Lec 15 Flashcards
(41 cards)
What geological processes have shaped Mars?
Based on its intermediate size, we expect Mars to be more geologically active than the Moon or Mercury but less active than Earth or Venus
impact cratering
-The differences in cratering tell us that the southern highlands are an older surface than the northern plains, which must have had
their early craters erased by other geological processes
volcanism
-most important process in erasing craters on the northern plains, although tectonics and erosion also played
-created by a long lived plume of rising mantle material that bulged the surface upward and provided the molten rock for the eruptions
that built the giant volcanoes
tectonics and deep valleys
-Parts of the canyon are completely enclosed by high cliffs on all sides, so neither flowing lava nor water could have been responsible.
-However, extensive cracks on its western end run up against the Tharsis Bulge, suggesting a connection between the 2
-Perhaps Valles Marineris formed through tectonic stresses accompanying the uplift that created Tharsis, cracking the surface and leaving the tall cliff walls of the valleys
erosion
-closer examination shows extensive evidence of erosion by liquid water
-Regardless of the specific mechanism, water is the only substance that could have been liquid under past martian conditions and that is sufficiently abundant to have created such extensive erosion features
What geological evidence tells us that
water once flowed on Mars?
-In most places and at most times, Mars is
so cold that any liquid water would immediately freeze into ice.
Even when the temperature rises above freezing, as it often does at midday near the equator, the air pressure is so low that liquid water would quickly evaporate
When we combine the clear evidence of water erosion with the fact that liquid water is unstable on Mars today, we conclude that Mars must once have had a very different
climate—with warmer temperatures and greater air pressure that would have allowed water to flow and rain to fall—than it does today
Geological evidence indicates that this warmer and wetter period must have ended long ago
Orbital Evidence for Ancient Water Flows
Strong evidence that Mars had rain and surface water in the distant past comes from both orbital and surface studies
-notice the indistinct rims of many large craters and the relative lack of small craters
-both facts argue for ancient rainfall, which would have eroded crater rims and erased
small craters altogether
Some studies even suggest that the northern plains may once have held a vast ocean, though the evidence is less definitive than the evidence for smaller lakes.
–the evidence for the ocean comes from features that look like an ancient shoreline.
–radar data also suggest that the rock along the proposed shoreline is sedimentary rather
than volcanic, just as we would expect if there had once been an ocean; this rock may even contain water ice in its pores and cracks
Surface Evidence for Ancient Water Flows
The most recent rover landed on Mars in 2021, carrying a set of scientific instruments, along with a small helicopter
–the instruments include a chemical analysis camera capable of detecting “biosignatures” (meaning molecules that might indicate
past or present life) that organisms might have left behind in the rock layers,
–a spectral analysis tool to study carbon bearing molecules and their possible biological roles,
–an instrument package for testing a system to generate oxygen from local materials, which would be important for future
human exploration of Mars
All of the rovers have found abundant mineral evidence of past liquid water on the surface.
-however, the character of the water appears to have differed at different times in
Mars’s deep past.
What geological processes have shaped
Venus?
Venus’s thick cloud cover prevents us from seeing through to its surface with visible light, but we can study its geological features with radar mapping, which bounces radio
waves off the surface and uses the reflections to create 3D images
impact cratering
-has small number of impact craters, indicating a young surface on which more ancient craters have been erased by other geological processes
-moreover, while Venus has a few large craters, it lacks the small craters that are most common on other worlds, probably because the small objects that could make such craters burn up completely as they enter Venus’s thick atmosphere
volcanic and tectonic features
-Some mountains are shallow-sloped volcanoes, indicating that they were built by eruptions in which the lava was about as
runny as that which formed Earth’s Hawai‘ian Islands.
-But it also has lava plains, which must have formed from a runnier lava, as well as a few steeper-sided volcanoes, indicating eruptions of a thicker lava.
