Big vapor plume on Saturn moon

Scientists explain big vapor plume on Saturn moon.
Scientists on Wednesday said they have an explanation how one of Saturn's moons can spew out a giant plume of water vapor, adding to evidence a source of life -- water -- lies beneath the moon's frozen surface.

Using a computer model, German researchers showed the temperature at the bottom of surface cracks on Enceladus has to be about 0 degrees Celsius, the so-called triple point of water where vapor, ice and liquid water all can coexist.



"This makes this moon very interesting for further study because there is a connection between liquid water and life," Sascha Kempf, a physicist at the Max Planck Institut in Heidelberg, said in a telephone interview.

"This is the kind of thing planetary scientists hope for."

The scientists published their findings in the journal Nature.

Mars, Jupiter's moon Europa and Enceladus are the only places in the solar system with direct evidence of water. Finding organisms different from those on Earth may provide scientists with answers to questions ranging from where diseases come from to how our sun and planets formed more than 4.5 billion years ago.

Scientists have taken an especially close look at Enceladus because it seems to have a smooth surface -- suggesting recent geological activity that, in turn, could mean liquid water.

They are also intrigued by the plume itself, a gigantic geyser of water vapor and tiny ice particles. One mystery was how the dust particles slowed down to keep the plume restrained by the gravity of the moon, said Kempf, who worked on the study.

Their model showed most of the dust particles collide with the walls of the surface crack as they are ejected and constrain the gas flow to keep the plume close to the surface rather than shooting into the atmosphere, Kempf said.

The team used images of the plume and properties of the escaping gas and dust particles to run their model. They found it only reproduced the plume when the temperature was at zero degrees at the bottom of the cracks, implying water exists there in liquid form.

The density of the gas jet inside the cracks is so high that the small dust particles should have the same speeds to escape from the gravity of the moon," he said. "If this was true you wouldn't see plumes. You would see a long jets expanding into the system."

Saturn has at least 47 moons and at least seven rings. The joint U.S.-European Space Agency Cassini mission, launched in 1997, is spending four years examining Saturn.

Cassini is scheduled to fly 50 kilometers (31 miles) over the moon's surface in March, which will provide more information on the precise chemical composition of the particles and water vapor as scientists try to better understand the plume, Kempf said.

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Liquid Water Near Saturn:



Almost two weeks ago, Cassini mission scientists published a host of results pertaining to Saturn’s tiny moon Enceladus. The world had been given a hint of what to expect from the image on the right released last November that revealed a geyser of material spewing away from the south polar region of the moon. We were treated to a literal feast of fantastic results, summarized in this perspectives article by Jeffrey Kargel (a hydrologist at University of Arizona). Because of the importance of these new papers, the journal Science is offering free access to the 11 articles published in the March 10th issue relating to Cassini and Enceladus. You can spend $10 to buy the newsstand edition, gotta love free and open science!

As much as I would love to review all of the Enceladus science, so many important discoveries could not receive the attention or care they deserve in a single blog entry. Instead I will focus just on the discovery of watery geysers emanating from Enceladus’ south polar region. I will focus on the two papers detail the discovery of water ice in that jet of material, though two additional papers provide the important linkage between that jet and relatively warm surface features. The so-called tiger-stripes seen on Enceladus occur on a geologically very recent portion of the moon’s crust (Porco and others), and are shown to be about 70 degrees warmer than the surrounding crust. This makes the tiger stripes about 145 Kelvin, or -128 Celsius, while the surrounding crust is a chilly 70 Kelvin.

Synopsis
The plume picture above was taken on an Enceladus flyby last December, but three previous flybys had confirmed the presence of the plume and identified that, in fact, it was composed mostly of water ice. Two independent instruments on Cassini, the Ultraviolet-Visible-Infrared Spectrometer (UVIS) and the Ion and Neutral Mass Spectrometer (INMS), both confirmed the presence of water in the geyser.

The paper by Hansen and others describes the discovery of water in the plume using UVIS. The figure on the left shows the geometry of two flybys of Enceladus during which a sufficiently bright star was occulted by the moon. Occultations are important to astronomers and planetary scientists because in order to remotely sense the composition of an atmosphere a light source must shine through the atmosphere (more on this in the next paragraph). Part A of this figure depicts the first occultation, of the star Lambda Scorpii on the Feburary 17th, 2005 flyby. Notice that Lambda Scorpii passes behind Enceladus well above the south pole. The next occultation, of gamma Orionis (Bellatrix), occurred during the third and closest flyby on July 14th, 2005. This time, Bellatrix crossed behind the moon very near its south pole and emerged near the equator (shown in C and D).

This figure shows the spectrum of Bellatrix (blue line) along with the spectrum of light from the star seen through the plume of material near the south pole (red line). Note that prior to this detection, it was not known that Enceladus had any sort of atmosphere, even a temporary one. So the detection of absorption of the light from Bellatrix indicated that something must be happening around Enceladus.

