"Our results allow the detailed reconstruction of the cooling history of an asteroid in the early solar system. Small planets were effectively heated by an internal heating source, most probably by the decay energy of the short lived isotope 26Al. Hence, planetesimal formation must have been fast, within a few millions of years." This is Dr. Trieloff's summary of a study published in this week's issue of NATURE (3 April), entitled : "Structure and Thermal History of the H-chondrite Parent Asteroid Revealed by Thermochronometry." The authors are Mario Trieloff (Mineralogical Institute of the University of Heidelberg), Elmar Jessberger (Institute of Planetology, University of Münster), Ingrid Herrwerth (Max Planck Institute of Nuclear Physics, Heidelberg), Jens Hopp (Mineralogical Institute, University of Heidelberg) and Christine Fiéni, Marianne Ghélis, Michèle Bourot-Denise and Paul Pellas (Natural History Museum, Paris, France).
Our solar system was formed about 4,600 million years ago by the collapse of an interstellar cloud of gas and dust. After formation of the early proto sun, it remained enclosed by an envelope of gas and dust for about 10 million years. This was the material from which first small bodies and later the larger planets formed. Between the planets Mars and Jupiter, the accretion process, i.e. the coagulation of larger bodies from smaller bodies or dust stopped when the bodies were some few hundred kilometres in diameter. These small planets, the asteroids, undergo collisions with each other and produce larger and smaller fragments that today reach the Earth as meteorites. Using geo-scientific laboratory techniques we can study these meteorites, and accordingly the early stage of planetary formation "frozen" in these rocks.
We know meteorites from small asteroidal bodies that heated up to high temperatures, melted and formed metallic iron-nickel cores and a mantle of silicate minerals, in a similar manner to what happened on the large terrestrial planets Mercury, Venus, Earth and Mars. On the other hand, we also have meteorites originating from asteroids that were heated but did not melt. They did not form a metallic core. These meteorites are called undifferentiated. They even contain primary millimetre-sized rocky melt droplets directly formed by the melting of dust grains in the early solar nebula. These round fragments are called "chondrules" and the undifferentiated meteorites in which they occur are called "chondrites".
We know of several chondrite classes that differ, for example, in the degree of iron oxidation. One class with a relatively high amount of metallic iron is that of the so-called H chondrites, where H stands for High metallic iron. These originate from an asteroid formed as a single body in the early solar system. H chondrites are very similar chemically, but we can classify different types (so called "petrologic types", expressed as numbers ranging from 3 to 6). These different types are due to the fact that, after accretion, the parent body heated up and reached different maximum temperatures at different depths. For example, type H4 chondrites reached only relatively low temperatures of about 650°C, while type H6 chondrites reached about 850°C.
We have studied H chondrites of different petrologic types with highly precise radiometric dating methods that use the decay of natural radioactive elements such as 40K (decaying to 40Ar) or 244Pu. This element was only active in the early solar system, due to its short half-life of 80 million years. The important point is that the "ages" determined in this way do not mean that the rocks came into existence at that specific time. These "ages" are "cooling ages", i.e. the time when the temperature of the rocks fell below a specific temperature. The rock may well have been formed earlier, but if it cooled down more slowly we thus measure a younger "age". For example, if the K-Ar age of the mineral feldspar is 4,400 million years, this means that at this time the temperature decreased below 280°C. This is the so called "closure temperature", the temperature at which the decay product of 40K, i.e. 40Ar, is effectively retained in the mineral feldspar.
Our results are a detailed reconstruction of the cooling curves of different types of H chondrites: Type 6 H chondrites (the most strongly heated, up to 850°C) needed about 160 million years to cool down to 120°C. Type 4 H chondrites (the most weakly heated, up to 650°C) cooled down in only a few million years. This cooling behaviour is in perfect agreement with what we expect if an asteroid is heated by an internal heat source originating from the rocks themselves. The central parts (H6 chondrites) are heated to higher maximum temperatures and need more time for cooling than the outer layers (H4 chondrites) close to the surface (this is nothing other than a basic principle of the physics of heat: heat is lost best at the cool surface, hence rocks at the surface remain coolest and cool down faster than the interior regions. These are better insulated and thus become hotter and cool down more slowly).
The question of which heat source provided the energy for warming up small asteroids (some of them high enough to melt and form iron cores) has long been a mystery. For the larger terrestrial planets, scientists considered heat sources like heating by late large colliding bodies or long-lived natural radioactive elements such as 40K, 238U, 235U or 232Th. When these elements decay, they produce not only new isotopes but also heat, the so called "decay energy", well known in nuclear physics. However, small asteroidal bodies need a stronger heat source to melt because small bodies lose heat more effectively and more quickly. Astrophysicists know that short-lived radioactive isotopes are present in star-forming environments. Meteorite specialists have found the decay products of the isotope 26Al in some meteorite minerals. This element has an only very short half-life of 0.72 million years. 26Al could have been an effective internal heat source, but only if asteroid sized bodies accreted fast enough, i.e., before most of it had decayed. This is precisely the point that was uncertain. There was also discussed of other heat sources being responsible, such as external heating, e.g., by illumination in an extremely luminous phase of the early proto sun or induction heating by early ion particle wind coming from the sun.
However, such a heat source would yield asteroids in which the surface layers were heated most (thereby producing H6 chondrites) and cooled down most rapidly, as heat from the surface is lost most effectively. The inner regions would have been heated weakly (thereby producing H4 chondrites) and should have cooled very slowly. However, our results did not confirm such cooling behaviour. Accordingly, this is an argument against an external heat source. Our results favour an internal heat source, such as decay energy from the decay of short-lived 26Al. This conclusion is corroborated by a calculation modelling the cooling at different depths within an asteroid that was heated by 26Al after fast accretion. The model cooling curves are good approximations of the actual measurements/reconstructions of cooling curves derived from the H chondrites of different petrologic types. This is the first time that 26Al heating has been demonstrated as the source heating one specific small body in the early solar system. These results also imply that accretion to 100 km-size bodies occurred within a few million years (otherwise 26Al would have disappeared). Fast accretion, on the other hand, is an important result that gives us information about the probability of terrestrial-type planets around other stars. Around most young stars, dust for the formation of terrestrial planets is only available for about 10 million years. If accretion was fast enough in our early solar system, then it can also be expected to happen around other stars. Thus formation of planets may occur quickly, making the existence of other terrestrial planets likely.
Please address any inquiries to
Dr. Mario Trieloff
University of Heidelberg
Dr. Michael Schwarz
Press Officer of the University of Heidelberg