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Origins Program Frequently Asked Questions

 Understanding Origins

Origins Science Roadmap


What role did gravity play in the emergence of galaxies from the almost perfectly smooth particle sea of the early universe?

Astronomers have formulated a paradigm for how gravity, acting on weak ripples in an otherwise smooth distribution of matter left by the Big Bang, acted in a hierarchical fashion to build larger and larger units. Discrete masses the size of modern galaxies began to appear somewhere between 100 million and 1 billion years after the Big Bang; soon after came the first of generations of stars that would produce the heavy chemical elements necessary for the formation of planets and the existence of life. Our best guess is that the ordinary "atomic" matter that makes up stars, planets, and people is only a small fraction of the matter in the universe. The majority, perhaps a sea of "non-baryonic," exotic particles, is likely to have played the key role in assembling the first galaxy-sized masses. Aspects of this picture have already been confirmed. The first evidence of ripples in the early matter distribution has been detected by the COBE satellite and ground-based radio telescopes. Observations of the earliest galactic-sized stellar systems have been pushed back to within 2-3 billion years of the Big Bang. (Origins Science Roadmap)

Does the birth and aging of a galaxy influence the chemical composition that is available to stars, planets, and living organisms?

It is entirely likely that the binding together of galactic-sized units was a necessary step in building the abundance of heavy elements to the point where the formation of Earth-like worlds and life as we know it became possible. Ices from the C-N-O family of elements and rocks made of silicon, magnesium, and iron group elements are the raw materials for planet formation, and life, at least as we know it, depends critically on the complex chemistry of organic matter compounds built around carbon atoms. We have learned that the lightest chemical elements were synthesized in the Big Bang, but that the heavier elements were made in stars. When the more massive stars exploded as supernovae, they enriched the material out of which subsequent generations of stars would be made with an ever-increasing amount of these heavy elements. The manner in which stars produce new elements from old is one of the great triumphs of science during this century. However, we know relatively little of the overall enrichment process for gaseous material in the universe. (Origins Science Roadmap)

Are planetary systems forming around young stars today?

Observations at infrared and longer wavelengths have done much in the past two decades to reveal the stages that interstellar gas and dust pass through along the way to forming stars and planetary systems. We now have observational evidence relating to most of the stages of star formation: dense cores that are on the verge of gravitational collapse in molecular clouds, newly-formed protostars characterized by continuing infall of gas and dust, young stellar objects associated with strong, bipolar winds suggesting that they are still accreting mass from the disks that surround them. Finally, we have observations of pre-main-sequence stars with evidence of lingering circumstellar disks. Such disks are observed in association with most, if not all, young stars. Disk properties can be deduced from observations at infrared to millimeter wavelengths, and millimeter-wave interferometers are beginning to probe their structure on scales relevant to planetary system formation. (Origins Science Roadmap)

 

Are there life-sustaining planets revolving around other stars in the solar neighborhood?

Research during the latter half of this century has led to the development of a generally accepted concept of how stars and planetary systems form. The concept suggests that objects similar in mass and composition to Earth may exist in many, perhaps most, planetary systems. Detection of such planets is a key outcome of the search for planetary systems in general. However, in order to ascertain whether any of these planets may be life-sustaining, we need to be able to investigate the composition of their atmospheres. Liquid water is a basic requirement for life as we know it, and it is the key indicator that will be used to determine whether planets revolving around other stars may be life-sustaining. These issues will only be addressed when we have conducted a survey of a statistically significant number of nearby stars for the presence of planetary systems and observed them sufficiently to infer the masses and orbital properties of the planets in those systems. (Origins Science Roadmap)

How are scientists using knowledge of life on Earth to develop a theory for the origin of life in general?

Recent evidence indicates that life began at least 3.85 billion years ago, soon after the violent period of planetary accretion. What were the first microorganisms and their habitat like, and how did they arise from inanimate matter? Insight into the answers to these questions will come from a reconstruction of the first billion years of Earth history. Progress toward that objective has already been made during the past 30 years, and new knowledge can be gained on several fronts. Contributions will come from advances in theoretical modeling of how Earth's surface environment changed over time. These advances, coupled with an environmental history derived from investigations of the earliest geological record, will provide a self-consistent model of the planet during this seminal epoch in its history. It is within this environmental context that pathways for the prebiotic synthesis of biomolecules and the origin of living systems can be formulated and assessed most fruitfully. Together, studies of the paleontology and phylogeny of Earth's earliest biosphere can provide new knowledge of the timing and sequence of evolutionary milestones in biology. In turn, this chronology holds promise of opening a perspective, backward in time, on the nature of Earth's earliest organisms and their habitat. (Origins Science Roadmap)

Did life arise elsewhere in the solar system and beyond and does it exist there at the present time?

