today in astronomy 111: mercury
TRANSCRIPT
Today in Astronomy 111: MercuryToday in Astronomy 111:
Mercury
The properties of Mercury Comparison of Mercury
and the Moon Water on Mercury and the
Moon Ice in the polar craters of
Mercury and the Moon Tidal locking and
Mercury’s eccentric orbit Moment of inertia and
differentiation in terrestrial planets
Mercury’s vital statistics 26
8
-3
12
Equatorial radius 2.4397 10 cm (0.383 )
Average density 5.427 gm cm Albedo 0.12 Average surface
442.5 K temperature
Orbital eccentri
58.785 days rotation period
Visits to Mercury
There have been two: NASA’s Mariner 10 conducted
three close fly-bys in 1974-1975. Since 17 March 2011, the NASA
MErcury Surface, Space ENvironment, Geochemistry and Ranging (MESSENGER) satellite has been in orbit around Mercury and will return detailed imaging, topographic, compositional, and magnetic- field data for about a year thenceforth.
MESSENGER displays its scientific instrument complement as it passes by (NASA/JHU-APL).
Moon vs. Mercury× ×
Mass (gm) 7.349 10 3.302 10
Radius (cm) 1.7381 10 2.4397 10
Mean density (gm cm ) 3.350 5.427 Albedo 0.12 0.12 Mean temperature (K) 274.5 442.5 Orbital
3.844 10 5.791 10 semimajor axis (cm) Orbita
° °
× -3
l eccentricity 0.0549 0.2056 Obliquity to Solar orbit 1.54 0.04 Tidally-locked rotation? Yes Yes Mean surface
0 2.2 10 magnetic field (G) Atmosphere None None Surface visited? Yes No Image by Tunç Tezel
Moon vs. Mercury (continued) From the appearance, albedo, and
reflectance spectra, we conclude that the surfaces are similar in composition: feldspar and pyroxene – bearing rocks.
From the density, we conclude that Mercury has more than its share of iron, in the form of a core with radius about ¾ that of the planet.
From the magnetic field, we conclude that Mercury’s core is probably liquid, for a dynamo mechanism to operate.
Diagrams by UCAR (U. Michigan)
Tectonic activity: Mercury vs. Moon
From observations of rilles and scarps, we conclude that both have a “plastic” mantle, but that Mercury has been tectonically active and the Moon has not. Recently, volcanoes have
been seen on Mercury, within the giant Caloris impact basin. The moon has none.
MESSENGER (JHU-APL/NASA)
Water on airless Solar system bodies
It is easy for hydrogen to escape from small bodies like the Moon and Mercury, and easy for sunlight to dissociate water into H and O. So is there any water at or just underneath the surface of Mercury and the Moon? There are two good reasons to think there should be, despite the difficulties: 1. The solar wind. The Sun is constantly spewing out about
in the form of the Solar wind, mostly as protons and electrons. The protons, travelling at high speeds, bury themselves
several cm below the surface of any airless body they encounter.
It doesn’t take long for each proton to become H, and for two of these to make water with a nearby O.
15 September 2011 Astronomy 111, Fall 2011 7
14 110 yearM− −
Water on airless Solar system bodies (continued)
This is probably the origin of the widespread water recently detected on the Moon by the NASA Deep Impact and ISRO Chandrayaan-1 spacecraft.
Quantity: about one liter per ton of rock or regolith. What’s closest to the surface seems to come and go with darkness and sunlight .
15 September 2011 Astronomy 111, Fall 2011 8
Temperature (left) and water concentration (right) on the Moon, a quarter of a lunar day apart. From Jessica Sunshine and the EPOXI team.
Polar ice: Mercury vs. Moon
2. Comet and asteroid impacts. In principle comets and asteroids could deliver larger amounts of water to the Moon and Mercury than the Solar wind can. It wouldn’t last long in the sunshine on either body – it
would be evaporated, dissociated, and in short order the hydrogen would escape.
But because the Moon and Mercury both have very small obliquity (tilt of rotation axis from the orbital axis), there are permanently-shaded parts of craters near the north and south pole, where some of the delivered water can last – in Mercury’s case, practically forever.
And thereby hangs a tale.
15 September 2011 Astronomy 111, Fall 2011 9
15 September 2011 Astronomy 111, Fall 2011 10
Polar ice: Mercury vs. Moon (continued)
The BMDO Clementine satellite passed over the poles of the Moon in early 1994, and took images that were interpreted as showing surface ice in polar craters. This would be of asteroidal/cometary
origin but the required amount was surprising large – typically 5 m deep, 105 m2 in area– and the results remained controversial.
