1
00:00:03,200 --> 00:00:06,300
This is geology B with Oliver 
stromwell. 

2
00:00:07,700 --> 00:00:11,200
Following the Apollo missions to
the moon in the late 1960s and 

3
00:00:11,200 --> 00:00:15,900
early 1970s, it became widely 
accepted that the moon formed as

4
00:00:15,900 --> 00:00:18,200
a result of a cataclysmic 
Collision. 

5
00:00:18,600 --> 00:00:21,300
When the Earth was forming about
four and a half billion years 

6
00:00:21,300 --> 00:00:24,600
ago. 
But a puzzle has remained as to 

7
00:00:24,600 --> 00:00:28,800
just what sort of collision must
have occurred in order to result

8
00:00:28,800 --> 00:00:31,100
in the earth-moon system. 
We see today. 

9
00:00:32,000 --> 00:00:36,600
How big was the impacting body? 
What was its composition and 

10
00:00:36,600 --> 00:00:38,800
what happened in the aftermath 
of the Collision? 

11
00:00:40,200 --> 00:00:43,600
Sarah Stewart is a professor in 
the department of Earth and 

12
00:00:43,600 --> 00:00:47,300
planetary Sciences at the 
University of California Davis. 

13
00:00:48,000 --> 00:00:51,400
She conducts, lab experiments in
conjunction with numerical 

14
00:00:51,400 --> 00:00:55,300
modeling and out-of-the-box 
thinking to develop new 

15
00:00:55,300 --> 00:00:58,100
scenarios for a moon-forming 
impact. 

16
00:00:58,100 --> 00:01:01,300
And the subsequent dynamical 
Evolution, that appears to 

17
00:01:01,300 --> 00:01:04,700
resolve many of the outstanding 
difficulties associated with 

18
00:01:04,700 --> 00:01:07,800
previous Solutions. 
Sarah Stewart. 

19
00:01:07,900 --> 00:01:11,500
Welcome to geology B. 
You Oliver for inviting me. 

20
00:01:12,400 --> 00:01:15,700
Why do planetary scientists 
believe it was a giant impact 

21
00:01:15,700 --> 00:01:19,000
that created the moon. 
It's the best idea to explain 

22
00:01:19,000 --> 00:01:22,500
the present day rotation of the 
earth, and the orbit of the moon

23
00:01:22,600 --> 00:01:24,400
along with the composition of 
the moon. 

24
00:01:25,000 --> 00:01:28,000
But it wasn't until after the 
Apollo missions that scientists 

25
00:01:28,000 --> 00:01:30,400
realized how common giant 
impacts would have been during 

26
00:01:30,400 --> 00:01:33,700
Planet formation. 
And right now, we know that 

27
00:01:33,700 --> 00:01:37,200
giant impacts were very likely 
events as planets were growing. 

28
00:01:37,900 --> 00:01:41,400
So what was the traditional Moon
formation scenario? 

29
00:01:41,400 --> 00:01:45,500
That prevailed before the Apollo
missions to the Moon before the 

30
00:01:45,500 --> 00:01:49,800
Apollo missions, we didn't have 
a single best idea for how the 

31
00:01:49,800 --> 00:01:53,300
moon formed there were actually 
multiple competing ideas. 

32
00:01:53,700 --> 00:01:57,100
The moon could have been 
captured or a created alongside 

33
00:01:57,100 --> 00:02:00,500
the Earth the whole time or 
there was even the idea that the

34
00:02:00,500 --> 00:02:03,700
moon spun out of the Equator of 
the Earth in a process called 

35
00:02:03,700 --> 00:02:10,000
fission, but the analysis of the
Apollo lunar samples changed our

36
00:02:10,000 --> 00:02:14,700
ideas, the data from Apollo 
ruled out, all of the ideas that

37
00:02:14,700 --> 00:02:16,900
scientists had for the origin of
the Moon. 

38
00:02:17,800 --> 00:02:21,900
The moon's core was too small to
explain the moon. 

39
00:02:21,900 --> 00:02:24,900
If it had been captured from 
somewhere else in the inner 

40
00:02:24,900 --> 00:02:28,400
solar system, the moon's 
composition is similar to 

41
00:02:28,400 --> 00:02:31,800
Earth's mantle but different 
enough that it doesn't explain 

42
00:02:31,800 --> 00:02:37,800
the idea that the moon just grew
along The Earth together and 

43
00:02:37,800 --> 00:02:41,100
fission had so many problems 
that people really didn't take 

44
00:02:41,100 --> 00:02:42,100
it. 
Seriously. 

45
00:02:42,500 --> 00:02:46,800
It didn't explain today's length
of day or the total angular 

46
00:02:46,800 --> 00:02:49,100
momentum of the Earth, Moon 
system. 

47
00:02:50,300 --> 00:02:53,200
So the Apollo Mission sent 
everyone back to the drawing 

48
00:02:53,200 --> 00:02:57,000
board on the question of how did
the moon form the giant impact 

49
00:02:57,000 --> 00:03:00,900
idea only emerged in this 
aftermath where we were looking 

50
00:03:00,900 --> 00:03:05,900
for new ideas and it was at that
time, that the Studies on how 

51
00:03:05,900 --> 00:03:09,500
all the planets grew together 
around the Sun realized that, 

52
00:03:09,500 --> 00:03:13,200
there would have been these 
giant hugely energetic 

53
00:03:13,200 --> 00:03:16,200
collisions, and before that 
people had really been thinking 

54
00:03:16,200 --> 00:03:19,900
that the energy of Planet 
accretion involve much smaller 

55
00:03:19,900 --> 00:03:24,200
bodies colliding. 
So how does the giant impact 

56
00:03:24,200 --> 00:03:28,900
scenario result in a Moon Earth 
system that we see today? 

