SOLAR ACTIVITY AND SOLAR WEATHER IN THE FRAMEWORK
OF LIFE'S ORIGIN AND EVOLUTION ON EARTH
M. Messerotti 1 and J. Chela-Flores 2, 3
(1)
INAF-Trieste Astronomical Observatory, Loc. Basovizza n. 302, 34012 Trieste, Italy
(2)
The Abdus Salam ICTP,Strada Costiera 11, 34014 Trieste, Italy
(3)
Instituto de Estudios Avanzados, IDEA, Caracas 1015A, Venezuela
ABSTRACT/RESUME
The effects of solar activity and solar weather have
played a leading role in the origin and evolution of life
on Earth. Our insights into this subject have improved
considerably in recent years, It is the purpose of our
work to reconsider two joint factors that together
contribute to the origin of life. Firstly, we consider
chemical and biological evolution on Earth and
chemical evolution on the small bodies of the Solar
System. Secondly, as our star approached the main
sequence, the continually evolving early Sun was
providing necessary energy requirements for the
emergence of life. We discuss possible favorable, and
also inhibiting effects on the origin of life that were
provided by the influence of solar evolution on space
climate and space weather. By not restricting ourselves
exclusively to solar effects, but rather to space weather
effects (with possible inputs from galactic events), we
gain a wider perspective on the importance of improved
understanding and predictions of solar activity.
considerable number of young stars with remnants of
accretion disks show energetic winds that emerge from
the stars themselves.
Similar ejections are still currently observed from
our Sun. For this reason it is believed that some of the
early Solar System material must keep the record of
such emissions [4]. Information on the energetic
emission of the Sun during this period can be inferred
from data on X ray and UV emission (larger than 10
eV) from pre-main-sequence stars [5]. We may
conclude that during pre-main-sequence period, solar
climate and weather presented an insurmountable
barrier for the origin of life anywhere in the solar
system. In the Hadean, conditions may still have been
somewhat favorable, especially with the broad set of
UV defense mechanisms that are conceivable. The high
UV flux of the early Sun would, in principle, cause
destruction of prebiotic organic compounds due to the
presence of an anoxic atmosphere without the presentday ozone layer [6-7]. Some possible UV defense
mechanisms have been proposed in the past, such as
atmospheric absorbers, prebiotic organic compounds,
and still some others.
1. INTRODUCTION
1.1 How early could life have emerged on Earth?
1.2 UV defense mechanisms contributed to the first
steps of life on Earth
During the early stages of the study of the origin of life
[1-2] not enough attention was paid to the question of
the correlation of chemical evolution on Earth and the
all-important evolution of the still-to-be understood
early Sun [3]. Today, due to the advent of a significant
fleet of space missions and the possibility of performing
experiments in the International Space Station (ISS), a
meaningful study of the factors that may have led to an
early onset of life on Earth begins to be possible.
We can understand general trends of the influence of
space climate and weather on the evolution and
distribution of life. As suggested in the previous section
solar climate during the first Gyr of the Earth was
radically different. The earliest relevant factor was
excessive solar-flare energetic particle emission, a
phenomenon that has been recorded in meteorites.
These extraterrestrial samples provide information on
events that took place during this early period after the
collapse of the solar nebula disk. Gas-rich meteorites
have yielded evidence for a more active Sun. A
Oceanic UV defense mechanisms increase the suite of
possible planetary habitable zones. Nature may have
provided UV defense mechanisms in the scenarios for
an origin of life in hydrothermal vents [8], benthic
regions [9] and in deep subsurface environments [10].
Most attempts to deal with this problem have involved
atmospheric absorbers such as SO2 [11], and organic
hazes, where it has been shown that UV absorption by
steady-state amounts of high-altitude organic solids
produced from methane photolysis may have shielded
ammonia sufficiently to maintain surface temperatures
above freezing [12].
