Astronomy 150 Online
R‐Plots
"R" , as given on the y‐axis, represents the fraction of a surface that has been cratered. The individual diameters of the craters found on that surface are represented on the x‐axis and usually given in kilometers
relationship between the size of the crater and that blanket of ejecta surrounding it:
"The material from the deepest layer below the impact that has been ejected from the crater is from a depth equal to 1/10th the crater diameter." oldest material near rim
Moons K value is
.393 almost homogenous
Rocks from the moon revealed that there are five stages of planetary evolution, which are:
1. Origin or the initial accretion of the world starting around 4.5 Byrs ago followed quickly by 2. Differentiation ‐ when material in the new, hot, molten world segregated based on its density 3. Late Heavy Bombardment - remember that all of the rocks from large impact basins are the same age, indicating that there was a period in time ‐ about 3.8 Byrs ago ‐ when large objects pummeled the surface of the Moon 4. Geologic Activity - a period from about 3.8‐3 Byrs ago where it appears all of the mare regions on the Moon were formed. 5. The Big Chill - the final stage where geologic activity
You have discovered a planet that has twice the gravity of Earth (Fgp = 2Fg⊕), but is only 1/2 the size of Earth (Rp = 1/2R⊕). How would the mass of the planet compare with the mass of Earth (Mp = XM⊕)?
1/2 as much
You have discovered a planet that has the same gravity as Earth (Fgp = 1Fg⊕), but is only 1/9th the mass of Earth (Mp = 1/9M⊕). How would the radius of the planet compare with the radius of Earth (Rp = XR⊕)?
1/3 as much
Typical planet size in diameter
1000's of km in diameter.
Complex impact craters are
10s to 100s kilometers formed from a more energetic impact than simple impact craters. The masses of the impacting objects are larger and therefore impart more energy to the surface. The surface deforms and rebounds in response to the tremendous amount of energy released, creating central peaks, like little, miniature mountains, in the middle of the craters as seen here. central peaks are very old
The gravity of the Moon is about 1/6 that of the Earth. For example, an astronaut weighing 180 pounds on the Earth would weigh about 30 pounds on the Moon. If you were to double the distance between the Earth and the Moon, how much would our 180 pound astronaut weigh on the Moon?
30 pounds - changing the distance between the two worlds goes not change the gravity on the Moon!
age of solar system
4.5 Byrs
If you decrease the size of the Earth by a half (R = 1/2R⊕) and double the mass of the Earth (M = 2M⊕), how much would it change the gravity on the Earth (Fg = XFg⊕)?
8 times as much
You have discovered a planet that is one quarter the radius of Earth (Rp = 1/4R⊕) and one half as massive (Mp = 1/2M⊕). How does the gravity on the surface of this planet compare to the gravity on the surface of Earth (Fgp = XFg⊕)?
8 times as much
If you were standing on the equator at noon on the spring equinox, the Sun would appear to be ______ degrees above the horizon.
90
Pressure given in
ATM or atmospheres
AU
AU is equivalent to the average distance between the Sun and the Earth Astronomical Unit
Moment‐of‐ Inertia Factor, typically given as the uppercase letter K
Between 0.0 to 0.4 Unitless Tells how mass is distributed in interior of world 0.4 is evenly distributed 0.32 concentrated in core
Earth-Moon System Theories
Co-Accretion Capture Fission Giant Impact
differentation
Dense materials such as metals tended to sink to the center of this non-solid material, while less dense rock-forming minerals floated at the surface. This process of separating materials by their density is called differentiation.
Gravitational Force Equation / Force of gravity equation
Fg = GMm/R^2 Simplified into: Fg = M/R^2 R can be diameter Where: Fg represents the surface gravity of the world in Newtons (N) M is the mass of the world in kilograms (kg) m is the mass of the object on the surface of the world in kilograms (kg). R is the radius of the world in meters (m). G is the gravitational constant, approximately equal to 6.67 x 10^-11 Nm^2kg^-2.
Highlands
High albedo many craters old impacts
Density reference numbers
Ice - 1 g/cm^3 Rock - 3 g/cm^3 Iron - 8 g/cm^3 Element that make up ice most abundant
Ages of particular surfaces on Mercury
If we count craters to determine crater densities on the highlands, mare, and impact basins of Mercury, we begin to sketch out the past evolutionary history of the planet. What we see is a world with an ancient history; a history that is very Moon‐like with: mare only slightly older than the Moon's (~3.5 Byrs) highland (~4.0 Byrs) and impact basin (~3.9 Byrs) ages basically the same as the Moon's.
Meridian
Imaginary line separating east and west, this line runs from due north through a point directly overhead, called the zenith, to a point due south.
What is the most wide spread (common) process shaping the surface of the Moon today?
