Astrobiology: an Introduction to the Field.

The origin of stars and planetary systems, and the possibility of life elsewhere in the Universe, have long fascinated mankind. There is a broad understanding that life originated from simple precursor molecules and proceeded via more complex molecules to self-replicating, metabolising entities capable of independent existence and subsequent evolution. However, the stages and mechanisms that comprise these processes are still poorly understood. The biogenic elements (H, C, N, O, S, P) and organic matter are some of the major constituents of the universe. Nucleosynthesis of heavy elements in stars, such as carbon, allowed the formation of organic molecules, which are widespread in our Galaxy and beyond. The discoveries of protoplanetary disks around other stars suggested that our solar system is not longer the only known example of a planetary system in the Universe. The hunt for planets resulted in the detection of more than 50 planets to date circling other stars. Future instrumentation in planetary detection might eventually show the existence of Earth-like planets capable of sustaining life.
The geological record tells us that life on Earth began very quickly. The first evidence for microbial life is provided by microfossils which are ~ 3.5 x 10⁹ years old (planet Earth was formed ~ 4.5 x 10⁹ years ago). During the first billion years our planet provided very hostile conditions for life to develop. Volcanic eruptions from the heated interior and external heavy bombardment by small bodies may have extinguished emerging life on a rapid timescale.

Where and how did life originate ?

Numerous theories for the origin of life exist which are based either on a terrestrial or an extraterrestrial origin. For decades it has been suggested that organic chemistry in a reducing atmosphere in Earth's early history triggered the formation of prebiotic molecules. To date an idea of life's origin in the ocean is more favoured. The cycle of water through ocean floor volcanic systems produces reducing conditions which may lead to the formation of complex organic molecules. Another theory claims that impacting prebiotic matter from comets and asteroids could have been the first step to complex life. More than 120 major craters found on Earth prove the importance of violent impacts from space. The progress in RNA chemistry has strongly helped in reconstructing the "genetic tree" and in revealing steps towards simple self-replicating systems. It has also been shown that life can flourish in quite extreme environments. The research of hydrothermal systems, permafrost, icy lakes and their inhabitants are a major focus in the field of astrobiology and provide at the same time constraints for the search of life on other planets in our solar system.
The major requirement for life - as we know it - are water, biogenic elements and a source of energy (such as the Sun, geothermal energy, weathering of volcanic rocks etc.). Apart from planet Earth there are currently three objects in our solar system which may harbor ingredients for life, namely Mars, Europa and Titan. The current Mars environment is too cold (and the atmosphere is too thin) to retain liquid water on its surface. However, data from the Mars Pathfinder, which landed successfully on Mars in July 1997 suggested widespread flowing water in the previous history of Mars. Water could also be trapped as underground ice on planet Mars. The moons Europa and Titan are major targets in the search for liquids in the Solar System. Jupiter's moon Europa probably hosts a subsurface water ocean beneath its outer ice crust. What geological processes create the ice rafts and other ice-tectonic processes that are at the origin of prominent surface features on Europa are currently strongly debated. Saturn's moon Titan is of interest because of its atmospheric organic chemical activity. Titan may also harbor a liquid hydrocarbon ocean.

Astrobiology is a new research field, which studies the origin, evolution, distribution and destiny of life in the Universe. This new discipline has been put forward through the establishment of astrobiology centers and grants in the US and Europe.
In a series of workshops, the aims and goals of the US astrobiology program have been defined and can be found at http://astrobiology.arc.nasa.gov.
Astrobiology is a multidisciplinary research field, combining astronomy, astrophysics, biology, biochemistry, chemistry, geology, climate research, and specific fields such as palaeobiology, organic chemistry, geomicrobiology, ecogenomics (genome evolution) and others. To understand life's origins in the context of planetary environments, numerous space missions and space- and Earth-based experiments are currently being carried out, or planned for the near future. Several space missions organized by both NASA and ESA are in progress, or are well into the planning stage, that have key objectives concerning the nature of extraterrestrial organic chemistry and the search for traces of past or present life. These include MARS-EXPRESS (to Mars), CASSINI-HUYGENS (to Saturn and Titan), ROSETTA (to comet Wirtanen). Future space missions, such as more advanced Infrared satellites (SIRTF, FIRST, NGST), an orbiter (or lander) to investigate Jupiter's moon Europa (EUROPA-Orbiter) or space-based telescopes to search for Earth-like planets (DARWIN, TPF-terrestrial planet finder) will build on and extend current mission objectives for life search strategies. Answering the questions how life originated on Earth is one of the main philosophical challenges of mankind and also of vital importance in the frame of recent planetary detections and the possible emergence of life elsewhere.
Astrobiology is a research field that is currently underemphasized in the Netherlands compared to other large European countries and the US. It is therefore that we organize a meeting to introduce this new research discipline on a national basis, to stimulate collaborations and to form a young generation working on future projects in this research field.

dr. Pascale Ehrenfreund,
Sackler Laboratory for Astrophysics, Leiden Observatory.

More links to astrobiology web sites are found at the Link page.