57
00:03:29,500 --> 00:03:34,000
So, the way the giant impact was
proposed is that the energy of 

58
00:03:34,000 --> 00:03:38,700
the event would Part of Earth's 
mantle and send it into orbit in

59
00:03:38,700 --> 00:03:43,000
a disk around the earth and the 
moon would grow out of that disc

60
00:03:43,000 --> 00:03:48,100
of hot debris and in the 
conceptual phase, this could 

61
00:03:48,100 --> 00:03:51,800
explain why the Moon looked 
similar to the Earth. 

62
00:03:51,800 --> 00:03:55,600
But not exactly like the Earth 
because it was a hot disc and 

63
00:03:55,600 --> 00:04:00,300
the moon didn't grow with any 
water or what we call moderately

64
00:04:00,300 --> 00:04:04,400
volatile elements elements, 
like, potassium and sodium that 

65
00:04:04,400 --> 00:04:06,600
vapor. 
Her eyes at intermediate 

66
00:04:06,600 --> 00:04:10,300
temperatures, like, 1,000 
degrees and so these 

67
00:04:10,300 --> 00:04:13,100
distinctions could be explained 
by a giant impact. 

68
00:04:13,100 --> 00:04:16,899
In addition to the large, 
massive, our moon, our moon is 

69
00:04:17,100 --> 00:04:21,300
1.2 percent, the mass of the 
Earth, and that's much larger by

70
00:04:21,300 --> 00:04:24,500
fraction compared to the moon's 
that we see in the outer solar 

71
00:04:24,500 --> 00:04:27,300
system. 
And finally, the giant impact 

72
00:04:27,300 --> 00:04:31,800
gave us a way to explain why we 
have a 24-hour day today along 

73
00:04:31,800 --> 00:04:33,500
with where the Moon is in its 
orbit. 

74
00:04:33,700 --> 00:04:36,800
So there are a lot of great 
features in the idea of the 

75
00:04:36,800 --> 00:04:40,500
giant impact from our earlier 
conversation. 

76
00:04:40,500 --> 00:04:45,800
It seems, as if there are three 
main characteristics of today's 

77
00:04:45,900 --> 00:04:50,200
earth-moon system, all of which 
a successful model has to be 

78
00:04:50,200 --> 00:04:55,900
able to replicate, these are the
compositional similarity of the 

79
00:04:55,900 --> 00:04:58,700
Moon and the Earth that you 
already referred to. 

80
00:04:59,300 --> 00:05:04,800
The tilt of the earth spin with 
respect to the ecliptic, which 

81
00:05:04,800 --> 00:05:09,300
is the plane of the Earth's 
orbit around the sun and the 

82
00:05:09,300 --> 00:05:14,000
inclination of the moon's orbit.
It's been difficult to develop a

83
00:05:14,000 --> 00:05:17,700
single model that can reproduce 
all these characteristics at 

84
00:05:17,700 --> 00:05:21,800
once. 
Why is that there are so many 

85
00:05:21,800 --> 00:05:27,300
pieces to the puzzle on the 
origin of the Moon, and they 

86
00:05:27,300 --> 00:05:31,700
involve both understanding the 
Eric's of collisions, the 

87
00:05:31,700 --> 00:05:35,000
chemistry of what occurs after 
those collisions and the 

88
00:05:35,000 --> 00:05:41,500
dynamical evolution of the lunar
orbit and the giant impact 

89
00:05:41,500 --> 00:05:44,200
hypothesis. 
Gave us the bulk 

90
00:05:44,200 --> 00:05:47,200
characteristics, like the mass 
of the Moon. 

91
00:05:47,200 --> 00:05:50,700
The fact that its core was so 
small because most of the debris

92
00:05:51,000 --> 00:05:54,900
was formed from the rocky 
mantels of the colliding bodies,

93
00:05:55,700 --> 00:06:00,900
but the way the original 
hypothesis worked out In terms 

94
00:06:00,900 --> 00:06:04,700
of, where did the material come 
from that went into the moon 

95
00:06:04,700 --> 00:06:07,900
forming disc. 
Most of that material came from 

96
00:06:08,100 --> 00:06:10,700
the body that struck the young 
Earth, and we gave that body a 

97
00:06:10,707 --> 00:06:12,400
name. 
Its name is Thea. 

98
00:06:12,800 --> 00:06:16,800
So the moon would look 
compositionally a lot like this 

99
00:06:16,800 --> 00:06:19,400
other body. 
And we know, from studying 

100
00:06:19,400 --> 00:06:22,600
meteorites that no two planets 
in the solar system are the 

101
00:06:22,600 --> 00:06:25,600
same, no, two asteroids, you're 
identical. 

102
00:06:25,600 --> 00:06:30,400
And so we would expect there to 
be different In its chemistry 

103
00:06:30,400 --> 00:06:33,600
and in the Isotopes of the 
different elements in the body. 

104
00:06:34,000 --> 00:06:37,200
What we learned from the Apollo 
samples is that the moon was 

105
00:06:37,200 --> 00:06:42,400
identical to the Earth in many 
ways and that really called into

106
00:06:42,400 --> 00:06:46,000
question, the giant impact 
hypothesis, were the moon form 

107
00:06:46,000 --> 00:06:49,500
mostly from this other body. 
We tried to come up with an 

108
00:06:49,500 --> 00:06:54,100
explanation of how to make the 
Moon look more like the Earth. 

109
00:06:55,000 --> 00:06:59,800
And the solution was to mix the 
two bodies to a greater degree 

110
00:06:59,900 --> 00:07:02,100
than in the original giant 
impact. 