Impact cratering the moon is geologically dead
If you were to triple the size of the Earth (R = 3R⊕) and double the mass of the Earth (M = 2M⊕), how much would it change the gravity on the Earth (Fg = XFg⊕)?
It's 2/9 as much.
Craters of Mars
L We think the energy released in the impact radiates outward essentially melting the material surrounding the impact site. On Mars, the surge and subsequent flow of that material, much like flowing mud, quickly solidifies in the cold Martian soil, essentially freezing the impact in time! We think impactors are hitting a surface potentially rich in ices, much like permafrost on Earth.
Where do you look for water on the moon and mercury?
Large amounts of water‐ice deposits were found hiding in the shadowed craters in the polar regions
Mare
Low albedo few craters young lava flows mostly on backside
M*P^2 = 4pi^2/G*a^3
M is the sum of the masses of the two objects involved P is the orbital period in hours, assume it's equal to the mass of the largest object G is the universal gravitational constant, equal to 8.642 x 10^-13 km3/(kg hr2) a is the semi-major axis of the orbit in kilometers
In Fg = M/R^2, M is...
Mass of the world in kilograms (kg)
Cumulative crater density
N (10) The total number of craters equal to or larger than 10 lm per million square km If we count up all of the craters on a Lunar surface of a given area and plot that cumulative number versus the absolute ages of those surfaces obtained from the Lunar samples, we get the figure on this slide. What this means is we can count up the number of craters in a given area on the surface of any world to get that surface's age from this relation!
density and K factor
Not related Keep in mind that two objects can have the same density, meaning they are made out of the same materials like the two spheres in this slide, but can have very different K factors if the distribution of that material is different in their interiors
∝
Porportionality
radioactive decay
Radioactive decay is a one‐way process. the radioactive decay of elements within a world releases heat and this heat contributes substantially to the world's overall heat budget. The three main contributors to the amount of heat a world has are potassium (K) 40, uranium 238, and thorium 232. These unstable isotopes will decay according to their individual half‐lives to stable isotopes. Once an isotope has decayed to a stable element, the heat production from that atom ceases. Therefore heat production in the interior of a world is always decreasing. The total amount of heat produced is related to the amount of radioactive material a world has to start out with. How much radioactive material a world gets is related to how much material it accreted when forming or basically how big that world got. The larger the world, the more radioactive isotopes it has, and the more heat it will produce.
In Fg = M/R^2, R^2 is...
Radius of the world in meters (m).
Impact Breccia
Rock composed of fragments of minerals/rock cemented together
Why can we use Crater-Density N(1) diagrams (ages) for all planets?
R‐Plots, or the cratering populations of mare regions on the Moon, Mercury, Venus and Mars are all essentially the same. Ditto for the highland regions on these worlds. If they are all the same, then the period in time (or their ages) and the rates at which these worlds were cratered should also match!
Gap between inner and outer planets is called a _? Is how many AU?
Snow line 3 AU
Co-accretion Theory
Suggests that the Moon and Earth were formed together from a cloud of gas and dust
In Fg = M/R^2, Fg is...
Surface gravity of the world in Newtons (N)
Evidence for water on Mars: What are some examples? Where are we more (or less) likely to find liquid water or evidence of past liquid water? Why was liquid water present only in Mars' distant past? How is the loss of liquid surface water tied to Mars' present atmosphere?
The craters are much like the moon and mercury, but what's odd are the raised features surrounding the craters. the energy of the impact likely radiated outward melting the material surrounding the impact then quickly solidified again there are also what look to be large outflow channels and teardrop‐shaped "islands" look just like the channels and flow patterns of water and catastrophic floods, as seen on Earth. Recently, high resolution imaging and spectra of what appear to be seasonal flows of a liquid trickling down the walls of craters during the Martian summer revealed themselves to indeed be liquid water. The water is actually a briny mixture of water and salts. The salts lower the freezing point of the water, allowing it to remain in its liquid phase even at temperatures below 0o F.
Tidal forces are caused by
The difference between the strength of the gravitational pull of the Moon and Sun on either side of the Earth.
There are basically three long period processes at work that you should be aware of - eccentricity, precession, and obliquity.
The eccentricity, or how far from circular the shape of the Earth's orbit is, is nearly zero ‐ meaning that the Earth's distance from the warmth of the Sun doesn't change dramatically throughout the year. The Earth is spinning like a top, and thanks to the Moon, the Earth's spin is slowing down - you can read about the details of this on pages 181‐182 of your text., but much like a top, as the Earth spins more slowly, the Earth wobbles or precesses more on its spin axis. Today the Earth's north pole points directly at a star named Polaris or "the north star". However, in about 12,000 years, precession will point the north pole to a nearby star called Vega. The Earth's obliquity, or axial tilt of about 23.5 degrees from the normal, is minor and changes very little over the course of 41,000 years ‐ varying between about 22 and 24 degrees. The tilt of the Earth is responsible for the seasons. If the Earth had no axial tilt, for example (that's an obliquity of zero degrees), you'd experience the same season at your latitude year round. The gravitational pull from the Moon keeps the Earth's obliquity stable. This variation in axial tilt may seem small, but even this 41,000 year cycle is believed to bring on really dramatic climate changes resulting in ice ages! This minor change in tilt changes the angle at which the light from the Sun is incident on the Earth, which can really mess with global temperatures!
