Venus and Mars - Once-Earthlike Worlds

at first glance this may look like something out of the Mojave Desert in California in fact it's Mars Gale Crater to be exact so today we're gonna look at Venus and Mars and understand some of their general characteristics let's make some quick comparisons venus is closer to the Sun than Earth and Mars is farther from the Sun the orbital periods are of course in accordance with Kepler's third law Venus takes a little less than a year to orbit whereas Mars takes nearly two years to make one orbit around the Sun and while a day on Mars is just a little bit longer than a day on earth Venus has a 243 day and it's backward it's rotating in the opposite direction this is because of Venus's obliquity it's been knocked over on its side and it is now essentially a hundred and seventy-eight degrees tipped over and this is why it rotates backward chances are Venus must have suffered from a major collision while it was forming thus knocking the planet off on its side Venus is almost as massive as Earth but Mars is only 11% of Earth's mass the gravitational pull at the surface is almost the same on Venus as it is on earth on Mars however you're weighing quite a bit less just 38% the densities are pretty comparable between Earth and Venus but Mars on the other hand is just a little bit less dense but the atmospheres of these two worlds could not be any more different from one another let alone any more different than Earth the atmosphere at Venus is 90 times the pressure that you would feel at the surface of Earth Mars on the other hand is nearly a vacuum at just 0.7 percent of Earth's the surface temperatures on Venus are a broiling 735 Kelvin that's almost 900 degrees Fahrenheit whereas Mars is freezing it's well actually it's way below freezing at only 226 Kelvin that's equivalent to negative 53 degrees Fahrenheit Elton John had a great line in the song rock he's saying that Mars ain't the kind of place to raise a kid in fact it's cold as hell man was he right now Venus is covered in clouds that means that we have to use radar in order to inspect its surface this was done by the Magellan spacecraft in the 1990s about 75% of Venus's surface are lowland Plains there's a few large continents there's no evidence of plate tectonics that is to say there are no subduction zones or and there don't appear to be any rift zones but there are several stress fractures this was due to convection rising from the interior stressing the surface so even though Venus doesn't seem to have any plate tectonics Venus does seem to have evidence of tectonic activity namely tectonic stresses Venus has relatively few craters per square kilometer and those craters that are found are relatively fresh that tells us that Venus is still geologically active its surface is can still its surface is still being renewed most likely by volcanic activity now the number of craters is governed by several factors such as tectonics whether or not you have a liquid or a dry surface the atmosphere that you have how thick it is namely and whatever erosion processes that exist venus is a volcanic world SIF Mons is Venus is the largest volcano at five hundred kilometers across and three kilometers high it's capped by two caldera a caldera is a crater but it's not an impact crater rather it's formed by the top of the volcano caving in as heat is released but the volcanic activity on Venus creates what are called pancake domes these are lava outflows that spread out in all directions and if we take the radar data and we put it into perspective you can get an idea as to what it might be like if you could walk around out there and Venus does show evidence of tectonic activity it's not plate tectonics but nevertheless we do detect ridges and cracks and lowlands this is due to heat from the interior convecting upward stressing the surface pulling it in some directions compressing it in others but Mars is home to the largest volcano anywhere in the solar system this is olympus mons it's so big it would actually cover the state of Arizona if it were dropped here on earth and Olympus Mons Dwarfs every mountain and every volcano in the solar system but how is it that Mars can have such large volcanoes well it has a lower surface gravity so volcanoes don't weigh as much as they would here on earth and they also don't have plate tectonics so volcanoes are free to grow without having to worry about being broken up and subducted or spread apart due to moving plates now these volcanoes were once upon a time very active but since then Mars has gone pretty quiet if we count the number of craters that we see we find that most Valken ism' ceased about a billion years ago that being said Olympus Mons has very few impact craters so that suggests that maybe there was volcanic activity as recently as a hundred million years ago maybe there still is some lingering heat in there to this day but why would Mars be such a dormant planet well simply because it lost its internal heat and the reason it lost its internal heat is because it is a much smaller planet than Venus and Earth remember whenever we have a small planet there is less mantle available to insulate the core remember smaller planets have less mantle to insulate their cores this allows heat to escape faster shutting down volcanic activity but when Mars did have heat it drove wedges in the surface creating Valles Marineris this is the largest canyon system anywhere in the solar system as a matter of fact our entire Grand Canyon would fit comfortably inside just one of these tributaries that you see this was all formed by tectonic fracturing again not plate tectonics but the stressing and pulling apart and shearing away at the surface due to convection from inside and Mars demonstrates a fair amount of erosion erosion is essentially the tearing down of taller features and the filling in of low-lying features and water and wind do this very effectively Mars is covered in dunes after all has a lot of loose dust on surface and so the wind on Mars thin as it may be will produce sand dunes and we can watch the sand dunes forming and blowing and changing their shape over time but could water have been flowing