111
00:07:03,000 --> 00:07:07,100
And the big idea that we had at 
the time was that the 

112
00:07:07,100 --> 00:07:11,400
moon-forming impact did not lead
to our 24 hour a day, but 

113
00:07:11,400 --> 00:07:16,400
actually made the earth spin 
much faster and in giant impacts

114
00:07:16,400 --> 00:07:19,800
were the Earth spins faster. 
The two bodies mix to a greater 

115
00:07:19,800 --> 00:07:23,400
degree and you could make the 
disc out of an equal portion 

116
00:07:23,400 --> 00:07:25,800
mixture as what went To the 
Earth. 

117
00:07:26,100 --> 00:07:29,400
And this was one way to explain 
how the chemistry and the 

118
00:07:29,400 --> 00:07:33,700
Isotopes of each element were 
more similar between the Earth 

119
00:07:33,700 --> 00:07:36,100
and the moon than the original 
giant impact model. 

120
00:07:36,700 --> 00:07:40,500
The catch though, is that you 
still have to get to today's 

121
00:07:40,500 --> 00:07:45,400
24-hour day and you must do that
during the lunar tidal evolution

122
00:07:45,700 --> 00:07:48,400
by transferring angular momentum
away. 

123
00:07:49,200 --> 00:07:53,300
Probably through a resonance 
with the Sun that slows down the

124
00:07:53,300 --> 00:07:57,500
spin of the Earth and Leads to 
the path that gets to our 24 

125
00:07:57,500 --> 00:08:01,700
hour day. 
So if so, to speak aloud to have

126
00:08:01,700 --> 00:08:06,000
a high angular momentum impact 
in which the Earth starts out 

127
00:08:06,000 --> 00:08:09,700
spinning very fast. 
How does that help us with? 

128
00:08:09,700 --> 00:08:12,700
The question of the same 
composition in the earth and 

129
00:08:12,700 --> 00:08:16,900
moon and ending up with the 
right orbital characteristics 

130
00:08:16,900 --> 00:08:22,100
today compared to the original 
giant impact model these high 

131
00:08:22,100 --> 00:08:28,600
angular momentum Giant Facts 
were also more energetic and led

132
00:08:28,600 --> 00:08:33,600
to more vaporization and more 
complete mixing between the two 

133
00:08:33,600 --> 00:08:36,100
bodies. 
However, when they were first 

134
00:08:36,100 --> 00:08:43,000
proposed in 2012, there were 
only a few very specific impact 

135
00:08:43,000 --> 00:08:48,600
scenarios that could lead to the
exactly the same mixture in the 

136
00:08:48,600 --> 00:08:52,000
moon, as in the earth and the 
response from the scientific 

137
00:08:52,000 --> 00:08:54,600
Community was to have some 
skepticism. 

138
00:08:54,700 --> 00:08:58,400
Mm, you know, is that really 
correct that you would need? 

139
00:08:58,500 --> 00:09:02,600
Perhaps a special case giant 
impact to explain the Earth and 

140
00:09:02,600 --> 00:09:05,300
the moon. 
And so there was some pushback 

141
00:09:05,300 --> 00:09:07,300
and it seemed like we were still
missing something. 

142
00:09:07,800 --> 00:09:10,600
Okay, so what did you do? 
Next? 

143
00:09:11,300 --> 00:09:16,600
It took several years of working
on these high energy, high angle

144
00:09:16,600 --> 00:09:20,200
momentum giant impacts to try 
and understand them and they 

145
00:09:20,200 --> 00:09:24,000
were just very different from 
what anyone had studied before 

146
00:09:24,100 --> 00:09:28,200
because Previously, the entire 
Community was focused on 

147
00:09:28,200 --> 00:09:32,300
explaining the 24-hour day that 
we have today, but once we were 

148
00:09:32,300 --> 00:09:37,400
free to explore a larger 
parameter space, we made giant 

149
00:09:37,400 --> 00:09:41,300
impacts that had outcomes that 
we hadn't seen before. 

150
00:09:42,700 --> 00:09:49,200
And it turns out that giant 
impacts can transform a planet 

151
00:09:49,200 --> 00:09:53,300
into a completely different type
of astronomical object. 

152
00:09:53,600 --> 00:10:00,900
And we we figured this out 
around 2016 by looking at the 

153
00:10:00,900 --> 00:10:06,000
computer calculations of these 
giant impacts and realizing that

154
00:10:06,000 --> 00:10:08,700
the object in the middle. 
Just didn't look like a planet 

155
00:10:08,700 --> 00:10:11,300
anymore. 
So there's a very specific 

156
00:10:11,300 --> 00:10:15,600
definition for a planet that the
international astronomical Union

157
00:10:15,600 --> 00:10:18,500
wrote down when we tried to 
decide, if Pluto was still a 

158
00:10:18,500 --> 00:10:23,600
planet planet has to orbit the 
sun, it has to clear its Zone 

159
00:10:23,600 --> 00:10:27,800
around. 
In its orbit around the sun and 

160
00:10:27,800 --> 00:10:32,100
it needs to have a spherical 
shape that represents what we 

161
00:10:32,100 --> 00:10:36,900
call hydrostatic equilibrium. 
And so, any rotating body will 

162
00:10:36,900 --> 00:10:39,600
have a slightly oblate 
spheroidal shape. 

163
00:10:39,600 --> 00:10:42,600
That's a balance between 
gravitational and pressure 

164
00:10:42,600 --> 00:10:46,800
forces within the body. 
And what happens in a giant 

165
00:10:46,800 --> 00:10:51,600
impact is that the planet 
becomes so hot that it partially

166
00:10:51,600 --> 00:10:54,600
vaporizes and puffs up and it's 
radius. 

