Homes Away from Home: Map II
The exploration of our solar system is founded upon the pursuit of three simple yet profound questions:Where do we come from? Where are we going? Are we alone?
This three-part series, "Homes Away from Home," highlights what is known today, and hoped to be discovered, about our solar neighborhood. The story of life elsewhere is told beginning with the very large and distant places, as they might have seeded what today is very small and near. Our solar system began to take shape about 4.6 billion years ago, as the primordial solar nebula of dust and gas began to coalesce around the infant Sun. Within the first billion years or so, the planets formed and life began to emerge on Earth -- and perhaps elsewhere.
Watering Holes in Space
In our solar system, the formative and evolutionary processes that acted on the planets made at least one of them a platform for the development of life. Was this an inevitable outcome of solar system evolution, and therefore potentially a common phenomenon, or was it merely an accident of chemistry, dynamics, and timing, unlikely to be reproduced elsewhere? And why did it happen so quickly -- the first signs of life on Earth possibly emerging just a few hundred million years after the planet cooled? Where else in our solar system were the conditions right for the formation and development of life?
|Jupiter eclipsed by relatively tiny passing moon.
The essential requirements for life are a source of usable energy and basic nutrients, organic material, and liquid water. There is strong evidence that these ingredients have been present and in contact with one another on bodies other than Earth. Water and organics appear to have been originally condensed or acquired in the outer reaches of the solar nebula where low temperatures favored their retention. Transported aboard cometary and asteroidal materials that were accreted by the planets, these essential ingredients of life were then effectively incorporated into the forming planetary environments. The planetary system we know today -- and the questions of habitability -- are thus intimately linked to the original distribution and transportation of water, other volatiles, and organic material.
Primordial Soup: Too Hot?
Most models of the solar nebula suggest that the conditions within it were too hot, at the time and place of Earth's formation, to retain the relatively large proportion of volatiles seen in the current Earth. Delivery of volatile-rich material from more distant, colder parts of the solar system is commonly invoked to explain this discrepancy. There are reasons to believe that even Jupiter received much of its volatile inventory in this way. However, the total quantity of volatile material, the relative proportion delivered from different possible sources, and the time period over which it was delivered all remain uncertain.
Today's short-period comets are the dynamical survivors of the much larger original population of comets that played a role in volatile distribution, and thus knowledge of their composition is crucial to understanding this process and its results. We know that comets delivered volatiles and organics to the inner planets, contributing to the formation of Earth's hydrosphere, atmosphere, and biosphere. Since comets spend the vast majority of time far from the Sun, their surfaces preserve accessible remnants of the primordial chemical constituents from which the entire solar system formed.
The abundance of water in the deep interior of Jupiter is a key to understanding the processes by which volatile materials were added to the planet as it formed. Water ice carried other condensed volatiles with it as planetesimals were accreted by Jupiter. However, the water abundance in Jupiter's deep atmosphere and interior remains highly uncertain, because the Galileo atmospheric probe descended in a dry, downdraft region where the water content was not representative of the planet as a whole.
Since the outer reaches of the original solar nebula were relatively cool, a variety of volatile compounds could condense from the nebular gas as the solar system formed. Of particular importance were ices containing carbon, nitrogen, and sulfur, as well as organic materials. The outer solar system was thus far richer in organic compounds, essential for prebiotic chemistry as we understand it, than was the inner solar system. Planetesimals that formed in this region probably delivered such materials to the moons of the outer planets and to the inner planets. Comets are volatile-rich and organic-rich samples from reservoirs in the outer solar system, including the Kuiper Belt beyond Neptune and the more distant Oort cloud. By determining the chemical composition of comets and Kuiper objects we can directly study chemical building blocks that may have laid the foundation for life.
Saturn's moon Titan is an organic-rich world that is of tremendous importance to our study of prebiotic chemistry. Data from Voyager, as well as from other observations and experiments, suggest that the pathways and products of long-term organic evolution on Titan may bear similarities to those that existed on the early Earth. The atmosphere and surface of Titan are a virtual time machine, presenting us with unique opportunities for studying photochemistry and chemical reactions that are no longer observable on our planet due to the pervasive effects of biology. In 2004 the Cassini-Huygens mission will initiate an intensive study of Saturn and Titan that is expected to revolutionize our understanding of complex organic chemical processes in the solar system.
Basins to Hold Hot Soup
Although similar in size, mass, and solar distance, Venus and Earth could hardly be more different. When and why did they take such divergent evolutionary paths? Venus has experienced what is sometimes called a "runaway greenhouse effect," rendering the planet hot, toxic, and lifeless. Earth's atmosphere today bears little resemblance to the atmosphere of the early Earth, in which life developed; it has been nearly reconstituted by the bacteria, vegetation, and other life forms that have acted upon it over the eons.
|Layered sediment-like remnants on Mars.