-entire surface appears to have been extensively contorted and fractured by tectonic forces
-some features, including large circular coronae provide strong evidence for mantle convection beneath the lithosphere
-the coronae were probably pushed upward by hot rising plumes of rock in the
mantle in much the same way that hot spots in Earth’s mantle
remains geologically active today,
since it should still retain nearly as much internal heat as Earth
weak erosion
-expect Venus’s thick atmosphere to produce strong erosion, but the view both from orbit and on the surface suggests otherwise
–can trace the lack of erosion on Venus to two simple facts
–1) Venus is far too hot for any type of rain or snow on its surface
–2) Venus’s slow rotation—once every 243 days—means it has very little surface wind.
Does Venus have plate tectonics?
Venus shows no evidence of Earth-like plate tectonics
-venus shows evidence of a very different type of global geological change.
-on Earth, plate tectonics reshapes the surface gradually, so different regions have different
ages
-in contrast, Venus’s relatively few impact craters are distributed fairly uniformly over the entire planet, suggesting that the surface is about the same age everywhere
The lack of plate tectonics on Venus therefore suggests either that it has weaker mantle convection or that its lithosphere somehow resists fracturing.
–the 1st possibility seems unlikely, because Venus’s similarity to Earth in size and density leads us to expect it to have a similar level of mantle convection
–most scientists therefore suspect that Venus’s lithosphere resists fracturing into plates because it is thicker and stronger than Earth’s
earths rampant erosion by water and wind
Earth’s rampant erosion by water and wind is explained by the combination of our planet’s size, distance from the Sun, and rotation rate:
Earth is large enough for volcanism and outgassing to have produced an atmosphere,
Its distance from the Sun allowed water vapor to condense and fall to the surface as rain, and Earth’s moderately rapid rotation drives wind and other weather.
How is Earth’s surface shaped by plate
tectonics?
The term plate tectonics refers to the scientific theory that explains much of Earth’s surface geology as a result of the slow motion of plates that essentially “float” over the mantle, gradually moving over, under, and around each other as convection moves Earth’s interior rock.
Earth’s lithosphere is broken into more than a dozen plates
discovery of continental motion
German meteorologist and geologist Alfred Wegener: continental drift, the idea that continents gradually drift across the surface of Earth.
Wegener got his idea in part from the puzzle-like fit of continents such as South America and Africa
-also noted that similar types of distinctive rocks and rare fossils were found in eastern South America and western Africa, suggesting that these two regions once had been close together.
-suggested that Earth’s gravity and tidal forces
from the Sun and Moon were responsible, but other scientists quickly showed that these forces were too weak to move entire continent
others discovered mid-ocean ridges along which mantle material erupts onto the ocean floor, pushing apart the existing seafloor on either side
–this seafloor spreading helped explain how the continents could move apart with time
–in addition, as more fossil evidence was gathered, it became clear that the continents really were arranged differently in the past, and Wegener’s idea of a “continental fit” for Africa and South America ultimately gained acceptance
Seafloor Crust and Continental Crust
Another key piece of evidence for plate tectonics came with the discovery that Earth’s surface has two distinct types of crust, one type found on seafloors and the other on continents.
Seafloor crust is thinner, denser, and younger than continental crust.
-seafloor crust made primarily of the relatively high-density rock called basalt.
-Further evidence of the young age of seafloors
comes from studies of impact craters.
-Large impacts should occur more or less uniformly over Earth’s surface, and the
oceans are not deep enough to prevent a large asteroid or comet from making a seafloor crater.
-However, we find far fewer large craters on the seafloor than on the continents, which means that seafloor crust must have been created more recently.
Continental crust
-made mostly of rock (such as granite) with lower density than seafloor crust.
No other planet shows evidence of such distinct differences in crust from place to place
conveyor belt of plate tectonics
Over millions of years, the movements involved in plate tectonics act like a giant conveyor belt for Earth’s lithosphere.
Mid-ocean ridges occur at places where mantle material rises upward, creating new seafloor crust and pushing plates apart.