Microbial life forms on Earth are found in acid-rich hot springs, alkaline-rich soda lakes and saturated salt beds. Additionally, microbial life has been found in the Antarctic living in rocks and at the bottoms of perennially ice-covered lakes. It is found in deep sea hydrothermal vents at temperatures of up to 120oC. Recently, bacteria have been discovered in deep (1 km) subsurface ecosystems deriving energy from basalt weathering. Some microorganisms survive ultraviolet radiation, while others tolerate extreme starvation, low nutrient levels, and low water activity. Surprisingly, spore-forming bacteria have been revived from the stomachs of wasps entombed in amber dated at 25-40 million years old. Clearly, life is remarkably diverse, tenacious, and adaptable to extreme environments. (Origins Science Roadmap)

Some of these environments are similar to past or even possibly present environments on other planets in the solar system. There were probably hydrothermal systems and ice-covered lakes on Mars in the past as well as subsurface aquifers there today. Studies of the icy satellite Europa by the ongoing Galileo mission have provided tantalizing, but unproven evidence of a substantial liquid-water ocean beneath that ice. The existence of life on other planets in the solar system remains an open question, waiting to be addressed by further exploration. These prospects are dramatically high-lighted by the recent claim of evidence for life in the Martian meteorite ALH84001. The profound implications of life on Mars warrant an extraordinary effort to assess the validity of the evidence for life in this and other Martian meteorites.

How do supernovae explosions contribute to the chemical evolution of the universe?

All elements heavier than boron are thought to be produced in the nuclear furnaces that power stars, or in supernovae, the explosive events that mark the end of many stars' lives. These explosions, along with the ejection of novae shells and the slow winds from evolved stars, send newly made heavy elements into the interstellar and intergalactic medium where they become the raw materials for new stars, planetary systems, and perhaps life. The conflagration of white dwarf cores in binary systems Type 1a supernovae are important producers of Fe peak elements and are superb distance mileposts.

The creation and dispersal of these heavy elements can be measured by observing the rates of supernovae (which should be observable to redshifts of z > 10, well within a billion years of the Big Bang), the strengths of spectral features around ancient star-forming regions, and in the integrated galactic light of established stars at redshifts z > 2. By comparing these data with the expected elemental yields from the stars producing the ultraviolet and far-infrared emission, also potentially observable to epochs beyond z > 10, we can reconstruct a coherent and consistent chronology of the formation and release of heavy elements in the universe. (Origins Science Roadmap)

What is baryonic matter and what role does it play in the evolution of the universe?

Current observations of the abundance of the light elements (H, D, He, and Li) indicate that our universe is one in which baryons matter made with the building blocks of protons, and neutrons contribute only a small fraction of the matter that is needed to halt the expansion of the universe. Spectroscopic observations of deuterium abundances in the Lyman-alpha forest of absorption lines, by the Keck telescope and the Hubble Space Telescope (HST), suggest that most of the baryons in the epoch 1 < z < 4 were still in the form of ionized clouds. Today, the inventory of baryonic matter appears seriously incomplete; most of it does not appear in either the stars or gas now residing in galaxies.

This lack of observational evidence has led us to theorize on many aspects: Was a significant fraction of baryons locked up in the formation of long-lived, low-mass stars or brown dwarfs? Are the black holes that reside in centers of active galactic nuclei perhaps just the most massive examples of a more numerous population? How much of the baryonic matter is hidden in the low-luminosity, low-density galaxies that are difficult to detect against the bright-sky background imposed on Earthbound telescopic observations? Is most baryonic matter still in the form of hot gas, as in the centers of clusters of galaxies, or cold molecular hydrogen, easily detected in galaxies with active star formation, but almost "invisible" in quiescent galaxies? What is the ionization history of the unclustered gas? New observational techniques and a refined cosmological model established by further research on the cosmic microwave background promise to complete our understanding of the fate of baryonic matter. (Origins Science Roadmap)

What wavelengths are best for trying to observe baryonic matter?

Probing the baryonic content of the universe as it evolves requires a broad imaging and spectroscopic capability from the ultraviolet to the infrared. For objects like brown dwarfs and dwarf galaxies, sensitive imaging in the infrared with moderate spatial resolution (D ~ 0.15") over wide fields of view (many square arcminutes) will be needed. Studies of black holes will benefit from even better spatial resolution, but only modest spectral resolution (R ~ 1000) is required.

Tracing the gas content of the universe requires spectroscopic observations over a wide wavelength range, from the ultraviolet to the infrared, with low spatial resolution (D ~ 1"); both the ultraviolet and thermal infrared require space missions. Modest spectral resolution (R ~ 1000) is required for studying emission from gas clouds, but R 2 x 104 is needed for many studies of absorbing gas, like the Lyman-alpha clouds that we observe in the light from distant quasars. Hot baryonic gas surrounding galaxies and in clusters is best studied in the 1-10 kev X-ray band this continues to be one of the best methods of studying the distribution of all matter, baryonic and non-baryonic. High spatial resolution observations in the millimeter band could also reveal the gas conditions in the vicinity of dust-enshrouded, star-forming regions in the early universe. (Origins Science Roadmap)

How is current galaxy structure related to very early galaxies and the history of galaxy formation?

