Not a week passes that a new astronomical discovery is announced in the popular as well as the scientific press. Our vision and appreciation of our living and cosmic environment is continually changing. Old, long-held views – for example, that our solar system is the only example of planetary systems – continue to be replaced by new ideas and concepts. During the 1960s and 1970s, we witnessed a similar paradigm shift from the notion of a very solid, relatively stable Earth, on which oceans and continents would periodically subside or emerge from beneath sea-level, to a vigorous Plate-Tectonic Earth, on which oceans are continually created and removed. We now know that light and buoyant continents are essentially carried as ‘passengers’ on the ever-circulating, dense, underlying mantle. It took decades before most (but not all) earth scientists were convinced by the evidence for this concept.
New astronomical techniques
Recent astronomical discoveries that challenge former theories are particularly fascinating. Looking out into the far reaches of our Universe, researchers have now detected a radio signal from the first stars formed. Because such distant red-shift signals fall in the FM radio electromagnetic spectrum, Judd Bowman, of the Arizona State University’s discovery team, had to use the Murchison Radio-astronomy Observatory in the ultra-quiet region of Western Australia, to detect the weak signal. This discovery, as indeed most astronomical information, comes to us via exploration of the whole electromagnetic spectrum.
In contrast, a whole new way of exploring the universe by observing gravitational waves (first reported in April 2016) has recently been proved. Andrew Grant has called this the beginning of multi-messenger astronomy (Physics Today, October 2017). Seconds after the gravity wave recorders (LIGO and VIRGO) detected gravitational waves, the Fermi Gamma-Ray Space Telescope spotted a gamma-ray burst. Many observatoriesconfirmed this event. The observations suggest that gamma-ray bursts result from colliding ultra-dense neutron stars, the enormous energy release from which is sufficient to create heavy elements like gold and uranium via the fusion of lighter elements.
In the past year, there have also been breakthroughs in the study of cosmic rays that continually bombard the earth. While most are light protons, some are ultra-high energy cosmic rays made of iron nucleii that may originate from supermassive black holes in the centre of distant galaxies. In addition to cosmic rays, our planet is immersed in charged particles emanating from the Sun in what is termed the solar wind. This reacts with the Earth’s magnetic field and creates numerous transitory phenomena including the aurorae, sprites and lightning. The solar wind controls spaceweather and climate. On the other hand, retention of the Earth’s atmosphere is generally attributed to theplanet’s strong magnetic field which prevents widespread stripping of the volatile gasses by the solar wind. A review of the Earth’s electromagnetic environment was published by Catherine Constable in 2016 (Surveys in Geophysics37, p27-45).
When some massive stars are at the end of their life cycle they become supernovae that violently explode, expelling gas at high speed into space. These explosions are strong enough to hurl matter across vast distances into neighbouring galaxies. Our own Milky Way galaxy has apparently grown significantly by capturing material from its satellite galaxies, namely the Small and Large Magellanic Clouds (A. Woodward, New Scientist, August 2017). We clearly live in a complex interactive universe, but the time scales are vast in comparison to the brevity of human experience.
Collapse of a single planetary paradigm
The old paradigm of planetary origins based on our Solar System has collapsed with the discovery of hundreds of other planets with very different orbits. An article by Ann Finkbeiner (Planets in Chaos: Nature, July 2014) describes this astronomical puzzle. Planets in their early years collided, interacted, migrated and grew in size, before ultimately settling into some relatively stable arrangement. This turbulent and chaotic impact history during the early assemblage of planets resulted in the expulsion of some planets into the cold emptiness of space, away from their parent stars. Some stars, like our own Sun, have large families of planets. As of December 2017, researchers have identified 3,567 exoplanets. Two telescopes planned to be launched in 2018 will search for signs of other planets by observing their crossings in front of parent stars (J.N. Winn, Scientific American, March 2018, p26-33).
Meteors and impacts on Earth
Recent observations confirm that meteorite impact on Earth is a dynamic process that continues and directly influences our environment. The planet is continually showered by meteors of various sizes from microscopic space dust and visible sand-sized meteorite grains, up to fragments, some kilometers in diameter, of rarer asteroids or comets.
Some asteroids with unusual fractured shapes and compositions have recently whizzed past our planet on hyperbolic (i.e. non-returning) orbits, and continued onto other stellar rendezvous (K. Cooper, New Scientist, February 2018). The estimated number of such interstellar objects may be in the thousands. More relevant of course, have been those visitors that have made direct hits on Earth. Much of Earth’s history since Proterozoic times has in fact been shaped by catastrophic meteorite impacts that have triggered mass extinctions of living creatures and other biological effects. The immense impact some 63 million years ago that spelt the extinction of dinosaurs, as well as ammonites and other marine creatures, is well known, as perhaps is the atmospheric explosion at Tunguska, in central Siberia only a century ago (1908). Recognition of the importance of major meteorite impacts on Earth history has been yet another paradigm shift in our understanding.
A cometary catastrophe
Less well known is the discovery that fragments of a disintegrating ~100km-diameter comet collided with the Earth some 12,800 years ago in what is known as the Younger Dryas period (named after a signature Arctic flower). The collision triggered a rapid return to glacial conditions which lasted about 1,400 years, interrupting the gradual warming of the planet after the Last Glacial Maximum around 20,000 years ago. In a two-part publication in 2018, Wendy Wolbach (and 31 co-authors) presented a detailed analysis of evidence of this most unusual climatic episode gathered over the last decade (W.S. Wolbach et al.,Journal of Geology126, p165-184 & p185-205). Data was gathered from ice-cores in Greenland, Russia and Antarctica as well as from lake, marine and terrestrial sediments. Contemporaneous layers of charcoal and dust in these geographically dispersed cores confirm this cosmic impact event. These specific layers are enriched in platinum and other impact-related elements. They also contain glassy spherules and nano-diamonds, and are anomalously high in ammonia, nitrate, and other compounds that represent a major period of extensive biomass burning. Sea levels rose a few metres due to major melting of the North American Ice Cap and this surge of fresh water disturbed the oceanic circulation that began a period of cooling.
Evidence points to numerous fragments of a disintegrating comet detonating above and/or colliding with ice-sheets, oceans, and land on at least four continents centred on North America. The radiant and thermal energy from multiple explosions triggered extensive wildfires that are estimated to have burned about 10% of the planet’s biomass, considerably more than that accompanying the meteorite impact that caused the demise of the dinosaurs. The burning created long-lived atmospheric soot, blocking most sunlight and creating an ‘impact winter’ and acid rain. The reduced vegetation caused a major crisis in the ecosystem and may have contributed to many megafaunal extinctions including mammoths, mastodons, ground sloths and American horses, along with many birds and smaller mammals. Human population declined for about a thousand years and the demise of the Clovis hunters ensued. This synchronicity of multiple events makes the Younger Dryas interval one of the most unusual climatic/ecological episodes during the last two million years. It also raises the importance of supporting the Near Earth Asteroid Survey in defence of future serious impacts on our planet.
Our changing paradigms
The rapid acquisition of new and exciting knowledge about astronomy and Earth history requires paradigm shifts in our thinking and interpretation. We should always be prepared for new scientific observations and revelations and continue to adapt our ideas and concepts to better explain our cosmic and earthly environments.
Dr Vic Gostin