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Glossary of Astrobiology
Terms
ALH84001 - Martian
meteorite discovered in the Allan Hills ice field in Antarctica
in 1984. It has been suggested that this meteorite contains evidence
of martian microbial life.
Amino Acid - Simple organic molecules
containing an amino group (NH2) and a carboxyl group (COOH). Amino
acids link together in chains to form proteins.
Archaea - One of three domains
of life, along with the Bacteria and the
Eucarya. The archaea are prokaryotic
organisms which were shown to be a distinct group from the bacteria
by molecular phylogeny using 16S ribosomal
RNA. The archaeal domain includes many extremophilic
organisms.
Archaebacteria - An alternative
name for the domain archaea.
Archean - The period in the history
of the Earth from 4000 million years ago to 2500 million years ago,
part of the Precambrian. The Archean
is preceeded by the Hadean and followed by
the Proterozoic. The earliest fossil
evidence for life on Earth appears during the Archean period.
ATP - Adenosine triphosphate - The molecule
which is the source of energy for most metabolic processe in living
organisms.
Bacteria - One of three domains
of life, along with the Archaea and the Eucarya.
The bacterial domain includes all prokaryotic
organisms not classified as archaea.
Carbonaceous Chondrite
- A rare type of meteorite which is
rich in organic material. The Murchison meteorite is a famous example.
Cenancestor - An alternative term
for the Last Common Ancestor
of all life on Earth.
Chirality - A chiral molecule is
a molecule with an asymmetric structure which can exist in two mirror
image forms (or enantiomers). In living
organisms such molecules are usually found in only one of the two
possible enantiomers (homochirality). Thus amino
acids are normally in the left-handed or L-enantiomer, while
sugars are in the right-handed or D enantiomer.
Chloroplast - An organelle
found in the cells of green plants in which photosynthesis
occurs. According to the endosymbiosis
theory, chloroplasts are descended from photosynthetic bacteria.
Cyanobacteria - A class of bacteria
which make use of oxygen producing photosynthesis.
Commonly referred to as blue-green algae.
DNA - Deoxyribonucleic Acid - The molecule
which constitutes the genome of living cells.
DNA molecules have a double stranded helical structure built from
a sugar phosphate backbone and a set of four bases (adenine, guanine,
cytosine and thymine). The sequence of bases specifies the genetic
information.
Domain - The highest taxonomic division
in the classification of living organisms. The three domains are
the Archaea, the Bacteria
and the Eucarya. Domains are subdivided into
kingdoms. While the three domain model is widely used in astrobiology,
some biologists prefer other schemes such as the Five-Kingdom system.
Doppler Shift - The shift in
wavelength of light or other electromagnetic radiation due to the
motion of the source relative to the observer. If the source is
moving away from the observer the wavelength of light is increased
(a red shift) whereas for a source moving towards the observer the
wavelength is decreased (a blue shift). By precise measurements
of the observed wavelength of spectral lines astronomers can determine
the velocity of a star with accuracies of a few metres per seconds.
Such observations are used to detect extrasolar
planets.
Drake Equation - An equation
proposed by SETI pioneer Frank Drake to calculate
the number of civilizations in the galaxy. It takes the form:
N = R fp ne fl fi fc L
where R is the birthrate of suitably long lived stars, fp
is the fraction of stars with planets, ne is the number of
earth-like planets per solar system, fl is the fraction of
earth-like planets on which life actually starts, fi is the
fraction of planets with life on which intelligence emerges, fc
is the fraction of intelligent civilizations which have the technology
and incentive to communicate over interstellar distances, and L
is the lifetime of such technological civilizations.
Ediacaran Fauna - A fossil
fauna of soft bodied organisms with an age of about 560 million
years, first found in the Flinders Ranges of South Australia. They
are the earliest known animals.
Enantiomer - The name given to
the two mirror image forms of a chiral
molecule.
Endosymbiosis - Symbiosis in
which one organism lives within the body of the other. More specifically
refers to the theory that eukaryotic cells
arose from the symbiosis of a number of organisms, with the organelles
such as mitochondria and chloroplasts
being descended from bacteria. This theory
has been confirmed by molecular phylogeny.
Enzyme - A protein
that acts as a catalyst. Most chemical processes in living cells
are enzyme catalysed.
Eubacteria - An alternative name
for the domain bacteria (or true bacteria)
as distinct from the archaebacteria
or archaea.