167
00:10:54,700 --> 00:10:59,600
Has become so big that it can't 
rotate all together anymore and 

168
00:10:59,600 --> 00:11:05,200
maintain that spheroidal shape. 
And at that time it transitions 

169
00:11:05,200 --> 00:11:09,800
to an object that forms a disk 
that's constantly changing and 

170
00:11:09,800 --> 00:11:12,400
shape. 
So it's no longer hydrostatic 

171
00:11:12,900 --> 00:11:15,800
and it no longer satisfies the 
definition of a planet. 

172
00:11:16,600 --> 00:11:20,300
And so we had discovered in our 
computers, this entirely new 

173
00:11:20,300 --> 00:11:24,600
object and so we had to give it 
a name we call these objects. 

174
00:11:24,700 --> 00:11:28,700
It's siness Diaz and we derive 
the word from the Goddess. 

175
00:11:28,700 --> 00:11:31,500
Hestia is the goddess of the 
Hearth and Home. 

176
00:11:31,500 --> 00:11:33,000
We were studying the origin of 
the earth. 

177
00:11:33,000 --> 00:11:36,400
It was hot. 
And we attach the prefix in to 

178
00:11:36,400 --> 00:11:39,800
emphasize that the disc that the
Moon is forming in was actually 

179
00:11:39,800 --> 00:11:44,700
part of the earth. 
And we proposed that the moon 

180
00:11:44,700 --> 00:11:49,100
formed basically inside the 
Earth, when the Earth had been 

181
00:11:49,100 --> 00:11:53,700
transformed to this huge object 
that we call a synesthesia Can 

182
00:11:53,700 --> 00:11:57,400
you describe what exactly a 
semester is? 

183
00:11:58,500 --> 00:12:02,700
When you think of a planet, you 
think of a spherical body that 

184
00:12:02,700 --> 00:12:08,900
rotates around altogether and 
the shape of that body is 

185
00:12:08,900 --> 00:12:13,000
spherical by a balance of forces
that we call hydrostatic 

186
00:12:13,000 --> 00:12:17,300
equilibrium. 
But if you make the body larger 

187
00:12:17,600 --> 00:12:22,700
by heating it up and vaporizing,
the outer layers, and making it 

188
00:12:22,700 --> 00:12:27,500
spin a little faster, increases,
the size of the Equator of that 

189
00:12:27,500 --> 00:12:33,300
object, And there's a certain 
distance from the center where 

190
00:12:33,900 --> 00:12:37,100
the body can no longer rotate 
altogether. 

191
00:12:37,900 --> 00:12:43,500
And any material outside of that
special distance, rotates more 

192
00:12:43,500 --> 00:12:51,700
slowly in a disk, like region 
around the Central Mass that 

193
00:12:51,700 --> 00:12:56,900
transition, we call the co 
rotation limit for a planet and 

194
00:12:56,900 --> 00:13:00,900
it depends on what the It is 
made out of and how massive it 

195
00:13:00,900 --> 00:13:04,500
is exactly what that critical 
distance is. 

196
00:13:05,900 --> 00:13:09,900
But once it's exceeded the shape
of the object is no longer a 

197
00:13:09,900 --> 00:13:15,900
nice spheroid but instead makes 
a fin disk that's connected to 

198
00:13:15,900 --> 00:13:21,400
the interior mass and that 
object is called a Celestia but 

199
00:13:21,900 --> 00:13:27,400
is the Sonesta just the bit 
beyond the rigid part When a 

200
00:13:27,400 --> 00:13:32,000
body exceeds the co rotation 
limit, There's no distinction 

201
00:13:32,000 --> 00:13:35,700
between what is on the inside of
that distance and on the 

202
00:13:35,700 --> 00:13:40,100
outside, all of the material is 
connected and interacting in the

203
00:13:40,100 --> 00:13:44,400
same fluid system. 
And so the name Sonesta 

204
00:13:44,400 --> 00:13:48,800
represents the entire system. 
All of the mass because there is

205
00:13:48,800 --> 00:13:52,800
no distinction between a planet 
in a disk anymore. 

206
00:13:52,900 --> 00:13:56,400
It's all one body so that'd be 
no sir. 

207
00:13:56,500 --> 00:14:00,300
Suffice to stand on when a rocky
planet. 

208
00:14:00,300 --> 00:14:04,400
Like the Earth has turned into a
synesthesia by a giant impact. 

209
00:14:04,700 --> 00:14:09,400
It's so hot that the gas and the
outer layer becomes a high 

210
00:14:09,400 --> 00:14:13,600
pressure gas and a supercritical
fluid as you go deeper and 

211
00:14:13,600 --> 00:14:16,100
deeper at higher and higher 
pressures. 

212
00:14:16,600 --> 00:14:22,100
And there's no liquid surface 
layer present at these extreme 

213
00:14:22,100 --> 00:14:25,100
energy conditions. 
So that's right. 

214
00:14:25,100 --> 00:14:27,300
There would be nowhere to float.
About. 

215
00:14:27,800 --> 00:14:31,400
I can see that this would give 
you a framework where you would 

216
00:14:31,400 --> 00:14:35,700
get an awful lot of mixing which
would solve the problem of the 

217
00:14:35,700 --> 00:14:38,200
Moon having the same composition
as the Earth. 

218
00:14:38,800 --> 00:14:43,300
But does it result in the 
orbital characteristics that we 

219
00:14:43,300 --> 00:14:45,200
see in the earth-moon system 
today? 

220
00:14:46,200 --> 00:14:50,500
There are still big requirements
on what happens after the Moon. 

221
00:14:50,500 --> 00:14:55,000
Accretes from that debris before
we can get to the present day. 

