AMANDA (Antarctic Muon and Neutrino Detector Array) top      see publications
With a total of 677 downlooking phototubes in a cylindrical array of radius 100 m, AMANDA is a working neutrino observatory installed in South Pole ice at depths 1500 to 2300 m. It detects neutrinos passing upward through the entire earth. AMANDA can answer fundamental questions in astrophysics (the nature of blazars and of gamma ray bursters), cosmology (the nature of the dark matter that comprises more than 90% of the mass of the universe), cosmic ray physics (origin of the highest energy cosmic rays), and particle physics (properties of neutrinos including their oscillations and their behavior in a gravitational field). The 19 strings of AMANDA optical modules, still in operation are part of the growing IceCube observatory.

IceCube Neutrino Observatory top      see publications
Congress has funded IceCube, which will be the largest detector in the world, with 80 strings each containing 60 ten-inch phototubes extending from 1450 to 2450 m depth in ice at the South Pole. The collaboration includes more than 150 scientists from universities and research institutes in the U.S., Europe, and Asia. The first string of 60 phototubes was successfully deployed on January 29, 2005. Below the lowest phototube of this string, Ryan Bay attached a dust logger we designed to read out depths of volcanic ash layers. By matching up those layers with layers to be recorded in other boreholes, we will be able to construct isochrones corresponding to the same eruptions. These contours will provide us with a three-dimensional map of optical properties of ice over the entire cubic kilometer of IceCube. As more strings are added, IceCube will take data in conjunction with the 19 strings of AMANDA detectors. IceCube and AMANDA together will form a single gigantic neutrino observatory, taking data for decades. See IceCube.karle.ppt for a recent talk by Albrecht Karle.

Glaciology top      see publications
Models of the flow of glacial ice in response to an applied stress (usually the downhill component of the gravitational force of overlying ice) are still phenomenological. Laboratory experiments have the double disadvantage that they must be carried out at higher stress than is typical in nature, and that the volume of ice is unrealistically small. Observations in nature have had the double disadvantage that they are almost always limited to temperate ice, in order to see observable flow on a reasonable time-scale, and that only the strain rate, not the stress, is observed. Dima Chirkin and undergraduate student Jeff Allen are using the AMANDA database of downward-going muons to map the three-dimensional strain rate of deep ice in a volume of ~0.02 km3. Their early result shows no significant shear strain after one year at a depth corresponding to a temperature of -29^oC. In future, the group plans to collaborate with Neil Humphrey in a long-term study of strain rate as a function of stress (to be measured with stress cells frozen into ice at various depths as IceCube is built). That experiment would instrument a cubic kilometer of ice at temperatures from -50^o to -20^o. Spinoffs of IceCube for glaciology include use of dust loggers to measure age vs depth and to map wind speed in the past 100,000 years.

Acoustic Detection of Neutrino-induced Cascades top      see publications
Despite its huge size, IceCube will not be large enough to detect the extremely low flux of neutrinos with ultrahigh energies (≤ 10¹⁷ eV) predicted to come from cosmic ray interactions with the cosmic background radiation and from hypothetical processes in the early universe. A relatively inexpensive way to search for ultrahigh-energy-produced cascades is to look for hydrodynamic waves that result from conversion of ionization energy into heat. Price has calculated the scattering and absorption of acoustic waves resulting from ultrahigh-energy neutrino interactions at various depths in South Pole ice. Graduate student Justin Vandenbroucke has participated in design and construction of three test strings of acoustic modules that will be deployed at depths down to ~400 m in three of the upcoming IceCube boreholes and will measure frequency-dependent acoustic noise and make the first estimates of acoustic attenuation in the ice. We are also participating in plans for a future hybrid array consisting of the IceCube optical array, a radio array, and an acoustic array.

Optical Properties of Glacial Ice top      see publications

Our group was the first to recognize that scattering and absorption by dust and volcanic ash are responsible for the optical properties of glacial ice at wavelengths 200 ≤ λ ≤ 500 nm where Cherenkov radiation from muons and cascades is most effective. Initially we and our colleagues mapped optical properties of South Pole ice as a function of depth and wavelength using light emitters on strings in one borehole and phototubes on strings in other boreholes. With dust loggers in strings 21 and 50 of IceCube, we have measured variations in optical properties with a resolution of ~1 cm at those two locations and will use a reusable logger in several additional holes in order to produce a 3-dimensional map of optical properties in IceCube

Paleoclimatology and Volcanology top      see publications
The dust logger has been a fantastic success. At the 1000-meter borehole at Siple Dome, Antarctica, the dust logger produced a record of air bubbles, of glacial and interglacial dust concentrations, and of volcanic ash layers as thin as a few mm in vertical extent. Several abrupt jumps in the dust record signify abrupt climate changes in the region around Siple Dome. The dust log of the depths of occurrence of volcanic ash has made it possible for volcanologists to find those same layers in ice cores for chemical and isotopic study, even when they are almost invisible to the eye. At the 3054-meter borehole at Summit, Greenland, both the top half, containing air bubbles, and the bottom half, containing a superb record of dust concentrations, provide paleoclimate information that is immune to fractures of the segments of ice cores removed from the borehole. Dramatic changes in climate occurring within a few decades are recorded in the dust record. In the Price group, Ryan Bay has found evidence for correlations of the dust and volcanic record between the southern and northern polar regions. He has proposed a mechanism by which volcanoes emit particles rich in soluble iron which provide critical nutrients for phytoplankton in the southern oceans; they grow rapidly and extract carbon dioxide from the atmosphere, thus reducing this major greenhouse gas and triggering global cooling.

