Proc. Neutrino Astrophysics - MPP Theory Group

6
What Killed The Dinosaurs?
Arnon Dar 1,2
1 Max-Planck-Institut für Physik (Werner-Heisenberg-Institut)
Föhringer Ring 6, 80805 München, Germany.
2 Department of Physics and Space Research Institute,
Technion, Israel Institute of Technology, Haifa 32000, Israel.
The early history of life during the Precambrian until its end 570 million years (My) ago
is poorly known. Since then the diversity of both marine and continental life has increased
exponentially. Analysis of fossil records shows that this diversification was interrupted by five
massive extinctions and some smaller extinction peaks [1]. The largest extinction occurred
about 251 My ago at the end of the Permian period. The global species extinction ranged then
between 80% to 95%, much more than, for instance, the Cretaceous-Tertiary extinction 64 My
ago which killed the dinosaurs and claimed ∼ 47% of existing genera [2]. In spite of intensive
studies it is still not known what caused the mass extinctions. Many extinction mechanisms
have been proposed but no single mechanism seems to provide a satisfactory explanation of
the complex geological records on mass extinctions [3]. These include terrestrial mechanisms
such as intense volcanism, which coincided only with two major extinctions [4] or drastic
changes in sea level, climate and environment that occurred too often, and astrophysical
mechanisms, such as a meteoritic impact that explains the iridium anomaly which was found at
the Cretaceous/Tertiary boundary [5] but has not been found in any of the other extinctions,
supernova explosions [6] and gamma ray bursts [7] which do not occur close enough at a
sufficiently high rate to explain the observed rate of mass extinctions.
The geological records, however, seem to indicate that an accidental combination of drastic
events [3] occurred around the times of the major extinctions. For instance, the dinosaur
extinction coincides in time with a large meteoritic impact, with a most intensive volcanic
eruption and with a drastic change in sea level and climate. The origin of these correlations is
still unclear. Meteoritic impacts alone or volcano eruptions alone or sea regression alone could
not have caused all the major mass extinctions. An impact of a 10 km wide meteorite with
a typical velocity of 30 km s −1 was invoked [5] in order to explain the Cretaceous-Tertiary
(K/T) mass extinction 64 My ago, which killed the dinosaurs, and the iridium anomaly
observed at the K/T boundary. But neither an iridium anomaly nor a large meteoritic
crater have been dated back to 251 My ago, the time of the Permian/Triassic (P/T) mass
extinction, which was the largest known extinction in the history of life [3] where the global
species extinction ranged between 80% to 95%. The gigantic Deccan volcanism in India that
occurred around the K/T boundary [4] and the gigantic Siberian basalts flood that occurred
around the P/T boundary have ejected approximately 2×10 6 km 3 of lava [4]. They were more
than a thousand times larger than any other known eruption on Earth, making it unlikely
that the other major mass extinctions, which are of a similar magnitude, were produced
by volcanic eruptions. Although there is no one-to-one correspondence between major mass
extinctions, large volcanic eruptions, large meteoritic impacts, and drastic environmental
changes, there are clear time correlations between them. We propose that near encounters
of Earth with “visiting planets” from the outer solar system are responsible for most of the
mass extinctions on planet Earth and can explain both the above correlations and the detailed
geological records on mass extinctions [8].