Even in the absence of atmospheric shielding it is
possible to conceive that sufficient UV absorbers were
present in the ocean to allow for the accumulation of
organic material [13], such as organic polymers from
electric discharges and HCN (hydrogen cyanide)
polymerizations, solubilized elemental sulfur, and
inorganics such as Cl, Br, Mg2+, and SH, or Fe2+.
There would have been a complete UV defense
mechanism if there was a frozen ocean [14], since these
authors have shown that bolide impacts between about
3.6 and 4.0 Gyr BP could have episodically melted an
ice-covered early ocean. Thaw-freeze cycles associated
with bolide impacts could have been important for the
initiation of abiotic reactions that gave rise to the first
living organisms. Oil slicks or large amounts of organic
foams provide other examples of possible UV defense
mechanism [15].
In addition, both biological and physical aspects
have motivated other UV defense mechanisms.
Cyanobacteria are represented by some of the oldest
fossils from the Archean, although the date of such
fossils remains a controversial subject [16-17]. There
seems to be little evolutionary change in these
prokaryotic microorganisms. But we are certain that the
atmosphere of the Archean Earth was anoxygenic. We
expect that the early microbes must have used various
means of avoidance of radiation damage. Some of these
are attenuation by the water column of their aquatic
habitats by the presence of some UVR absorbing
substance. It is known that water itself does not protect
life. Indeed, UVR is known to penetrate the water
column up to at least 50 meters. But if the water
contains iron, or nitrogenous salts, UVR is efficiently
screened (cf., [18] for references to the literature). On
the other hand, UV absorbing pigments may provide an
alternative UV screening strategy. One well-known
example is scytonemin [19]. A physically motivated UV
defense mechanism has been suggested in the past [20].
2. THE RELEVANCE OF SPACE WEATHER ON
ASTROBIOLOGY IN THE SOLAR SYSTEM
The incidence of non-ionizing UVR on the early surface
of Earth and Mars to a large extent can be inferred from
observations. Ionizing radiation, mainly due to nuclear
and atomic reactions is relevant: X-rays are emitted
spanning the whole spectrum of hard X-rays to soft Xrays (0.01-10 nm); gamma rays are present too. The
primary components affecting space climate are:
galactic cosmic rays and solar cosmic radiation.
To these we should add events that contribute to
solar weather, such as solar particle radiation that
consisted of the low-energy solar-wind particles, as well
as more energetic solar burst events consisting of solar
particles that arise from magnetically disturbed regions
of the Sun. These events vary in frequency according to
the eleven-year cycle. However, scenarios for an early
onset of life that have been proposed in the past have to
deal with space weather that was radically different in
the early Sun.
Knowledge of the prehistory of solar particle
radiation can be approached with a combined effort
from observations of present-day emissions, together
with studies of energetic solar particles recorded in
extraterrestrial materials, notably the Moon material that
became available with the Apollo missions, as well as
with the study of meteorites.
Life emerged on Earth, during the Archean (3.8 - 2.5
billion years before the present, Gyr BP). We should
first of all appreciate the magnitude of the ionizing
radiation that may have been present at that time.
According to some theoretical arguments the origin of
life may be traced back to the most remote times of this
eon (3.8 Gyr BP). Indeed, isotopic and geologic
evidence suggest that photosynthesis may have been
already viable by analysis of the biogeochemical
parameter delta 13 C [21]. Besides in the Archean the
atmosphere was to a large extent anoxic. As a result the
abundance of ozone would not have acted as a UV
defense mechanism for the potential emergence of life.
UVB (280–315 nm) radiation as well as UVC (190–280
nm) radiation could have penetrated to the Earth's
surface with their associated biological consequences
[22-23]. If the distribution of life in the solar system
took place by transfer of microorganisms between
planets, or satellites, knowledge of solar weather
becomes fundamental during the early stages of its
evolution, to have some constraints on the possible
transfer of microorganisms, as investigated extensively
by Horneck and co-workers [24].
The most radiation resistant organism known at
present exhibits a remarkable capacity to resist the
lethal effects of ionizing radiation. The specific
microorganism is a non-spore forming extremophile
found in a small family known as the Deinococcaceae.