How do galaxies form and grow? Do they form hierarchically through a process of agglomeration of similar units, or do they form through a large-scale gravitational instability? If the former is the case, the diversity in morphological types of galaxies, and their luminosities, mass, and size, would be a consequence of the stage in the agglomeration process a given galaxy stopped growing. If the latter is the case, this diversity must be a reflection of widely varied initial conditions at the onset of the gravitational instability.

The answer to the question of galaxy formation requires measurement of the luminosity distribution and structure of galaxies, through cosmic time and back to the emergence of galaxy-size units in the early universe. Because of the need to look far back in time, and therefore to significant redshifts, the answer will be found in visible and near-to-mid infrared high sensitivity, high spatial resolution (D ~ 0.1") observations. (Origins Science Roadmap)

How and where in the solar neighborhood should one look for other life-sustaining planets?

We must conduct an extensive census of mature systems to determine the orbital characteristics and gross physical properties of extrasolar planets. In particular, we need to make a complete inventory of all nearby stars to find all Jovian-mass planets, and as many Neptune-like (ice-rock) planets as possible. This will require radial velocity precision of 1 m/s or better, astrometric precision of 50 microarcseconds, and the ability to image planets a few tenths of an arcsecond from a star. The planets will be about a factor 109 fainter than the stars in reflected visible light and a factor of 106 fainter in thermal infrared emission. A survey of carefully selected samples of more distant stars will reveal the dependence of planet formation on stellar properties such as mass, heavy element abundance, angular momentum, and magnetic field, and may well tell us when in the history of the galaxy the earliest planets formed. Determination of the frequency of Earth-like planets, and whether they orbit within the habitable zone of their central star, is of particular importance. While this is beyond the capability of radial velocity techniques, it can be achieved on nearby stars by precise photometry, and on even more distant stars by gravitational microlensing.
(Origins Science Roadmap)

What are the minimum requirements of planetary atmospheres that might either support life or indicate the presence of life?

According to our present, limited, understanding, the minimum prerequisites for life are liquid water, certain elements, and a source of energy to drive complex chemical reactions. Beyond our solar system, the search for such conditions is limited to examining the range of distances around nearby stars called the "habitable zone," where the surface temperature on a planet can support liquid water with appropriate atmospheric pressures. The spectral signatures of molecules can be used to infer the composition and physical conditions in the atmospheres of planets. The mid infrared wavelengths (7-17 µm) contain strong spectral features of key gases, including CO2, H2O, and O3, that can be detected with modest spectral resolution. The distance of the planet from its parent star, its total infrared brightness and temperature, and the strength of the CO2 and H2O lines together can provide a rich characterization of a planet's surface conditions.

Also, the presence of large quantities of O2 in a planetary atmosphere would be an indicator of life, since, in normal chemical equilibrium, O2 is quickly incorporated into other molecules. However, while O2 is almost impossible to detect spectroscopically, O3 is a good proxy for O2. Detection of O3 in a dense, warm atmosphere would be a strong, but not certain indicator of a planet inhabited by aerobic life. Finally, there are a variety of molecules, more difficult to detect than CO2, H2O, and O3, that would provide still more certain indicators of life. Detection of CH4 and NO2 is an important long range goal for Origins, just as or potentially more important than the imaging of planets themselves regarding the question of life on these worlds. (Origins Science Roadmap)

How did the early history of the Earth contribute to the creation of life as we know it?

As a consequence of advances in modeling planet formation processes, Earth is now thought to have formed hot, rather than cold, with core formation occurring during accretion. The atmosphere and oceans formed early, and although the climate was probably hot early on, the variation of surface temperature over time is largely unknown. Depending on the redox state of the mantle and that of impacting bodies, the atmosphere could have been weakly or strongly reducing overall, with perturbations caused by impacts. Because we now know that prebiotic synthesis of organic compounds is difficult in the former case, the composition of the atmosphere prior to the origin of life remains an important unresolved issue. (Origins Science Roadmap)

Accretionary impacts could have constrained the time and place of origin of the first sustainable ecosystems. Evidence exists that asteroids and comets would have contributed organic compounds to the prebiotic inventory, but how much and in what forms remains uncertain. The discovery of submarine hydrothermal systems and their role in recycling oceans through the crust suggests they controlled the composition and, therefore, the prebiotic chemistry of seawater early on. Consequently, knowledge of early seawater composition is critical. The short time scale for such recycling could have placed bounds on how much time was available for prebiotic evolution in seawater. Instead of hundreds of millions of years, it could have been as little as tens of millions of years.

The site of the origin of life remains problematic, but life could have arisen in hydrothermal systems where it would have been least affected by impacts. These implications are currently reshaping theories of the origin of life and pointing to new directions for study. Despite the progress already made, most of the earliest history of the planet remains unknown. Additional knowledge will come from theoretical modeling of early planetary processes and studies of the geological record tracing the history of habitable environments backward in time. Answers to key questions would have a strong bearing on the future development of theory.  (Origins Science Roadmap)



The Origins Education Forum is the central node for the education and outreach activities of the Origins Theme of NASA's Office of Space Science. The Origins Education Forum is managed for NASA by the Space Telescope Science Institute.

origins@stsci.edu
Last Modified: March 28, 2000