This spirited history of public television offers an insider's
account of its topsy-turvy forty-year odyssey. James Day, a founder
of San Francisco's KQED and a past president of New York's WNET,
...provides a vivid and often amusing behind-the-screens history. Day
tells how a program producer, desperate to locate a family willing
to live with television cameras for seven months, borrowed a
dime-and a suggestion-from a blind date and telephoned the Louds of
Santa Barbara. The result was the mesmerizing twelve-hour
documentary An American Family. Day relates how Big Bird
and his friends were created to spice up Sesame Street when test
runs showed a flagging interest in the program's "live-action"
segments. And he describes how Frieda Hennock, the first woman
appointed to the FCC, overpowered the resistance of her male
colleagues to lay the foundation for public television. Day
identifies the particular forces that have shaped public television
and produced a Byzantine bureaucracy kept on a leash by an
untrusting Congress, with a fragmented leadership that lacks a
clearly defined mission in today's multimedia environment. Day
calls for a bold rethinking of public television's mission,
advocating a system that is adequately funded, independent of
government, and capable of countering commercial television's
"lowest-common-denominator" approach with a full range of
substantive programs, comedy as well as culture, entertainment as
well as information. This title is part of UC Press's Voices
Revived program, which commemorates University of California
Press's mission to seek out and cultivate the brightest minds and
give them voice, reach, and impact. Drawing on a backlist dating to
1893, Voices Revived makes high-quality, peer-reviewed scholarship
accessible once again using print-on-demand technology. This title
was originally published in 1995.
Volatile elements have a fundamental role in the evolution of planets. But how budgets of volatiles were set in planets, and the nature and extent of volatile-depletion of planetary bodies during the ...earliest stages of Solar System formation remain poorly understood. The Moon is considered to be volatile-depleted and so it has been predicted that volatile loss should have fractionated stable isotopes of moderately volatile elements. One such element, zinc, exhibits strong isotopic fractionation during volatilization in planetary rocks, but is hardly fractionated during terrestrial igneous processes, making it a powerful tracer of the volatile histories of planets. Here we present high-precision zinc isotopic and abundance data which show that lunar magmatic rocks are enriched in the heavy isotopes of zinc and have lower zinc concentrations than terrestrial or Martian igneous rocks. Conversely, Earth and Mars have broadly chondritic zinc isotopic compositions. We show that these variations represent large-scale evaporation of zinc, most probably in the aftermath of the Moon-forming event, rather than small-scale evaporation processes during volcanism. Our results therefore represent evidence for volatile depletion of the Moon through evaporation, and are consistent with a giant impact origin for the Earth and Moon.
Stochastic Late Accretion to Earth, the Moon, and Mars Bottke, William F; Walker, Richard J; Day, James M.D ...
Science (American Association for the Advancement of Science),
12/2010, Volume:
330, Issue:
6010
Journal Article
Peer reviewed
Core formation should have stripped the terrestrial, lunar, and martian mantles of highly siderophile elements (HSEs). Instead, each world has disparate, yet elevated HSE abundances. Late accretion ...may offer a solution, provided that ≥0.5% Earth masses of broadly chondritic planetesimals reach Earth's mantle and that approximately 10 and approximately 1200 times less mass goes to Mars and the Moon, respectively. We show that leftover planetesimal populations dominated by massive projectiles can explain these additions, with our inferred size distribution matching those derived from the inner asteroid belt, ancient martian impact basins, and planetary accretion models. The largest late terrestrial impactors, at 2500 to 3000 kilometers in diameter, potentially modified Earth's obliquity by approximately 10°, whereas those for the Moon, at approximately 250 to 300 kilometers, may have delivered water to its mantle.