In fact, Deinococcus radiodurans is a Gram-positive,
red-pigmented, non-motile bacterium. It is resistant to
ionizing and UV radiation. Various groups have studied
these microorganisms [25-27]. Members of this bacteria
taxon can grow under large doses of radiation [up to 50
grays (Gy) per hour]. They are also known to recover
from acute doses of gamma-radiation greater than
10,000 Gy without loss of viability. Out of the several
closely related species only the radio-resistance of D.
radiodurans appears to be the result of an evolutionary
process that selected for organisms that could tolerate
massive DNA damage. Such microorganisms
demonstrate that life could have survived at earlier
times when the Earth surface was more exposed to solar
radiation. They are also examples of living organisms
that could survive in the extreme environments that the
magnetosphere of Jupiter provides to the surface of the
icy Galilean satellites. This aspect of the microbiology
of the Deinococcaceae is significant, since the Jovian
system does provide on its icy moons environments
where life could have emerged [28]. We shall return to
this topic in the Discussion.
3. THE RELEVANCE OF THE INTERSTELLAR
ENVIRONMENT ON THE ORIGIN OF LIFE
Solar systems originate out of interstellar dust, namely
dust constituted mainly out of the fundamental elements
of life such as C, N, O, S, P and a few others
(Ehrenfreund and Charnley, 2000). Just before a star
explodes into its supernova stage, all the elements that
have originated in its interior out of thermonuclear
reactions are expelled, thus contributing to the
interstellar dust. Recent work has some implications on
the question of Space Weather.
Two sets of
experiments have established that UV radiation plays a
significant role in the synthesis of some of the
precursors of the biomolecules, especially the amino
acids [29-30]. The expanding dust and gas form typical
'planetary nebulae at a later stage of stellar evolution.
Stars whose mass is similar to that of the Sun remain at
the red giant stage for a few hundred million years. The
star itself collapses under its own gravity compressing
its matter to a degenerate state, in which the laws of
microscopic physics eventually stabilize the collapse.
This is the stage of stellar evolution called a 'white
dwarf '. Stellar evolution of stars more massive than the
Sun is far more interesting: after the massive star has
burnt out its nuclear fuel a catastrophic explosion
follows in which an enormous amount of energy and
matter is released. These 'supernovae' explosions are the
source of enrichment of the chemical composition of the
interstellar medium. This chemical phenomenon, in
turn, provides new raw material for subsequent
generations of star formation, which leads to the
production of planets.
Our solar system may have been triggered over 5
Gyr BP by the shock wave of a supernova explosion.
Indeed, there is some evidence for the presence of
silicon carbide (carborundum, SiC) grains in the
Murchison meteorite, where isotopic ratios demonstrate
that they are matter from a type II supernova [31].
Around 4.6 Gyr BP on the nascent planet, the organic
compounds may have arisen in the following manner:
all of the volatiles to the early biosphere. The
Delsemme model based on the assumption of the
cometary origin of the biosphere argues that an intense
bombardment of comets has brought to the Earth most
of the volatile gases present in our atmosphere and most
of the carbon extant in the carbonate sediments, as well
as in the organic biomolecules [32]. With the measuring
ability that was available throughout the 20th century
molecules have been identified by their spectra in a
wide range of wavelengths, and even by in situ mass
spectrometry during spacecraft flybys of comet
P/Halley. These measurements provided composition of
parent volatiles and dust, properties of the nuclei and
physical parameters of the coma. Besides the Halley
comet already mentioned, two other comets also likely
to have originated in the Oort cloud (as the Halley
comet) were particularly useful objects for remote
sensing measurements:

The earliest geologic period (the lower Archean) may
be considered virtually as a 'heavy bombardment
period'. During that time various small bodies, including
comets, collided frequently with the early planet
carrying precursors of the biomolecules that eventually
started off the evolutionary process on Earth and its
oceans. The mechanism of cometary bombardment
appears not to be restricted to our solar system. In
addition, comets may be the source of other volatiles in
the biosphere, as well as the biochemical elements that
were precursors of the biomolecules. Collisions of
comets, therefore, are thought to have played a
significant role in the formation of the hydrosphere and
atmosphere of habitable planets, such as the Earth. The
source of comets is the Oort cloud and Kuiper belt.
