In this activity, students will explore the science behind an intriguing feature on the dwarf planet Ceres and a discovery on Mars. Students will dissolve various salts into water, make predictions about what will happen when the solutions are left to dry, then make observations and provide explanations about what they observed.
- Students can work individually or in groups. Individuals or groups can be assigned to make solutions using different salts or salt-to-water ratios for comparison.
- Be sure to make note of the different ratios listed in Step 1. Table salt, Epsom salt, and baking soda have different saturation points in water. While table salt and Epsom salt are similar enough to use the same salt-to-water ratios, less baking soda can be dissolved in water before it becomes saturated.
- The time needed for solutions to dry will vary. This activity can be split into two sessions as needed to allow for drying time.
- Precipitated salts in the petri dish will be easier to see if the petri dish is placed on a dark surface.
Even before Dawn arrived at Ceres, scientists had observed bright regions on the dwarf planet through telescopes, but their origin remained a mystery. The Dawn spacecraft's close-up view allowed scientists to gain a better understanding of how the hundreds of bright regions, known as faculae (meaning bright areas) came to be.
The most reflective of these faculae are found in crater floors, with Occator Crater being home to two of the most distinct examples, named Cerealia Facula and Vinalia Faculae. Using data returned from Dawn, scientists discovered that reflective salt compounds (sodium chloride chemically bound with water and ammonium chloride) are concentrated in Cerealia Facula, creating the six-mile wide bright region. These compounds made their way to the surface when briny water from underground reservoirs percolated up. As the brine dehydrated over hundreds of years, the salts were left behind as evaporite minerals, creating the bright areas we observe on Ceres.
The processes we see on Ceres is a common process elsewhere in the solar system. Here on Earth, it results in the formation of evaporite minerals, such as halite (sodium chloride, or table salt) and gypsum (calcium sulfate, used industrially as plaster). On Mars, the discovery of evaporated salts in depressions that were once shallow ponds indicates the presence of water as recently as two billions years ago — about a billion years more recent than previously thought.
- Instruct students to dissolve salt into warm water to create saline solutions using one of the following methods: 1) Use different ratios of the same type of salt; 2) use the same ratio of different types of salt.
Table salt and epsom salt ratios:
1:4 (1.0 Tbsp salt to 0.25 cup water)
1:8 (1.5 tsp salt to 0.25 cup water)
1:16 (0.75 tsp salt to 0.25 cup water)
Baking soda ratios:
1:12 (1.0 tsp baking soda to 0.25 cup water)
1:16 (0.75 tsp baking soda to 0.25 cup water)
1:24 (0.5 tsp baking soda to 0.25 cup water)
- Tell students they will apply a small amount of each solution to a separate petri dish and allow the solution to dry. Ask them to predict what will happen when the solutions dry. They can verbally explain their predictions but should also draw or write what they expect to observe on a piece of paper or the observation worksheet. As appropriate based on student abilities, make sure students consider the different salt ratios or salt types in their predictions.
- Instruct students to use a pipette, eye dropper, or measuring spoon to place 5 ml (approximately 1 tsp) of each solution into the petri dishes. Allow time for drying. This process can be sped up by placing the petri dishes in direct sunlight or near another heat source. Be sure to not place the petri dishes so close to a heat source that they melt.
- When the solutions have dried, students should draw or write a description of what they observe. If available, they can use a hand lens to make up-close detailed observations.
- Ask students to explain to one another or to the class what they think occurred. They should add this to their observation drawings or writings. If students don’t make the connection, explain that as the water in the solution evaporated, the salts that were dissolved were left behind in the petri dish and formed what are known as evaporite minerals, or simply evaporites.
- Where on Earth do you think the process of solids being dissolved in water or water evaporating and leaving behind salts could be occurring? What evidence might we look for if we were searching for these deposits of evaporite minerals?
Examples might include dried or drying salty lakes (e.g., Salton Sea, Bonneville Salt Flats) irrigated farmland and hard water deposits on plumbing fixtures. Evidence might include bright/white crusty or rocky materials.
- Do you think it’s possible that these processes are occurring elsewhere in the solar system?
Discuss the bright regions on Ceres, the percolation of briny subsurface water to the surface, and the bright salts left behind as the water evaporates. Show students images of Ceres and its bright spots. Share that scientists recently discovered evaporated salts on Mars, indicating the presence of liquid water in the past. Show images of Mars indicating the existence of salts on the surface.
- Students may notice the crystalline structures of their evaporites look different from what they see in images of Ceres. Ask them to explain why there are differences.
Responses may include: Students are getting a close-up view of the evaporites compared to Dawn’s view from orbit. There is so much more material on Ceres than in the dish, and it's very thickly layered in Cerealia Facula. Even if there are voids or gaps in a layer, the material above or below is still very bright.
- What do you think would happen if water were added to the salt crystals that formed in the petri dish?
Students should predict that the salts will dissolve into the water. (If students dissolved more than one type of salt) are there differences in the evaporite minerals left behind that were observed?
Evaporite minerals from different salts will have different crystal structures and thereby take on a different appearance.
- Other than salts, do you think there might be other dissolved solids in water? How could you find out?
Students may be encouraged to look up Total Dissolved Solids (TDS) from their local water district’s annual reporting, or learn about hard water deposits and their cause.
- If it is possible that there are dissolved solids in water, how could you purify water?
This may lead to a discussion about the differences between filtered water, purified water, and distilled water. See Explore More below for a water filtration activity.
- Students should make reasonable predictions about what will happen as the solutions dry based on their background knowledge and experience. Students' drawn or written predictions should accurately reflect their thinking.
- Students’ drawn or written descriptions of their observations should accurately reflect the phenomenon they observed.
- Student explanations about the observed phenomenon should make use of appropriate vocabulary based on their knowledge and experience (e.g., drying, evaporation, precipitates, etc.)
- Students can read more about the Dawn mission’s scientific discoveries by visiting the Dawn mission science page for Ceres.
- Students may be curious about what the results would look like if they added more salt to their solution. This is an opportunity to discuss solubility and saturation in greater detail and allow students to try dissolving higher concentrations of salt in their solutions if time allows.
- Students can dissolve different types of salts into a single solution, make predictions about what will occur when the water evaporates, and compare their predictions to observations made once the solution has evaporated.
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