This work studies the synthesis of organic molecules that occurs as a result of meteorite impacts into planets with icy surfaces, such as Europa. Meteorite impacts into icy surfaces cause a large zone of fracturing under the impact crater. Very large voltages are generated during this fracturing, and that energy is, in effect, "stored" in the ice as electrostatic charges spread over a large area of the ice for substantial periods of time. Over time, the electrostatic charges can accumulate until a critical level ("the breakdown potential of ice") is reached, at which time electrical arcing occurs. In the presence of water-ice, methane, and ammonia, this arcing serves as the energy source for the synthesis of organic molecules. Spark experiments performed on ices that simulate conditions on Europa have produced large organic molecules, which have become semiconducting.
The classic experimental apparatus for spark-discharge experiments designed by Miller/Urey/Sagan/Khare consisted of a glass sphere with two electrodes connected to an external high-voltage source protruding into its cavity. The sphere cavity was filled with ammonia, methane, and water vapors, and an electrical arc was induced between the electrodes. This simple system generated complex molecules known as tholins. Experiments with extending this model to simulated meteorite impacts into ice have shown that impacts can generate voltages sufficiently high to serve as the energy source for the synthesis of organics in ice.
An ice cylinder (25 by 61 centimeters (cm) long) is formed in a Teflon tube. The thick-walled (2.5 cm) Teflon contains the ice from exploding sideways at impact and cushions the ice, thus simulating a larger ice field. Four electrodes are embedded in the ice cylinder at 5, 20, 38, and 53 cm from the top surface of the ice. Three magnetic coils are wound on the circumference of the tube, and a photodiode monitor is placed about 15 cm from the top of the ice. A quarter-inch-diameter solid aluminum sphere serves as the simulated "meteor."
The Ames Vertical Gun Facility was used to launch the sphere at 5.6 kilometers per second
(km/sec) into the ice cylinder cooled to -150 degrees Centigrade (°C). Recent test results from this system showed that the voltage created in the ice at the uppermost electrode was greater than 300 volts (the channel saturated). Additionally, several saturated spikes were noted in the oscilloscope trace, indicating that arcing was occuring in the ice during the impact. Large oscillatory (10-kilohertz) magnet signals occurred in the same time frame as the impact, showing that large currents were flowing in the ice, in turn suggesting that the ice became conductive at impact of the projectile.
The second series of experiments had photodiodes monitoring the ice both from the top and from the side about 15 cm down from the top through a light pipe. The impact velocity of eighth- to quarter-inch aluminum spheres was 5 to 6 km/sec, and the energy of the projectile from 200 to 3000 joules with the ice at a temperature of -170 °C. At projectile impact, light emission, high voltage, and a large magnetic field that lasted about 5 milliseconds (ms) were recorded; then after a pause of 280 ms, a secondary light, voltage, and magnetic field occurred. The voltage and light had four spikes in a
10-ms timeframe, and the emissions correlated with each other, indicating that arcing was occurring in the fracture zone under the impact.
The fracture zone under a meteorite impact can be thought of as having two regions (like the plates of a capacitor) that will be charged either positive or negative. The charging of the two regions or plates occurs because of the breaking of molecular bonds in the fracture zone and/or crystalline ice that is subjected to the piezolectric effect. Breaking of molecular bonds would produce free electrons and positive and negative ions, which would produce the large electrostatic charge imbalances, and would result in the formation of new organic molecules.
Another recent ice experiment was conducted in the laboratory using an ice block with two electrodes imbedded in the ice organic mixture at -200 °C. The results showed that the organics were generated and the ice mixture became semiconducting. The semiconducting organics caused the spark to cease, and the ice mixture became liquid at an outside temperature of -200 °C. Identification of the organics produced is under way.
The result of the above experiments have shown that large (tholin type) organic molecules can be formed in the fracture zones produced by meteorite impacts on the ice surface of Europa.
Point of Contact: B. Khare
(650) 604-2465
bkhare@mail.arc.nasa.gov
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