Clementine was the DoD’s first try at the “faster– better – cheaper ” philosophy of spacecraft development.
Images of the Moon’s south pole, from Clementine
Polar ice: Mercury vs. Moon (continued)
In 1998, NASA’s Lunar Prospector satellite repeated the imaging ob- servations, added neutron spectro- scopy, and apparently confirmed the Clementine findings. Again plausible, but the low-
energy neutron method of detecting water (hydrogen in the ice) had never been tried before under any conditions, and thus was also controversial.
Lunar Prospector was one of NASA’s first tries at “faster – better – cheaper.” Like Clementine there were difficulties in calibrating and interpreting its measurements.
Lunar Prospector neutron- spectrometer scans during an orbit over both poles (Feldman et al 1998).
Polar ice: Mercury vs. Moon (continued)
In 1999, radar astronomers at Arecibo Observatory made a long- wavelength, radar-polarimetric study of the north pole of Mercury, and found that the perpetually- shadowed crater floors were very reflective in a way most easily explained by clean water ice. For ice detection, this technique
is less controversial than the means employed by the Lunar satellites. But it was shocking nonetheless: ice on Mercury?
Radar-reflectivity image of the north pole of Mercury (Harmon et al. 2001)
Polar ice: Mercury vs. Moon (continued)
So Arecibo tried to confirm the Clementine and Lunar Prospector results. They were able to show that there isn’t any surface ice in the Moon’s polar craters. Recently confirmed from space by
the JAXA Kaguya/SELENE and NASA LRO satellites.
Clementine-Prospector results were confounded by steep illumination angles, calibration difficulties.
The “you get what you pay for” satellite-development philosophy is making a comeback.
Radar image of the Moon’s south pole (N.J.S. Stacy, Ph.D. dissertation, Cornell U.)
Polar ice: Mercury vs. Moon (continued)
What about subsurface ice, mixed with dirt, in the Lunar polar craters? To expose this was the job of NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS): Make a small crater within a
permanently-shadowed region, with the impact of a spent rocket stage.
Analyze the composition of the ejecta, from measurements by satellites right overhead.
15 September 2011 Astronomy 111, Fall 2011 14
A permanent shadow on the moon (yellow contour), compared to neutron signal (blue) and the LCROSS impact site (red dot). (NASA and ISR/RAS)
Polar ice: Mercury vs. Moon (continued)
Results show water ice present at the level of about 50 liters of water per ton of ejecta, in this polar-crater floor. This would be on the high end of soil
water content for the driest deserts on Earth, like the Sahara and Atacama.
There’s as much molecular hydrogen (H2) in the soil as water ice, molecule for molecule. Much of the neutron signal was from this H2.
The water ice comes with very large amounts of other volatiles like CO, calcium, magnesium and mercury.
15 September 2011 Astronomy 111, Fall 2011 15
Species Abundance in ejecta (percent by mass)
H2O 5.4
H2 1.4
CO 5.7
Hg 1.2
Ca 1.6
Mg 0.4
Polar ice: Mercury vs. Moon (continued)
This is far less water than was claimed on the basis of the Clementine and Lunar Prospector results, and is consistent with the Arecibo radar observations. There’s enough to be scientifically interesting, and the
mixture of volatiles will usefully constrain models of delivery and the thermal history of the crater floors.
An entire permanently shaded region contains 106-107
gallons, which would fill a large municipal water tower. This would go a long way toward the needs of a Lunar base. Has to be laboriously extracted and purified, though.
Coming soon: MESSENGER observations of the shadows, topography and composition of the polar-crater floors on Mercury. (Not going to create any craters, though.)
15 September 2011 Astronomy 111, Fall 2011 16
Tidal locking and Mercury’s orbit
In celestial mechanics, tidal locking means that the heat dissipated during an orbit, by variation in tidal forces, is minimized. Tidal forces are the gravitational force differences between
one end and another of one body, owing to different distances away from a second body, and from one side and another of one body, owing to different directions toward the second body.
The result is a stretch along the line between the two bodies, and a compression in the perpendicular directions (see next page).
We will derive formulas for tidal forces – and much more on tides – later this semester.
15 September 2011 Astronomy 111, Fall 2011 18
Imagine yourself in orbit around Earth, feeling tidal forces
Earth pulls all things toward its center, and pulls harder on nearer things.
