Christopher F. Chyba

Voyager 2 at Neptune: Imaging Science Results

Voyager 2 images of Neptune reveal a windy planet characterized by bright clouds of methane ice suspended in an exceptionally clear atmosphere above a lower deck of hydrogen sulfide or ammonia ices. Neptunes atmosphere is dominated by a large anticyclonic storm system that has been named the Great Dark Spot (GDS). About the same size as Earth in extent, the GDS bears both many similarities and some differences to the Great Red Spot of Jupiter. Neptunes zonal wind profile is remarkably similar to that of Uranus. Neptune has three major rings at radii of 42,000, 53,000, and 63,000 kilometers. The outer ring contains three higher density arc-like segments that were apparently responsible for most of the ground-based occultation events observed during the current decade. Like the rings of Uranus, the Neptune rings are composed of very dark material; unlike that of Uranus, the Neptune system is very dusty. Six new regular satellites were found, with dark surfaces and radii ranging from 200 to 25 kilometers. All lie inside the orbit of Triton and the inner four are located within the ring system. Triton is seen to be a differentiated body, with a radius of 1350 kilometers and a density of 2.1 grams per cubic centimeter; it exhibits clear evidence of early episodes of surface melting. A now rigid crust of what is probably water ice is overlain with a brilliant coating of nitrogen frost, slightly darkened and reddened with organic polymer material. Streaks of organic polymer suggest seasonal winds strong enough to move particles of micrometer size or larger, once they become airborne. At least two active plumes were seen, carrying dark material 8 kilometers above the surface before being transported downstream by high level winds. The plumes may be driven by solar heating and the subsequent violent vaporization of subsurface nitrogen.

Energy for microbial life on Europa

The planet Jupiters moon Europa may harbour a subsurface water ocean, but estimates of the available free energy have not been encouraging for supporting life. Here I show that disequilibrium chemistry in the oceans ice cover, driven by charged particles accelerated in Jupiters magnetosphere, should produce enough organic and oxidant molecules to fuel a substantial Europan biosphere. Microbial life could exist in concentrations detectable by surface landers able to filter meltwater from Europas ice.

Terrestrial mantle siderophiles and the lunar impact record

A simple analytical fit to the lunar cratering record, scaled from final to transient crater diameters, then to impactor masses, implies a total mass ∼1.0 × 1020kg incident on the Moon subsequent to the solidification of the lunar crust ∼4.4 Gyr ago. About half this mass would be retained, and a comparable lunar mass would be eroded. These results are in good agreement with geochemical estimates of the meteoritic component mixed into the lunar crust, which give (0.4−1.5) × 1020 kg. Gravitationally scaling to Earth, and taking account of the statistical probability that the largest impactors incident on Earth were more massive than the largest incident on the Moon, gives an estimate of 1.5 × 1022 kg of material accumulated by Earth subsequent to 4.4 Gyr ago. This result is in excellent accord with geochemical estimates of post-core formation meteoritic input. These estimates, based on abundances of highly siderophile elements in the terrestrial mantle, lie in the range (1–4) × 1022 kg. The significant result is the approximate agreement of the lunar cratering record scaling with both lunar and terrestrial geochemical constraints, numerous uncertainties render exact comparisons pointless. Nevertheless, the close agreement suggests the model developed here may credibly be used to estimate exogenous volatile and prebiotic organic delivery.

Proliferation Rings: New Challenges to the Nuclear Nonproliferation Regime

of the Democratic People’s Republic of Korea (DPRK), Iran, and Pakistan provide the most visible manifestations of three broad and interrelated challenges to the nuclear nonproliferation regime. The arst is so-called latent proliferation, in which a country adheres to, or at least for some time maintains a facade of adhering to, its formal obligations under the Nuclear Nonproliferation Treaty (NPT) while nevertheless developing the capabilities needed for a nuclear weapons program.1 That country can then either withdraw from the NPT and build actual weapons on short notice, or simply stay within the NPT while maintaining the latent capability for the rapid realization of nuclear weapons as a hedge against future threats. This was the path followed by the DPRK with its plutonium program and one that is likely being followed by Iran and more subtly by others. The second broad challenge is arst-tier nuclear proliferation, in which technology or material sold or stolen from private companies or state nuclear programs assists nonnuclear weapons states in developing illegal nuclear weapons programs and delivery systems.2 The third challenge— the focus of this article—is second-tier nuclear proliferation, in which states in Proliferation Rings