These two components of the outer solar system seem to
be common for other solar systems. Noble gases are
released from the interior of the planet at oceanspreading centres (i.e., the mid-oceanic ridges) when
new crust is formed, as well as from volcanoes and hot
springs. Examples are argon (Ar), xenon (Xe) and
krypton (Kr), which are inert and accumulate in the

At the end of accretion organic compounds would
have been incorporated or delivered by small
bodies, both comets and meteorites
Planetary processes, such as those that may have
occurred close to hydrothermal vents, may have
synthesized organic compounds.
4. THE ROLE OF COMETS
FORMATION OF THE BIOSPHERE
IN
THE
Comets develop gaseous envelopes when they are in the
close vicinity of the Sun. In fact, as the comet
approaches the Sun, the higher temperature of
interplanetary space induces the cometary ice to begin
sublimating. Gas leaves the comet carrying some of the
dust particles. On the other hand, the Earth’s biosphere
is that part of this planet where life can survive; it
extends from a few kilometres into the atmosphere to
the deep-sea vents of the ocean, as well as into the crust
of the Earth itself. It is generally agreed that in the
evolution of the Solar System, comets deliver part if not
 Comet Hale-Bopp was discovered in 1995, 21 months
before its approach to pre-perihelion, which allowed
for careful preparation. Its closest approach to Earth
was at 1.3 astronomical units (AU). Its gas and dust
output were much greater than those for Comet
P/Halley.
 Comet Hyakutake was discovered in 1996. Its closest
approach to Earth was 0.1 AU the same year [33].
These comets led to the detection of HCN, methane,
ethane, carbon monoxide, water, as well as a variety of
biogenic compounds.
5.
THE
ORIGIN
OF
HYDROAND
ATMOSPHERE, ENVIRONMENTS FAVORABLE
TO LIFE
atmosphere. On the other hand, noble gases are present
in the Earth’s atmosphere with mixing ratios ranging
from almost 1 percent in the case of argon to about 1
part per billion (ppb) for xenon. The heavy noble gases
are primitive, since they were captured from the solar
nebula when the Earth formed 4.5 Gyr BP. The high
value of the ratio Xe / Kr in meteorites argue in favour
of bombardment of the early Earth by comets, rather
than the meteorites themselves. Besides, the relative
abundance of the isotopes in atmospheric xenon is
different from those in meteorites. For these reasons
comets are probably better candidates for supplying the
heavy noble gases than meteorites [34-35].
Additional indirect evidence for cometary
contributions to the volatiles of the Earth biosphere
come from observations of the noble gas content in
Jupiter’s atmosphere by the Galileo mission. Comets
that contributed to Jupiter’s mass were formed at
temperatures lower than those predicted by present
models of giant- Additional indirect evidence for
cometary contributions to the volatiles of the Earth
biosphere come from observations of the noble gas
content in Jupiter’s atmosphere by the Galileo mission.
Comets that contributed to Jupiter’s mass were formed
at temperatures lower than those predicted by present
models of giant-planet formation [36]. This conclusion
is based on the observed abundance of Ar, Kr and Xe.
The noble gas abundances in Jupiter are significantly
different from those in the Sun (and the solar nebula).
The enrichment in these noble gases is the result of
comets contributing a significant fraction to Jupiter’s
inventory of elements heavier than hydrogen and
helium.
In order for comets to be able to trap sufficient noble
gases to produce the observed abundances in Jupiter,
they had to have formed at temperatures below 30 K. In
big bang cosmology, after the ‘moment of decoupling of
matter and radiation’, hydrogen atoms were formed and
helium atoms arose from the combination of deuterium
with itself. However, fast cosmic expansion did not
allow a thermonuclear steady state being formed. This
generated a fraction of deuterium. It is estimated that
this cosmic ratio of D / H had an upper bound of some
30 parts per million (ppm). D cannot be created de novo.