To you, it seems as if you are stretched in the direction of the earth’s center, and squeezed in the other direction.
Figure from Thorne, Black holes and time warps
15 September 2011 Astronomy 111, Fall 2011 19
Tidal locking and Mercury’s orbit (continued)
A body in circular orbit, with rotation period equal to revolution period, suffers no change in tidal forces as it travels its orbit. Thus there is no heat dissipated from stretching and shrinking.
The Moon’s orbit about Earth has a fairly small eccentricity (0.055), so it can be considered circular to good approximation.
The Moon’s rotation and revolution periods are identical, so it is tidally locked.
15 September 2011 Astronomy 111, Fall 2011 20
Tidal locking and Mercury’s orbit (continued)
( )
f a c a
Orbit comparison
Drawn to scale, in units of semimajor axis length (a); coordinate axis origin on one focus.
0 (Circle) ε =
0.055 (Moon) ε =
0.2056 (Mercury) ε =
Tidal locking and Mercury’s orbit (continued)
Since the distance between Sun and Mercury varies by so much through an orbit, it’s not possible to have no change in tides.
Nevertheless, astronomers used to think that Mercury’s rotation and revolution periods were equal (at 88 days), because that’s what observations seemed to indicate, and because it agreed with naïve expectations based on circular orbits.
2
Tidal locking and Mercury’s orbit (continued)
In the mid-1960s, though, radar astronomers at Arecibo Observatory showed that the rotation period of Mercury is actually closer to 59 days.
Shortly thereafter it was realized by theoreticians that the minimum in tidal heating for Mercury’s eccentric orbit was reached for an orbital period exactly 3/2 times the rotation period.
In the mid 1970s the fly-bys by Mariner 10 yielded the rotation period much more precisely and bore out this prediction. Mercury rotates exactly three times, every two orbits.
Tidal locking and Mercury’s orbit (continued)
Like the Moon, the tidal forces to which it is subjected has made Mercury slightly oblong. The orbit is arranged so that the long axis always points toward the Sun at perihelion. This keeps the tidal stretching to a minimum, though the minimum isn’t zero.
Other trivia: the perihelion of Mercury’s orbit precesses slowly in the prograde direction. Long a mystery, the precise accounting for this orbital precession by warping of space by the Sun’s gravity was one of the first triumphs of Einstein’s general theory of relativity (1915).
15 September 2011 Astronomy 111, Fall 2011 25
Moment of inertia and planetary differentiation
For a given spin rate, different distribution of a given mass within the bounds of a planet would lead to different values of the planet’s spin angular momentum. The relation between total angular momentum L and angular frequency ω is
The moment of inertia can be measured from the fine details of a planet’s gravitational field, as we will discuss in detail later this semester.
2where
Moment of inertia and planetary differentiation (continued)
Here are some simple spherical configurations of mass – all with mass M, radius R – and the corresponding moment of inertia. Constant-density sphere:
Spherical shell, with constant mass per unit area:
Density maximum at center, declining linearly to zero at r = R:
The more strongly concentrated the mass is at the center, the smaller the leading factor in the expression for I.
22 5
I MR=
22 3
I MR=
Moment of inertia and planetary differentiation (continued)
Results: The Moon has , close enough to
that its density must be nearly constant. Mercury has significantly smaller than the
uniform-density value, and probably reflective of a much larger density in the center than at the surface. An iron core, of the sort described above, would work fine. The moment of inertia and the average density can be used to calculate, fairly confidently, the properties of the core.
A planet that is not concentrated at the center, but tends to have uniform density, is called undifferentiated. The process of concentration of heavier minerals and metals at the center is called differentiation.