The geochemistry of Martian meteorites provides a wealth of information about the solid planet and the surface and atmospheric processes that occurred on Mars. The degree to which Martian magmas may ...have assimilated crustal material, thus altering the geochemical signatures acquired from their mantle sources, is unclear. This issue features prominently in efforts to understand whether the source of light rare-earth elements in enriched shergottites lies in crustal material incorporated into melts or in mixing between enriched and depleted mantle reservoirs. Sulphur isotope systematics offer insight into some aspects of crustal assimilation. The presence of igneous sulphides in Martian meteorites with sulphur isotope signatures indicative of mass-independent fractionation suggests the assimilation of sulphur both during passage of magmas through the crust of Mars and at sites of emplacement. Here we report isotopic analyses of 40 Martian meteorites that represent more than half of the distinct known Martian meteorites, including 30 shergottites (28 plus 2 pairs, where pairs are separate fragments of a single meteorite), 8 nakhlites (5 plus 3 pairs), Allan Hills 84001 and Chassigny. Our data provide strong evidence that assimilation of sulphur into Martian magmas was a common occurrence throughout much of the planet's history. The signature of mass-independent fractionation observed also indicates that the atmospheric imprint of photochemical processing preserved in Martian meteoritic sulphide and sulphate is distinct from that observed in terrestrial analogues, suggesting fundamental differences between the dominant sulphur chemistry in the atmosphere of Mars and that in the atmosphere of Earth.
Low estimated lunar volatile contents, compared with Earth, are a fundamental observation for Earth-Moon system formation and lunar evolution. Here we present zinc isotope and abundance data for ...lunar crustal rocks to constrain the abundance of volatiles during the final stages of lunar differentiation. We find that ferroan anorthosites are isotopically heterogeneous, with some samples exhibiting high δ(66)Zn, along with alkali and magnesian suite samples. Since the plutonic samples were formed in the lunar crust, they were not subjected to degassing into vacuum. Instead, their compositions are consistent with enrichment of the silicate portions of the Moon in the heavier Zn isotopes. Because of the difference in δ(66)Zn between bulk silicate Earth and lunar basalts and crustal rocks, the volatile loss likely occurred in two stages: during the proto-lunar disk stage, where a fraction of lunar volatiles accreted onto Earth, and from degassing of a differentiating lunar magma ocean, implying the possibility of isolated, volatile-rich regions in the Moon's interior.
Basaltic lavas erupted at some oceanic intraplate hotspot volcanoes are thought to sample ancient subducted crustal materials. However, the residence time of these subducted materials in the mantle ...is uncertain and model-dependent, and compelling evidence for their return to the surface in regions of mantle upwelling beneath hotspots is lacking. Here we report anomalous sulphur isotope signatures indicating mass-independent fractionation (MIF) in olivine-hosted sulphides from 20-million-year-old ocean island basalts from Mangaia, Cook Islands (Polynesia), which have been suggested to sample recycled oceanic crust. Terrestrial MIF sulphur isotope signatures (in which the amount of fractionation does not scale in proportion with the difference in the masses of the isotopes) were generated exclusively through atmospheric photochemical reactions until about 2.45 billion years ago. Therefore, the discovery of MIF sulphur in these young plume lavas suggests that sulphur--probably derived from hydrothermally altered oceanic crust--was subducted into the mantle before 2.45 billion years ago and recycled into the mantle source of Mangaia lavas. These new data provide evidence for ancient materials, with negative Δ(33)S values, in the mantle source for Mangaia lavas. Our data also complement evidence for recycling of the sulphur content of ancient sedimentary materials to the subcontinental lithospheric mantle that has been identified in diamond-hosted sulphide inclusions. This Archaean age for recycled oceanic crust also provides key constraints on the length of time that subducted crustal material can survive in the mantle, and on the timescales of mantle convection from subduction to upwelling beneath hotspots.