222
00:14:56,100 --> 00:15:00,900
Initially, any giant impact 
makes a disc in the equator of 

223
00:15:00,900 --> 00:15:04,800
the new post-impact body. 
And so we expect the moon's 

224
00:15:04,800 --> 00:15:09,400
orbit to be in the equator of 
the young Earth but as the Moon 

225
00:15:09,400 --> 00:15:14,800
tidally migrates away from the 
Earth it transitions to having 

226
00:15:14,800 --> 00:15:19,300
its orbit be dominated by 
interactions with the sun and 

227
00:15:19,300 --> 00:15:23,200
the ecliptic plane of the solar 
system that the sun defines. 

228
00:15:24,500 --> 00:15:29,400
And in that transition, we 
expect the moon's inclination to

229
00:15:29,400 --> 00:15:34,100
end up perfectly in that plane. 
And if the moon were in that 

230
00:15:34,100 --> 00:15:37,400
same plane, then we would have a
solar eclipse every month. 

231
00:15:37,500 --> 00:15:41,200
The moon would line up perfectly
with the Sun, but we don't, and 

232
00:15:41,200 --> 00:15:44,700
we don't, because the Moon is 
tilted by 5 degrees from the 

233
00:15:44,700 --> 00:15:47,700
ecliptic plane. 
And that's not what we expect, 

234
00:15:47,700 --> 00:15:49,800
if the moon formed in a giant 
impact. 

235
00:15:50,000 --> 00:15:53,000
So there's still another major 
puzzle to be solved. 

236
00:15:53,700 --> 00:15:56,200
So how did you go about 
resolving that? 

237
00:15:56,800 --> 00:16:02,100
We went back again to the idea 
of a high angular momentum giant

238
00:16:02,100 --> 00:16:04,600
impact. 
It turns out that there are many

239
00:16:04,600 --> 00:16:07,800
ways to make a siness TIA, most 
giant impacts have a lot of 

240
00:16:07,800 --> 00:16:12,300
angular momentum, and many 
different kinds of giant impacts

241
00:16:12,300 --> 00:16:15,300
can make a synesthesia. 
And so, we now have more choices

242
00:16:15,300 --> 00:16:20,000
for the kind of giant impact, 
but we proposed a path that 

243
00:16:20,000 --> 00:16:23,200
could lead to the five degree 
inclination. 

244
00:16:23,200 --> 00:16:26,300
That Involves a lot of angular 
momentum. 

245
00:16:26,700 --> 00:16:31,900
More than two, two and a half 
times what we have today and 

246
00:16:31,900 --> 00:16:37,400
that the giant impact tilted, 
the axis of the new Earth over 

247
00:16:38,000 --> 00:16:44,200
very far, maybe 65 70 degrees 
from the celestial North. 

248
00:16:44,600 --> 00:16:50,600
Today, we're at a 23-degree tilt
and a giant impact has equal, 

249
00:16:50,600 --> 00:16:54,700
probability of the Earth's, spin
axis pointing On the sky after 

250
00:16:54,700 --> 00:16:57,700
the giant impact. 
So it is plausible that the 

251
00:16:57,700 --> 00:17:02,300
Earth's spin axis was tilted 
over but then something has to 

252
00:17:02,300 --> 00:17:06,700
happen to change the tilt of the
earth and that's part of the 

253
00:17:06,700 --> 00:17:10,500
title, migration of the Moon. 
If the Earth starts tilted, over

254
00:17:10,500 --> 00:17:14,599
in the process of the Moon, 
migrating away from the earth, 

255
00:17:14,599 --> 00:17:19,000
there's an interaction with the 
Sun that leads to the Earth's 

256
00:17:19,000 --> 00:17:21,400
Tilt being restored to where it 
is today. 

257
00:17:21,400 --> 00:17:27,400
But leaves the Moon in a high, 
inclination orbit tilted away 

258
00:17:27,400 --> 00:17:31,700
from the plane of the solar 
system high enough so that 

259
00:17:31,700 --> 00:17:35,500
during its title Evolution, it 
doesn't reach zero today. 

260
00:17:36,400 --> 00:17:39,700
So this is a brand new idea that
my colleagues and I have 

261
00:17:39,700 --> 00:17:44,300
published as a way of explaining
this 5 degree tilt and it's just

262
00:17:44,300 --> 00:17:48,900
been launched into the community
to try and work on. 

263
00:17:49,500 --> 00:17:53,500
But currently it is at least a 
self-consistent way to make. 

264
00:17:53,600 --> 00:17:56,900
Our flavor Moon and its peculiar
orbit. 

265
00:17:57,200 --> 00:18:01,000
I understand you figured this 
out by running some numerical 

266
00:18:01,000 --> 00:18:05,600
simulations on a computer. 
But do you have a qualitative 

267
00:18:05,600 --> 00:18:09,700
understanding of how this 
inclination becomes reduced, 

268
00:18:10,000 --> 00:18:13,400
while the Earth's Tilt gets back
to what it is today? 

269
00:18:13,900 --> 00:18:17,700
And we settled down into this 
stable orbit that we have now 

270
00:18:19,200 --> 00:18:23,200
During the lunar tidal, 
migration outwards there are a 

271
00:18:23,200 --> 00:18:28,000
couple of dynamical resonances 
that occur and you can think of 

272
00:18:28,000 --> 00:18:32,000
these resonances being times 
where the orbit is a bit 

273
00:18:32,000 --> 00:18:35,800
unstable and very sensitive to 
large changes. 

274
00:18:36,200 --> 00:18:39,100
And so if people intuitively 
want to understand what is 

275
00:18:39,100 --> 00:18:43,900
happening with the moon's orbit 
and the Earth's Tilt, the first 

276
00:18:43,900 --> 00:18:48,900
major process has to do with 
that transition from the Moon. 