20.394I MR= 22 5
Mercury’s vital statistics
Water on airless Solar system bodies
Water on airless Solar system bodies (continued)
Polar ice: Mercury vs. Moon
Polar ice: Mercury vs. Moon (continued)
Polar ice: Mercury vs. Moon (continued)
Polar ice: Mercury vs. Moon (continued)
Polar ice: Mercury vs. Moon (continued)
Polar ice: Mercury vs. Moon (continued)
Polar ice: Mercury vs. Moon (continued)
Polar ice: Mercury vs. Moon (continued)
Tidal locking and Mercury’s orbit
Imagine yourself in orbit around Earth, feeling tidal forces
Tidal locking and Mercury’s orbit (continued)
Tidal locking and Mercury’s orbit (continued)
Orbit comparison
Moment of inertia and planetary differentiation
Moment of inertia and planetary differentiation (continued)
Moment of inertia and planetary differentiation (continued)
The properties of Mercury Comparison of Mercury
and the Moon Water on Mercury and the
Moon Ice in the polar craters of
Mercury and the Moon Tidal locking and
Mercury’s eccentric orbit Moment of inertia and
differentiation in terrestrial planets
Mercury’s vital statistics 26
8
-3
12
Equatorial radius 2.4397 10 cm (0.383 )
Average density 5.427 gm cm Albedo 0.12 Average surface
442.5 K temperature
Orbital eccentri
58.785 days rotation period
Visits to Mercury
There have been two: NASA’s Mariner 10 conducted
three close fly-bys in 1974-1975. Since 17 March 2011, the NASA
MErcury Surface, Space ENvironment, Geochemistry and Ranging (MESSENGER) satellite has been in orbit around Mercury and will return detailed imaging, topographic, compositional, and magnetic- field data for about a year thenceforth.
MESSENGER displays its scientific instrument complement as it passes by (NASA/JHU-APL).
Moon vs. Mercury× ×
Mass (gm) 7.349 10 3.302 10
Radius (cm) 1.7381 10 2.4397 10
Mean density (gm cm ) 3.350 5.427 Albedo 0.12 0.12 Mean temperature (K) 274.5 442.5 Orbital
3.844 10 5.791 10 semimajor axis (cm) Orbita
° °
× -3
l eccentricity 0.0549 0.2056 Obliquity to Solar orbit 1.54 0.04 Tidally-locked rotation? Yes Yes Mean surface
0 2.2 10 magnetic field (G) Atmosphere None None Surface visited? Yes No Image by Tunç Tezel
Moon vs. Mercury (continued) From the appearance, albedo, and
reflectance spectra, we conclude that the surfaces are similar in composition: feldspar and pyroxene – bearing rocks.
From the density, we conclude that Mercury has more than its share of iron, in the form of a core with radius about ¾ that of the planet.
From the magnetic field, we conclude that Mercury’s core is probably liquid, for a dynamo mechanism to operate.
Diagrams by UCAR (U. Michigan)
Tectonic activity: Mercury vs. Moon
From observations of rilles and scarps, we conclude that both have a “plastic” mantle, but that Mercury has been tectonically active and the Moon has not. Recently, volcanoes have
been seen on Mercury, within the giant Caloris impact basin. The moon has none.
MESSENGER (JHU-APL/NASA)
Water on airless Solar system bodies
It is easy for hydrogen to escape from small bodies like the Moon and Mercury, and easy for sunlight to dissociate water into H and O. So is there any water at or just underneath the surface of Mercury and the Moon? There are two good reasons to think there should be, despite the difficulties: 1. The solar wind. The Sun is constantly spewing out about
in the form of the Solar wind, mostly as protons and electrons. The protons, travelling at high speeds, bury themselves
several cm below the surface of any airless body they encounter.
It doesn’t take long for each proton to become H, and for two of these to make water with a nearby O.
15 September 2011 Astronomy 111, Fall 2011 7
14 110 yearM− −
Water on airless Solar system bodies (continued)
This is probably the origin of the widespread water recently detected on the Moon by the NASA Deep Impact and ISRO Chandrayaan-1 spacecraft.
Quantity: about one liter per ton of rock or regolith. What’s closest to the surface seems to come and go with darkness and sunlight .
15 September 2011 Astronomy 111, Fall 2011 8
Temperature (left) and water concentration (right) on the Moon, a quarter of a lunar day apart. From Jessica Sunshine and the EPOXI team.
Polar ice: Mercury vs. Moon
2. Comet and asteroid impacts. In principle comets and asteroids could deliver larger amounts of water to the Moon and Mercury than the Solar wind can. It wouldn’t last long in the sunshine on either body – it
would be evaporated, dissociated, and in short order the hydrogen would escape.
But because the Moon and Mercury both have very small obliquity (tilt of rotation axis from the orbital axis), there are permanently-shaded parts of craters near the north and south pole, where some of the delivered water can last – in Mercury’s case, practically forever.
And thereby hangs a tale.
15 September 2011 Astronomy 111, Fall 2011 9
15 September 2011 Astronomy 111, Fall 2011 10
Polar ice: Mercury vs. Moon (continued)
The BMDO Clementine satellite passed over the poles of the Moon in early 1994, and took images that were interpreted as showing surface ice in polar craters. This would be of asteroidal/cometary
origin but the required amount was surprising large – typically 5 m deep, 105 m2 in area– and the results remained controversial.
