Traditionally astronomers look out in space while geologists look down at Earth. But at the Astrogeobiology Laboratory we do astronomy by looking down deep into Earth’s sedimentary record. We try to link the history of Earth and its life to the astronomical realm. Whereas other research groups study ancient sediments for animal or plant fossils, we search for material of extraterrestrial origin. In our laboratory we have developed the first methods to reconstruct the micrometeorite flux to Earth in the distant past. In our approach we dissolve in strong acids tons of sedimentary rocks that formed on sea floors up to 3.5 billion years ago. From the rocks we extract rare, but very resistant, microscopic minerals, so called spinels, that once resided in micrometeorites that fell on Earth and became buried in sediments. Micrometeorites, up to two millimetres large, originate from colliding bodies in different parts of the asteroid belt between Mars and Jupiter. By reconstructing what types of micrometeorites that reached Earth at different times in the past, we can link the history of life to otherwise hidden aspects of solar-system history. The micrometeorites also contain a record of the flux of cosmic rays in the solar system. Such cosmic rays originate from exploding supernovas; hence we can establish a record of when or if the Earth has passed near such a supernova while the solar system travelled on its 225-million-year orbit around the centre of our galaxy. In another project we search for larger meteorites, 1-20 cm in diameter, that fell on a sea floor during the mid- Ordovician period, 470 Myr ago, in what is now southern Sweden. The meteorites are preserved in the sedimentary limestone as fossils, just like animal fossils. They are found during quarrying of the ancient sea floor to produce e.g. sawed floor tiles. Since 1993, together with the owners of the quarry, the Thor family, we have searched about 50,000 m² of the ancient sea floor for meteorites. More than 140 meteorites have been found, representing almost all fossil meteorites known to science. These meteorites tell a story about one of the most dramatic collisional events in the asteroid belt the last three billion years, the breakup of the circa 150 km large so-called L-chondrite parent body. For the past two hundred years geologists have tended to look on the Earth as an almost closed system. This general perception was changed through a major discovery by Walter and Luis Alvarez in the early 1980’s. By means of a thin iridium-rich clay layer distributed worldwide they were able to show that Earth’s Mesozoic fauna, including all the dinosaurs, was wiped out 66 million years ago because Earth collided with an about 10 km large asteroid. Today we know that the impacting body created the ca. 200 km in diameter Chicxulub crater on the Yucatan peninsula, Mexico. The inspiration to develop this type of research grew during my post doc stay with the group of Luis Alvarez at the Lawrence Berkeley Laboratory in 1988-1989. The title of the Alvarez paper that created the field of astrogeobiology was “Extraterrestrial Cause for the Cretaceous-Tertiary Extinctions- Experimental results and theoretical interpretations”. To me the second part of the title conveys an important message: Hypotheses about Earth’s ancient past must be testable and rely on robust and high-quality empirical data.
Lund – April 4, 2025
Birger Schmitz