Eucarya (or Eukarya) - One of three
domains of life along with the Archaea
and the Bacteria. The Eucarya domain covers
all organisms with cells which have a nucleus and organelles
(Eukaryotic cells). The Eucarya domain
includes the animals, plants, fungi and protists.
Eukaryote - An organism belonging
to the domain Eucarya. Eukaryotes have cells
with the genetic material contained in a nucleus and with separate
organelles. Eukaryotic cells are typically
about 10 times bigger in each dimension than prokaryotic
cells.
Extrasolar Planet - A planet
orbiting a star other than the sun.
Extremophile - An organism adapted
to life in extreme conditions. For example thermophiles
and hyperthermophiles are adpated
to high temperatures.
Gas Giant - A massive planet such
as Jupiter or Saturn composed mainly of gases with no solid surface.
Genetic Code - The set of rules
by which three letter "words" in a DNA
or RNA sequence describe an amino
acid to be incorporated into a protein.
Genome - The complete set of genetic
information for a particular organism.
Gravitational Lensing -
The effect on the appearance of an astronomical source due to
the bending of light by the gravitational field of an intervening
object.
Habitable Zone - The region
around a star in which an orbiting planet could maintain conditions
suitable for life.
Hadean - The period from the formation
of the Earth (about 4550 million years ago) to 4000 million years
ago. The first division of the Precambrian.
The Hadean period is followed by the Archean.
Heavy Bombardment - During
the first few hundred million years of the solar system the earth
and other planets were subject to an intense bombardment by the
debris left over from the formation of the solar system. It is during
this heavy bombardment phase that most of the craters on the moon
were formed. The emergence of life on Earth appears to roughly coincide
with the end of the heavy bombardment.
Hot Jupiter - A massive planet
orbiting very close to a star, such as the planet of 51 Pegasi with
an orbital period of only four days. Many of the extrasolar
planets so far found are of this type.
Hyperthermophile - An organism
adapted to life at very high temperatures. Hyperthermophiles have
optimum growth temperatures above 80 degrees C, and a number can
grow at temperatures above 100 degrees C.
Impact Frustration - The
hypothesis that during the heavy bombardment
phase, impacts of massive objects may have wiped out emerging life
several times before it was able to permanently establish itself
on Earth.
Interferometer - An instrument
which makes use of the phenomenon of interference of light. Astronomical
interferometers work by combining the light of two or more telescopes
to achieve the resolving power equivalent to a much larger telescope.
Interferometers are widely used in radio astronomy, but the technique
is more difficult to apply in the optical because of the disturbing
effects of the earth's atmosphere. Space interferometers avoid these
difficulties and may play an important role in the search for extrasolar
planets.
Interstellar Molecule -
Molecules in interstellar space are most commonly detected by
means of radio frequency emission lines coming from the gas in molecular
clouds. Molecules can also be detected from the infrared spectra
emitted from dust. Well over a hundred molecular species have been
detected by these methods. Some of the more complex molecules found
include acetic acid, acetone and ethanol.
Last Common Ancestor - The
last common ancestor of all organisms living today. The root of
the tree of life.
Lateral Gene Transfer
- The transfer of genes between different species. Lateral gene
transfer may have been widespread in the early stages of life on
Earth and this complicates the interpretation of the tree
of life.
LUCA - Another term used for the Last
Common Ancestor of all living organisms. Acronym for Last Universal
Common Ancestor.
Martian Meteorites - Meteorites
which originate from Mars, also known as SNC
meteorites. Their martian origin is demonstrated by bubbles
of gas trapped within them which have identical composition to the
atmosphere of Mars.
Meteorite - A meteor that reaches
the earth's surface. There are many types of meteorites, the most
important for astrobiology being the carbonaceous
chondrites which contain organic material, and the martian
meteorites.
Microlensing - Gravitational
lensing of a star indicated by the brightening of the star as
the lensing object moves in front of it. Analysis of the light curve
of a microlensing event can reveal the presence of an extrasolar
planet.
Miller-Urey Experiment
- An experiment carried out by Stanley Miller and Harold Urey
in 1952 which demonstrated the synthesis of amino
acids in conditions simulating a thunderstorm on the early Earth.
The experiment can be seen as marking the beginning of the experimental
study of the chemistry of life's origins.