So the variable presence of D / H is a marker for
understanding various aspects of the evolution of solar
systems. Since D can react easily inside stars, it is not
surprising that its abundance in interstellar matter be
smaller than the original cosmic abundance. Yet, in
Jupiter, for instance, the value is higher than in
interstellar matter, reflecting its abundance in the
protoplanetary nebula about 5 Gyr BP. (The Jupiter
abundance is of the order of 20 ppm, closer to the
expected cosmic abundance.) In the Earth's seawater
this ratio is about 8 times the value of the solar nebula.
The D / H ratio is known in three comets: Halley,
Hyakutake and Hale-Bopp. This work on the D / H ratio
suggests that cometary impacts have contributed
significantly to the water in the Earth's oceans.
However, because the D / H ratio observed in comets is
twice the value for Earth’s oceans, it may be argued that
comets cannot be the only source of ocean water.
6. FRONTIERS BETWEEN ASTRONOMY AND
ASTROBIOLOGY
6.1 Solar constraints on the process of life
Meteorites are a possibility for delivering the precursors
of the amino acids and nucleotides. This possibility has
been amply demonstrated by the organic chemistry
analysis of the Murchison meteorite [37], as well as in
other meteorites [38]. There is a reasonable explanation
for the source of the biomolecule precursors that were
found in Murchison and other meteorites. Models of
interstellar grain mantles [39-40] support the view that
organic compounds were synthesized in interstellar
space, which in turn were to be delivered on planetary
surfaces during the late accretion period. Experiments
have shown that Space Weather has a significant role to
play in this process, since UV radiation will drive the
synthesis of photochemical processes [29]. The
oxygenation of the Earth atmosphere as well as the
chemical and biological evolution of life was due to a
large extent to solar UV radiation [41]. Several
processes were at play with intensities was proportional
to the flux emitted by the Sun. We emphasize that the
Standard Solar Model predicts that the solar flux of the
young Sun was lower than the present one, whereas
high-precision solar evolutionary models [42] provide a
value significantly higher than the present one. The 'dim
Sun' hypothesis should be ascertained by observation
and experiment. This hypothesis is crucial since it plays
a fundamental role in identifying (via reverse modeling)
the evolution of the atmosphere and hydrosphere. The
relevant factors to clarify are, firstly, those that may
have catalyzed chemical evolution of life. Secondly,
other relevant factors are those that had an opposite
effect, namely, factors that may have biased biological
evolution. Furthermore, observational evidence on
solar-like stars at different evolutionary stages indicate
that during the whole time span relevant to the
appearance of life on Earth (4.5 to 3.5 Gyr BP), the
early Sun might have experienced a highly active phase
in its particle and radiation environment [43]. In any
case the focus of experimental work has been on the
synthesis of the main three organic compounds, either
by incorporation at the end of accretion, or by delivery
of the precursors of the biomolecules that may have
evolved on early planets that support life.
6.2 Prebiotic synthesis of amino acids
The Stanley Miller experiment assumed that the
prebiotic synthesis of biochemical was due to a process
of chemical synthesis that took place in conditions that
resembled the early Earth [44]. Self-assembly into
living cells included the following three classes of
molecules: polymeric nucleic acids (information
coding), polypeptides for the formation of proteins, and
complex lipids for the formation of semi-permeable
membranes.
Experimentally the Miller assumptions are that
chemical evolution occurred in an atmosphere of a
mixture of methane, ammonia, hydrogen and water
vapor. Solar energy was imitated by an electric
discharge. It is well known that this scheme produces
amino acids. Subsequent work along these lines led to
the synthesis of nucleic acid polymers. It is possible that
the primitive atmosphere did not have much free
hydrogen or even hydrogen compounds, as assumed in
the Miller experiment. Since the late 1970s it became
clear that the early atmosphere was composed of carbon
dioxide and nitrogen, rather than the reducing
atmosphere assumed in the Miller experiment [45]. But
in a weakly reducing atmosphere, where little free
hydrogen was present UV photons may interact with
carbon dioxide, carbon monoxide and nitrogen with
only a small amount of hydrogen. This process will
produce hydrogen cyanide, HCN, which has been
shown to lead to amino acids in prebiotic experiments
imitating the primitive oceans [46].