277
00:18:49,000 --> 00:18:52,900
In being an orbit around the 
earth's equator and dominated by

278
00:18:52,900 --> 00:18:57,500
interaction with the tilt of the
Earth, to the Moon, precessing 

279
00:18:57,500 --> 00:19:01,300
around the ecliptic plane and 
that transition is called the 

280
00:19:01,300 --> 00:19:06,800
LaPlace plain transition and 
when the earth begins tilted 

281
00:19:06,800 --> 00:19:11,800
over there's a big instability 
during that transition that 

282
00:19:11,900 --> 00:19:16,800
changes the tilt of the earth 
and changes the length of day of

283
00:19:16,800 --> 00:19:18,800
the earth to get us on a path to
today. 

284
00:19:19,800 --> 00:19:23,600
And leaves the moon and a high 
inclination orbit. 

285
00:19:23,900 --> 00:19:27,500
And then as the moon continues 
to migrate away from the earth, 

286
00:19:27,600 --> 00:19:31,700
it feels special Tides with the 
Earth that high inclination, 

287
00:19:31,700 --> 00:19:35,400
that lower the inclination, but 
not all the way to 0, which is 

288
00:19:35,400 --> 00:19:37,900
what we want. 
It doesn't seem intuitive at all

289
00:19:37,900 --> 00:19:41,500
to me, but I'm sure the computer
models are correct. 

290
00:19:42,100 --> 00:19:45,700
I can see that this is something
that was surprising to the 

291
00:19:45,700 --> 00:19:48,300
community. 
Did you expect to get this 

292
00:19:48,300 --> 00:19:49,000
result? 
Out. 

293
00:19:49,900 --> 00:19:53,500
Now, I have been surprised at 
pretty much every stage of my 

294
00:19:53,500 --> 00:19:58,200
work on the origin of the Moon, 
and it's partly because we live 

295
00:19:58,200 --> 00:20:03,000
in a world where our textbooks 
have given us an explanation of 

296
00:20:03,000 --> 00:20:06,600
a mars-sized object called FEA. 
That struck the Earth and gave 

297
00:20:06,600 --> 00:20:11,000
us the angular momentum that 
leads to 24 hours, but the 

298
00:20:11,000 --> 00:20:15,100
details were missing and there 
were some serious observations 

299
00:20:15,100 --> 00:20:19,100
that, couldn't be explained by 
this specific giant impact 

300
00:20:19,100 --> 00:20:22,300
event. 
And and what has happened with 

301
00:20:22,300 --> 00:20:25,900
the angular momentum work? 
And these new title models is 

302
00:20:25,900 --> 00:20:29,400
there's actually a lot of new 
ideas, a lot of changes, the 

303
00:20:29,400 --> 00:20:34,200
dust has not settled at all. 
I'm trying to explain the lunar 

304
00:20:34,200 --> 00:20:38,700
orbit and the Isotopes. 
So it's a very exciting time in 

305
00:20:38,700 --> 00:20:40,200
trying to understand the origin 
of the earth. 

306
00:20:40,200 --> 00:20:43,100
And moon, was that mentioned in 
the introduction. 

307
00:20:43,500 --> 00:20:45,900
You also conduct lab 
experiments? 

308
00:20:46,500 --> 00:20:51,100
Can you tell us about those? 
The experiments are amazing 

309
00:20:51,500 --> 00:20:54,600
because we can now reach in the 
lab. 

310
00:20:54,600 --> 00:20:58,800
All of the pressures and 
temperatures that are generated 

311
00:20:58,800 --> 00:21:03,200
during Planet formation, even in
these humongous giant impacts, 

312
00:21:04,500 --> 00:21:06,100
And what's hard to appreciate 
is? 

313
00:21:06,100 --> 00:21:09,400
How much power is in a giant 
impact. 

314
00:21:09,500 --> 00:21:12,700
You calculate, the total kinetic
energy of the two colliding 

315
00:21:12,700 --> 00:21:15,900
bodies over the time scale of 
the event. 

316
00:21:16,200 --> 00:21:19,300
The power of a giant impact is 
like all of the energy coming 

317
00:21:19,300 --> 00:21:24,200
out of the Sun and so all of 
that energy going into a planet 

318
00:21:24,900 --> 00:21:28,300
leads to nearly complete melting
and partial vaporization. 

319
00:21:29,300 --> 00:21:34,200
And we can recreate that 
thermodynamic path in the lab. 

320
00:21:34,300 --> 00:21:37,900
How on Earth could you do that? 
No pun intended. 

321
00:21:39,200 --> 00:21:43,800
We do what are called Shockwave 
experiments in the lab and we 

322
00:21:43,800 --> 00:21:49,900
can generate these huge pulses 
of pressure that lead to 

323
00:21:49,900 --> 00:21:54,000
temperatures. 
Tens of thousands of degrees by 

324
00:21:54,000 --> 00:21:59,700
using lasers or huge capacitors.
That can launch metal plates up 

325
00:21:59,700 --> 00:22:05,800
to 20, 30, 40 kilometers per 
second and that's close to the 

326
00:22:05,800 --> 00:22:08,700
speed that the Earth is orbiting
around the Sun. 

327
00:22:09,600 --> 00:22:15,200
So these facilities that can 
reach these extreme conditions 

328
00:22:15,200 --> 00:22:17,500
were built by the department of 
energy. 