Mitochondria - The organelles
in eukaryotic cells in which aerobic respiration
takes place, providing a supply of chemical energy (in the form
of ATP) for the cell.
Molecular Cloud - A cloud
of cool gas in interstellar space. These clouds provide the raw
material out of which new stars form and contain many types of interstellar
molecules.
Nucleic Acid - The molecules
which carry genetic information, DNA and RNA.
Organelle - Membrane enclosed structures
in eukaryotic cells such as the mitochondria
and chloroplasts.
Panspermia - The hypothesis that
life has spread from planet to planet throughout the galaxy and
therefore did not originate on Earth. The term originates with Svante
Arrhenius in the 1900s. More recently the most prominent supporters
of this view have been Fred Hoyle and Chandra Wickramasinghe. Most
scientists find no evidence to support panspermia and think it unlikely
that life could survive a journey between stars. There is somewhat
more support for the idea that life could have spread between planets
within the solar system (planetary
panspermia)
Peptide - A compound linked together
by peptide bonds such as a protein.
Photosynthesis - The chemical
process carried out in photosynthetic bacteria and green plants
in which light energy is used to synthesize carbohydrates from water
and carbon dioxide.
Phylogeny - The evolutionary relationships
between different species of organisms as represented in a phylogenetic
tree. In molecular phylogeny these relationships are determined
by analysis of the differences in the sequences of genes common
to the various species.
Planetary Panspermia - The
spreading of life between planets within the solar system. In particular
the hypothesis that life might have originated on Mars and been
carried to Earth - a suggestion which is supported by the existence
of martian meteorites.
Precambrian - The period in Earth's
history from its formation up to the beginning of the Cambrian period
about 540 million years ago. The Precambrian is subdivided into
the Hadean, the Archean,
and the Proterozoic.
Pre-RNA World - A hypothetical
early stage in the development of life which preceeded the RNA
World and used some other genetic material in place of RNA or
DNA.
Prokaryote - An organism belonging
to the domains archaea or bacteria.
Prokaryotic cells lack a nucleus and organelles
and are typically much smaller than eukaryotic
cells.
Protein - A polymer built from amino
acid monomers. Living organisms use many different proteins
acting as catalysts (enzymes) and for structural
and other roles.
Proterozoic - The period in Earth's
history from 2500 million to 540 million years ago. The last division
of the Precambrian.
Ribosome - A structure composed of
protein and RNA molecules
which reads genetic information from messenger RNA and synthesises
the corresponding protein.
Ribosomal RNA - The RNA
components of a ribosome. One of these components,
the small sub-unit ribosomal RNA (also known as 16S ribosomal RNA
in prokaryotes or 18S ribosomal RNA in
eukaryotes) has been widely used to determine
the tree of life.
Ribozyme - An RNA
molecule which acts as a catalyst. The discovery of RNA catalysis
led to a Nobel prize for Sidney Altman and Thomas Cech, and to the
RNA World concept.
RNA - Ribonucleic acid - A molecule which
can carry genetic information in a similar way to DNA.
In modern cells RNA molecules are important in the process of protein
synthesis, in the form of messenger RNA, ribosomal
RNA and transfer RNA.
RNA World - A hypothetical early
stage in the development of life in which RNA
molecules provided both the genome and the catalysts, roles which
subsequently were taken over by DNA and proteins.
SETI - The Search for Extraterrestrial
Intelligence.SETI projects aim to detect radio or other signals
from advanced civilizations elsewhere in the galaxy.
SNC meteorites - Another term
for martian meteorites. The name
derives from the initials of three examples, Shergotty, Nakhla and
Chassigny.
Snowball Earth - Name given
to events in which the Earth was glaciated and the oceans frozen
from pole to pole. Snowball Earth events have been suggested as
occuring at about 2.3 billion years ago and between 750 and 580
million years ago.
Stromatolite - Layered structures
built by colonies of microorganisms which are commonly found in
the Archean and Proterozoic
fossil records. Modern examples can be found in sites such as Shark
Bay in Western Australia.
Terrestrial Planet - Rocky
planets such as the Earth and other inner planets, as distinct from
the Gas Giants.
Thermophile - An organism adapted
to life at high temperatures.
Tree of Life - A phylogenetic
tree covering all groups of life on Earth. The term is commonly
used for the tree derived by molecular phylogeny
using small sub-unit Ribosomal RNA
as pioneered by Carl Woese in the 1970s.
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