In fact, hydrogen cyanide polymers are
heterogeneous solids varying in color from yellow to
orange, from red to black may be among the organic
macromolecules most readily formed within the solar
system, for instance the black crust of comet Halley
might consist largely of such polymers [47].
The main sources of energy available for the
processes that led to the first living cell are not only
solar radiation, but also some alternatives to the Miller
scenario were possible such as hydrothermal vents [48].
Another possible source of energy was volcanic
lightning [49]. Also, energy arising from collisions of
meteorites and comets with the surface of the early
Earth is a possible source of energy to trigger the
synthesis of the early biomolecules that would
eventually constitute the first living cell [50]. There is
general agreement that oxygen was absent from the
early atmosphere. After the appearance of the first cells,
filamentous cyanobacteria living in the Archean in
colonial mats (stromatolites) began to oxygenate the
atmosphere. But solar UV radiation can break water
molecules leading to oxygen and ozone, which will
eventually protect the early stages of evolution from the
high-energy UV radiation.
6.3 Prebiotic synthesis of lipids
Keeping in mind the relevance of amphiphilic lipids for
the formation of semipermeable membranes, the
chemical evolution that began in a primordial pond must
take into account the formation of lipids that would
eventually encapsulate the complex organic molecules
both amino acids that would form proteins, as well as
nucleotides that would form nucleic acids (RNA and
DNA). Amongst the lipids phospholipids have been
shown to form liposomes consisting of a single
molecular bilayer surrounding an aqueous core. Such
models of the primitive cell have remarkable properties
such as the encapsulation of biomolecules [51]. This
approach illustrates the possible pathway to the first
cell. The self-assembly of all the molecules would lead
to a primitive protocellular structure.
6.4 Prebiotic synthesis of nucleic acids and the RNA
world
RNA synthesis in a Miller type of experiment remains
as a challenge. Yet some significant RNA analogs have
been discussed with promising results [52]. In spite of
these difficulties and preliminary successes, the first
living system of the early Earth has been conjectured to
be RNA. Walter Gilbert referred to this possibility as an
RNA world [53]. The main motivation for supporting
the RNA world is that not only has RNA along with
DNA, the possibility of codifying genetic information,
but some RNAs have catalytic properties [54]. This
property has been confirmed even in modern organisms
by the fact that in ribosomes protein synthesis is
catalyzed by RNA [55]. It still remains as a challenge to
get deeper insights into the transition from chemical to
biological evolution. The role of Space Weather
remains critical for the preservation of life from its
earliest microbial stages. Two factors are of paramount
importance, the stability of both Sun and the Earth
magnetic field. By the analysis of the late stages of the
evolution of the Sun, Space Weather is likely to play an
increasingly more relevant role in understanding the
preservation and eventual inhibition of life on Earth.
Similar conditions are expected to occur in exoplanetary
systems between the star and its orbiting terrestrial-like
planets. Hence the application of the above analysis to
exoplanetary environments should hold upon eventual
modifications that would be forced upon us by the
specific stellar and exoplanetary characteristics.
7. WOULD IMPROVED UNDERSTANDING OF
SOLAR
ACTIVITY
PROVIDE
DEEPER
INSIGHTS INTO THE ORIGIN OF LIFE?