Energy, Chemical Disequilibrium, and Geological Constraints on Europa

Europa is a prime target for astrobiology. The presence of a global subsurface liquid water ocean and a composition likely to contain a suite of biogenic elements make it a compelling world in the search for a second origin of life. Critical to these factors, however, may be the availability of energy for biological processes on Europa. We have examined the production and availability of oxidants and carbon-containing reductants on Europa to better understand the habitability of the subsurface ocean. Data from the Galileo Near-Infrared Mapping Spectrometer were used to constrain the surface abundance of CO(2) to 0.036% by number relative to water. Laboratory results indicate that radiolytically processed CO(2)-rich ices yield CO and H(2)CO(3); the reductants H(2)CO, CH(3)OH, and CH(4) are at most minor species. We analyzed chemical sources and sinks and concluded that the radiolytically processed surface of Europa could serve to maintain an oxidized ocean even if the surface oxidants (O(2), H(2)O(2), CO(2), SO(2), and SO(4) (2)) are delivered only once every approximately 0.5 Gyr. If delivery periods are comparable to the observed surface age (30-70 Myr), then Europas ocean could reach O(2) concentrations comparable to those found in terrestrial surface waters, even if approximately 10(9) moles yr(1) of hydrothermally delivered reductants consume most of the oxidant flux. Such an ocean would be energetically hospitable for terrestrial marine macrofauna. The availability of reductants could be the limiting factor for biologically useful chemical energy on Europa.

Comets as a Source of Prebiotic Organic Molecules for the Early Earth

Life on Earth originated during the final throes of the heavy bombardment, in which the Earth—Moon system, as well as the rest of the inner solar system, was subjected to an intense bombardment of comets and asteroids. This bombardment may have rendered the Earth’s surface inhospitable for life for hundreds of millions of years subsequent to terrestrial formation. It may also have delivered to the Earth’s surface the bulk of the current terrestrial volatile inventory, in the form of a late-accreting impact veneer. Delivering intact prebiotic organic molecules of interest for the origins of life is much more difficult. However, several mechanisms seem likely to have been delivering exogenous organics to the surface of the Earth, or shock-synthesizing them in impacts. In an early carbon dioxide-rich terrestrial atmosphere, these mechanisms would have quantitatively rivaled or exceeded terrestrial organic synthesis in situ. In an early reducing (methane-rich) atmosphere, the exogenous sources would have been quantitatively unimportant compared to atmospheric production.

Solid organic residues produced by irradiation of hydrocarbon-containing H2O and H2O/NH3 ices: Infrared spectroscopy and astronomical implications

Methane clathrate (CH4 nH2O)--expected in cometary nuclei, in outer Solar System satellites, and perhaps in interstellar grains--as well as ices prepared from other combinations of CH4, C2H6, or C2H2 with H2O (and sometimes with NH3) were irradiated at 77 degrees K by plasma discharge. CH4 clathrate and other H2O/hydrocarbon ices color and darken noticeably after a dose approximately 10(8) to approximately 10(9) erg cm-2 over a period of 1-10 hr. Upon evaporation of the now yellowish to tan irradiated ices, a colored solid film adheres to the walls of the reaction vessel at room temperature. Transmission measurements of these organic films were made from 2.5 to 50 micrometers wavelength. The residue left after CH4 nH2O irradiation exhibits IR bands which we tabulate and identify with alkane, aldehyde, alcohol, and perhaps alkene and substituted aromatic functional groups. Aldehydes are especially well indicated, and may be related to recent claims of polyoxymethylene (H2CO)n in the coma of Comet Halley. Spectra presented here are compared with previous studies of UV or proton-irradiated, nonenclathrated hydrocarbon-containing ices may be useful for interpreting infrared features found in the spectra of comets and interstellar grains.

