Gamma Ray Photons and Neutrons from Mars: Student Reading

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Ernest Casey
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Gamma Ray Photons and s from Mars: Student Reading Introduction The Mars Gamma Ray Spectrometer (GRS) is currently orbiting Mars aboard the Mars Odyssey spacecraft.

1 Gamma Ray Photons and s from Mars: Student Reading Introduction The Mars Gamma Ray Spectrometer (GRS) is currently orbiting Mars aboard the Mars Odyssey spacecraft. GRS, shown on its extended boom in the image to the right, detects neutrons and gamma ray photons coming from the surface Mars. In this activity, you will learn why Mars gives off these particles and find out how we can use them to learn about what the surface of Mars is made of. s and Gamma Ray Photons First, let s talk about what neutrons and gamma ray photons are. A neutron is a particle that is similar in mass to a proton but with no charge. The center, or nucleus, of an atom is made of protons and neutrons. A gamma ray photon is a high-energy packet of light, or electromagnetic radiation. Just as our eyes or visible cameras can detect visible photons, gamma ray detectors such as Mars GRS can detect gamma ray photons. The two main reasons that gamma rays and neutrons are given off by Mars are described below. Radioactive Decay One way that gamma rays are produced at the surface of Mars is through radioactive decay. When unstable radioactive elements such as potassium, thorium, and uranium decay into other elements, they can give off gamma ray photons with specific energies (see diagram below). For example, when potassium-40 decays into argon-40, a gamma ray photon of a specific energy is released. We will call this a decay event. Detection of gamma ray photons with this energy as Mars GRS flies over a part of Mars indicates the presence of potassium in that region. Radioactive potassium-40 nucleus DECAYS into an argon-40 nucleus and gives off gamma ray photon Potassium-40 Argon-40 + Gamma ray photon

2 Cosmic Ray Particle Bombardment s and gamma ray photons can also be made when the surface of Mars is bombarded by cosmic ray particles. Cosmic ray particles are high-energy hydrogen and helium nuclei that race throughout outer space at exceptionally high speeds. When one of these particles smashes into the surface of Mars, the atoms that it strikes give off fast moving neutrons. These fast neutrons can then either escape from the planet or interact with other atoms at the surface of the planet. The cartoon sketches below show these interactions. In the first drawing, a cosmic ray particle collides with a target atom (in this case iron) and gives off a fast neutron. We will call this a collision event. In the second drawing, a fast neutron bounces off of a target atom (in this case silicon) and slows down to a medium neutron. We will call this a scatter event. A gamma ray photon of a specific energy is given off. In the final drawing, a slow neutron is captured by and excites a target atom (in this case chlorine), which then gives off a gamma ray photon of a specific energy. We will call this a capture event. Cosmic ray particle COLLIDES and gives off fast neutron Fast neutron SCATTERS and gives off gamma ray photon Slow neutron is CAPTURED and gives off gamma ray photon Cosmic Ray Particle Iron Fast Silicon Medium Slow Chlorine Fast Gamma Ray Photon Gamma Ray Photon Figuring Out What Mars Is Made Of As Mars GRS orbits Mars, it detects neutrons and gamma ray photons given off by the processes described above. Because every element (for example, iron, silicon, chlorine, hydrogen) gives off gamma ray photons of specific energies, we can figure out the concentration of these elements based upon the energies of photons that are detected. Also, because we understand how neutrons interact with these elements, we can learn about the composition of the surface based upon the energies of the neutrons we detect.

3 Gamma Ray Photons and s from Mars: Student Questions Answer the following questions based upon the student reading. 1) Give two characteristics for each of the following particles: Particle Characteristic 1 Characteristic 2 Gamma Ray Photon Cosmic Ray Particle 2) List the four types of events described in the reading? 3) Which of these types of events will NOT be affected by an increase in the number of cosmic ray particles striking the surface of Mars. Justify your answer. 4) If Mars GRS detects an increase in gamma ray photons specific to hydrogen when it is orbiting over a particular region of Mars, what might this tell us about the composition of the surface in that region?

4 Gamma Ray Photons and s from Mars: Simulation Instructions We will now simulate the production of neutrons and gamma ray photons at Mars. A simulation of the Martian surface has been set up in which you will re-enact decay events, scatter events, and capture events. These particles will then be collected to simulate detection by Mars GRS. Materials: 1. You need a pencil, pen, or marker to complete this activity. Activity Instructions Obtain a Nuclear Event Card to find out if you will simulate a decay event or a cosmic ray particle event: Decay Event 1. To simulate radioactive decay, find a radioactive atom and follow the instructions on the atom card. Cosmic Ray Particle Event 1. To simulate cosmic ray particle collision, find a non-radioactive atom and follow the instructions on the atom card listed under Cosmic Ray Particle. You will leave this atom either with a Cosmic Ray Particle card or a card. 2. Move to another non-radioactive atom and follow the instructions on the atom card based upon the type of particle card you are carrying (Cosmic Ray Particle or ). 3. Continue moving from atom to atom, following the instructions listed under the type of particle you are carrying (Cosmic Ray Particle or ). If the card instructs you to Go to Mars GRS, carry any particles that you are still holding to the Mars GRS collection bins that your instructor has set up. Depending upon the directions from your instructor, either obtain another Nuclear Event Card and repeat the process or proceed to data analysis. After instructor stops the activity, the particles collected in the Mars GRS collection bins must be analyzed. Record your results and answer questions on the sheet titled Gamma Rays and s from Mars: Simulation Analysis.

5 Gamma Ray Photons and s from Mars: Simulation Analysis 1) Following the first simulation, count up the number of particles collected in the GRS collection bins and record your results in the table below: s Number of Particles Collected Iron Gamma Ray Photons Particle Simulation 1 Silicon Gamma Ray Photons Chlorine Gamma Ray Photons Hydrogen Gamma Ray Photons Potassium Gamma Ray Photons 2) Did you deposit the same types and numbers of neutrons and gamma rays each time you went to the GRS collection bin? Why or why not? 3) If the Mars Gamma Ray Spectrometer (GRS) collects gamma rays corresponding to silicon, iron, chlorine, and potassium as it orbits around Mars, what does that tell us about what Mars is made of?

6 4) If time allows, you will redo the simulation after your instructor changes the composition of the simulated Mars surface by adding or removing Atom Cards. Use the table below to record your results from these various simulations. s Iron Gamma Ray Photons Number of Particles Collected Particle Simulation 2 Simulation 3 Silicon Gamma Ray Photons Chlorine Gamma Ray Photons Hydrogen Gamma Ray Photons Potassium Gamma Ray Photons 5) What happened to the types of neutrons and gamma ray photons collected by GRS after your instructor changed the composition of the simulated Martian surface? 6) If GRS orbits over a place with a large amount of water ice (H 2 O), will it collect more or less gamma rays with energies specific to hydrogen? The diagram below shows sample data collected by Mars GRS. For each energy listed along the bottom axis, the graph shows the number of photons collected for that energy. Note that several peaks appear on the graph. Each of these corresponds to a unique energy of the gamma ray photon released by unique atoms at the surface of Mars. For more information about Mars GRS and the Mars Odyssey mission, visit the following websites: and

Gamma Ray Photons and Neutrons from Mars: Student Reading

Gamma Ray Photons and Neutrons from Mars: Student Reading Gamma Ray Photons and s from Mars: Student Reading Introduction The Mars Gamma Ray Spectrometer (GRS) is currently orbiting Mars aboard the Mars Odyssey spacecraft. GRS, shown on its extended boom in the