329
00:22:17,600 --> 00:22:20,600
They're housed, Lawrence 
Livermore, National Lab Sandia, 

330
00:22:20,600 --> 00:22:24,900
National Lab, they were built to
try and Achieve nuclear fusion 

331
00:22:25,400 --> 00:22:31,100
by generating intense pressures.
And so we use the same platforms

332
00:22:31,100 --> 00:22:34,600
to study natural processes. 
Like giant impacts and 

333
00:22:34,600 --> 00:22:38,400
understand how planetary 
materials respond to very strong

334
00:22:38,400 --> 00:22:41,100
shock waves. 
So you managed to get some time 

335
00:22:41,100 --> 00:22:45,300
on these giant. 
Laser Fusion machines, these 

336
00:22:45,300 --> 00:22:49,200
world-class facilities that were
built by the department of 

337
00:22:49,200 --> 00:22:54,700
energy, all have basic science 
programs and so scientists from 

338
00:22:54,700 --> 00:22:58,200
all over the world can apply to 
conduct. 

339
00:22:58,600 --> 00:23:02,200
Experiments on these facilities 
and we compete with each other 

340
00:23:02,200 --> 00:23:05,000
for time. 
And so we have successfully 

341
00:23:05,000 --> 00:23:11,000
gotten precious days on these 
massive machines to make giant 

342
00:23:11,000 --> 00:23:14,700
impacts in the lab. 
So what do the results of your 

343
00:23:14,700 --> 00:23:20,700
experiments tell you? 
Because we could reach the exact

344
00:23:20,700 --> 00:23:23,900
pressures obtained, during a 
giant impact. 

345
00:23:23,900 --> 00:23:28,200
Millions of times more than 
atmospheric pressure on the 

346
00:23:28,200 --> 00:23:32,900
earth, we could determine how 
hot geologic materials got 

347
00:23:32,900 --> 00:23:36,200
during an impact event. 
And we developed a special 

348
00:23:36,200 --> 00:23:41,900
technique to find the boiling 
points of rocks and metals that 

349
00:23:41,900 --> 00:23:44,900
are reached during giant 
impacts. 

350
00:23:44,900 --> 00:23:48,100
And so we could calculate the 
action of the earth and the 

351
00:23:48,100 --> 00:23:51,900
fraction of the bodies that 
collided with us, what fraction 

352
00:23:51,900 --> 00:23:55,600
became gas and what fraction 
melted into magma. 

353
00:23:55,600 --> 00:24:00,200
Oceans does this mean you 
exploring new parts of the phase

354
00:24:00,200 --> 00:24:04,900
diagram of earth materials at 
high temperatures and pressures?

355
00:24:05,800 --> 00:24:09,300
That's right, we can't reach 
these conditions in static 

356
00:24:09,300 --> 00:24:12,500
experiments when we think of 
phase diagrams. 

357
00:24:12,500 --> 00:24:16,100
When does a material melt and 
when does it turn into a gas for

358
00:24:16,100 --> 00:24:18,700
many materials? 
Like water, we can reach all of 

359
00:24:18,700 --> 00:24:22,400
those conditions in the lab by 
controlling the temperature and 

360
00:24:22,400 --> 00:24:26,100
pressure. 
But for the main minerals in 

361
00:24:26,100 --> 00:24:31,000
planets, the temperatures are 
just too high for us to be able 

362
00:24:31,000 --> 00:24:34,100
to hold the material at those 
temperatures and study them. 

363
00:24:34,400 --> 00:24:38,300
So the only way to get to the 
boiling points of many of the 

364
00:24:38,300 --> 00:24:44,500
phases in the earth is to do it 
transiently during a shock wave 

365
00:24:44,500 --> 00:24:48,600
experiment. 
And then we We can generate a 

366
00:24:48,600 --> 00:24:53,600
little parcel of rock, that's 
boiling and study it and the 

367
00:24:53,600 --> 00:24:58,800
whole experiment could last tens
of NS over, what sort of scale, 

368
00:24:58,800 --> 00:25:00,900
how big a piece can you 
experiment on? 

369
00:25:01,200 --> 00:25:05,100
And is it a piece of Olivine? 
You stick inside these machines?

370
00:25:06,100 --> 00:25:10,300
We've been studying the major 
minerals in planets that 

371
00:25:10,300 --> 00:25:15,300
includes Olivine and Bridgeman 
night type compositions, which 

372
00:25:15,300 --> 00:25:20,900
is the main mineral in the lower
mantle and our samples are tiny 

373
00:25:21,000 --> 00:25:25,600
because we can only take a small
volume to these high pressures 

374
00:25:25,600 --> 00:25:29,100
in a controlled way so that we 
have time to study it. 

375
00:25:29,400 --> 00:25:32,800
So the sample thicknesses are 
only a few hundred microns 

376
00:25:32,800 --> 00:25:35,700
thick. 
So how do you use the results of

377
00:25:35,700 --> 00:25:39,400
your Lab experiments to try and 
figure out what happened during 

378
00:25:39,400 --> 00:25:44,300
this Collision. 
We use our lab data to extract, 

379
00:25:44,300 --> 00:25:48,400
the thermodynamic properties of 
the major minerals in the earth.

380
00:25:49,400 --> 00:25:53,300
And we construct a model for 
that material that is called an 

381
00:25:53,300 --> 00:25:58,600
equation of state and that model
is fed into the computer codes 

382
00:25:58,800 --> 00:26:03,800
that calculate the giant impacts
and so because of our lab data, 

383
00:26:04,600 --> 00:26:09,200
we could believe the 
temperatures being calculated in

384
00:26:09,200 --> 00:26:15,100
the codes and Just that the 
amount of vapor that was being 

385
00:26:15,100 --> 00:26:21,800
calculated was real. 
And that's when we realized 

386
00:26:21,800 --> 00:26:27,300
that, these huge vaporous 
planets were as large as the 

387
00:26:27,300 --> 00:26:31,600
codes were calculating, that 
this was a, an accurate result 

388
00:26:32,000 --> 00:26:37,200
and that lent support to the 
idea that this new object called

389
00:26:37,200 --> 00:26:41,700
hacen, esto is being created. 
So if we adopt your soon, Your 

390
00:26:41,700 --> 00:26:46,600
model and the results of your 
dynamical simulations for the 

391
00:26:46,600 --> 00:26:49,300
subsequent evolution of the 
Earth and Moon orbits. 