7.1 There are many outstanding questions in spite of
considerable progress
We have seen in Sec. 6 that there are many outstanding
questions in the frontier of astrobiology and astronomy,
in spite of considerable progress since Oparin took his
first steps 1. Fundamental factors such as space climate
and space weather should set additional constrains on
possible theories for the origin of life. They should also
make further additions to our understanding of the early
evolution of life. For instance, the Sun changed
considerably in time its temperature and luminosity, key
factors for astrobiology that require better
understanding, so that we would be in a position to
predict with a certain degree of confidence what were
the conditions like during the first Gyr of the evolution
of the Solar System. The violent eruptions of solar flares
during its earliest stage (T-Tauri), soon gave way to a
very broad range of physical conditions that have to be
clarified by further research. There are several reasons
to raise the question: Why do we need improved
understanding, and predictions of solar activity? One
reason arises from theoretical modeling of the earliest
organisms. Predictions range broadly in their claims.
Improved understanding of solar activity in the first
billion years of the Earth would provide essential clues.
Additional aspects of astrobiology, such as the question
of the distribution of life in the Solar System, also
depend on further research outside the frontier of
astrobiology, namely
by acquiring
improved
understanding of solar activity, such as the preliminary
information that has been possible to retrieve from the
ISS.
7.2 Space climate and space weather are factors in
constraining theories for the origin of life.
Space climate and space weather are also fundamental
for understanding the early evolution of life. The DNA
repair mechanisms that extremophilic organisms
evolved in response to a continuously changing space
environment. The Sun changed considerably in time its
temperature and luminosity, key factors for astrobiology
that require better understanding, so that we would be in
a position to predict with a certain degree of confidence
what were the conditions like during the first Gyr of the
evolution of the Solar System. The violent eruptions of
solar flares during its earliest stage (T-Tauri), soon gave
way to a very broad range of physical conditions that
have to be clarified by further research. There are
several reasons to raise the question: Why do we need
improved understanding, and predictions of solar
activity? One reason arises from theoretical modeling of
the earliest organisms. Predictions range broadly in their
claims. Improved understanding of solar activity in the
first billion years of the Earth would provide essential
clues. Additional aspects of astrobiology, such as the
question of the distribution of life in the Solar System,
also depend on further research outside the frontier of
astrobiology, for instance, improved understanding of
solar activity with the preliminary information that has
been possible to retrieve from the ISS.
7.3 What about the future?
Much progress has been achieved since the 1990s
missions for studying the Sun. Firstly, remarkable
progress has been possible with the Solar and
Heliospheric Observatory (SOHO), a joint NASA-ESA
spacecraft. SOHO is concerned with the physical
processes that form and heat the Sun's corona, maintain
it and give rise to the expanding solar wind. A second
mission launched in the last decade is Ulysses with
measurements of the Sun from a polar orbit. It is also
dedicated to interplanetary research especially with
interplanetary-physics investigations, including close
(1992) and distant (2004) Jupiter encounters. More
recently, the Transition Region and Coronal Explorer
(TRACE) is giving us information on the threedimensional magnetic structures that emerge through
the photosphere, defining both the geometry and
dynamics of the upper solar atmosphere. In this respect,
the Solar Terrestrial Relations Observatory (STEREO)
to be launched in February 2006 will study the nature of
coronal mass ejections (CME), which in spite their
significant effects on the Earth, their origin, evolution or
extent in interplanetary space remains as a challenge.
We need improved understanding so as to be able to
make predictions on this contemporary phenomenon.
Such knowledge will allow us to extrapolate into the
past and investigate which constraints CME poses on
the origin and evolution of life, and its possible
distribution in the solar system by spreading through
interplanetary space. The Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI) is also extending
the information we have on the basic physics of particle
acceleration and large energy release in solar flares.
Solar-B with a coordinated set of optical, EUV and Xray instruments, will investigate the interaction between
the Sun's magnetic field and its corona, to yield an
improved understanding of the driving force behind
space weather. Together with NASA's Solar Dynamics
Observatory (SDO). in the foreseeable future we will
have a much better insight into space weather and its
influence on Earth and near-Earth space by studying the
solar atmosphere in many wavelengths simultaneously.
