Chemistry and Biochemistry

High Velocity Impacts of Bacteria Spores

 

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Introduction

Phobos is the larger and inner moon of Mars. The regolith of Phobos may consist of up to 250 ppm materials transported from Mars. These materials landed on Phobos with the impact velocity of at least 1.08 km/s. In this case if there are any viable microorganisms on the surface of Mars, they can be transported to Phobos via this process and survive upon landing. Previous researches demonstrated some bacterial spores survive an impact with the velocity up to 400 m/s. In this project, we have gradually increased the impact velocity and investigate the viability of bare microorganisms under this condition.

 

Methods

The figure below describes the schematic of the high-velocity impact instrument. An electrospray ionization source is used to produce electrically charged, gas-phase microbes, which are desolvated and introduced into vacuum. After the initial pressure-reducing orifice, an aerodynamic accelerating tube provides the final velocity of 300 m/s. Then a two-stage image charge detector is used to record the charge and velocity of each particle. A collection vessel placed on the last chamber allows the recovery of any microbes passing through the charge detector. Assessments on bio-viability of microbes on high-velocity impact can be performed afterward.

 

Reaching Higher Velocities

Previously, we used a 30 cm long metal tube with the diameter of 4 mm. To optimize the design of the beam tube, we have used Star-CCM+, a computational fluid dynamic simulation software, to simulate the aerodynamic mechanism of the accelerating tube. With our original tube design, we can only achieve a sub-sonic gas flow accelerating bacterial spores to about 340 m/s.

Two alternative solutions have been examined with Star-CCM+. First, we have used Helium and Hydrogen instead of air as the accelerating gas. Since the speed of sound is much higher in both Helium and Hydrogen than in air, higher velocities can be achieved.

Second, we have added a supersonic nozzle at the end of the metal tube shown below. With this modified design, we are able to achieve a gas flow with the flow rates of 517.25 m/s (air), 1288.9 m/s (Helium), and 1941.6 m/s (Hydrogen).

 

Results

As the first step of improving the flow rate, we used two gas-supply tubes to create an atmosphere of Helium near the inlet.

Results from the two-stage image charge detector show the velocity of the polystyrene beads with Helium is 10% faster than with air. One possible reason explaining lower than expected velocities is that the background gas is still a mixture of helium and air. This mixture slows the flow rate since the heavier gases in air affect the speed of sound though a gas more than the lighter Helium gas.

 

Future Work

Higher flow rate of Helium and a more enclosed inlet will be promising to create a purer background ga. A newly designed supersonic tube will be applied as the next step to achieve the required velocity. When we achieve the higher velocities, B. Subtilis samples will be electrosprayed and collected to test their survivability.