Friday, July 29, 2005

Press Release: Cassini Finds an Active, Watery World at Saturn's Enceladus

JPL has put out a press release describing the results from July 14 Enceladus flyby. In addition to the CIRS, MAG, and UVIS results mentioned here in other posts from today, INMS and CDA results are described. INMS, which normally looks at Titan in order to measure the composition and density of its upper atmosphere, found that Enceladus' atmosphere is composed of 65% Water vapor and 20% molecular hydrogen, with trace amounts of methane, carbon dioxide, and a combination of nitrogen and Carbon Monoxide (both molecules have the same mass so INMS can't distinguish between the two). The presence of Water vapor in its molecular form and the absense of atomic oxygen (also seen by UVIS), suggests that Enceladus' atmosphere is created from outgassing or evaporation at areas of elevated temperatures, and not from sputtering or impacts, like Europa's atmosphere. INMS also found spatial variations, both between regions near the south polar region and away from, and in layers, suggesting that there is a localized source of the atmosphere in the south pole, results that jive well with the MAG and UVIS data.

The CIRS data is the kicker. CIRS made temperature measurements both in dedicated scans and as ride-along observations with ISS. In both measurements, elevated temperatures were found in the south polar region near the tiger stripes and particularly within the tiger stripes. Color temperatures suggest that some areas in the tiger stripes could be as warm as 140K, indicating that the tiger stripes are regions of escaping internal heat. Such observations are indications that Enceladus is one of only three moons in the outer solar system with active volcanism (though the jury is still out with Titan, no elevated surface temperatures have been found there).

The CDA instrument found that dust kicked off by micrometeorite impacts are the source for the E-ring. Now, I should clear up some confusion brought up when comparing the atmospheric and dust results. Gas particles that make up the atmosphere appear to be generated by evaporation of warm ice or outgassing. The impact-generated cloud that forms the source of the E-ring consists of dust-sized particles, which are different from the atmosphere discussed earlier. The gases produced during a micrometorite must not be generated at a fast enough rate to be a significant contributor to Enceladus' atmosphere.

Off-topic: Tenth Planet Found

Busy day today, news wise. First we have active venting found on Enceladus announced. And now, a new planet! News reports have been circulating today regarding a newly found Trans-Neptunian object called 2003 EL61. Early reports, from a team of astronomers led by Jose Ortiz, suggested that the object was perhaps as large as Mars. However, a team led by Mike Brown announced that results from Spitzer suggested that 2003 EL61 is much smaller, maybe 1500 km across, smaller than Pluto. In addition, a much smaller moon was found orbiting 2003 EL61. By measuring the orbital period (49 days) and the orbital distance, Brown's team found the mass of the system to be only 30% of Pluto.

But that's not what I am talking about. I am talking about another discovery made by Mike Brown's team, 2003 UB313. A lower limit of the size for this object is a little larger than Pluto, meaning this world is definitely larger than Pluto. Size estimates assuming an albedo of 0.25, or around the albedo of other large bodies in the Kuiper Belt suggest a size around that of Mars.

So definitely an exciting discovery. Again, because this is off-topic from the discussion of outer planet satellites, I will limit discussion of 2003 UB313 to this post, unless of course a moon is found...

UPDATE: 07/29/2005 6:25pm: JPL has a press release now on their site about 2003 UB313. The new planet appears to be 2600 km across with bounds of around 2300 km (assuming it reflects all the light it receives from the sun, an unlikely proposition) and 3000 km (because it failed to be detected by Spitzer). Spectroscopy of 2003 UB313 indicates the presence of methane ice on the surface. In addition, Mike Brown's group has a website about the new planet that might be worth monitoring for the next few days, as well as the Wikipedia page.
UPDATE: 07/29/2005 8:08 pm: Planet not named Lila nor is the proposed name Lila. Please disregard.