But, with so much present and future work, we
would like to add a small contribution on two topics that
affect especially the interface between the solar sciences
and astrobiology. It concerns the possible space weather
conditions that may be favorable for the origin and
evolution of life. Besides the early Earth, the most likely
scenarios for early life are Mars and the Jovian moon
Europa. Ulysses has demonstrated two relevant aspects
in this context, namely knowledge of the heliosphere
vulnerability to cosmic rays, and to the interaction of
interstellar gas.
7.4 Why should we persevere with further solar
missions?
A first reason for preserving Ulysses type of missions is
related with the 30 year-old problem of the interaction
of the solar system with a stream of neutral helium
atoms. This cloud is flowing into the solar system,
coming from the direction of the constellation
Sagittarius. Because the atoms in the stream are
uncharged, the heliosphere does not interact with them.
The Ulysses spacecraft GAS instrument were able to
sample directly the flow. Currently, SOHO is
monitoring the helium stream. Besides, the NASA's
Extreme Ultraviolet Explorer (EUVE) and Advanced
Composition Explorer (ACE) are collaborating in this
endeavor. The stream's temperature, density and
velocity are relevant parameters. All the data supports
the view that the solar system is colliding with a vast
interstellar cloud. The cloud's abundant hydrogen does
not easily penetrate the heliosphere because hydrogen
atoms in the cloud are ionized by interstellar ultraviolet
radiation. Like cosmic rays the hydrogen atoms are
charged and, thus, are prevented to enter the interior of
the solar system.
The numbers retrieved by Ulysses are important to
understand the size and leakiness of the heliosphere.
Once again, persevering with the solar missions like
Ulysses (or Ulysses itself) should be useful, especially
in the future when we should address an important
question: whether in the past 2 - 3 Gyrs similar events
may have produced space climate and weather harmful,
or beneficial for the evolution of life in the solar system.
There is another reason for persevering with
Ulysses-type of missions. The focus of space weather
from the point of view of astrobiology gives us a hint.
Jupiter's moon Io is emitting volcanic particles at
passing spacecraft. The dominant source of the jovian
dust streams is Io's volcanoes [56]. In September 2004
Io emitted dust particles whose impact rate was
recorded by the Cosmic Dust Analyzer on board of
Ulysses. The discovery of this phenomenon dates back
to 1992 when, a stream of volcano dust hit Ulysses as it
approached within 1 AU from Jupiter [57]. Cassini's
dust detector is more capable than the instrumentation
on Ulysses when faced with a similar event [58]. In
addition to mass, speed, charge and trajectory, Cassini
measured elemental composition finding sulfur, silicon,
sodium and potassium, whose origin is volcanic. This
raises a question that will deserve further attention in
the future: There is much spectroscopic evidence for the
presence of non-ice substances on the surface of Io's
neighboring moon Europa [59]. In particular, Galileo
Near-Infrared Mapping Spectrometer (NIMS) evidence
for the presence of sulfur compounds has been
discussed in detail [60-61]. Prior to the discovery of the
dust stream, it had been suggested that the sulfur
contamination of the icy surface of Europa was due to
the implantation of S from the Jovian magnetosphere
[62]. In fact, volcanic material from Io's more than 100
volcanoes escapes and disperses through the jovian
system painting the other satellite surfaces. However,
since the sulfur on the Europan icy surface is patchy and
not uniform, and based on combined spectral reflectance
data from the Solid State Imaging (SSI) experiment, the
NIMS and the Ultraviolet Spectrometer (UVS), it has
been argued that that the non-water ice materials are
endogenous in three diverse, but significant terrains
[63]. Effusive cryovolcanism is clearly one possible
endogenous source of the non-water-ice constituents of
the surface materials [64].
8. CONCLUSIONS
With the technologies that are currently available it is
possible to obtain further insights into some key factors
for the eventual understanding of the origin of life in the
Solar System The arguments in this paper have aimed
to demonstrate the importance of viewing the study of
solar activity, space weather and astrobiology within a
unified framework. This approach has led us to the
suggestion of exploiting the instrumentation of
somewhat dissimilar sciences (astronomy and
astrobiology) with a unified objective.
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