Lecture 8 -- Earth-Venus-Mars: The Evolution of Landscape
Thursday, June 13
Mariner 10 in 1974 -- only spacecraft to visit
Very moonlike, old surface, many craters
Mercury has a large iron core 3/4 diameter
Exciting news-- may be ice at the poles, in the shadows of craters that never see light. Ice would have stayed after a comet impact. The same thing may even be true of our Moon
After Mercury had collected craters, caused by leftover planetesimals, it apparently shrank 2-4 km in diameter. Stresses on its crust--made huge faults where one part of the crust tried to ride up onto another, almost like plate tectonics. Due to cooling of planet and formation of core
Venus: Earth's twin--almost same size, composition, gravity, distance from Sun
Venus spins backwards--maybe due to a giant impact flipping it over
Day is 117 days, but this is its synodic period --noon to noon
Sidereal day - one rotation with respect to stars, is 243 days
It's year is 225 days. Year is less than its day
Very, very hot, heavy atmosphere, covered in clouds
CO2, N2 atmosphere
Venus is way too hot to have liquid or solid water -- but there's a tiny bit in the atmosphere. So it doesn't have oceans
But it has large, continent like features, and low lying volcanic plains
Recall that there is a compositional difference between oceanic and continental crust
Is the same true for Venus--are the highlands lighter rocks floating on heavier rocks?
Is there plate tectonics on Venus?
Cobbled together from old spare Voyager parts, the spacecraft that had been sent out earlier to Jupiter, Saturn, Uranus and Neptune
Had only one instrument, an imaging radar system
Radar has wavelength of 4 cm, and can see through the cloud
Mapped the whole planet, things as small as 200 m
Sends out radio pulses
They bounce off rocks, some rocks reflect more than others
Pulses are received, computer reconstruction of surface
Radar is not light -- bright means rough or electrically conductive
Dark means smooth or an insulator
Here's an image of the LA basin, using radar. Can see right through clouds, but things don't look quite right. Mountains
Have to keep this in mind when we're looking at Magellan Radar images
Bright and rough continental area, Alpha Regio. Looks like scrunched up crust
Dark, patches around craters that may be related to dust kicked up from the impacts
Highest mountains -- 12 km high (40,000 ft). Very reflective, could be solidified metal, spewed out from volcanoes
Going E around Venus, we see the beginnings of a huge continent-like area, called Aphrodite. Shot through with enormous chasms that look like giant rift zones. At first we thought that it might have been plate tectonics, but there is no evidence anywhere for subduction zones
Large circular regions may be partial subduction zones, or they may be places where huge plumes came up from the mantle, and spread out under the lithosphere. They are called coronae
Belt of fractures seem to continue around the planet, and often where chasms join, there are huge volcanoes
Smooth volcanic plains above and below the belts
Venus is actually 80% basaltic plains
Recall basalt is the kind of rock that comes directly from the mantle, and is heavier, like ocean bottoms
In addition to 10's of enormous volcanoes, (600 km across, 5X bigger than Moana Loa, biggest volcano on Earth), may 10,000s of small shield volcanoes, 20 km across.
There are lots of regions of scrunched up crust (more violent than on the Earth), called tessera. There are very few impact craters on the tessera, indicating that the surface has been so recently that the craters have been wiped out. We call this a young surface, like the Earth
There are about 910 craters on Venus, none smaller than 1.5 km. This is because the atmosphere is so thick that any impactor smaller than 150 m across gets burned up. These craters formed on the volcanic plains. Notice the bright impact ejecta--rough, blocky material thrown out of the craters
Notice the central peak, found in larger craters due to the rebound of the initial crater walls. Notice the large cracks near this crater
There are very few craters, so the surface must have been reworked by volcanism covering them up, or cracks destroying them
But this must have been fairly recently, because only 910 remain
We know about how often impacts are expected to happen, so we know that the average surface age is about 1/10 age of the planet, or 500 my
Largest impact crater on Venus, 280 km across.
Named after Margaret Mead--all feature on Venus named after women
Except Maxwell, after James Clerk Maxwell, studied light and radio
Another strange feature of Venus' impact craters is that only a handful of them are flooded with lava. Since Venus collects impacts continuously, that means that volcanism couldn't have been going on continuously, because then we would see lots of craters partly covered with lava.