Clementine was the DoD’s first try at the “faster– better – cheaper ” philosophy of spacecraft development.
Images of the Moon’s south pole, from Clementine
Polar ice: Mercury vs. Moon (continued)
In 1998, NASA’s Lunar Prospector satellite repeated the imaging ob- servations, added neutron spectro- scopy, and apparently confirmed the Clementine findings. Again plausible, but the low-
energy neutron method of detecting water (hydrogen in the ice) had never been tried before under any conditions, and thus was also controversial.
Lunar Prospector was one of NASA’s first tries at “faster – better – cheaper.” Like Clementine there were difficulties in calibrating and interpreting its measurements.
Lunar Prospector neutron- spectrometer scans during an orbit over both poles (Feldman et al 1998).
Polar ice: Mercury vs. Moon (continued)
In 1999, radar astronomers at Arecibo Observatory made a long- wavelength, radar-polarimetric study of the north pole of Mercury, and found that the perpetually- shadowed crater floors were very reflective in a way most easily explained by clean water ice. For ice detection, this technique
is less controversial than the means employed by the Lunar satellites. But it was shocking nonetheless: ice on Mercury?
Radar-reflectivity image of the north pole of Mercury (Harmon et al. 2001)
Polar ice: Mercury vs. Moon (continued)
So Arecibo tried to confirm the Clementine and Lunar Prospector results. They were able to show that there isn’t any surface ice in the Moon’s polar craters. Recently confirmed from space by
the JAXA Kaguya/SELENE and NASA LRO satellites.
Clementine-Prospector results were confounded by steep illumination angles, calibration difficulties.
The “you get what you pay for” satellite-development philosophy is making a comeback.
Radar image of the Moon’s south pole (N.J.S. Stacy, Ph.D. dissertation, Cornell U.)
Polar ice: Mercury vs. Moon (continued)
What about subsurface ice, mixed with dirt, in the Lunar polar craters? To expose this was the job of NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS): Make a small crater within a
permanently-shadowed region, with the impact of a spent rocket stage.
Analyze the composition of the ejecta, from measurements by satellites right overhead.
15 September 2011 Astronomy 111, Fall 2011 14
A permanent shadow on the moon (yellow contour), compared to neutron signal (blue) and the LCROSS impact site (red dot). (NASA and ISR/RAS)
Polar ice: Mercury vs. Moon (continued)
Results show water ice present at the level of about 50 liters of water per ton of ejecta, in this polar-crater floor. This would be on the high end of soil
water content for the driest deserts on Earth, like the Sahara and Atacama.
There’s as much molecular hydrogen (H2) in the soil as water ice, molecule for molecule. Much of the neutron signal was from this H2.
The water ice comes with very large amounts of other volatiles like CO, calcium, magnesium and mercury.
15 September 2011 Astronomy 111, Fall 2011 15
Species Abundance in ejecta (percent by mass)
H2O 5.4
H2 1.4
CO 5.7
Hg 1.2
Ca 1.6
Mg 0.4
Polar ice: Mercury vs. Moon (continued)
This is far less water than was claimed on the basis of the Clementine and Lunar Prospector results, and is consistent with the Arecibo radar observations. There’s enough to be scientifically interesting, and the
mixture of volatiles will usefully constrain models of delivery and the thermal history of the crater floors.
An entire permanently shaded region contains 106-107
gallons, which would fill a large municipal water tower. This would go a long way toward the needs of a Lunar base. Has to be laboriously extracted and purified, though.
Coming soon: MESSENGER observations of the shadows, topography and composition of the polar-crater floors on Mercury. (Not going to create any craters, though.)
15 September 2011 Astronomy 111, Fall 2011 16
Tidal locking and Mercury’s orbit
In celestial mechanics, tidal locking means that the heat dissipated during an orbit, by variation in tidal forces, is minimized. Tidal forces are the gravitational force differences between
one end and another of one body, owing to different distances away from a second body, and from one side and another of one body, owing to different directions toward the second body.
The result is a stretch along the line between the two bodies, and a compression in the perpendicular directions (see next page).
We will derive formulas for tidal forces – and much more on tides – later this semester.
15 September 2011 Astronomy 111, Fall 2011 18
Imagine yourself in orbit around Earth, feeling tidal forces
Earth pulls all things toward its center, and pulls harder on nearer things.
To you, it seems as if you are stretched in the direction of the earth’s center, and squeezed in the other direction.
