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Lightning is a fascinating phenomenon that can have dramatic and powerful effects here on Earth.. Scientists have also observed lightning on other planets within our Solar System, and are currently studying whether this phenomenon can happen on exoplanets (planets orbiting around other stars). In order to predict if lightning is possible on these planets, we first need to understand how and why lightning happpens.
In this inquiry lesson, students will conduct experiments that will investigate what conditions a planet would need for lightning to occur. Students will learn how to create a lightning spark and then experimentally explore which conditions are necessary for lightning to form. They will then present their findings to their peers, ending with a guided class discussion on the implications of lightning research within exoplanet science.NOTE: This activity can be introduced or followed by "Clouds on other planets"
Experiment equipment (one per group):
Downloadable resources (see attachments):
Figure 1: Materials needed for the activity
Introduction
Lightning is a fascinating physical phenomenon with many interesting effects on a planet’s atmosphere and beyond. Learning about lightning can help students understand electrical charge and electrical fields in a novel and exciting context. Learning about lightning in space can help to show students some of the applications of scientific concepts such as electric charge outside of what may be expected.
Lightning storms are common occurrences in most planetary atmospheres with clouds. As you are likely aware, lightning storms occur regularly on earth.
Figure 2: Lightning photographed by ESA astronaut Paolo Nespoli from on board the International Space Station. Credits: ESA/NASA
Lightning also occurs on planets in our solar system aside from Earth. Lightning storms are particularly active on Jupiter and Saturn. They are also found on Uranus and, most recently, Venus. These lightning storms have been observed by space crafts such as Galileo (Jupiter) and Cassini (Saturn).
In addition to lightning that is found in our solar system, it is also predicted that many of the planets that we have found outside of our solar system (exoplanets) could also have lightning. We have discovered over 5000 exoplanets, and many of these have the right conditions for clouds to form, and astronomers are searching for chemical signatures in exoplanet atmospheres that scientists believe could be caused by lightning in those clouds.
Lightning may also be found occurring in clouds in space outside of planetary atmospheres. There are currently exoplanet scientists who are investigating the causes and effects of lightning within protoplanetary disks (the dusty disks around stars where planets form).
How lightning occurs
Lightning occurs when there is a charge separation with in a cloud, leading to one side of the cloud being negatively charged and the other being positively charged. When this charged side of the cloud is brought near to another conductive object (such as the ground or another cloud), the charged side of the cloud will attract the opposite charge in the conductive object, and a potential difference will build up in the gap between them. When this potential difference is large enough, it can overcome the non-conductive air between the objects and discharge a high energy, high temperature stream of electrons, ionising the channel of air as it travels. This is what we see as lightning. The thunder that accompanies lightning is the sound of the air around this channel of electrons being heated so dramatically and so quickly that the expansion of the air causes a shock wave.
The charge separation in the cloud initially happens though a combination of factors and effects such as gravity, triboelectric charging and air currents. Clouds are made up of a combination of types of particles including small liquid droplets and ice crystals and larger soft hail.
During a storm, the particles in a cloud will undergo lots of turbulence causing the particles to collide frequently. This causes triboelectric charging of the particles. The small liquid droplets and ice crystals tend to lose electrons in this, resulting in them becoming positively charged and the larger hail tends to attract electrons, resulted in them becoming negatively charged. Gravity and air currents bring the larger negatively charged particles to the bottom of the cloud, and the smaller positively charged particles to the top of the cloud. This results in a cloud with two sides with a very strong charge, ready for lightning to come from them.
Triboelectric Charging
Triboelectric charging is an important part of lightning formation. Triboelectric charging is also known as ‘static charge’. You may have experiences this in everyday life, for example when taking clothes from the dryer and finding them cling together, or when you brush your hair with a plastic comb. This charging is due to the transfer of electrons from one material to another, leaving both materials with a net charge. Some materials are more likely to collect electrons, and some materials are more likely to lose electrons. Most atoms begin neutrally charged, and so have an equal number of protons and electrons, but when some materials are rubbed together, electrons will be pulled from one material onto the other. This leaves the material that lost electrons with a net positive charge, and the material that gained electrons with a net negative charge. The reason that the materials must be rubbed together for this to happen more effectively is that it allows maximum contact between the surfaces of the materials.