Electrical energy sources for organic synthesis on the early earth

In 1959, Miller and Urey (Science130, 245) published their classic compilation of energy sources for indigenous prebiotic organic synthesis on the early Earth. Much contemporary origins of life research continues to employ their original estimates for terrestrial energy dissipation by lightning and coronal discharges, 2 × 1019 J yr−1 and 6 × 1019 J yr−1, respectively. However, more recent work in terrestrial lightning and point discharge research suggests that these values are overestimates by factors of about 20 and 120, respectively. Calculated concentrations of amino acids (or other prebiotic organic products) in the early terrestrial oceans due to electrical discharge sources may therefore have been equally overestimated. A review of efficiencies for those experiments that provide good analogues to naturally-occurring lightning and coronal discharges suggests that lightning energy yields for organic synthesis (nmole J−1) are about one order of magnitude higher than those for coronal discharge. Therefore organic production by lightning may be expected to have dominated that due to coronae on early Earth. Limited data available for production of nitric oxide in clouds suggests that coronal emission within clouds, a source of energy heretofore too uncertain to be included in the total coronal energy inventory, is insufficient to change this conclusion. Our recommended valves for lightning and coronal discharge dissipation rates on the early Earth are, respectively, 1 × 1018 J yr−1 and 5 × 1017 J yr−1.

Toward Biological Security

THE ANTH RAX ATTAC KS on the United States in the autumn of 2001, and the fear and confusion that followed, made clear that the country lacks a comprehensive strategy for biological security-the protection of people and agriculture against disease threats, whether from biological weapons or natural outbreaks. Too often, thinking about biological secu rity has been distorted by misplaced analogies to nuclear or chemical weapons. An effective strategy must leave these analogies largely behind and address the special challenges posed by biological threats. A strategy for biological security must confront drug-resistant and emerging diseases-more than 30 of which have entered the human population over the past quarter-century. There is no good analogue to this naturally occurring threat in the realm of nuclear or chemical weapons. Moreover, diseases may be targeted against livestock or crops as well as against human populations. And outbreaks of deadly, contagious, and long-incubating diseases such as smallpox have to be detected and stopped rapidly wherever in the world they occur. Fortunately, once formulated, a sound strategy for biological security will help sustain itself because many of its core provisions will benefit public health even apart from acts of bioterror. In fact, many of the tools used to address natural disease threats will be needed to respond to an intentional attack. The U.S. response to the anthrax attacks has emphasized the importance of improving

Applicability of Information Theory to the Quantification of Responses to Anthropogenic Noise by Southeast Alaskan Humpback Whales

We assess the effectiveness of applying information theory to the characterization and quantification of the affects of anthropogenic vessel noise on humpback whale (Megaptera novaeangliae) vocal behavior in and around Glacier Bay, Alaska. Vessel noise has the potential to interfere with the complex vocal behavior of these humpback whales which could have direct consequences on their feeding behavior and thus ultimately on their health and reproduction. Humpback whale feeding calls recorded during conditions of high vessel-generated noise and lower levels of background noise are compared for differences in acoustic structure, use, and organization using information theoretic measures. We apply information theory in a self-referential manner (i.e., orders of entropy) to quantify the changes in signaling behavior. We then compare this with the reduction in channel capacity due to noise in Glacier Bay itself treating it as a (Gaussian) noisy channel. We find that high vessel noise is associated with an increase in the rate and repetitiveness of sequential use of feeding call types in our averaged sample of humpback whale vocalizations, indicating that vessel noise may be modifying the patterns of use of feeding calls by the endangered humpback whales in Southeast Alaska. The information theoretic approach suggested herein can make a reliable quantitative measure of such relationships and may also be adapted for wider application to many species where environmental noise is thought to be a problem.