The newly formed crust cools and contracts as it spreads sideways from the central ridge, giving seafloor spreading regions their characteristic ridged shape
Along the mid-ocean ridges worldwide, new crust covers an area of about 2 square kilometers every year, enough to replace the entire seafloor within about 200 million years—and thereby explaining the less-than-200-million-year age of seafloor crust.
subduction
Over tens of millions of years, any piece of seafloor crust gradually makes its way across the ocean bottom, then finally gets recycled into the mantle in the process we call subduction.
Subduction occurs where a seafloor plate meets a continental plate, which is generally somewhat offshore at the edge of a sloping continental shelf.
As the dense seafloor crust of one plate pushes under the less dense continental crust of another plate, it can pull the entire surface downward to form a deep ocean trench
faults
Places where plates slip sideways relative to each other are marked by what we call faults—fractures in the lithosphere.
The San Andreas Fault in California marks a line where the Pacific plate is moving northward relative to the continental plate of North America
The two plates do not slip smoothly
against each other; instead, their rough surfaces catch.
Stress builds up until it is so great that it forces a rapid and violent shift, causing an earthquake.
hot spot
Not all volcanoes occur near plate boundaries.
Sometimes, a plume of hot mantle material rises in what we call a hot spot.
The Hawai‘ian Islands are the result of a
hot spot that has been erupting basaltic lava for tens of millions of years
plate tectonics through time
we can use the current motions of the plate to project the arrangement of continents millions of yrs ago
-about 200 million years ago the present-day continents were together in a single “supercontinent,” sometimes called Pangaea
What are terrestrial planets like on the inside?
In order of decreasing density and depth, the
interior structure consists of core,
mantle, and crust. The crust and
part of the mantle together make up
the rigid lithosphere. In general, a
thinner lithosphere allows more
geological activity
What causes geological activity?
Interior heat drives
geological activity by causing
mantle convection, keeping the
lithosphere thin, and keeping
the interior partially molten.
All the terrestrial interiors were
once hot, but larger planets cool
slowly, retaining more interior
heat and staying geologically
active longer
■ Why do some planetary interiors create magnetic fields?
A planetary magnetic field requires three things:
an interior layer of electrically conducting fluid, convection of that fluid, and rapid rotation. Among the terrestrial
planets, only Earth has all three characteristics.
How do impact craters reveal a surface’s geological age?
More craters indicate an older surface. All the terrestrial worlds were battered by impacts when they were
young, so those that still have many impact craters must
look much the same as they did long ago. Those with
fewer impact craters must have had their ancient craters
erased by other geological processes
Why do the terrestrial planets have different geological histories?
Fundamental planetary properties, especially
size, determine a planet’s
geological history. Larger
worlds have more volcanism
and tectonics, and these
processes erase more of the
world’s ancient impact craters.
Erosion depends on a planet’s size, distance from the Sun,
and rotation rate.
What geological processes shaped our Moon?
The lunar
surface is a combination of
extremely ancient, heavily
cratered terrain and somewhat
younger lava plains called the
lunar maria. Some small
tectonic features are also
present. The Moon lacks
erosion because it has so little atmosphere.
■ What geological processes shaped Mercury?
Mercury’s
surface resembles that of the Moon in being shaped by
impact cratering and
volcanism. It also has
tremendous tectonic cliffs that
probably formed when the
whole planet cooled and contracted in size
What geological processes have shaped Mars?
Mars
shows evidence of all four
geological processes. It has
the tallest volcano and the
biggest canyon in the solar
system, evidence of a period
of great volcanic and tectonic
activity. It also has abundant
craters and evidence of erosion by wind and flowing
water.
What geological evidence tells us that water once flowed on Mars?
Images of dry river channels and eroded
craters, along with chemical
analysis of martian rocks,
show that water once flowed
on Mars. Any periods of
rainfall seem to have ended at
least 3 billion years ago. Mars
still has water ice underground and in its polar caps
and could possibly have
pockets of underground liquid
water