A list of materials ranked by their tendancy to lose or collect electrons can be found by looking up the ‘Triboelectric series’. This list of materials shows the relative tendency of a material's to build up a net charge. It does not tell us the materials conductivity or the speed at which the material will lose their charge.
The effects of lightning
In order for lightning to occur, there needs to be a planetary atmosphere with some type of clouds in it. There also needs to be turbulence or wind, and particles of different materials in order to cause the triboelectric charging needed. There is a potential for direct observations of lightning flashes on other planets (in our solar system and beyond) using radio telescopes, and these observations can tell us about a planets atmosphere.
Lightning is a very high-energy event in an atmosphere. The high energy and temperatures associated with a lightning flash can trigger chemical reactions that would normally not occur in the atmosphere of a planet, altering the planets atmospheric composition. This affects both the types of reactions possible on a planet, and will also affect the observations made my astronomers.
In the early stages of Earth's lifetime, lightning may have played a significant role in the synthesis of prebiotic molecules as first demonstrated by the Muller-Urey experiments in the 1950s. Due to this, it is not unreasonable to assume that, given the correct conditions, lightning could play a role in the origin of life on other planets.
To know more about these topics, please refer to the Tearcher's Guide (in attachment)
Lightning on other planets is divided in a set of activities that are fully described in the Teacher Guide, were you can also find exercises and questions to ask.Optional: Students can also be asked during the Activities to fill in the Student Fact sheet so that they can use it later as a reference or for evaluation (see Teachers guide and Students Factsheet).
In this activity, students will be introduced to the idea of lightning in space. The goal of this activity is to gauge students prior understanding to the topic and to engage them and elicit the students interest. The science behind the lightning will not be addressed in this first part, instead it will focus on where lightning can occur, what effects it can have, and why it is important for exoplanet scientists.
Follow the slideshow of true or false questions about lightning. For each question poll the students before revealing the answer or utilise methods such as ‘think, pair, share’. If there are many additional and follow up question from the students at this stage, it is suggested to write down these questions so that they can be addressed at the end of the lesson.
Discussion
Lightning in protoplanetary disks and exoplanets is an active research topic in the scientific community and astronomers are very interested in learning more about it and its effects.
As seen in the true or false questions, exoplanet researchers have not yet directly detected lightning flashes on exoplanets or in protoplanetary disks, but it is very probable that they are occurring and many astronomy researchers are focusing on making predictions for these. If lightning is observed in space, it is important to know what this scientific discovery tells us.
Exercise
What conditions do you think are needed to cause lightning?
There is no set answer for this question at this stage. The goal of this question is to pique the student's interest. The same question will be asked later in the evaluation section (see Teacher Guide).
Show a video demonstration of the lightning experiment (the video can be found at https://www.youtube.com/watch?v=sk2Uu2lygUA&ab_channel=sciencemuseumok).
It is important that you stop the video at 1:40 before the science behind the demonstration is explained. During this demonstration, it is important that students are only shown the effect and are not given the explanation yet of what is happening. This is to encourage the curiosity of the students and to encourage them to come up with their own hypotheses during the experiment and exploration phase of the lesson.
The students will be shown a video of a demonstration showing lightning. It is encouraged that they question how this is happening, but that answers are not provided by the teacher at this stage. The students will attempt to develop hypothesis themselves and test them in the following phases of the activity.
Students work as a class to determine the variables in this demonstration.
As a class, ask the students which variables are present in this demonstration. These should be written up on a board or screen that all of the students can see. The teacher may guide the students as needed. For each variable, try to match it to what the equivalent would be in an exoplanet’s atmosphere.
Results
With guidance from the teacher where necessary, the following variables should be identified by the class:
The variables with * are suggested to be tested in the next activity, so identification of these is highly recommended. The variables without * are useful for context and completeness and the identification of these will aide in class discussion. Your class may also identify other variables that are not on this list! Not all variables will have a direct parallel to exoplanet atmospheres. This is because all physical analogies and demonstrations will have limitations, as it is not an exact replica of an exoplanet atmosphere.
