William R. Kuhn

Ammonia photolysis and the greenhouse effect in the primordial atmosphere of the earth

Abstract Photochemical calculations indicate that in the prebiotic atmosphere of the Earth ammonia would have been irreversibly converted to N 2 in less than 40 years if the ammonia surface mixing ratio were ≤ 10 −4 . However, if a continuous outgassing of ammonia were maintained, radiative equilibrium calculations indicate that a surface mixing ratio of ammonia of 10 −5 or greater would provide a sufficient greenhouse effect to keep the surface temperature above freezing. With a 10 −4 mixing ratio of ammonia, 60 to 70% of the present day solar luminosity would be adequate to maintain surface temperatures above freezing. A lower limit to the time constant for accumulation of an amount of nitrogen equivalent to the present day value is 10 my if the outgassing were such as to provide a continuous surface mixing ratio of ammonia ≥ 10 −5 .

Evolution of a Nitrogen Atmosphere on Titan

Photochemical calculations indicate that if NH3 outgassed from Titan it should have been converted to a dense N2 atmosphere during the lifetime of the satellite. A crucial step in the process involves a gas phase reaction of N2H4 with H. The most favorable conditions for this step would be the intermediate production of a CH4-H2 greenhouse capable of raising the gas temperature to 150�K. Subsequently about 20 bars of N2 could have evolved. The pressure-induced opacity of 20 bars of N2 should suffice to explain the recently measured 200�K surface temperature. Unlike the situation on Jupiter, NH3 is not recycled on Titan by reactions involving N2 or N2H4. This may explain the failure of recent attempts to detect NH3 in the upper atmosphere of Titan.

Solar radiation incident on the Martian surface

SummaryCalculations indicate that the maximum daily solar radiation reaching the Martian surface is about 325 cal/cm2 during southern hemisphere summer at latitude of about 40°S. In the ultraviolet region of the spectrum, the radiation reaching the surface at wavelengths greater than 2800 Å is within 10% of the radiation incident on the atmosphere. There is significant extinction of radiation in the spectral region near 2500 Å in mid and high latitudes due to absorption of radiation by ozone; radiation reaching the surface may be reduced to one one-thousandth of that incident on the atmosphere during winter. Virtually no radiation of wavelengths less than 1900 Å reaches the surface because of absorption by the large column abundance of carbon dioxide. Daily and latitudinal distributions of radiation are presented for wavelengths of 3000, 2500 and 2000 Å.

The distribution of ammonia and its photochemical products on Jupiter

Abstract Altitude profiles of ammonia and its photochemical products are generated in the light of the new measurements of the Jovian temperature structure, eddy transport coefficient, improved chemical scheme, and rate constants. Realistic limits are placed on the concentration of hydrazine which may participate in the recycling of ammonia on Jupiter. The maximum hydrazine ice production rate is calculated to be about 1.3 mg m −2 /Jovian day. The distribution of nitrogen gas is presented with and without supersaturation of hydrazine. The nitrogen mixing ratio near the ammonia cloud top is estimated to be in the range of 10 −9 to 10 −11 . An appreciable latitudinal variation in the ammonia concentration is expected.

Precambrian climate: The effects of land area and Earth's rotation rate

Rotation rate and land area have changed significantly over the Earths history. The associated climate changes are examined with a general circulation model (GCM) and compared with the current climate state. Unlike previous GCM studies, clouds in the model interact with the radiation field. We show that rotation rate changes can affect mean air temperatures by strongly affecting the cloud field. For example, an Earth day length of 14 hours causes the global mean cloud fraction to decrease 20%. There are also large changes in the wind field; indeed, at some mid-latitudes the time-averaged surface wind changes from westerlies to easterlies. If the land masses are removed from the model and with present-day rotation, the global mean air temperature is 4 K higher than for the present-day simulation; if in addition the day length is 14 hours, the global mean air temperature is 5.5 K. Thus the early atmosphere may not have contained the very large concentrations of CO2 thought to be necessary to prevent a frozen planet higher.

Solar radiation incident on Mars and the outer planets: Latitudinal, seasonal, and atmospheric effects

Calculations of the daily solar radiation incident at the tops of the atmospheres of Mars and the outer planets and its variability with latitude and season are presented in a series of figures and tables similar to those for Earth in The Smithsonian Meteorological Tables. The changes in the latitudinal and seasonal distributions of daily surface insolation during the great Martian dust storm of 1971 (when Martian atmospheric optical depth increased from about τ = 0.1 to 2.0 were significant and dramatically illustrate the effect of atmospheric aerosols on surface insolation; i.e., the mean annual daily insolation at the poles decreased by more than a factor of 100 as τ increased from 0.1 to 2.0.