The same is true of craters split by fractures--there are very few
Here we see one of the many, many cracks, but no craters
So we have only 910 craters, with maybe 34 of them modified. Venus has wiped out all the older craters, but there is no real evidence of how they got wiped out--the modified ones aren't enough
This has yielded a theory called catastrophic resurfacing -- 500 my ago, the entire lithosphere of Venus became unstable, and reworked itself in 100 my. It turned over in one enormous burp, followed by the building up of volcanoes, and the wholesale cracking and buckling of the crust. But no plate tectonics -- that is a continual process.
This is looking down on one of the largest volcanoes on Venus--a huge shield that developed over a hot spot
By counting the craters in the region around the volcano, we think that volcanoes are the youngest thing on Venus. Maybe 150 My, maybe still active a little
You can see two peaks, and on a close up, you can see 2 huge volcanic calderas, or volcanic craters
Huge circular features, but ranging in size from 100 km to 2600 km. May be the surface expression of huge plumes from the interior, or maybe where crust is subducting slowly
Still, no global pattern of plates like we clearly see on the Earth
Dark, smooth lava floor. All the way from small ones like this (picture graininess) to ocean sized ones
Even stranger volcanic features, small volcanoes called arachnoids, because they look like spiders
You can even see a vent, and the intense tectonic fractures around it
Upwelling of thick, viscous lava, forming these pie shaped bubbles of magma under the surface
Highly organized cracks that extend for thousands of km. The whole surface of Venus experienced huge stresses
Another mystery of Venus is a network of channels, some 6000 km, and going uphill! They couldn't have formed from water, because it's too hot.
We know that it's been hot since the catastrophic event, because there are no small craters, the air has been thick, and the greenhouse effect has prevailed
Channels might be some kind of lava tube, but there are no lava tubes on Earth anywhere near this long--long as the longest rivers on Earth
Even though Venus has a very thick atmosphere, there are only weak winds at the surface. The reason is that the air conducts the heat very efficiently, and temperature differences from place to place, that drive winds, don't build up.
But there is some wind, and even some sand dunes. But we know it isn't too common, because we don't see sand all over the place, like in craters
At the end of the long channels, or canali, there are river delta features
It looks like the liquid that made the channels ended its journey here, but they look a lot like terrestrial river deltas
A scientist at USGS has come up with a suggestion to explain these river delta features
They could be formed by low temperature lavas, ones that stayed liquid for a long time on the hot Venus surface, and flowed like water
He even proposed that this stuff could still be liquid, in under ground rivers of liquid carbonate
Magellan took three kinds of data -- images, altitudes, and gravity.
We have been looking at the images
Gravity tells us the density of stuff beneath the crust, so that we can tell if some things are continents, or if volcanoes have deep roots
Altitude tells us how high things are
If we use a computer to combine the image and altitude data, we can simulate any perspective, even ones the spacecraft never had.
This is a volcano, where you can see recent bright flows that came off the summit. The dark, smooth area is old lava flows, maybe lava plains that existed before the volcano
Landscape -- a jumble of lava flows, mountains, and cracks
System of cracks near the crater
Bright, rough ejecta, central peak
Volcano in background
Notice how the peak is white. The white at a certain height seems to be everywhere on the peaks of Venus, like a frost line
But what would act like a frost on hot Venus?
Answer, there's a metal that condenses just at that temperature line
tellurium. Spewed out in tiny amounts, it is what rains out on the peaks and causes the frost line
The Soviets were much more aggressive in sending missions to Venus
They sent a number of landers, which sent pictures for one hour (1970s)
Here's what the surface looked like
Rocks, soil like surface, a little bit of sky
Image is distorted because of the camera optics.
Looks like a volcanic plain
Slate like slabs, a little sky
Venera landers also measured what the crust was made of
It turned out to be basalt. Very basalty basalt, like new Earth crust in the oceans
VIDEO: 'THE ASTRONOMERS', PROSPECTING FOR PLANETS (Second half) VIDEO: CLIPS OF FLYING OVER VENUS FROM 'FLYING BY THE PLANETS'