We think there are two. The third-densest atmosphere in the solar system is Io's. From observations with the Submillimeter Array, Arielle Moullet and three coauthors measured sulfur dioxide, sulfur monoxide, and sodium chloride in its air.
Yes, salt is a constituent of Io's atmosphere! Some of these gases, probably including all of the salt, is fed directly to Io's atmosphere by its constantly erupting volcanoes, but there's too much sulfur dioxide for all of it to be volcanic; some of it has to come from sublimating sulfur dioxide frost.
At the surface, the peak pressure is around 1. But it's still an atmosphere, and a variable one. Io's surface breathes over the course of its day, exhaling sulfur dioxide as it warms in sunlight, and pulling it back down to the surface as it cools at night. The volcanoes heat their local areas, making patches of denser atmosphere and out-flowing winds.
One of the weirdest things in Io's atmosphere is a consequence of it having no magnetic field while being embedded inside Jupiter's. Jupiter's magnetic field sweeps across Io's atmosphere once each Jupiter day, slamming into the back of Io's atmosphere.
The magnetic field moves the ions in Io's atmosphere, but not the neutral molecules, so the moving ions crash into the neutral molecules, moving as a current through Io's resistant neutral atmosphere. The interaction of Jupiter's magnetic field with Io's ionosphere has a visible manifestation as a glowing Io footprint within Jupiter's aurora. For further reading, I recommend this PDF of a PowerPoint presentation on Io's atmosphere that had no name on it -- please speak up in the comments if you know whose presentation it is!
Okay, so how about the fourth densest atmosphere? A new paper in press at Icarus by Nathanial Cunningham and five coauthors suggests that there is one more known collisional atmosphere among the solar system's moons.
The fourth densest moon atmosphere in the solar system is drum roll please :. Cunningham and coauthors used the Cosmic Origins Spectrograph on Hubble to detect the telltale ultraviolet emission of atomic oxygen and determine its abundance. Oxygen probably forms at Callisto when water ice molecules at the surface are split into hydrogen and oxygen atoms; some of the lightweight hydrogen escapes from the atmosphere entirely, leaving the heavier oxygen behind.
John Spencer who was second author on the paper helped me convert from atmospheric column abundances to surface pressures. Sunlight is quite dim on Titan, and climate is driven mostly by changes in the amount of light that accompanies the seasons. Data also suggests the presence of a liquid ocean beneath the surface , but it is still to be confirmed.
As more planets have been found outside of the solar system, Titan has served as a model of cloudy bodies. Examining the atmosphere of the moon has helped scientists to understand the atmospheres of these distant systems.
In , the Cassini spacecraft wrapped up its two-decade-long mission to Saturn. Launched on October 15, , the space craft arrived at Saturn on June 30, Huygens was equipped to study Titan by landing on the Saturn moon and achieved astounding results. For example, many mountains above 10, feet high have been identified on the moon. The Huygens probe landed via parachute on Jan. Because of Huygens's observations, Titan became a top priority for scientists.
During its primary and extended missions, Cassini was able to get fundamental data about Titan's structure and the complex organic chemistry of its atmosphere.
It is because of Cassini's findings that scientists suspect the presence of an internal ocean composed of water and ammonia. The spacecraft has also spotted seasonal change , such as when an ice cloud formed in Titan's southern hemisphere in suggesting that winter was going to be harsh in that zone.
The focus of the mission, as it relates to Titan, was to find signs of seasonal changes and volcanic activity. Titan played a dominant role in Cassini's planned ending. The massive moon provided the gravitational boost the spacecraft needed to thread between Saturn's rings in its final months, exploring a never-before-seen region. After this major boost, the spacecraft continued to use Titan to tweak its orbit, making its final close approach to Titan on September 12, Over its 13 years orbiting Saturn, Cassini made encounters with the moon, some close and others more distant.
It is thought that conditions on Titan could make the moon more habitable in the far future. If the sun increases its temperature 6 billion years from now and becomes a red giant star, Titan's temperature could increase enough for stable oceans to exist on the surface, according to some models.
This surface is coated with organic molecules that have rained or otherwise settled out of the atmosphere in the form of sands and liquids. The surface is hugged by a dense atmosphere. However, its atmosphere provides a clue. The surface of Titan is one of the most Earthlike places in the solar system, albeit at vastly colder temperatures and with different chemistry. Here it is so cold degrees Fahrenheit or degrees Celsius that water ice plays the role of rock. No other world in the solar system, aside from Earth, has that kind of liquid activity on its surface.
The "sand" in these dunes is composed of dark hydrocarbon grains thought to look something like coffee grounds. In appearance, the tall, linear dunes are not unlike those seen in the desert of Namibia in Africa. Titan has few visible impact craters, meaning its surface must be relatively young and some combination of processes erases evidence of impacts over time. Earth is similar in that respect as well; craters on our planet are erased by the relentless forces of flowing liquid water, in Earth's case , wind, and the recycling of the crust via plate tectonics.
These forces are present on Titan as well, in modified forms. In particular, tectonic forces—the movement of the ground due to pressures from beneath—appear to be at work on the icy moon, although scientists do not see evidence of plates like on Earth. Our solar system is home to more than moons, but Titan is unique in being the only moon with a thick atmosphere.
At the surface of Titan, the atmospheric pressure is about 60 percent greater than on Earth—roughly the same pressure a person would feel swimming about 50 feet 15 meters below the surface in theocean on Earth.
Because Titan is less massive than Earth, its gravity doesn't hold onto its gaseous envelope as tightly, so the atmosphere extends to an altitude 10 times higher than Earth's—nearly miles kilometers into space. Titan's atmosphere is mostly nitrogen about 95 percent and methane about 5 percent , with small amounts of other carbon-rich compounds.
The pieces of these molecules recombine to form a variety of organic chemicals substances that contain carbon and hydrogen , and often include nitrogen, oxygen and other elements important to life on Earth. Some of the compounds produced by that splitting and recycling of methane and nitrogen create a kind of smog—a thick, orange-colored haze that makes the moon's surface difficult to view from space. Spacecraft and telescopes can, however, see through the haze at certain wavelengths of light outside of those visible to human eyes.
And methane condenses into clouds that occasionally drench the surface in methane storms.
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