What causes tides?
The gravitational pull of the moon High and low tides are caused by the moon. The moon's gravitational pull generates something called the tidal force. The tidal force causes Earth—and its water—to bulge out on the side closest to the moon and the side farthest from the moon. These bulges of water are high tides.
Ages of particular surfaces on Mars
The northern regions are covered in mare and indeed have similar ages to the mare seen on the Moon and Mercury. Note that these "young" surfaces are still ancient at 3.5 Byrs! The southern highlands and the Hellas Impact Basin also reflect the ages of similar features on the Moon and Mercury. (highlands 4.0, impact basin 3.9?_ young region called called Tharsis. It is virtually devoid of craters, rendering a remarkably young age for this area. On average, its only about 0.5 Byrs old. it is a massive, elevated structure with several prominent volcanos, the biggest of which is Olympus Mons the largest volcano - by far - in the entire Solar System! It absolutely dwarfs the largest mountains and volcanos on Earth. it is a shield volcano, and is able to be so much taller than volcanos on earth because the gravity on mars is half that og earths. region is so heavy martian crust can't support it because of it's size its geologic activity lasted much longer than mercurys or the moons as evidenced by some of its mare and the relative youth of the Tharsis Region.
orbit
The path of an object as it revolves around another object in space
Fission Theory
The theory that the Moon formed from matter flung off the Earth because the planet was rotating extremely fast. the Moon "split off" of the Earth when both were still molten; explains the similar composition of Earth and Moon but does not explain low angular momentum of the system
Obliquity
The tilt of the Earth's axis the angle between a planet's spin axis and a line perpendicular to the planet's orbital plane
radiometric dating
We use the decay rate or half‐life of an element to determine how long that element has been decaying or "falling apart" since it was formed. This is how we get ages
tidal locking
When a satellite's period of rotation matches its period of revolution the one to one relationship between the rotation and revolution periods of an orbiting body that is created by tidal effects, such that the orbiting object always shows the sae side to the object being orbited
Equinox
a day when the hours of daylight and darkness are equal. the autumn equinox occurs approx. sept. 21, the spring equinox aprox. march 21
Average asteroids in diameter, appearance description
a few tens of km in diameter heavily cratered potato shaped
Ellipse
a geometric form in the shape of an oval or circle in which the sum of the distances from two points (foci) o every point on the ellipse is contant
Ecliptic
a great circle on the celestial sphere representing the sun's apparent path during the year, so called because lunar and solar eclipses can occur only when the moon crosses it.
Relative age vs. absolute age
age of object in comparison to another object vs numerical age of object
Apollo 11 and 12's impact breccia returned from those large impact basins was...
all the same age, around 3.8 billion years
Average pressure on earth typical room temp
atm 20 C
______ makes up the mare region?
basalt
When the lava flows solidify they become
basalt, one of the many sample‐types returned from the Moon
samples from apollo 11 and 12 were dominated by ____ ?
basalt. astronauts also picked up a lot of impact breccia
endogenic exogenic
coming from within/outside of system
Simple Impact Craters
diameters < 10 km bowl shaped raised rims gentle sloping walls circular less than a few km in diameter and do not have an uplifted centre Simple impact craters are found virtually everywhere on the Moon. Characterized by their simple shape and structure, they are usually surrounded by a blanket of material called ejecta. The Moon has no atmosphere so impactors of all sizes can strike the surface, the smallest of which create simple impact craters
Giant Impact Theory
early in the formation of the Solar System, as the Earth and other planets were still taking shape, a Mars‐sized object (dubbed Theia) collided at a shallow angle with the nascent Earth. Theia's iron core merged with the Earth's and the volatile‐depleted disk of debris from the mantles of both worlds eventually accreted into the Moon.
