Planetary ices

Support for this research was provided by NASA grant entitled Experimental Investigation of the RJieologies of Planetary Ices . The authors thank J. Pinkston (USGS, Menlo Park) for his technical support. 

Nellis WJ, Hamilton D C, Holmes N C, Radousky H B, Ree F H, Mitchell A C and Nicol M 1988 The nature of the interior of Uranus based on studies of planetary ices at high dynamic pressure Science 240 779... 

Another family of feedbacks arises because the radical differences in the albedo (reflectivity) of ice, snow, and clouds compared to the rest of the planetary surface, which causes a loss of the absorption of solar radiation and thereby cools the planet. Indeed, the high albedo of snow and ice cover may be a factor that hastens the transition into ice ages once they have been initiated. Of course, the opposite holds due to decreasing albedo at the end of an ice age. As simple as this concept may appear to be, the cloud-albedo feedback is not easy to quantify because clouds reflect solar radiation (albedo effect) but absorb... 

The Planetary Energy Balance [3] of Incoming Solar (340 W/m2) minus Reflected (101 W/m2) minus Radiated (238 W/m2) = 1 W/m2. This excess energy warms the oceans and melts glaciers and ice sheets. The GHG component is 2 W/m2. The amount of heat required to melt enough ice to raise sea level 1 m is about 12 Watt-years (averaged over the planet)—energy that could be accumulated in 12 years if the planet is out of balance by 1 W/m2 per year. 

They contain, by weight, only about 15-20% hydrogen and helium. The greater part of the planetary mass consists of rocky material and water ice (a mixture 0fH2O,NH3 and CII4). 

Significant economies of computation are possible in systems that consist of a one-dimensional chain of identical reservoirs. Chapter 7 describes such a system in which there is just one dependent variable. An illustrative example is the climate system and the calculation of zonally averaged temperature as a function of latitude in an energy balance climate model. In such a model, the surface temperature depends on the balance among solar radiation absorbed, planetary radiation emitted to space, and the transport of energy between latitudes. I present routines that calculate the absorption and reflection of incident solar radiation and the emission of long-wave planetary radiation. I show how much of the computational work can be avoided in a system like this because each reservoir is coupled only to its adjacent reservoirs. I use the simulation to explore the sensitivity of seasonally varying temperatures to such aspects of the climate system as snow and ice cover, cloud cover, amount of carbon dioxide in the atmosphere, and land distribution. 

Fireman, E. L., Carbon-14 Dating of Antarctic Meteorites and Antarctic Ice (Abstract), In Lunar and Planetary Science XI, Lunar and Planetary Institute, Houston, 1980, 288-290. 

Such a measurement can tell us about the chemical evolution of oxygen, such as whether the isotopes differentiated via a thermal cycle in which lighter leO fractionates from the heavier lsO, much as Vostok ice-core oxygen ratios reveal the Earth s prehistoric climate. From this fixed point of the Sun s oxygen ratios, we can then trace the history of water in other planetary bodies since their birth in the solar nebulae through the subsequent cometary bombardment [13]. In NASA s search for water on the Moon, important for the establishment of a future Moon base, such isotopic ratios will determine whether the water is a vast mother lode or just a recent cometary impact residue. 

Pluto and some moons of the giant planets contain considerable amounts of ices and deserve special mention. In some cases they even exhibit active or recent processes that liberate liquids and gases derived from ices, suggesting a tentative link with cometary activity. These bodies, which are too large to be called planetesimals, include former KBOs now relocated into the planetary region, as well as objects that probably accreted in the giant planet region. 

The Earth s oceans reveal an abundance of water that corresponds to —1/1000 of the planet s mass. Mars, too, once had liquid water that sculpted its surface, and water ice still resides at its poles and in its subsurface at high latitudes. The high D/H ratio in the atmosphere of Venus suggests that it once may have contained water in similar abundance to the Earth. Even Mercury, baking in the Sun s glare, appears to have water ice at its poles. The amounts of water in the terrestrial planets are modest, relative to the amounts of water in gas- and ice-rich planets in the outer solar system, but the importance of water for planetary habitability demands that we discuss how the inner planets got their water. 

Laboratory experiments on the lifetime of paramagnetic species in irradiated ice, dry ice (solid C02), solid SO and CH4 have been made during the consideration of the ambient temperature of outer planets. Materials used for ESR dating, dosimetry, microscopy and assessment of the environment in earth and planetary science are summarized at the end of the chapter. 

Dan L. Ji J. and Li Y. (2005). Climatic and biological simulations in a two-way coupled atmosphere-biosphere model (CABM). Global and Planetary Change, 47(2-4), 153-169. Dansgaard W. Johnsen S.J. Clausen H.B. Dahl-Jensen D. Gundestrup N.S. Hammer C.U. Hvidberg C.S. Steffensen J.P. Sveinbjornsdottir A.E. Jouzel J. and Bond G. (1993). Evidence for general instability of past climate from a 250-kyr ice-core record. Nature, 364, 218-220. 

In our model, cap carbonates are explained as a result of high carbonate alkalinity in an ice-covered ocean, regardless of the specific mechanism for melting the snowball Earth. Ice could melt for a variety of reasons (e.g., greenhouse gases, a positive perturbation in solar forcing, a decrease in planetary albedo), but cap carbonates are expected to be produced as a result of the transition from high to low oceanic carbonate alkalinity. 

This is, of course, a very Terra-centric categorization, but so far we only have the unitary example of Earth from which to draw examples. Furfaro et al. (2007) have developed a fuzzy-logic-based scheme that goes more toward the fundamentals of coupled physical/biological planetary systems, but still it takes a restricted view oriented toward Mars- and Earth-like planets. It is entirely possible that Earth has a thin biospheric veneer compared to potentially more massive biospheres on worlds with deep ice-covered oceans such as Europa we just don t know. 

Allamandola and Hudgins have considered the formation of complex organic species in ice matrices and provided a summary of the photochemical evolution on those ices found in the densest regions of molecular clouds, the regions where stars and planetary systems are formed 42 Ultraviolet photolysis of these ices produces many new compounds, some of which have prebiotic possibilities. These compounds might have played a part in organic chemistry on early Earth. 

Meteorites are divided into two broad categories chondrites, which retain some record of processes in the solar nebula and achondrites, which experienced melting and planetary differentiation. The nebular record of all chondritic meteorites is obscured to varying degrees by alteration processes on their parent asteroids. Some meteorites, such as the Cl, CM, and CR chondrites, experienced aqueous alteration when ice particles that co-accreted with the silicate and metallic material melted and altered the primary nebular phases. Other samples, such as the ordinary and enstatite chondrites, experienced dry thermal metamorphism, reaching temperatures ranging from about 570 to 1200 K. In order to understand the processes that occurred in the protoplanetary disk, we seek out the least-altered samples that best preserve the record of processes in the solar nebula. The CV, CO,... 

Earth and Mars clearly contain H2O. Venus s atmosphere is very dry, and composed mainly of CO2, but the high D/H ratio of the small amount of water present suggests Venus was once much wetter than today (Zahnle 1998). Mercury is perhaps too small and too close to the Sun to have acquired and retained water. Water may have been present in much of the material that accreted to form the Earth. Small amounts of water may have been adsorbed onto dust grains at 1 AU by physisorp-tion or chemisorption (Drake 2005). Once Jupiter formed, substantial amounts of water could have been delivered to the growing Earth in the form of planetesimals and planetary embryos from the Asteroid Belt (Morbidelli et al. 2000). It is also possible that Earth lay beyond the snowline at some point during the evolution of the solar nebula (Chiang et al. 2001) so that local planetesimals contained ice. 

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