Our robot emissaries have given us much information about our neighboring planets and solved many of the mysteries that shrouded them before we moved into space. But they have generated just as many questions from their findings. There are more mysteries in the solar system now than ever.
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The Inner Planets
Before we entered what Wiley Ley called ‘The Third Era of Astronomy’—that is, robot probes and men exploring the other worlds—there were all kinds of speculation about what the surface of Mercury and Venus was like. Many believed the surface of Mercury–that we thought at the time continuously faced the Sun– would be smooth and featureless, having been melted smooth by the extreme heat. (We now know it faces the Sun only 2/3rds of Mercury’s 88-day orbit)
The total cloud cover that wrapped Venus led many to conjure up a dense, lush tropical rain forest, perhaps harboring life.
Our robots have destroyed those visions. We know now that Mercury is pock marked with craters, just as is our moon, and the side that faces the Sun most of the time is no different than the 'dark' side. There is also evidence of volcanic activity.
Did volcanoes exist on Mercury at one time. Are they still active? We may answer those mysteries in 2011 when the Messenger spacecraft goes into orbit about the planet. And we may learn if there is water ice at the poles, which are always in shadow. Flybys have detected patches brighter than the rest of the surface at both poles. A sign of ice?
And the clouds shrouding Venus do not hide a lush tropical jungle. The clouds are almost all carbon dioxide. The surface of the planet is hot, dry and ragged, with a temperature exceeding 470 degrees C. The robots that have landed on the surface have lasted only minutes.
But Venus may not always have been this way. German researchers have identified rocks in some areas that look like granite. Granite is formed when basalt is forced beneath plates by tectonic forces, mixes with water and is pushed back to the surface by volcanic activity.
Did Venus once have oceans and continents? If so, why did they disappear and the atmosphere become carbon dioxide? It will take another robot (or more) to solve that mystery.
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The Martian Chronicles
At this point in the 21st century, we probably know more about Mars than any planet besides our own in the solar system. Yet, every robot we send finds new mysteries on the Red Planet. They’ve found unmistakable signs of water—in fact, ice. Yet, no signs of primitive life have yet been found. If there is water on Mars now, and there was more in the past, there had to have been some forms of life. Where is it?
But perhaps the biggest mystery on Mars is the dichotomy between the northern and southern hemispheres. The northern hemisphere consists of lowlands, scarred with some impact craters. The southern hemisphere is highland country. But surprisingly, the transition between the two is smooth and gradual.
Caltech scientists have developed a computer simulation that indicates a massive asteroid impact about four billion years ago could have created this dichotomy. Coincidentally, this is about the same time a massive asteroid impact may have torn the Moon from Earth.
Did an asteroid form Mars’ surface features? It will take more robots, or a manned expedition to decide.
Then of course there are Mars’ two tiny moons, Phobos and Deimos. Suspected as being captured asteroids, they look the part, being ragged, misshapen and small—Phobos is only 22 km in diameter, and Deimos just 12.6 km. Phobos orbits just 6000 km above its primary.
Were these stray asteroids captured when the giant crashed into the fourth planet during a rain of asteroids on the inner planets?
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The Asteroid Belt
We know asteroids slither out of their orbit in the asteroid belt when Jupiter’s massive gravitational field sometimes disturbs their orbits. They then tend to swing towards the inner planets, including Earth. But why are the asteroids there to begin with? Several robot spacecraft have flown through the belt, and the NEAR spacecraft even landed on the asteroid Eros. A Japanese robot landed on one, collected a sample, and took off to return the sample.
Yet, we still don’t know why this collection of rocks is at this location in the solar system. They occupy an orbit where a fifth planet should be. Did a planet try to form but could not because of Jupiter’s massive gravity, or did it form and then for some reason break apart?
If a fifth planet broke apart, or for some reason exploded about four billion years ago, did that cause the rain of asteroids that pummeled the inner planets to form Mars’ surface, create our moon, devastate Venus and leave Mercury pock marked with craters?