volatile
easily vaporized (transformed into a gaseous state) st relatively low temperatures; or a volatile substance
Mars is
highly differentiated mostly rock with a little iron varied terrain very thin atmosphere
Density
how much matter there is an a cubic centimeter
Mercury's Density is: Moment of inertia: Temperatures Atmosphere: Crater Density:
huge, highest in the solar system highly differentiated, massive iron core k-value is .34 are extreme, boiling during day freezing at night doesn't have one like the moon, but due to the extremes in temperature on its surface, it develops a tenuous, transient atmosphere of sorts which planetary scientists refer to as an exosphere. It's mostly just materials being scoured and boiled off the surface due to the intense radiation of the Sun. crater density is similar to the moon
"Geologic Activity Scales with Size"
just knowing that simple number - surface area / volume - will allow us to predict the level of and length of time that geologic activity will last on a world. Large worlds have a larger percentage of radioactive isotopes in their cores and therefore produce more heat. Their relatively small surface areas compared with their large volumes forces them to release that heat to space much more slowly. So large worlds stay warm and drive geologic activity on their surfaces in the form of volcanism, tectonics, and so on, much longer than small worlds.
heat‐driven geologic activity manifests itself on the surface of the world in the form of
lava flows (mare), sinuous rills (lava channels), and tectonic structures like rifts or fractures in the world's surface
The moment of inertia (K) of a differentiated world would be _______________ an undifferentiated world?
less than As a world becomes more differentiated (mass is more highly concentrated in the core) the K value drops.
Opposite the zenith lies the
nadir
Impact basins
over 300 km some 1000s km often filled in by mare very old
age of rocks determined by
radiometric
The Moon is believed to be a _____ force for the Earth. The tidal dance between the Earth and Moon has ______
stabilizing reduced potentially large orbital fluctuations to very minor, long period events.
refractory materials
substances that are capable of withstanding high temperatures without melting or being vaporized
Summer Solstice Winter Solstice
summer solstice - the day (approx. June 21 in the Northern Hemisphere) with the most hours of sunlight, when the sun appears highest in the sky winter solstice - the day (approx. Dec 21 in the Northern Hemisphere) with the fewest hours of sunlight, when the sun appears lowest in the sky
Stratigraphy tells us
that rocks from deep layers are from old layers, layers that were formed long ago, and those old layers tell us about the past history of the surface we are interested in! surfaces must overlap, for us to say something about their relative ages.
Surface Area / Volume
the SA/V ratio of a small world (like the Moon) will be a larger number than the SA/V ratio for a big world (like the Earth). This turns out to be a very powerful number to know. Worlds with small SA/V ratios lose their heat to space more slowly than worlds with large SA/V ratios. Which means the Earth's interior stays warmer longer than the interior of a smaller world like the Moon. important because it points to - "Geologic Activity Scales with Size"
accretion
the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter, in an accretion disk. Most astronomical objects, such as galaxies, stars, and planets, are formed by accretion processes.
Difference between mercury and the moon
the chemical composition of the mare and highlands on Mercury is very slight - the average albedo of the two surfaces is similar Unlike the Moon, its harder to distinguish between the mare and highland regions on Mercury. A feature that is unique to Mercury is this strange, jumbled terrain seen on only one side of the planet! this is a consequence of having such a large iron core. The large Caloris Basin impactor struck the surface with so much force that the shock wave made the planet ring like a bell! The large iron core focused the shockwaves on the antipode - the opposite side of the planet from the impact site. The shock jumbled the terrain on that side wrinkles called scarps seen all over the surface of Mercury and they too are unique to this little world. These features are also a consequence of having a big ol' iron core. Iron shrinks when it cools. Mercury's iron core is so big that the overlying crust gets warped and shifted as it responds to the shrinking core beneath it! The result is a wrinkled appearance to the crust evidence for recent volcanism. unlike earth volcanism is actually rare in the solar system
Vernal Equinox and Autumnal Equinox
the equinox in spring, on about March 20 in the northern hemisphere and September 22 in the southern hemisphere. day and night 12 hrs autumnal is beginning of autumn
Capture Theory
the idea that the moon was created at a different location in the solar system and subsequently captured by earth's gravity
Anorthosite, or sometimes referred to as Pristine Highland Rock was also returned from Apollo 11 and 13 and represents material from ____? and is _____ years old?
the original lunar crust 4.6 to 4.3 billion years Anorthosite is material that formed between 4.6 and 4.3 Byrs ago! This rock is primordial - it is the lowest density (about 2.8 g/cc) material on the Moon that floated to the surface of what we believe was a magma ocean after the Moon fully accreted. This means that the Moon had to have been completely molten for this kind of differentiation to have taken place as demonstrated in the graphic on the lower half of the slide!
Albedo
the percentage of light that reflects off a surface. a number between 0.0 and 1.0. albedo of 0.0 reflects 0% of the sunlight, appears black albedo of 1.0 then, reflects 100% of the sunlight , appears bright white to your eyes.
sidereal period
the period of rotation or revolution of an astronomical body relative to the stars
since its formation the moon has changed ______ ?
very little. Rock samples from the highland surfaces are as old as the Solar System itself!
