Presentation: the impacted worlds

The Apollo 16 mission to the moon--good times. But was there a scientifically sound reason to go to the moon? (Video link: "John Young's Lunar Salute on Apollo 16.")

After the previous presentation on Earth, we now turn to the terrestrial planets most different from Earth: Mercury, and the moon (not a planet), which are very similar in some aspects, and also very different. In the next presentation we will discuss the terrestrial planets most similar to Earth: Venus and Mars.

First: similarities and differences in their surface features.

At first glance, the moon and Mercury appear much alike, with similar colors, and covered with impact craters.

Taking a closer look at the moon, there are the lighter-colored highlands, which are completely covered with impact craters upon impact craters; and the darker-colored flat lava plains (maria, due to their passing resemblance to oceans), which fill in large impact crater basins, and the maria are themselves covered with a sprinkling of small impact craters.

Which feature on the moon is the youngest?
(A) Craters partially filled in with flat lava plains.
(B) Craters on top of flat lava plains.
(C) Flat lava plains.
(D) (There is a tie.)
(E) (Unsure/guessing/lost/help!)

What did you observe in this slide that tells you that your answer is correct? Video link: "Moon Images Shot by the Onboard HDTV of the KAGUYA.")

This was a major goal of the Apollo moon missions--to verify this hypothesis of which features on the moon are oldest to youngest. Landings occurred on the maria to gather and bring back rocks for analysis on Earth, with side trips to the small impact craters located there, and then to the highlands in later landings.

Now for a close-up on Mercury, where there are also highlands saturated with impact craters, lava-filled lowlands (which are more nearly the same color as the highlands than on the moon), and interestingly long, curving ridges (sometimes called rupes or rilles that pass through the highlands and lowlands.

Which feature on Mercury is the youngest?
(A) Lava-filled lowlands.
(B) Large crater basins.
(C) Long curving ridges.
(D) (There is a tie.)
(E) (Unsure/guessing/lost/help!)

What did you observe in this slide that tells you that your answer is correct? How do you know that the long curving ridges are neither midocean rises nor subduction zones associated with tectonic motion?

Although we have never landed on any terrestrial planet beyond Earth, we have a very good understanding of which features on Mercury are oldest to youngest because of the groundwork established for similar structures on the moon. Video link: "A...Movie of Mercury's Surface.")

Second, cores and catastrophes.

If the moon and Mercury had formed in the same manner as Earth, then they would be expected to have the proportionally the same size cores as Earth (shown outlined in white in these cross-sections). The sizes of the cores of the moon and Mercury can be measured from studies of the gravitational fields, and it is found that the moon has a core that is smaller than expected for its size (making it nearly all crust), while Mercury has a core that is larger than expected for its size, meaning that its crust is quite small.

Recall from the "turkey/cornish hen effect" that small objects cool off faster than larger objects, such that the cores of the moon and Mercury will have cooled off faster than all the other terrestrial planets. This explains their lack of geological activity today, and why most of their impact craters are still intact, unchanged by the brief amount of lava flows when they were still geological active.

The very thin crust of Mercury suggests that the long curving ridges on its surface are the result of its large core cooling and shrinking, such that the crust wrinkles up, much like the skin of an apple as its insides dry out and shrink.

But what made the core of the moon so small, and Mercury's core so large?

The hypothesis best supported by evidence so far for the origin of the moon involves a large impact between two planetesimals. The larger object on the right is not really Earth, and the smaller object on the left is not really the moon. But together as they collide in the early stages of our solar system formation, these two bodies will become Earth and the moon. (Video link: "Planetary Smash-Up.")

The bulk of these two objects will combine to form Earth, with most of their cores and the surrounding crust melting, then coalescing. (Video link: "Planetary Smash-Up.")

However, the outer layers the vaporized off of both of these objects contains mostly crust material, and very little core material. This is the stuff that will eventually coalesce and form the moon in orbit around Earth, and the samples brought back from the moon by the Apollo missions best support this scenario over many other historical theories of the formation of the moon. (Video link: "Planetary Smash-Up.")

And early planetesimal collisions in the early solar system may have been very common, as a similar scenario has the cores of two objects colliding forming Mercury's core. The difference here is that the outer layers that are vaporized mostly escape, due to the weaker gravitational pull of proto-Mercury, such that there was no crust material captured to give Mercury a moon, and even the crust that was captured was not very much, giving proto-Mercury a very thin layer around its sizable core. However, other evidence suggests that Mercury could not have formed from a large impact--as its crust material is volatile (easily vaporized), such that it would not have stuck around to reform a crust after a large impact event. How to resolve two contradictory theories, each supported by different pieces of evidence...gather more evidence?