The Moon: Internal Structure & Magma Ocean

Transcription

1 The Moon: Internal Structure & Magma Ocean 1

2 Lunar Magma Ocean & Lunar Interior 2 Two possible views of the Moon s interior:

3 The Moon: Internal Structure 3 Like Earth, the Moon is a differentiated body. However, the exact nature of the interior structure of the Moon is still debated. The Moon is believed to possess a rather small Fe-rich core that is surrounded by a thick, solid mantle. The exact nature of the core (specific composition, size, liquid or solid) is still debated.

4 Planetary Dynamo Thermal & compositional gradients in liquid outer core result in convection (Fe-rich outer core), which in turn produces a magnetic field. 4

5 The Lunar Dynamo Magnetic measurements of Apollo samples indicate that the Moon had a very intense magnetic field from ~ Ga, and it then declined substantially by 3.3 Ga. 5 The relatively long-lived nature of this dynamo suggests crystallization of a core. [from Weiss & Tikoo, Science, 2014] Fig. 6. Paleointensity measurements of the lunar magnetic field using modern methods and estimated lifetimes of various lunar dynamos. Each point represents measurements of a single Apollo sample. Circles represent actual paleointensities, downward arrows represent upper limits on paleointensity, and right arrows represent upper limits on age. Green and blue points were measured using the IRM and Thellier-Thellier methods, respectively, and the upper limits were derived from the ARM method. Note the datum at <7 Ma at the extreme right of the figure.the shaded green box encompasses the mean paleointensity value for the period 3.56 to 4.25 Ga (central green line) and its estimated 2 SD uncertainty (upper and lower lines) (see table S6). The paleointensity value for (leftmost green point) is currently not well constrained due to spurious demagnetization effects. Vertical dashed lines show estimated maximum lifetimes of various proposed lunar dynamo mechanisms: purely thermal convection in a dry mantle (with and without an early thermal blanket surrounding the core); impact-driven mantle rotational changes, purely thermal convection in a wet mantle; precession; and thermochemical core convection driven by core crystallization. The horizontal line shows the maximum lunar surface field as estimated from Eq. 2. The lifetimes of the precession and thermochemical dynamos are highly uncertain. Paleointensity data set is from (14, 18, 50, 59 61, 63, 66)andisalsolistedintableS6.

6 The Moon: Internal Structure 6 Recent re-interpretation of seismic data from the Apollo missions suggests the presence of a solid inner core, a liquid outer core, and a partially molten zone at the base of the mantle. This structure is a bit more similar to Earth than previous ideas, but it remains to be seen if it is accurate. Data from the NASA GRAIL (gravity) mission will help to answer these questions. We only have rocks from the Moon s crust, not the mantle, but some volcanic glasses (the green glasses) are believed to be very primitive (chemically speaking) and inform us about the mantle composition.

7 Magma Cooling & Crystallization 7 Different minerals have different crystallization (or melting) temperatures; as a magma cools, minerals will crystallize out in a certain order. Different minerals have different densities, and some mineral crystals can settle to the bottom of a magma chamber because they are more dense than the magma. When a mineral crystallizes, the elements that go into that mineral will be removed from the magma, thus the magma will become depleted in those elements and enriched in others.

8 Lunar Magma Ocean 8 Olivine and Pyroxene (orthopyroxene, to be precise) are the first minerals to crystallize. These minerals are dense, sink to the bottom, & accumulate in a thick, dense zone: these are the cumulate rocks These minerals are rich in Fe and Mg, so the residual melt/magma is depleted in these elements. Mg & Fe go so Ca & Al go (in a relative sense) The increase in Ca and Al causes plagioclase minerals to crystallize. This plagioclase is less dense than the magma and floats to the top. The plagioclase accumulates at the top to form the anorthositic crust, about 4.46 billion years ago. The small amount of melt remaining was very enriched in elements that didn t want to go into olivine, pyroxene, or plagioclase. This included potassium (K), the Rare Earth Elements (Th, U, Ba, Zr, etc.), and phosphorous (P). The KREEP rocks imply significant amounts of initial magma in order to get this kind of concentration, and they are ~4.36 billion years old, so melting was large-scale, and crystallization was early & fast.

9 Lunar Magma Ocean 9 The magma ocean hypothesis is able to explain: the thick anorthositic crust, the different types of mare basalts, the KREEP rocks, and it is consistent with the ages of the lunar samples. Hf-W measurements show that the Moon s core formed about ~30 million years after solar system formation. Other isotopes show that most of the magma ocean crystallized by 4.4 Ga and the final KREEP melt solidified by 4.36 Ga: it all happened early in the solar system!

10 Lunar Magma Ocean 10 Based on the chemistry of lunar rocks, it appears that at least 50% of the entire Moon was once in a molten state. This has been referred to as the Lunar Magma Ocean hypothesis. The voluminous anorthositic crust (rich in plagioclase feldspar) requires a mechanism to concentrate signficant amounts of Al in the crust. Incompatible elements (ones that tend to remain in the magma) are highly concentrated at the lunar surface, and these rocks are very old, so the concentration process happened fast.

11 Lunar Magma Ocean 11 A consequence of the crystallization sequence of minerals and the cooling of the lunar magma ocean is that very dense minerals tend to form last, especially from the later melts that have a lot of titanium. Therefore, the upper part of the mantle has a layer that is denser than the material below. It has been suggested that this was not stable, and that the dense, Ti-rich material at the top sank back down to the bottom of the mantle ( magma ocean overturn ). Depending on various assumptions about the Moon s composition and internal structure/ temperature, simulations show that this can in fact happen under certain conditions, but it s not yet clear that it actually did happen. This is an important idea because these Ti-rich melts also contained a lot of the heat-producing elements that produced long-lived volcanism.

12 Clementine: Crustal Thickness 12

13 GRAIL: Gravity & Crustal Thickness Average crust thinner than previously recognized! Al content likely similar to bulk silicate Earth. 13

14 GRAIL: Gravity & Crustal Thickness 14 As we learn more about the Moon (new missions!), our estimates of crustal thickness have decreased over time average crustal thickness 60 crustal thickness (km) A12/14 A16 Apollo zone seismic crustal thickness Apollo zone Apollo zone A16 A15 A12 Apollo zone A publication date [Taylor & Wieczorek, 2014]

15 GRAIL: Oceanus Procellarum 15 What is the origin of Oceanus Procellarum? Why are maria concentrated on the near side? In addition to being an area of thinner crust, there is also a high Th anomaly here (heat producing elements).

16 GRAIL: Oceanus Procellarum 16 The gradient in the GRAIL gravity data indicate large fracture (dike) systems. These are due to uneven cooling of the crust and act as conduits for magma. Thermal stress, not a giant impact! [Andrews-Hanna et al., 2014]

17 GRAIL: Oceanus Procellarum 17 There is a similar feature in the South Polar Terrain of Enceladus (moon of Saturn). Can this cooling/thermal stress process also explain some of the features observed on that icy body? [Andrews-Hanna et al., 2014]