These are mysteries of the inner planets. Our robots have found even more intriguing mysteries among the outer giants.
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Beyond the asteroid belt the giant planets hold sway. But for the most part, it is their moons that hold the mysteries. There are moons that may have oceans of water beneath their crust, oceans that could support life. There is a moon with a substantial atmosphere, heavy with organic compounds that could mean life forms on the surface.
And there is a planet lying on its side, so we can't tell which is its north and south pole.
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The Moons That May Have Hidden Oceans
As fascinating as the two giants of the solar system are—Jupiter and Saturn—their moons are even more intriguing. Three of them show strong evidence that they have oceans, oceans of water—H2O—beneath their crusts.
Jupiter’s Europa has surface features, photographed by the Galileo spacecraft, that show strange characteristics. Small blocks of the crust float like icebergs over something—perhaps an invisible sea? Europa is in a gravitational tug of war with its huge primary, and its icy surface is pulled and flexed several times a day. The crust cannot move so much, but a liquid beneath that crust could rise and fall, causing ice volcanoes or geysers to shower the surface.
This scenario is not simply speculation. The Cassini spacecraft photographed just that phenomenon on Saturn’s Enceladus. The image to the right shows enormous ice jets erupting from Enceladus’ subsurface ocean.
And Saturn’s Titan may also have an ocean beneath its crust. The Huygens probe that landed on the moon in 2005 seemed to detect a subsurface ocean, but the data was not conclusive. Since then the Cassini spacecraft has detected a slight variation in Titan’s rotation rate that could be caused by a subsurface ocean.
Titan interests scientists in another way as well. It has a significant atmosphere, 60 percent thicker than Earth’s. The “air" is mostly nitrogen, just as is our atmosphere, but there is no oxygen. There appears to be heavy concentrations of organic compounds such as methane however. And that opens the question of some form of life existing on the surface.
But on all three moons, the possibility that they may have subsurface oceans opens the possibility of life in those oceans. Do the moons of the giant planets harbor life? It will take a highly sophisticated robot to find out.
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The Planet Lying on its Side
The seventh planet in our solar system harbors its own mystery. Uranus is tilted on its axis 98 degrees. All the other planets are within a few degrees of their orbital plane. Earth for instance is tilted about 23 degrees. Uranus is virtually lying on its side as it makes its way around the sun.
How did it get this way? The planets formed from the dust and gas belt surrounding the sun after its formation. They all formed in the plane of the ecliptic and all should have formed with similar axial tilts, dictated by the rotation of the primordial cloud.
Many scientists believe Uranus was knocked for a loop by a collision with an Earth-sized planet. But where did the clandestine planet come from? And where is it now?
Uranus has a couple of other mysteries hidden in its hazy atmosphere. When Voyager flew by it, its pole was pointed directly at the sun, Yet the equater still was warmer than the pole. The physics just do not add up.
And about those poles, With the planet laying on its side, we're not sure which is the north pole, and which is the south!
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Another Moon With An Atmosphere (Sort of)
Neptune’s moon Triton, like Titan, also has an atmosphere, but it is unique to say the least. Triton’s surface temperature is just a tad above absolute zero—an estimated 35 degrees K.
Its atmosphere is tenuous, consisting of nitrogen ice and…water ice. In fact, Triton’s density indicates it is perhaps as much as 45 percent water. And there are indications of ice geysers just as on Enceladus. In fact, Triton's upper atmosphere is considerably warmer than its surface—-temperatures there are about 95 degrees K.
Triton is also turned on its side, with an axial tilt of 157 degrees to its primary’s orbit. It also orbits Neptune in retrograde—backwards to normal motion. These facts are clear evidence Triton is a captured body, most likely a fugitive from the Kuiper belt (see Part 2).
Beyond Neptune is what once was the ninth planet—or was it? And perhaps now there is a tenth planet—or is there? And then a strange coruscating cloud of comets and rocks. And beyond that—something?