Students are split into groups and are provided with the equipment needed to perform the demonstration. Each group is given/chooses one variable to systematically adjust and record the results of.
Building the experiment
The students should be split into groups of 3 to 5 students for this activity and follow the instructions of the video, also descibed below.
Step 1 Pierce a thumb tack through the bottom of the aluminium foil tray. Push the eraser on the end of a pencil onto the thumb tack. The pencil will now act as a handle and allow you to lift the tray up without touching it. Place this aside for now.
Step 2 Rub a dry piece of wool against the bottom of a polystyrene tray for two minutes. A little pressure can be applied, but not hard enough to break the container. Once the tray is charged enough, it should lift up when the wool is lifted. It may help to wear rubber gloves for this stage of the experiment to minimise the grounding of charge through your body.
Step 3 Set the piece of wool aside and leave the tray, upside down, on the table. Do not touch the top of the tray.
Step 4 Using the pencil to lift it, place the aluminium foil tray on top of the polystyrene tray
Step 5 Slowly bring a metal fork towards the rim of the aluminium tray. Once it is close enough, if you watch carefully you might see a flash of lightning between the prong of the fork and the rim of the tray. You may also hear a small ‘cracking’ noise from the electrical discharge. This is analogous to the thunder that accompanies lightning.
Figure 3: different steps of the experiment
Tips
The static can be removed from the polystyrene tray by rubbing your hand or a damp cloth over it a few times. Ensure to dry the polystyrene tray thoroughly before restarting the experiment.
It is also very important to ground both the probe and the foil tray before restarting the experiment. This can be done by holding them in your hand (assuming you are not wearing thick soled trainers), or placing them against a metal table leg or something similar).
Ensure that there are at least a few layers of wool material between your hand and the tray when rubbing them together. You may also choose to try using rubber gloves for this step.
Exercise: investigating the variables
Each group will be asked to choose a variable to test. If they have difficulty selecting a variable you may allocate them one of the following suggested variables:
The optimum combination of variables is:
When a group is changing one of the variables, it should be suggested that the group keep the other control variables at the optimum values stated above.
Recording Results
Students can record their results in any way that they see fit. The clearest result will be if students can hear/see a lightning flash or not. Students can note down if the flash was visible to the human eye. They may also note down how loud accompanying noise was. As this is subjective, it is useful to have multiple people viewing and recording this information. If a group finishes testing their initial variable, they may experiment further by exploring a second variable or changing the variable in smaller increments.
Figure 4: A student doing the experiment
To conclude this activity the students will present their findings and can have an open discussion where they break down the steps in the experiment to understand what conditions are needed for clouds to form on a planet.
In this activity students are encouraged to think independently and practice their problem solving skills. Students are allowed to come up with their own hypothesis’ and test them using methods that they deem appropriate and helpful. It is important during this activity that the students are documenting their results in an appropriate and accurate manner.
As an extension to the practical part of this activity, students can also make a group presentation.
After the students have presented their findings, a discussion can be had as a class to determine what they have found out collectively and try to devise a list of the conditions needed for lightning to occur on a planet.
See the teacher notes for more details (available in the attachments).
In this activity, students will break down the steps in the experiment to understand the conditions needed for lightning to occur on a planet, leraning how atoms are made of charges and map the transfer of charge in lightning formation.
The conditions that cause lightning to occur are the same regardless of which planet you are on. The laws of physics still apply, even though there may be differences, such as variations in atmospheric composition, pressure, or temperature.
Details about the physics that causes lightning can be found in the attached Teacher's Guide, in the slides and slide notes, which are available for download.
If running this activity as an inquiry lesson, let the students summarise their findings themselves to encourage learner agency. As the teacher, you may add to or guide their summarising of results. To aid in this, the following paragraph summarises the science learned in this lesson. For more details, refer to the teacher guide and the final section of the attached slides.
The conditions that you require for lightning to occur are as follows;
During this activity there are questions list in the Teacher Guide to be asked during and after the activity. See "Teacher Guide" in the attachments section for more information.
NASA Video - Exoplanet Clouds: 'Jewels' of New Knowledge
ESA resources
Extra information
CHAMELEON Marie Curie Exoplanet Research Network