New Enceladus Image: Warm Fractures on Enceladus

CICLOPS and the Composite InfraRed Spectrometer (CIRS) team have released this view of Enceladus in coordination with today's press release. During the flyby on July 14, CIRS took spectral measurements at wavelengths between 9 and 16.5 microns to measure the surface temperature of Enceladus, often coinciding with images taken by ISS. A string of ride-along measurements are shown here, superimposed on a ISS false color image taken at the same time as the measurements. The boxes represent the location of the measurements and the numbers above represent the temperature measured in Kelvin. Most of the surface was found to be in the expected range of 75K, however regions in the south polar ridged terrain showed higher temperatures. A couple of measurements, seen here, were a much higher temperatures, between 87 and 91 Kelvin, which is difficult (but not impossible) to explain by solar heating. When looking at the color temperatures measured, which looks at the shape of the spectrum seen by CIRS to determine the temperatures of several sub-pixel components. A two-temperature fit to the spectrum results in at least 1 percent of the 91 K pixel having a temperature of 140 K, further supporting the hypothesis that the increased temperatures seen over this area of the tiger stripes is caused by heat escaping from Enceladus' interior.

The ISS view is a false color version of the clear filter image released earlier this week and has a resolution of 122 m/pixel. This image uses an ultraviolet filter for blue, the clear filter for green (the normal green filter image was summed), and a near-infrared filter for red. The area surrounding the tiger stripe fracture is thought to be bluer because of coarse-grained water ice that has not been covered by the fine-grained ice that has painted the rest of Enceladus' surface.

UVIS Directly Measures Enceladus' atmosphere

After not sensing the atmosphere during an occultation in February, UVIS finally observed the atmosphere first observed by the magnetometer instrument. The UVIS detection came during another stellar occultation, this time of gamma Orionis, or Bellatrix. This star turned out to be a much better one to use than the star used in February, Lambda Sco. During the occultation, UVIS measured the intensity of the star as it passed behind Enceladus and its atmosphere, then reappeared following the occultation. Measurements during ingress (or approaching occultation) indicated a UV absorber between Bellatrix and Cassini over Enceladus south polar region, indicating an atmosphere composed of water vapor. Measurements during egress (or following the occultation) over nightside equatorial regions of Enceladus did not show nearly as strong of a UV absorber. This measurements fits well with the MAG models indicating that the atmosphere is not uniform over the surface of Enceladus and is likely concentrated over the south pole. The lack of a detection during egress could also result from the atmosphere "freezing out" at night.

The presence of water vapor in Enceladus' atmosphere and the lack of atomic oxygen indicates that the atmosphere is produced from the venting or evaporation of water ice from the surface rather than from magnetospheric sputtering, as is the case for the rarefied atmospheres of Europa and Ganymede. Instead the atmosphere has more in common with Io.

Hot Spot found on Enceladus

The CIRS instrument released this view of Enceladus. The image on the left is based on a model assuming a sub-solar temperature of 80 Kelvin, then cooling as you approach the terminator. The view on the left is the brightness temperatures actually measured by CIRS at wavelengths between 9 and 16.5 microns. The equatorial temperatures were as expected, again with temperatures around 75 Kelvin. Surface temperatures then cool to around 65 Kelvin as you approach the south polar region. However, in the south polar region, the temperature dramatically increases, to brightness temperatures of around 85 Kelvin, 15 degrees warmer than expected. The detector footprint at this distance was 25 km. Color temperatures from this data reveal small regions on the surface at well over 110 Kelvin.

The temperatures found in the south polar terrain are too warm to be caused by heating from sunlight alone, though exotic sunlight-trapping processes have not been ruled out. This means that heat from Enceladus' interior is escaping in the south polar region. The temperature would also be warm enough to produce evaporation of water ice at the surface, consistent with UVIS and MAG measurements of a concentration of Enceladus' atmosphere in the region.

This data makes Enceladus only the third body in the solar system (including the Earth and Io) where heat escaping from the body's interior has been found.

Press Release: Magnetometer instrument confirms Enceladus' atmosphere

The Cassini Magnetometer team, using data from the very close flyby on July 15, have confirmed the atmosphere first seen during the two earlier flybys in February and March. Based largely on the MAG data obtained during those flybys, the altitude of the July flyby was lowered from 1000 km to a scant 175 km. With this new data in hand, and with improved modeling of the earlier data, the MAG team was also better able to constrain the nature of the atmosphere. Rather than a uniform atmosphere across the surface of Enceladus, the Magnetometer results are most consistent with a neutral cloud of gas concentrated over the south polar region of Enceladus. ISS images of the region showed a very young surface with numerous tectonic fractures and strange "chunky peanut butter-like" terrain. Away from the south polar region, the atmosphere is much thinner. These results indicate that Enceladus' atmosphere is similar to that of Io's, where point sources, in Io's case volcanoes, pump gases into space which then spreads out across the surface or escapes the satellite's gravitational pull entirely. This leads to significant differences in atmospheric density across the surface. The press release compared the concentration to that of a cometary jet, though there is no evidence that it needs to be THAT concentrated of a source. Perhaps there are several sources spread out over a similar area (20 degree half-width).