392
00:26:50,000 --> 00:26:53,500
Does that managed to replicate 
all the compositional and 

393
00:26:53,500 --> 00:26:57,200
orbital characteristics. 
We observe in the Earth and Moon

394
00:26:57,700 --> 00:27:02,700
without positing a contrived 
low-probability scenario for the

395
00:27:02,700 --> 00:27:06,000
nature of the moon-forming 
impact. 

396
00:27:06,600 --> 00:27:08,100
Everything about planet 
formation. 

397
00:27:08,100 --> 00:27:11,600
That we know, today tells us 
there were lots of giant impacts

398
00:27:11,600 --> 00:27:15,500
and most of them are energetic 
and have a lot of angular 

399
00:27:15,500 --> 00:27:17,400
momentum. 
And many of them will make 

400
00:27:17,400 --> 00:27:20,800
Celestia's. 
And so I think the idea of a 

401
00:27:20,800 --> 00:27:23,300
Sonesta is not a low probability
event. 

402
00:27:23,300 --> 00:27:27,500
There's a lot of work though, 
that still needs to be done in 

403
00:27:27,500 --> 00:27:31,800
order to fill in all of the 
puzzle pieces for what we see 

404
00:27:31,800 --> 00:27:34,200
today. 
And one of the biggest 

405
00:27:34,200 --> 00:27:37,500
challenges that we have is 
Linking. 

406
00:27:37,600 --> 00:27:40,400
Our physical model of a giant 
impact. 

407
00:27:40,400 --> 00:27:43,800
Where usually were only 
calculating the first 24 hours 

408
00:27:43,800 --> 00:27:48,700
for The First 48 hours. 
We now have to link that to data

409
00:27:48,700 --> 00:27:52,800
that we have on the Moon from 
four and a half billion years 

410
00:27:52,800 --> 00:27:56,200
later and in between a lot has 
happened. 

411
00:27:56,200 --> 00:28:00,600
There's that core formation on 
the earth and the moon exactly 

412
00:28:00,600 --> 00:28:04,200
how did the moon form out of the
disk, the title evolution of the

413
00:28:04,200 --> 00:28:07,800
Moon, volcanism on the moon and 
all of Is affect the chemical 

414
00:28:07,800 --> 00:28:09,900
signatures that were using to 
guide us. 

415
00:28:10,900 --> 00:28:15,600
And so the challenge that we 
have is to make specific 

416
00:28:15,600 --> 00:28:18,400
chemical predictions from our 
physical model. 

417
00:28:18,400 --> 00:28:20,600
And to date, that's been really 
difficult. 

418
00:28:21,000 --> 00:28:24,000
We've only just gotten the 
temperatures right, all right? 

419
00:28:24,000 --> 00:28:27,900
And so now we're going to go 
another step to understand the 

420
00:28:27,900 --> 00:28:33,100
chemistry, the number and 
diversity of exoplanetary 

421
00:28:33,100 --> 00:28:39,700
systems has exploded of late. 
Does your work provides some new

422
00:28:39,700 --> 00:28:42,300
predictions? 
About what we might expect to 

423
00:28:42,300 --> 00:28:47,500
see in these systems. 
All of our calculations to date.

424
00:28:48,000 --> 00:28:53,900
Indicate that siness Diaz are a 
common outcome of giant impacts.

425
00:28:54,400 --> 00:28:57,900
And what we know about planet 
formation is that every Star 

426
00:28:57,900 --> 00:29:00,900
forms with a disc that generates
planets. 

427
00:29:01,400 --> 00:29:04,300
And we think that all of those 
systems had giant impacts. 

428
00:29:04,800 --> 00:29:08,700
So, the prediction at the moment
is that almost every star in the

429
00:29:08,700 --> 00:29:14,700
universe has had Celestia's one 
or more around it, but the catch

430
00:29:15,000 --> 00:29:16,800
Is that they didn't last very 
long. 

431
00:29:17,600 --> 00:29:21,600
They cool off very quickly, 
because in most planetary 

432
00:29:21,600 --> 00:29:25,800
systems, it's not hot enough to 
sustain Rock Vapor, that would 

433
00:29:25,800 --> 00:29:27,800
only occur very close to the 
star. 

434
00:29:27,900 --> 00:29:33,600
So, most would cool off quickly 
and the gas would rain down onto

435
00:29:33,600 --> 00:29:36,300
the planet to form a magma 
ocean. 

436
00:29:37,000 --> 00:29:41,100
And we would have to be very 
lucky to see that in action. 

437
00:29:41,700 --> 00:29:47,500
Now we've seen the debris from 
Giant packs around other stars 

438
00:29:47,800 --> 00:29:52,200
and so we can and astronomers 
are looking for other odd 

439
00:29:52,200 --> 00:29:56,400
features around young stars 
during the planet formation 

440
00:29:56,400 --> 00:30:00,600
stage and we have made 
predictions for what a 

441
00:30:00,600 --> 00:30:04,200
synesthesia should look like, 
around another star but we 

442
00:30:04,200 --> 00:30:07,200
haven't found one yet. 
Sarah Stewart. 

443
00:30:07,500 --> 00:30:11,100
Thank you very much. 
Thank you for having me for 

444
00:30:11,100 --> 00:30:14,000
illustrations that support this 
podcast. 

445
00:30:14,300 --> 00:30:16,400
You can go to geology B.com