Paleoamospheric temperature structure

Abstract Radiative equilibrium and radiative convective temperature profiles for the Earths evolving atmosphere been have calculated. If the atmosphere evolved from one rich in carbon on dioxide, and deficient in oxygen, to its present composition, the temperature structure showed considerable change. The models of 3 to 4 billion years ago display steadily decreasing temperatures with altitude, being 185°K at pressures associated with the present-day upper stratosphere. A lapse rate feature similar to the present-day tropopause is not indicated until about 1 billion years ago; but the stratospheric region is approximately 15°K colder than presently found at comparable pressures. Surface temperatures approximately 10°K warmer than at present existed until nearly 1 billion years ago. When the oxygen content exceeded roughly 0.1 times the present level, surface temperatures began to decrease. If biological processes are important to carbon dioxide—ozone variations, such as has been suggested during the Ice Ages, then estimates of surface temperature should include the effects of both gases.

Temperatures in a runaway greenhouse on the evolving Venus: implications for water loss

Abstract Recent work has established that Venus once had at least 100 times its present complement of outgassed water. An original complement of water comparable to the earths is not inconsistent with our present knowledge (and is worthy of consideration since it involves no special assumptions concerning the differences in origin of the two planets). We use a one-dimensional calculation to investigate temperatures in the “runaway greenhouse” which may have ensued if Venus once had more than a few percent of the earths water complement. We find that the atmosphere must exhibit an unusual structure, with condensation and presumably cloud formation taking place at high altitudes, while deep in the atmosphere the gas is strongly unsaturated with respect to water vapour. The necessity of including clouds introduces considerable uncertainty into the calculation of surface temperatures. However, for certain reasonable values of the cloud parameters, very high temperatures can be sustained, approaching the temperature of the basalt solidus. We speculate that such high temperatures may have promoted water-rock reactions, releasing free hydrogen and “burying” oxygen. A plastic or molten surface could have promoted rapid exposure of fresh rock, significantly easing the problem of disposing of the oxygen released by the dissociation of water.

Incorporation of multiple cloud layers for ultraviolet radiation modeling studies

Cloud data sets compiled from surface observations were used to develop an algorithm for incorporating multiple cloud layers into a multiple-scattering radiative transfer model. Aerosol extinction and ozone data sets were also incorporated to estimate the seasonally averaged ultraviolet (UV) flux reaching the surface of the Earth in the Detroit, Michigan, region for the years 1979-1991, corresponding to Total Ozone Mapping Spectrometer (TOMS) version 6 ozone observations. The calculated UV spectrum was convolved with an erythema action spectrum to estimate the effective biological exposure for erythema. Calculations show that decreasing the total column density of ozone by 1% leads to an increase in erythemal exposure by approximately 1.1-1.3%, in good agreement with previous studies. A comparison of the UV radiation budget at the surface between a single cloud layer method and a multiple cloud layer method presented here is discussed, along with limitations of each technique. With improved parameterization of cloud properties, and as knowledge of biological effects of UV exposure increase, inclusion of multiple cloud layers may be important in accurately determining the biologically effective UV budget at the surface of the Earth.

The response of selected terrestrial organisms to the Martian environment: A modeling study

Abstract An energy balance model has been developed to investigate how the Martian atmospheric environment could influence a community of photosynthetic microorganisms with properties similar to those of a cyanophyte (blue-green algal mat) and a lichen. Surface moisture and soil nutrients are assumed to be available. The model was developed to approximate equatorial equinox conditions and includes parameters for solar and thermal radiation, convective and conductive energy transport, and evaporative cooling. Calculations include the diurnal variation of organism temperature and transpiration and photosynthetic rates. The influences of different wind speeds and organism size and resistivity are also studied. The temperature of organisms in mats less than a few millimeters thick will not differ from the ground temperature by more than 10°K. Water loss is actually retarded at higher wind speeds, since the organism temperature is lowered, thus reducing the saturation vapor pressure. Typical photosynthetic rates lead to the production of 10 −6 to 10 −7 mole O 2 cm −2 day −1 .