalidenstraße 43
    10115 Berlin
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  • Prof. Dr. Harry Becker
    Freie Uni­ver­sität Berlin
    In­sti­tute of Ge­o­log­i­cal Sci­ences
    Telephone: +49 30 838-70668
    Malteserstr. 74-100
    12249 Berlin
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