The press release also mentions that other Cassini instruments found an atmosphere at Enceladus, though it didn't go into specifics. UVIS had a stellar occultation at closest approach designed to look for the atmosphere and other fields-and-particles instruments, like INMS, were on during the encounter.

DPS 2005 Abstracts online

Abstracts for the Division of Planetary Sciences (DPS) meeting in Cambridge, England in early September are now online. DPS abstracts are much shorter than LPSC abstracts (which are practically mini-papers) and are notorious for saying everything and nothing. However, there are some interesting talks and posters described here, that I would view, if I were going... For vanity purposes, the abstracts I am a co-author, and thus will be very disappointed if you don't go to if you are attending the conference (I'm not going, but I'll know, trust me, I'll know).

Tuesday, July 26, 2005

New Enceladus Image: Zooming in on Enceladus

CICLOPS has released this huge mosaic of the surface of Saturn's moon Enceladus. This mosaic was produced using 21 "footprints" of images taken during last week's encounter. Each footprint consists of images taken in a filter sensitive to ultraviolet light at 338 nanometers, a filter sensitive to infrared light at 930 nanometers, and either a filter sensitive to green light at 568 nanometers or a clear filter. This image contains much of the higher resolution images taken by Cassini during this encounter. The mosaic resolution is 134 meters/pixel, but it contains images that have original resolutions of up to 67 m/pixel.

In addition to the large mosaic, produced over the weekend by yours truly, is a fantastic movie zooming in on the mosaic, onto the highest resolution images taken during the UVIS occultation. The highest resolution images were taken below (in the large mosaic) the left most tiger stripe in a belt of tectonic grooves. You can see the location in the movie.

Press Release: Close Cassini Flyby Hints at Unusual Tectonic Activity on Enceladus

CICLOPS has now posted some initial results from images taken during last week's flyby of Saturn's strange icy moon Enceladus. Whereas the previous two encounters highlighted features seen in the equatorial regions of Enceladus, this encounter focused on terrain near the south pole of Enceladus. Previous images of the region taken by Cassini showed some interesting features, but it wasn't until now that these features were seen at geologically useful resolutions.

From the press release:
New detailed images taken by NASA's Cassini spacecraft of the south polar region of Saturn's moon Enceladus reveal distinctive geological features, and the most youthful terrains of any seen on Enceladus. These findings point to a very complex evolutionary history for Saturn's brightest, whitest world.

Cassini's flyby on July 14 brought it within 175 kilometers (109 miles) of the surface of the icy moon. The close encounter revealed a landscape near the south pole littered with house-sized ice boulders, carved by tectonic patterns unique to Enceladus, and almost entirely free of impact craters. These features set the southerly region apart from the rest of the moon.
Several lines of evidence are discussed that point toward a youthful age for Enceladus. While absolute ages have yet to be determined due to a lack of understanding of the cratering function for the Saturnian system, circumstantial evidence such as the lack of craters larger than 1 km and the paucity of craters smaller than that and the rough appearance at very high resolution, including building-sized boulders, the lack of fine grained material. Enceladus should be an interesting but challenging world to geologically map, but Enceladus has thrown us a bone. Many of the geologic regions are topographically and morphologically distinct, such as the south polar region with its wavy boundary with several y-shaped features filled with folded terrain. Dr. Paul Helfenstein suggests that suggest that such a boundary, located at around 60 degrees South latitude, could be the result of changes in the global stress pattern due to despinning. Such geologic features he contends "may tell us a very interesting story about the way Enceladus has evolved over time and what might have provided the energy to power the geologic activity that has wracked this moon".

UPDATE 12:30 pm: Usually I post links to news articles of news organizations covering releases like this. Unfortunately, I have found that only even has a copy of the press release, let alone did a story of their own. Everything today is about the Shuttle launch.

New Enceladus Image: Boulder Strewn Surface

CICLOPS has released several processed views using the highest resolution images taken thus far of Saturn's moon Enceladus. In the combined view above, the image at lower left is an expanded view of the wide-angle image and the inset image is the narrow angle view taken at the same time. Both images were taken when Cassini was at an altitude of 208 km above Enceladus' icy surface, but because Cassini was looking at the limb at the time, the actual distance to this area on the surface was 319 km. Thus the resolution of the narrow angle view was 3.7 m/pixel while the wide angle view has a resolution of 37 m/pixel.

