First, I'd like to applaud your curiosity in the matter. Second, when it comes to any sort of scientific discussion, there is absolutely nothing which can truly be considered random (and by extension coincidental). Consider this graph of planetary velocity provided by: http://www.bbc.co.uk/education/guides/zk8hvcw/revision/4 Notice any similarities between this and your line of best fit for averaged density as a function of distance? The first part of my answer refers to the formation of our own little piece of the universe; when our solar system was formed, it is theorized that virtually all matter ended up taking on the form of a massive spinning disk. The distribution of matter as well as gravitational forces within the forming system and the rest of the universe would all have contributed to how these stellar bodies formed. Why doesn't earth's atmosphere, composed of gaseous nitrogen, oxygen, carbon dioxide, and other gasses simply dissipate into the vastness of space? Yes, partially this is due to the electromagnetic field blocking radiation from the sun from torching our little blue paradise, but forget that for a moment. It's gravity isn't it? At the same time, the atmosphere isn't being pressed against the surface of the earth into an insanely thin layer (relatively speaking it is quite thin). This is due to pressure; the gasses repel each other and seek to dissipate. However, the further away those gasses get, the weaker the gravitational force gets. At a mere 14,000 km from the center of earth, acceleration due to gravity is a mere 2 meters per second squared. Little more than a fifth what it is at the surface. However, even from this distance from our star (nearly 150,000,000 km), the sun's gravitational pull is only reduced to .06% of the strength of earth's gravity. Simply put, in order for matter to remain confined to earth, acceleration due to the gravitational pull of earth must exceed the acceleration of the matter due to any other forces, such as pressure forcing gas outwards. How would this be effected if earth were closer to the sun? Well, not only would surface temperatures increase, but the relative strength of the sun's gravitational pull would also increase. Higher temperatures mean greater pressure, and greater pressure within an atmosphere would force gas away from the surface, where planetary gravity continues to lessen; at a point, the sun's gravity itself would be enough to siphon gas away from the planet. Going further away from the sun, thermal energy decreases, and the gravitational pull of the sun also lessens. In order for matter to exist in close proximity to the sun without simply being pulled into the sun, a combination of factors including momentum, relative gravitational field strength (compared with the sun), boiling point, and propensity to absorb or reflect solar radiation must be present. To boil it down, how dense is the matter, how does the matter react to applied energy, how is the density altered by this reaction, and what are the relative gravitational strengths of the body of matter (i.e. planet) and star? A gas giant with a composition such as that of Jupiter, in Mercury's orbit, for example, should not be capable of maintaining its gaseous atmosphere in the presence of the Sun's gravity and thermal energy. Now, the gravitational pull of the sun on atmospheres has less of an effect at the present point in time, but when the solar system was in its infancy, matter lacking adequate density to be bound within a stellar body's gravitational pull (or the absence of a massive body to provide such gravitational strength) would have been pulled into that of the soon-to-be star. There are a myriad of other factors, though I hope this helps to explain some of your confusion regarding the correlation between orbital distance and average density. The simplest way to try to wrap your head about such concepts is to consider all possible relationships. If you have distance on the independent access, what would happen if you tried plotting something like gravitational pull or solar radiation intensity per square meter on the dependent? Perhaps you could even create plots for atmospheric and planetary composition.
The Pull of the Planets is a 30-minute activity in which teams of children model the gravitational fields of planets on a flexible surface. Children place and move balls of different sizes and densities on a plastic sheet to develop a mental picture of how the mass of an object influences how much effect it has on the surrounding space. This activity should be conducted after Heavyweight Champion: Jupiter!, which allows the children to discover the force of gravity in the solar system. These concepts involve more advanced science than previous activities in Jupiter's Family Secrets, and they explore more deeply the science of the Juno mission and the rich information it will return to us. Facilitators who choose to undertake this activity should have a firm grasp of the scientific basis so that misconceptions are not introduced to the children. This series is appropriate for children ages 10 to 13. What's the Point?
MaterialsFor each group of up to 30:
For each group of four children:
For each child:
For the facilitator: Preparation
Activity1. Ask the children to connect what they have learned about gravity to the motions of objects in the solar system.
Facilitator's Note: There are many different misconceptions about gravity; children may think that it is related to an object's motion, its proximity to Earth, its temperature, its magnetic field, or other unrelated concepts. Guide conversations cautiously and listen carefully to what the children say to avoid supporting their misconceptions.
Explain that space can act much like the surface of the trampoline. The indentations made on the surface represent the "gravity wells" created by massive objects in space.
Facilitator's Note: Gravity is a universal force, like magnetism and electricity. However, it becomes important only at large scales. Gravity determines the interactions stars, planets, and moons. In the model, the balls are too small to exert a significant gravitational pull on each other. However, they are gravitationally pulled toward Earth! They move toward each other because the weights of heavier objects distort the sheet and lighter objects roll "downhill."
Facilitator's Note: The Play-Doh and Styrofoam balls used in steps 5–7 serve to create test "wells" on the sheets. They should remain stationary while the children roll the marbles to see how they move at each step. Encourage the children to only roll marbles, as the Play-Doh is sticky and will not model the motion accurately.
Remind the children that the gravitational pull of a planet depends on its mass and size. Saturn is large in size, but it does not have nearly as much mass packed into its volume as Jupiter does.
Facilitator's Note: Saturn does have plenty of mass, and as they explored in Heavyweight Champion: Jupiter!, it does have gravity. However, because it is not dense, a person standing in its cloud tops would only weigh about as much as they weigh on Earth. Saturn's cloud tops are far above the planet's bulky — and gravitationally strong — center.Because the force of gravity depends on both mass and distance, planets that are puffy and less dense have less gravity at their cloud-tops or surfaces, which are far above the bulk of the mass in their interiors. This is why planets like Saturn appear to have less gravity than Neptune, despite Saturn's greater mass. You may need to remind the children of what they learned in Dunking the Planets in order for them to understand these difficult concepts.
Ask the children to draw in their journals, based on their models, how deep a gravity well the Moon, Earth, and Jupiter each create in space. Have them describe how their differences in gravity relate to each object's size and mass.
Facilitator's Note: Children also may not understand that the planets are not being significantly pulled toward each other. They are strongly pulled toward the Sun, but since they are also moving, they move around the Sun in stable orbits. Smaller objects like comets and asteroids may have less circular orbits that cross the paths of planets — sometimes resulting in a collision. Be careful when identifying the objects in this activity not to introduce misconceptions regarding planets' orbits and collisions. ConclusionExplain that the Juno mission to Jupiter will experience Jupiter's gravity in much the same way as a very, very small marble might in our model. Show a picture or video animation of Juno orbiting Jupiter. (Juno will orbit Jupiter, however, rather than falling into it.) Juno's instruments will keep careful track of how Jupiter's pull on the spacecraft changes as the spacecraft passes over the planet's surface. In this way, Juno will be able to measure how Jupiter's gravity is different from place to place. By measuring the slight changes in Juno's trajectory, scientists will learn where exactly Jupiter keeps the bulk of its mass in its deep interior. Scientists can then infer details about the composition of Jupiter's unseen lower layers and core.
If possible, build on the children's knowledge by offering them a future Jupiter's Family Secrets activity. Invite the children to return to wrap-up their investigations of Jupiter by attending the concluding activity, My Trip to Jupiter, where they create scrapbooks to document their own journeys into Jupiter's deepest mysteries! |