Bay has now included ice core records of sulfate emissions from volcanoes with dust logger records and has shown that: the record of volcanism at Siple Dome, Vostok Station, and Dome C in Antarctica correlates with abrupt climate change in glacial periods at a 95% to 99.8% significance level; that the volcanic sequences in the two hemispheres match at levels > 3 sigma; and that these candidate global events were associated with abrupt cooling, often simultaneous with onsets or sudden intensifications of millennial cold periods. He and graduate student Robert Rohde have measured age vs depth over the last 120,000 years in South Pole ice and have used differences in centimeter-resolution dust logs in two IceCube strings to estimate wind speed over more than 40,000 years.

Life in Extreme Environments top      see publications
In A Habitat for Psychrophiles in Deep Antarctic Ice, Price showed that suitably adapted microbial life can exist in ice at temperatures down to about -90 degrees Celsius, depending on the presence of acids or salts that are insoluble in the solid ice and that form liquid phases with low eutectic temperatures. Within a network of ion-rich liquid veins at the triple junctions of ice grains, microbes can move, extract energy via reduction-oxidation reactions of the ions, and utilize elements necessary for life. The metabolic rate is such a strong function of temperature that the sparse nutrients present in veins can sustain a fairly large population of microbes, nearly dormant, for hundreds of thousands of years.

In a PNAS paper, Price and Todd Sowers studied the temperature dependence of metabolic rates for microbial growth, maintenance, and survival. On an Arrhenius plot of log rate vs 1/T (in Kelvins), we showed that metabolic rates per cell of microbial communities fall into three groupings: a rate for unlimited exponential growth; a far lower rate for maintenance of community size; and a still lower rate for immobilized communities to barely survive, using only enough energy to repair macromolecular damage. The temperature dependences of the three groupings were characterized by similar activation energies, with values at a given temperature in the ratio ~10⁶:10³:1. The rate continued to follow the same Arrhenius lines even down to -40ºC with no indication of a cutoff. The rate for repairing damage by means of DNA-repair enzymes and protein-repair enzymes is comparable to the rate of spontaneous molecular damage, from which we inferred that a microbial community immobilized in shale or ice may survive for geologic time.

In our study of microbes as a function of depth in the 3043-m Greenland ice core, we have mapped the vertical distribution of methanogens, using their F420-autofluorescence, and the distribution of all cells, using a Syto23 stain. At three depths where methane concentrations in the ice were a factor 10 above normal, we found huge excesses of methanogens and of all cells. This work provided evidence that methanogens might account for the excess methane on Mars.

Biospectrologger top      see publications
Graduate student Nathan Bramall has developed a series of biologgers including a miniaturized one that will fit into a 5-cm borehole in ice or rock. Using a 224-nm laser to excite fluorescence of tryptophan (an amino acid present in all biological cells), the device will map microbial concentrations down to 1 cell cm^-3. Using a 404-nm laser, the device can map the fluorophor F420 (a signature of methanogens) and chlorophyll. Graduate student Robert Rohde, with Price and Bay, have used a portable biologger to study microbes in ice cores at the National Ice Core Laboratory. They have discovered sharp, localized spikes of tryptophan in concentration at depths where there are abrupt increases in methane and nitrous oxide, and also in volcanic ash bands and in liquid veins. We have demonstrated by direct counts of microbial cells that methanogens account for the excess methane, and we next will look for excesses of microbes in the regions rich in nitrous oxide and volcanic ash. An advantage of the portable biologger is that it can rapidly scan many meters of ice, pinpointing microbe-rich regions that can be later studied by biologists.

Sizing of individual Bacillus spores top      see publications
Andrew Westphal and Price have used automated scanning microscopy to study various species of Bacillus, which sporulate (convert to spores) when conditions become hostile. In the first set of experiments, we found that Bacillus thuringiensis spores, despite being dormant, rapidly expand or shrink in size in response to an abrupt change in humidity. In followup work, we found that spores of different species of Bacillus have slightly different sizes and swelling amplitudes. On a plot of size at 0% relative humidity vs increase in size for an increase from 0% to 100% relative humidity, the species cluster in different regions of the plot. These encouraging results led us to suggest that automated measurements may some day make possible the identification of Bacillus anthracis in dust from air or from a central mail room.

Biographies top      see publications
The National Academy of Sciences publishes an annual volume of biographies of deceased members. See the biographies of John Reynolds, a former member of the Berkeley Physics Department, and of Bob Walker, a physicist with whom I collaborated in the 1960s.

Opening lectures at international conferences on nuclear tracks in solids top      see publications
Price is a co-founder of the technique of nuclear tracks in solids. He often gives the opening lecture and summary rermarks at the biennial international conferences on applications of nuclear tracks in solids.