The images themselves show a tectonically tortured region near the south pole of Enceladus. The lack of craters here, and the paucity of craters elsewhere in the region suggests that this area is geolgically young. In the narrow angle view, numerous boulders, 10-100 meters across, can be seen. How such boulders were produced in a relatively crater-free region will be a focus for study. The fact they have not been eroded by space weathering is another indication that this region is very young.

New Enceladus Image: Tiger Stripes Up Close

CICLOPS has released this high resolution frame showing the "tiger stripes" near Enceladus' south pole. These stripes were first seen as albedo features in images taken in May (though their northern ends could be seen in images taken in March). This image has a resolution of 122 meters/pixel and was taken from a distance of 20,720 km from Enceladus. It shows the tiger stripes to be fractures cutting across the Enceladean south polar region with ridges on each side of the fractures in many cases. The tiger stripes are distinctive in color observations as blue-green stripes. Blue-green terrain, thought to be coarse-grained water ice, has been observed in other region within bed-ice outcrops and canyon walls. Here, the blue ice appears to be a mattling, not just in fractures, but in relatively flat terrain as well. The cause for the difference in distribution is still under investigation.

New Enceladus Image: Enceladus In False Color

CICLOPS has released this processed, lower resolution color image of the full disk of Enceladus, as seen during last Thursday's flyby. This image was taken from a distance of 112,100 km and has a resolution of 670 m/pixel. It is centered at 42 degrees South, 167 degrees West, showing mostly the southern anti-Saturnian hemisphere of Enceladus. This image shows the south polar region in all its glory. Unlike the cratered region in the northern part of the disk, the south polar region has very few craters that can be resolved at this resolution. Instead the area is covered in fractures of all sizes. The south polar region is even seperated from the surrounding terrain by a wavy boundary characterized by tectonic gashes and by "'Y-shaped' tectonic patterns that appear to engulf folded regions of ridges and troughs.

This is a false color view created using image taken in an ultraviolet filter centered at 338 nm, a green filter at 568 nm, and an infrared filter centered at 752 nm (most other false color views use one centered at 930 nm).

New Enceladus Image: Craters and Cracks

CICLOPS has released this view of Enceladus show heavily modified craters and fractures along the northern edge of the South Polar tectonic region. This is the highest resolution image taken before the UVIS occultation ride-along image. Near the top of the image, numerous craters can be observed, most covered in tectonic fractures and cracks. Such morphology has been observed throughout this region of Enceladus, suggesting intense stresses following the initial formation of the south polar terrain. Also note the relative lack of rough terrain in the cratered region (outside of the thin fractures), quite different from the region of large boulders seen in the highest resolution images, also released today. In the lower part of the image, relatively few craters are seen and all are much less than a kilometer across.

This image has a resolution of 67 meters per pixel and was taken from a distance of 11,500 km.

Monday, July 25, 2005

New Scientist: Has Huygens found life on Titan?

Now, for the crank story of the day. New Scientist has a story online regarding the possible existence of hydrogen-breathing microbes on the surface of Titan. The story is based on a preprint of an article to be published in Icarus by Chris McKay and Heather Smith. They discuss the possibility that microbes on the surface of Titan could derive their energy from breathing hydrogen and "eating" acetylene that falls from the upper atmosphere on to the surface. They further suggest that depletions in both hydrogen and acetylene near the surface may betray the presence of such organisms. These types of measurements were made by the GCMS instrument on Huygens in January. Hasso Niemann, the PI for the GCMS instrument, stated that the hydrogen abundance is still being worked out and that an upper limit should be arrived at soon.

Of course this article fails to explain how you get around a 94 K surface temperature at the landing site....

New Prometheus Image: Lumpy Prometheus

CICLOPS has released this view of Saturn's small inner moon Prometheus. Prometheus orbits on the inner edge of the F ring. In this view, you can clearly see Prometheus' irregular and oblong shape. Prometheus is only 102 km across and is too small for its gravity to pull itself into a spherical shape. Several craters are visible in this view of Prometheus' southern anti-Saturnian hemisphere. This image has a resolution of 3 km/pixel (though it has been magnified 2x to aid visibility) and was taken from a distance of 438,000 km on June 7, 2005.