This is a very interesting question. Of course, as you noted, you have simplified things quite a bit; there are other factors besides temperature that affect habitability. Regarding Venus, you probably know that Venus is extremely hot at its surface not just because it is closer to the Sun, but because it has a thick CO2 atmosphere and is warmed by the greenhouse effect. There are, in fact, two things about Venus that would prevent it from being habitable wherever you put it. One is the lack of a magnetosphere, which is necessary to prevent ionizing radiation (particularly from the solar wind) from reaching the planet's surface. The lack of a magnetosphere appears to be due to the lack of a geodynamo on Venus, which has to do with the structure of its core. Second, Venus appears to lack tectonic plates, which you may know are responsible for earthquakes here on Earth. Interestingly, tectonic plates play a major role in controlling the buildup of CO2 in the atmosphere (see here for details). Lacking tectonic plates, Venus is doomed to have a large CO2 atmosphere wherever you put it, which would not make it a nice place to live. Mars, on the other hand, is a very different matter. It has both a magnetosphere (albeit it is very weak) and it likely has tectonic plates (although last I heard it is thought to only have two). One of the reasons that NASA has sent so many probes to Mars is that it was thought to be habitable at one point. It is thought that Mars's growth was stunted because of gravitation effects from Jupiter and Saturn. So, in another universe, Mars could have ended up very much like another Earth.
Updated April 23, 2018 By Kelly MacGregor
The answer to whether Mars or Venus is closer to the Earth is, "It depends." Mars and Venus are Earth's immediate neighbors in the Solar System. However, all three planets have nearly-circular orbits around the Sun and move at different speeds. So, sometimes Earth and Mars are close and Venus is on the other side of the Sun, and sometimes Venus is cozy with the Earth and Mars is the distant one.
Venus gets closer to the Earth than Mars or any other planet: 38.2 million kilometers (23.7 million miles).
At their closest, Mars is 55.7 million kilometers (34.6 million miles) from Earth but only 38.2 million kilometers (23.7 million miles) separates Venus and our planet. In relation to the sun, Venus is 108,200,000 kilometers (67,232,400 miles) away, Earth is 149,600,000 kilometers (92,957,100 miles), and Mars is 227,940,000 kilometers (141,635,000 miles) from the sun. For a more easily digestible size comparison, if you put the sun at one corner of the room, Venus would be two paces away, Earth just one half pace more, and Mars one and one half paces beyond that -- Pluto would probably take you outside your house, as it is 100 paces from the sun.
Venus spins in the opposite direction of Earth and has an atmosphere that destroys the NASA space probes in just a few minutes. The atmosphere is largely made of carbon dioxide and nitrogen. In terms of overall size, Earth and Venus are alike, but beyond that Venus is like an Earth where global warming has completely taken over. As the hottest planet in the solar system, the surface of Venus is 462 degrees Celsius (864 degrees Fahrenheit), and the planet is covered in volcanoes.
While Venus is a world on fire, Mars is cold -- temperatures range from -87 to -5 degrees Celsius (-125 to 23 degrees Fahrenheit). At approximately half the size of Earth, Mars has a desert surface but a very thin atmosphere. This tiny atmosphere is made up of carbon dioxide and nitrogen, like Venus, with argon added in. Mars may have had liquid water 3.5 billion years ago -- so much water, in fact, that there is evidence of giant floods on its surface.
Mars, Venus and Earth are all similar, as they are three of the four terrestrial planets -- Mercury is the fourth. Terrestrial planets are “Earth-like” as they all have a core, a mantle and a crust. Plate tectonics, erosion and volcanoes change the surfaces of Mars, Venus and Earth. Earth’s moon is sometimes studied with the terrestrial planets because its makeup is similar to Earth, but it is not a planet.
Using two ESA spacecraft, planetary scientists are watching the atmospheres of Mars and Venus being stripped away into space. The simultaneous observations by Mars Express and Venus Express give scientists the data they need to investigate the evolution of the two planets’ atmospheres.
Scientists call this work comparative planetology. Mars Express and Venus Express are so good at it because they carry very similar science instruments. In the case of the Analyser of Space Plasmas and Energetic Atoms (ASPERA), they are virtually identical. This allows scientists to make direct comparisons between the two planets. The new results probe directly into the magnetic regions behind the planets, which are the predominant channels through which electrically-charged particles escape. They also present the first detection of whole atoms escaping from the atmosphere of Venus, and show that the rate of escape rose by ten times on Mars when a solar storm struck in December 2006. By observing the current rates of loss of the two atmospheres, planetary scientists hope that they will be able to turn back the clock and understand what they were like in the past. “These results give us the potential to measure the evolution of planetary climates,” says David Brain, Supporting Investigator for plasma physics for Venus Express and Co-Investigator for ASPERA on Mars and Venus Express at the University of California, Berkeley.
The new observations show that, despite the differences in size and distance from the Sun, Mars and Venus are surprisingly similar. Both planets have beams of electrically charged particles flowing out of their atmospheres. The particles are being accelerated away by interactions with the solar wind, a constant stream of electrically charged particles released by the Sun. At Earth, the solar wind does not directly interact with the atmosphere. It is diverted by Earth’s natural cloak of magnetism. Neither Mars nor Venus have appreciable magnetic fields generated inside the planet, so each planet’s atmosphere suffers the full impact of the solar wind. Interestingly, this full-on interaction does create a weak magnetic field that drapes itself around each planet and stretches out behind the night-side in a long tail. Venus’s atmosphere is thick and dense, whereas that of Mars is light and tenuous. Despite the differences, the magnetometer instruments have discovered that the structure of the magnetic fields of both planets are alike. “This is because the density of the ionosphere at 250 km altitude is surprisingly similar,” says Tielong Zhang, Principal Investigator for the Venus Express magnetometer instrument at Institut für Weltraumforschung (IWF), Österreiche Akademie der Wissenschaften, Austria. The ionosphere is the surrounding shell of electrically-charged particles created by the impact of sunlight on the planet’s upper atmosphere.
The proximity of Venus to the Sun does create an important difference, however. The solar wind thins out as it moves through space so the closer to the Sun it is encountered, the more concentrated is its force. This creates a stronger magnetic field, making the escaping atmospheric particles move collectively like a fluid. At Mars, the weaker field means that the escaping particles act as individuals. “This is a fundamental difference between the two planets,” says Stas Barabash, ASPERA Principal Investigator on both Mars Express and Venus Express, Swedish Institute of Space Physics. Another illuminating difference between Mars and Venus is that Mars displays strong small-scale magnetic fields locked into the crust of the planet. In some regions, these pockets protect the atmosphere, in others they actually help funnel the atmosphere into space. The complexity of the different processes revealed at Venus and Mars means that planetary scientists do not yet have the full picture. “There will be many more results to come,” says Barabash. There is a lot to do because there are many different mechanisms that may cause the atmospheric particles to escape. Untangling it all will take time. “The longer the spacecraft work together, the longer we can watch and see what really happens,” says Brain.
These new results are presented in 19 papers to be published in a special issue of the journal Planetary and Space Science. Some out of these have been available online from 19 January 2008.
Stanislav Barabash, Swedish Institute of Space Physics
Tielong Zhang, Institut für Weltraumforschung (IWF), Österreiche Akademie der Wissenschaften, Austria
Håkan Svedhem, ESA Venus Express Project Scientist
Agustin Chicarro, ESA Mars Express Project Scientist
David Brain, University of California, Berkeley, USA |
Which planet is between Venus and Mars
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