The ratio of the red and blue lines in the previous figure are plotted in this graph as the thin solid line. Overlain on that curve as a darker solid line is the absorption spectrum of water. The water absorption spectrum explains the bulk of the total absorption seen near the south pole of Enceladus. Finer fluctuations are due to a number of a factors including the presence of other chemical species in smaller concentrations.

During the same flyby that UVIS was observing the occultation of Bellatrix by Enceladus, the INMS instrument detected something entirely unexpected: a sudden increase in particles near the south pole and surface of Enceladus. The INMS is, as its name says, a Mass Spectrometer (see this synopsis on the Huygens probe for an explanation of a similar instrument, the GCMS) that is capable of detected both ionized and neutral particles. The INMS is capable of detecting a wide range of chemicals including water, carbon dioxide, nitrogen, carbon monoxide, ammonia, and various hydrocarbons. It detected all of those compounds, some in very small quantities, in Enceladus’ plume.

The figure on the right illustrates how the density of water molecules detected by INMS changed as Cassini flew by on July 14th. A similar plume of dust was detected on the same approach using Cassini’s Cosmic Dust Analyzer (CDA). Notice that the peak of the CDA and INMS-Water plumes are offset slightly. This may indicate that the source of each plume is slightly different, or that the path between a common source and the detector differed for the dust and water. Also, notice that the peak of the water plume occurs very near to the south pole. This is strong evidence that the plume is originating from some sort of volcanic or vapor-geyser activity in that region. Combining this observation with those from UVIS and infrared maps of the surface makes a compelling case the the tiger stripes on Enceladus’ surface are the source of the plume. This, and the relatively frigid temperatures, led mission scientists to dub this plume “Cold Faithful.”

Synopsis
The plume picture above was taken on an Enceladus flyby last December, but three previous flybys had confirmed the presence of the plume and identified that, in fact, it was composed mostly of water ice. Two independent instruments on Cassini, the Ultraviolet-Visible-Infrared Spectrometer (UVIS) and the Ion and Neutral Mass Spectrometer (INMS), both confirmed the presence of water in the geyser.

The paper by Hansen and others describes the discovery of water in the plume using UVIS. The figure on the left shows the geometry of two flybys of Enceladus during which a sufficiently bright star was occulted by the moon. Occultations are important to astronomers and planetary scientists because in order to remotely sense the composition of an atmosphere a light source must shine through the atmosphere (more on this in the next paragraph). Part A of this figure depicts the first occultation, of the star Lambda Scorpii on the Feburary 17th, 2005 flyby. Notice that Lambda Scorpii passes behind Enceladus well above the south pole. The next occultation, of gamma Orionis (Bellatrix), occurred during the third and closest flyby on July 14th, 2005. This time, Bellatrix crossed behind the moon very near its south pole and emerged near the equator (shown in C and D).

This figure shows the spectrum of Bellatrix (blue line) along with the spectrum of light from the star seen through the plume of material near the south pole (red line). Note that prior to this detection, it was not known that Enceladus had any sort of atmosphere, even a temporary one. So the detection of absorption of the light from Bellatrix indicated that something must be happening around Enceladus.

The ratio of the red and blue lines in the previous figure are plotted in this graph as the thin solid line. Overlain on that curve as a darker solid line is the absorption spectrum of water. The water absorption spectrum explains the bulk of the total absorption seen near the south pole of Enceladus. Finer fluctuations are due to a number of a factors including the presence of other chemical species in smaller concentrations.

During the same flyby that UVIS was observing the occultation of Bellatrix by Enceladus, the INMS instrument detected something entirely unexpected: a sudden increase in particles near the south pole and surface of Enceladus. The INMS is, as its name says, a Mass Spectrometer (see this synopsis on the Huygens probe for an explanation of a similar instrument, the GCMS) that is capable of detected both ionized and neutral particles. The INMS is capable of detecting a wide range of chemicals including water, carbon dioxide, nitrogen, carbon monoxide, ammonia, and various hydrocarbons. It detected all of those compounds, some in very small quantities, in Enceladus’ plume.

The figure on the right illustrates how the density of water molecules detected by INMS changed as Cassini flew by on July 14th. A similar plume of dust was detected on the same approach using Cassini’s Cosmic Dust Analyzer (CDA). Notice that the peak of the CDA and INMS-Water plumes are offset slightly. This may indicate that the source of each plume is slightly different, or that the path between a common source and the detector differed for the dust and water. Also, notice that the peak of the water plume occurs very near to the south pole. This is strong evidence that the plume is originating from some sort of volcanic or vapor-geyser activity in that region. Combining this observation with those from UVIS and infrared maps of the surface makes a compelling case the the tiger stripes on Enceladus’ surface are the source of the plume. This, and the relatively frigid temperatures, led mission scientists to dub this plume “Cold Faithful.”

Synopsis
The plume picture above was taken on an Enceladus flyby last December, but three previous flybys had confirmed the presence of the plume and identified that, in fact, it was composed mostly of water ice. Two independent instruments on Cassini, the Ultraviolet-Visible-Infrared Spectrometer (UVIS) and the Ion and Neutral Mass Spectrometer (INMS), both confirmed the presence of water in the geyser.