Once delivered to the planets, volatiles may be sequestered in surface and interior reservoirs, partitioned into the atmosphere, or lost to space. The volatile evolution of the three large terrestrial planets -- Earth, Venus, and Mars -- apparently took radically different paths with fundamentally different outcomes. One key means of understanding these differences is to trace the volatile history of Venus, and in particular the processes that led to the loss of the water that should originally have been present. Pioneer Venus and Venera provided some insight into the composition of the atmosphere and surface, but more detailed measurement of the chemical and isotopic composition of Venus' surface and atmosphere is required if we are to fully understand its evolutionary history.
We know that carbon dioxide on Mars is cycled between the atmosphere and the winter polar caps. At the poles there is evidence of a long history of frozen volatiles, which may preserve evidence of different climatic regimes and possibly of life-supporting environments. Current missions such as Mars Global Surveyor and Mars Odyssey are revealing striking evidence of past and present reservoirs of water on Mars -- frozen at the poles and beneath the surface today, but possibly pooled in large ponds, lakes, or oceans in the past.
Recent discoveries suggest that life's "habitable zones" are defined not just by a planet's distance from its parent star, but by a complex relationship involving external and internal energy sources, chemical inventories, and geophysical processes. The chemical building blocks of life and complex organic chemistry are known to exist throughout our solar system, and there are tantalizing hints that liquid water may be present in a few key environments. This has significantly expanded our view of the number of solar system environments that might be or might have been conducive to life. Mars is widely regarded as a planet on which the conditions for habitability could have been met.
Other recently recognized potentially habitable environments are the inferred subsurface liquid oceans on several major satellites of Jupiter, especially Europa. Considered at least a theoretical possibility for nearly three decades, the existence of global liquid layers under the icy crusts of the larger moons has received major support from the Galileo mission's exploration of the jovian system. Although the evidence for its existence is as yet indirect, there is wide acceptance that Europa does today possess a subsurface global ocean of liquid water. Although explored less intensively by Galileo, both Ganymede and Callisto show indications of subsurface structure similar to that of Europa. If the formation of oceans is found to be a common phenomenon, the implications for life in the cosmos could be stunning.
Jupiter and its moons are in many ways like a miniature solar system. The gas giant Jupiter can be thought of as a star that didn't get quite large enough to ignite, and the four largest moons -- the Galilean satellites -- possess many of the characteristics of small planets. There is striking evidence that one of them -- Europa -- and possibly Callisto and Ganymede, hide oceans of liquid water beneath their frozen surfaces. The heat source keeping these oceans liquid, if they exist, is the constant tidal flexing caused by Jupiter's intense gravitational pull. Could this internal heating also power the same types of thermal vents that are known to support life on Earth's ocean floor?
Microbial life on Earth may have come into existence nearly 4 billion years ago, shortly after the end of the most violent phase of formation of the planet. Today it thrives wherever liquid water and usable energy exist together. Microbial life forms have been discovered on Earth that can survive and even thrive at extremes of high and low temperature and pressure, and in conditions of acidity, salinity, alkalinity, and concentrations of heavy metals that would have been regarded as lethal just a few years ago. These discoveries include the wide diversity of life near sea-floor hydrothermal vent systems, where some organisms live essentially on chemical energy in the absence of sunlight.
Wrecking Balls of Fire
|Close view of asteroid.
Once a source of life-giving organics and water, cosmic impacts also have the potential to wreak widespread destruction or even to extinguish much of life -- and these events occur regularly on planetary timescales. This sobering conclusion stems from the convergence of many lines of study, from geology to astronomy to paleontology.
Evidence continues to mount that the so-called Cretaceous-Tertiary mass extinction event 65 million years ago was caused by the impact of an extraterrestrial body about 10 kilometers in diameter. It has also become apparent that even much smaller objects, which impact Earth much more frequently, are capable of doing serious damage to modern industrialized society.
Although the impactor flux has declined greatly since the early days of the solar system, it is estimated that up to 50,000 objects of diameter at least 50-100 meters still exist in orbits near Earth. Of those, up to 1,500 may have diameters of 1 kilometer or larger. An impactor at the smaller end of this size range could wipe out a city or an entire coastal region; at the upper end of this range it could cause global devastation.
Considerable progress has been made in identifying and cataloguing near-Earth objects that could potentially pose a threat to Earth, and it is estimated that about 50% of the near-Earth objects larger than 1 kilometer have now been identified. Current projects are slated to identify and track at least 90% of the near-Earth objects greater than 1 kilometer in diameter by 2008.
Map III for "Homes Away from Home" will address the future mission profiles to explore other planets.
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