1. Lunar Interior Magnetic Sounding
Unanswered questions about the lunar interior
Did the Moon form from a collision between a Mars size object and the Earth? What was the impactor size? How much of the lunar material came from the Earth and how much came from the impactor? How deep and variable (in composition and depth) was the lunar magma ocean?
Does the Moon have a core? What is its size?
Electromagnetic induction studies from Apollo era surface and spacecraft magnetometers support the lunar magma ocean hypothesis but are ambiguous because of low signal/noise ratio.
Data from Lunar Prospector (see Figure 2, right, Hood et al. 1999, GRL) is supportive of a lunar core (radius ~ 400 km) but the results are marginal again because of the low signal/noise ratio of the data.
The unique two point simultaneous measurements from Artemis will allow us to separate the external (inducing field) and internal (the induction response) fields at a wide range of frequencies, uniquely and accurately, resulting in a much higher signal/noise ratio.
The shorter periods (minute to tens of minutes) will provide information on the crust and upper mantle of the moon, helping us determine the location of the magma ocean boundary.
The longer periods (several hours) will provide information on the size and conductivity of the lunar core (see Figure 1, right).
Only night side data will be used to minimize the compressional effect of the solar wind on the induction field.
Hood et al. 1999, GRL 1
Hood et al. 1999
Studies of the crust and upper mantle provide information on the past history of the Moon. The moon is believed to have formed from a collision of Mars size body and the Earth. The energy released from the giant impact melted the top several hundred kilometers of the Moon. . Subsequent differentiation of the magma ocean resulted in the formation of a plagioclase-rich thin crust (~ 50 km thickness) and a mafic mantle. Studies of the crust from induction measurements at high frequencies will provide information on the formation of the Moon.
Seismic measurements from the Apollo Seismometers were unable to identify any signals reflected from the core-mantle boundary. The only evidence for the core of the Moon is provided by the electromagnetic induction studies.
Figure 1 right shows the decay of the dipole moment induced by a square pulse in the ambient field from conducting sources in the crust and mantle (for various assumed values of conductivities and radii). After a few hours, the only field remaining would be that of the core whose dipole moment (dotted line) does not decrease appreciatively in this time.
Figure 2 shows the averaged induction signal from Lunar Prospector after the Moon entered the lobe of the earth’s magnetotail. The signal was obtained by averaging data over 7 orbits.