New tungsten isotope data for modern ocean island basalts (OIB) from Hawaii, Samoa, and Iceland reveal variable 182W/184W, ranging from that of the ambient upper mantle to ratios as much as 18 parts ...per million lower. The tungsten isotopic data negatively correlate with ³He/⁴He. These data indicate that each OIB system accesses domains within Earth that formed within the first 60 million years of solar system history. Combined isotopic and chemical characteristics projected for these ancient domains indicate that they contain metal and are repositories of noble gases. We suggest that the most likely source candidates are mega–ultralow-velocity zones, which lie beneath Hawaii, Samoa, and Iceland but not beneath hot spots whose OIB yield normal 182W and homogeneously low ³He/⁴He.
The Moon is depleted in volatile elements relative to the Earth and Mars. Low abundances of volatile elements, fractionated stable isotope ratios of S, Cl, K and Zn, high μ (238U/204Pb) and long-term ...Rb/Sr depletion are distinguishing features of the Moon, relative to the Earth. These geochemical characteristics indicate both inheritance of volatile-depleted materials that formed the Moon and planets and subsequent evaporative loss of volatile elements that occurred during lunar formation and differentiation. Models of volatile loss through localized eruptive degassing are not consistent with the available S, Cl, Zn and K isotopes and abundance data for the Moon. The most probable cause of volatile depletion is global-scale evaporation resulting from a giant impact or a magma ocean phase where inefficient volatile loss during magmatic convection led to the present distribution of volatile elements within mantle and crustal reservoirs. Problems exist for models of planetary volatile depletion following giant impact. Most critically, in this model, the volatile loss requires preferential delivery and retention of late-accreted volatiles to the Earth compared with the Moon. Different proportions of late-accreted mass are computed to explain present-day distributions of volatile and moderately volatile elements (e.g. Pb, Zn; 5 to >10%) relative to highly siderophile elements (approx. 0.5%) for the Earth. Models of early magma ocean phases may be more effective in explaining the volatile loss. Basaltic materials (e.g. eucrites and angrites) from highly differentiated airless asteroids are volatile-depleted, like the Moon, whereas the Earth and Mars have proportionally greater volatile contents. Parent-body size and the existence of early atmospheres are therefore likely to represent fundamental controls on planetary volatile retention or loss.
Noble gas isotope systematics of ocean island basalts (OIB) provide evidence for relatively undegassed and primitive mantle sources. These OIB sources partly derive from the deep mantle by virtue of ...their distinctiveness from mid-ocean ridge basalts (MORB), which dominantly sample upper mantle. New helium, neon and argon isotope data are presented for Canary Islands lavas, carbonatites and cumulate and mantle xenoliths confirming 3He/4He ratios that are the same or lower than MORB, but that are heterogeneous (∼3–9.5RA) within and between islands in the archipelago. Neon and Ar isotope systematics for lavas are mostly within the range of air compositions. Harzburgite xenoliths from Lanzarote, which are interpreted to represent ancient refractory mantle residues, have distinct He-Ne-Ar isotope systematics from lavas or cumulate xenoliths. The harzburgites are characterized by forsteritic olivine (>Fo91) with uniformly low-3He/4He (6.6 ± 0.2RA), high 40Ar/36Ar (630–4900), and have Ne isotope compositions that range between air values or that are similar or more nucleogenic than depleted MORB mantle (DMM). Similar refractory mantle peridotites have been discovered as xenoliths at other OIB localities and are likely to be distinct from continental lithospheric mantle. Refractory mantle (RM), which by virtue of its high melting temperature is difficult to partially melt, may be a significant component in the convecting mantle and has potential to impart a cryptic noble gas signature to partial melts in intraplate, divergent and convergent margin settings. In this sense, RM may represent a ‘sixth mantle component’ after DMM, high-µ (high 238U/204Pb; HIMU), enriched mantle endmembers (EMI, EMII) and the ‘focus zone’ (FOZO). An RM-type component may be presented in the older eastern Canary Islands and possibly in recent rejuvenated volcanism from Teide (Tenerife). Canary Island intraplate volcanism samples multiple mantle components, confirming that the most gas-rich mantle sources involved in magmatism dominate OIB noble gas compositions.