The paper by Hansen and others describes the discovery of water in the plume using UVIS. The figure on the left shows the geometry of two flybys of Enceladus during which a sufficiently bright star was occulted by the moon. Occultations are important to astronomers and planetary scientists because in order to remotely sense the composition of an atmosphere a light source must shine through the atmosphere (more on this in the next paragraph). Part A of this figure depicts the first occultation, of the star Lambda Scorpii on the Feburary 17th, 2005 flyby. Notice that Lambda Scorpii passes behind Enceladus well above the south pole. The next occultation, of gamma Orionis (Bellatrix), occurred during the third and closest flyby on July 14th, 2005. This time, Bellatrix crossed behind the moon very near its south pole and emerged near the equator (shown in C and D).

This figure shows the spectrum of Bellatrix (blue line) along with the spectrum of light from the star seen through the plume of material near the south pole (red line). Note that prior to this detection, it was not known that Enceladus had any sort of atmosphere, even a temporary one. So the detection of absorption of the light from Bellatrix indicated that something must be happening around Enceladus.

The ratio of the red and blue lines in the previous figure are plotted in this graph as the thin solid line. Overlain on that curve as a darker solid line is the absorption spectrum of water. The water absorption spectrum explains the bulk of the total absorption seen near the south pole of Enceladus. Finer fluctuations are due to a number of a factors including the presence of other chemical species in smaller concentrations.

During the same flyby that UVIS was observing the occultation of Bellatrix by Enceladus, the INMS instrument detected something entirely unexpected: a sudden increase in particles near the south pole and surface of Enceladus. The INMS is, as its name says, a Mass Spectrometer (see this synopsis on the Huygens probe for an explanation of a similar instrument, the GCMS) that is capable of detected both ionized and neutral particles. The INMS is capable of detecting a wide range of chemicals including water, carbon dioxide, nitrogen, carbon monoxide, ammonia, and various hydrocarbons. It detected all of those compounds, some in very small quantities, in Enceladus’ plume.

The figure on the right illustrates how the density of water molecules detected by INMS changed as Cassini flew by on July 14th. A similar plume of dust was detected on the same approach using Cassini’s Cosmic Dust Analyzer (CDA). Notice that the peak of the CDA and INMS-Water plumes are offset slightly. This may indicate that the source of each plume is slightly different, or that the path between a common source and the detector differed for the dust and water. Also, notice that the peak of the water plume occurs very near to the south pole. This is strong evidence that the plume is originating from some sort of volcanic or vapor-geyser activity in that region. Combining this observation with those from UVIS and infrared maps of the surface makes a compelling case the the tiger stripes on Enceladus’ surface are the source of the plume. This, and the relatively frigid temperatures, led mission scientists to dub this plume “Cold Faithful.”

Context
This raft of discoveries about Enceladus are important for a number of reasons. First, they provide an explanation for a key observation about Saturn’s E-Ring. Second, Encladus has now become only the third planetary body whose volcanic activity has been directly observed from space (Earth, Io, and Enceladus are now on the list). Third, the heat output from that geological activity is very likely enough to ensure a body of liquid water at Enceladus’ south pole; again the list of moons or planets thought to have liquid water is quite short (Earth, Mars, Europa, Enceladus).

Sustained geologic (and volcanic) activity requires only one thing, heat. Larger planets at least as large as Venus (and maybe as small as Mars) have two long-term heat sources: 1) when the planet coalesces a tremendous amount of heat is released because of the decrease in potential energy of the grains of material from their diffuse spread-out state to the planet-bound one, 2) each body has a fixed amount of radioactive isotopes that decay and give off heat over millions to billions of years. Smaller moons, like Enceladus, lose their initial gravitationally-released heat rapidly, but they have a potential additional source heat from the tidal motions caused by their parent planets. Our Moon, for instance, seems to possess a liquid core largely because of tidal heating from the Earth. Europa probably has a global liquid ocean thanks to Jupiter’s massive tidal pull. Io is flagrantly volcanic for the same reason.

Sustained sources of heat means that water can remain liquid at depth on at least 4 planetary bodies in our solar system (again, Earth, Mars, Europa, Enceladus). Liquid water is a key ingredient in our form of life, and it may be a key ingredient in all life because of its unique properties. Europa and Enceladus are too far from the Sun for photo-dependent life to emerge, but chemically-dependent (no double-entendre intended) life probably could develop there. So now the search for life in our solar system can expand to Enceladus.

I have not yet heard any mention of such a mission in planning, but it would seem to me that searching for life on Enceladus is far simpler than doing so on Europa. After all, the “Cold Faithful” geyser system would eject not only liquid water but any organisms thriving there. So, send a craft to enter orbit around Enceladus and pass through that geyser thousands of times with a suite of life-detection instruments. This mission would be cheap and fast; I’m sure we will hear news of this idea from NASA very soon!