on Mars the answer is overwhelmingly that it must have we see several dried up riverbeds and streams we find craters that are filled in part way other craters look like they kind of struck like almost like a muddy wet surface exactly what you would expect if you had water flowing on Mars and when you consider Mars as temperature range there is only one type of volatile that could flow on Mars and that would be water and Mars even has glaciers that means that once upon a time there is ice flowing around on Mars where is that ice today it's just beneath the surface the Mars Phoenix lander scooped up a sample of the Martian soil and there was ice the ice didn't last long it sublimated instantly once it was exposed to the vacuum of Mars's atmosphere but the thin layer of dust is keeping the ice under just enough pressure to prevent it from sublimating and so we believe that there's a layer of permafrost frozen water and some frozen carbon dioxide directly underneath the surface Venus and Mars have greenhouse effects as well but they're of different strengths Venus has the strongest greenhouse effect it raises its temperature by about 400 Kelvin whereas Mars having such a very thin atmosphere has a very weak greenhouse effect raising it only by about 5 Kelvin or so remember that volcanoes release water carbon dioxide and sulfur dioxide into the atmosphere the water rains out carrying with it the carbon dioxide and sulfur dioxide plants absorb the and I oxide and breathe out oxygen the rest of the greenhouse gases are dissolved into the ocean and the rest is brought into the tectonic plates themselves there subducted and brought underneath into the Earth's mantle some of those gases are re-released in subsequent expulsion ik eruptions and we have a kind of a cycle of carbon regulation so our atmosphere is mostly nitrogen and oxygen with trace amounts of water and carbon dioxide the rest remains sequestered and it's sequestered in forests and plants in the oceans rocks coral reefs wetlands oil and coal permafrost which by the way contains a great deal of methane and the rest is deeper still within the mantle so the idea is that geological activity of Earth are regulating the amount of carbon dioxide in the atmosphere and this prevents a runaway greenhouse effect on Venus however it's a very different story volcanoes release the same types of gases into its atmosphere as they do on earth however because Venus is closer to the Sun it's already experiencing a little extra warmth as a result the water does not fall out as a rain so there are no oceans and the carbon dioxide stays in the atmosphere and that right there will intensify the greenhouse effect but it gets worse because the sun's ultraviolet radiation will break apart some of that water that means that the hydrogen atoms which are the lightest in the universe are free to escape and the oxygen atoms combine with oxygen atoms to form ozone ozone z'n other greenhouse gas which further amplifies the greenhouse effect and the remaining water well that combines with sulfur dioxide to produce sulfuric acid it rains sulfuric acid on Venus it doesn't reach the ground of course because it's too hot it evaporates nevertheless all of these gases further warm the surface which release more carbon dioxide and other greenhouse gases into the atmosphere this is Venus's runaway greenhouse effect Mars on the other hand it's kind of a runaway cooling effect it's volcanic activity largely stopped billions of years ago Mars is farther from the Sun so it receives less heating water falls out as rain carrying with it carbon dioxide and sulfur dioxide they dissolve in the oceans but ultraviolet radiation from the Sun breaks apart some of those water and nitrogen molecules allowing the hydrogen nitrogen and oxygen atoms to escape and that's pretty easy because Mars already has a lower gravitational field so they achieve escape velocity with fewer greenhouse gases in the atmosphere Mars's greenhouse effect weakened this caused oceans to freeze further sequestering carbon dioxide with it and since Mars doesn't have any plate tectonics those gases remain today trapped in the permafrost so Mars underwent a runaway cooling effect now the atmospheres of Venus and Mars are almost entirely carbon dioxide in fact there are almost the exact same proportion so why did Mars get so cold and Venus gets so hot well it's not the percentage of carbon dioxide that you have it's the amount that's the key thing think of it this way you could have 95 percent of a dollar or you could have 95 percent of a million dollars what would you rather have Venus and Mars don't have any magnetic fields basically they're dynamos shut down for slightly different reasons Venus because it has such a slow rotation and Mars because it no longer has any thermal convection in its core so both planets are losing their atmospheres to the solar wind and of the two planets Mars is especially susceptible to these high-energy particles today Venus and Mars are both very inhospitable worlds but we believe that once upon a time they were very habitable maybe even earth-like so what happened to them well in the case of Venus remember Venus being closer to the Sun meant that it was more susceptible to the sun's brightening as the Sun grew older it brightened got a little bit hotter and started flooding Venus with a little more energy this led to the warming of Venus which eventually led to the evaporation of its oceans and more and more co2 being released into its atmosphere hence Venus is runaway greenhouse effect but Mars went through a slightly different evolution or devolution it lost its internal heat because it is a smaller planet and that allowed the greenhouse effect to essentially shut down so water escaped or froze into the permafrost and that sent Mars on a runaway cooling path so these combined effects turned these once earth-like planets into a modern-day hell on Venus and a cold desert of Mars

Channel: Launch Pad Astronomy

Views: 14,333

Rating: 4.9342723 out of 5

Keywords: Astronomy, Solar System, Terrestrial Planets, Venus, Mars, Greenhouse Effect

Id: KY70s0W5zH0

Length: 14min 10sec (850 seconds)

Published: Mon Apr 16 2018