When the first rocks came back from the Moon in 1969, the question geologists asked was "what has happened on the Moon in the past to make a rock like this?" We weren't able to do things on the Moon that we do on Earth to figure out how rocks form. For example, we weren't able to keep going back to the Moon, and we weren't able to excavate big pits to see what things look like deeper in the Moon's crust. So there was a big flurry of experiments on Earth to find out what kind of processes and events must have happened to get rock like the Moon rocks.

"Petrology" is the study of what rocks tell us about how they formed. The use of experiments to try to find how a rock formed is called "experimental petrology".

I am an experimental petrologist. NASA pays me to do experiments to figure out how rocks form on the Moon or other planets. Someday, when we establish colonies on the Moon or other planets, this understanding will help us know about our new world, find resources there to help us become economically independent of Earth, and anticipate the kinds of opportunities that other worlds offer us.

Here is a puzzle which you can solve that is similar in concept to the experimental petrology that I do.

Suppose that on some planet that is much colder than Earth, and the "rock" is made out of ice, you see a "volcano" that is erupting salty water. You figure out that the salty water comes from deeper in the planet where it got warm enough for a little pocket of ice to partly melt. The melted part erupted to the surface as watery "lava". The solid, icy part stayed behind inside the planet. (illustration)

Your question is this: Is the solid part left behind inside the planet JUST AS SALTY, or SALTIER, or NOT AS SALTY as the salty water "lava" that erupted to the surface?

(Salty water eruptions are believed to have occurred on the Moon's of Jupiter, Europa and Ganymede)

Hint: Think of an experiment you can do that will tell you whether more salt will go with the liquid part erupted to the surface, or with the solid part that remains behind.

Experimental design for measuring the distribution of salt between liquid and solid water

Materials: 4 paper cups (8 ounces work fine), 1 16 ounce cup or container, larger container about 6-7 inches in diameter and about 3-4 inches deep (such as a metal pan), crushed ice, salt, measuring cup, 1/4 teaspoon measurer, knife, spoon, and battery operated salinity tester (optional).

Measure 3/4 teaspoon of salt into 1 cup of water in the 16 oz cup. Mix until the salt is completely dissolved. Fill one of the 8 oz cups about 1/2 full with this water.

Mix crushed ice and 2 cups of salt in the metal pan to about 3/4 full. Add water just until the open spaces in the ice are filled. Place the 8 oz cup with salt water in the middle of the ice-salt mixture. Place a paper towel folded over several times over the cup (you may have to weight this down with something to keep the cup from bobbing up or falling over). Periodically stir the ice-salt mixture gently.

The ice-salt mixture has a temperature below the freezing point of water. It will cause the water inside the cup to begin to freeze (This is how making home-made ice cream works too!). Allow about half of the water in the cup to freeze (the ice will freeze from the outside in, wait until the layer of ice extends about 1/2 to 1/3 of the distance to the middle of the cup). This will take about 20-30 minutes, perhaps less.

Pour the remaining liquid from the cup into one of the other 2 8 oz cups and label it.

Let the cup with ice sit for a few minutes until a teaspoon or so additional water has formed from melting of the ice. Pour this water off into another 8 oz cup. (The purpose of this is to get rid of some bubbles of liquid water that got caught in the ice - this water should be more like the liquid you poured off first than like the ice)

Let the remainder of the ice melt (you can speed it up by heating in a microwave for 30 seconds or so). Pour this liquid into the empty 8 oz cup and label. This water is from the ice in your experiment.

Now, in the experiment, the first liquid is like the salt water lava in the eruption on our planet. The second liquid, from the ice, is like the ice left behind deeper in the planet. Your question is: Is the ice inside the planet saltier, less salty, or the same saltiness.

Each person in your group should taste each sample of water, from the first liquid and from the ice. Write down a "1" for the one you think is less salty and a "2" for the one you think is more salty. Compare your results to those of the rest of your group. Is one saltier than the other?

Now, what is the answer to the puzzle: Is the solid part left behind inside the planet JUST AS SALTY, or SALTIER, or NOT AS SALTY as the salty water "lava" that erupted to the surface?

Note: This experiment is qualitative, meaning you didn't measure the actual concentrations, only which one had more or less salt. You can make it quantitative by using a salinity tester to measure the actual concentration of salt in the water. However, you should be aware that little pockets of liquid water trapped in your ice contaminate your results and will make your results differ somewhat from the "true" distribution of salt between liquid and solid water.