Figure from Thorne, Black holes and time warps
15 September 2011 Astronomy 111, Fall 2011 19
Tidal locking and Mercury’s orbit (continued)
A body in circular orbit, with rotation period equal to revolution period, suffers no change in tidal forces as it travels its orbit. Thus there is no heat dissipated from stretching and shrinking.
The Moon’s orbit about Earth has a fairly small eccentricity (0.055), so it can be considered circular to good approximation.
The Moon’s rotation and revolution periods are identical, so it is tidally locked.
15 September 2011 Astronomy 111, Fall 2011 20
Tidal locking and Mercury’s orbit (continued)
( )
f a c a
Orbit comparison
Drawn to scale, in units of semimajor axis length (a); coordinate axis origin on one focus.
0 (Circle) ε =
0.055 (Moon) ε =
0.2056 (Mercury) ε =
Tidal locking and Mercury’s orbit (continued)
Since the distance between Sun and Mercury varies by so much through an orbit, it’s not possible to have no change in tides.
Nevertheless, astronomers used to think that Mercury’s rotation and revolution periods were equal (at 88 days), because that’s what observations seemed to indicate, and because it agreed with naïve expectations based on circular orbits.
2
Tidal locking and Mercury’s orbit (continued)
In the mid-1960s, though, radar astronomers at Arecibo Observatory showed that the rotation period of Mercury is actually closer to 59 days.
Shortly thereafter it was realized by theoreticians that the minimum in tidal heating for Mercury’s eccentric orbit was reached for an orbital period exactly 3/2 times the rotation period.
In the mid 1970s the fly-bys by Mariner 10 yielded the rotation period much more precisely and bore out this prediction. Mercury rotates exactly three times, every two orbits.
Tidal locking and Mercury’s orbit (continued)
Like the Moon, the tidal forces to which it is subjected has made Mercury slightly oblong. The orbit is arranged so that the long axis always points toward the Sun at perihelion. This keeps the tidal stretching to a minimum, though the minimum isn’t zero.
Other trivia: the perihelion of Mercury’s orbit precesses slowly in the prograde direction. Long a mystery, the precise accounting for this orbital precession by warping of space by the Sun’s gravity was one of the first triumphs of Einstein’s general theory of relativity (1915).
15 September 2011 Astronomy 111, Fall 2011 25
Moment of inertia and planetary differentiation
For a given spin rate, different distribution of a given mass within the bounds of a planet would lead to different values of the planet’s spin angular momentum. The relation between total angular momentum L and angular frequency ω is
The moment of inertia can be measured from the fine details of a planet’s gravitational field, as we will discuss in detail later this semester.
2where
Moment of inertia and planetary differentiation (continued)
Here are some simple spherical configurations of mass – all with mass M, radius R – and the corresponding moment of inertia. Constant-density sphere:
Spherical shell, with constant mass per unit area:
Density maximum at center, declining linearly to zero at r = R:
The more strongly concentrated the mass is at the center, the smaller the leading factor in the expression for I.
22 5
I MR=
22 3
I MR=
Moment of inertia and planetary differentiation (continued)
Results: The Moon has , close enough to
that its density must be nearly constant. Mercury has significantly smaller than the
uniform-density value, and probably reflective of a much larger density in the center than at the surface. An iron core, of the sort described above, would work fine. The moment of inertia and the average density can be used to calculate, fairly confidently, the properties of the core.
A planet that is not concentrated at the center, but tends to have uniform density, is called undifferentiated. The process of concentration of heavier minerals and metals at the center is called differentiation.
20.394I MR= 22 5
Mercury’s vital statistics
Water on airless Solar system bodies
Water on airless Solar system bodies (continued)
Polar ice: Mercury vs. Moon
Polar ice: Mercury vs. Moon (continued)
Polar ice: Mercury vs. Moon (continued)
Polar ice: Mercury vs. Moon (continued)
Polar ice: Mercury vs. Moon (continued)
Polar ice: Mercury vs. Moon (continued)
Polar ice: Mercury vs. Moon (continued)
Polar ice: Mercury vs. Moon (continued)
Tidal locking and Mercury’s orbit
Imagine yourself in orbit around Earth, feeling tidal forces
Tidal locking and Mercury’s orbit (continued)
Tidal locking and Mercury’s orbit (continued)
Orbit comparison
Moment of inertia and planetary differentiation
Moment of inertia and planetary differentiation (continued)
Moment of inertia and planetary differentiation (continued)