Water plumes over Europa Options

[tedstryk]

Tempting, but I've been trying to search a plume out, and I've been tempted to try to see something centered around the 8 o'clock point in this six-image stack. The result looks a lot like yours but is not in the same place.

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[ollopa]

Bob Pappalardo and an assembled cast of the usual suspects addressed this directly in JGR in 1999. "Does Europa have a subsurface ocean?" (Circa Page 31):

our analysis shows that this is a false double-exposure rather than a real phenomenon, probably resulting from incomplete closing of the camera shutter blades in combination with an imaging mode that does not include a preexposure erasure of the CCD array.

I'm not saying the Voyager observations are correct, but they are not disproven by this quick look.


Phil

[vexgizmo]

Bob Pappalardo and an assembled cast of the usual suspects addressed this directly in JGR in 1999. "Does Europa have a subsurface ocean?" (Circa Page 31):

our analysis shows that this is a false double-exposure rather than a real phenomenon, probably resulting from incomplete closing of the camera shutter blades in combination with an imaging mode that does not include a preexposure erasure of the CCD array.

That was in response to the Galileo double-exposure false-plume. There are other statements in that paper about the Cook et al "plume":

The best Voyager evidence for the presence of anomalously fresh frost on Europa's surface is from the controversial, 143° crescent "plume" image of Cook et al. [1982, 1983] (Figure 17). In addition to showing an off-limb feature that was inter¬preted as a volcanic event [Cook et al., 1982, 1983], the image shows a conspicuous bright spot on the surface at 34°, 337°. Helfenstein and Cook [1984] measured the photometric con¬trast of the bright feature relative to surrounding terrains and compared it with that measured of the same geographic region viewed at 13° phase. They found that, relative to the surrounding features, the brightness of the anomalous spot increased as the phase angle increased from 13° to 143° phase by more than seven standard deviations above the average surface change. Although Helfenstein and Cook [1984] interpreted this brightness change to be due to active emplacement of surface materials, a more conservative interpretation would be that the feature represents relatively transparent frost that was deposited on Eu¬ropa's surface in recent geological history [cf. Verbiscer et al., 1990; Verbiscer and Veverka, 1990; Verbiscer and Helfenstein, 1998]. McEwen [1986a] suspected that this bright region was not actually anomalous compared to other bright regions on Europa which had not been seen at high phase angles by Voyager. Indeed, in low-phase global-scale Galileo images, this area appears similar to other bright plains regions.

Figure 17. Unprocessed Voyager 2 clear filter image (FDS 20767.37) at a resolution of ~44 km/pxl and phase angle of 143°, argued by Cook et al. [1982, 1983] to show an active plume along Europa's bright limb. Stretched inset image shows detail of the bright limb and putative plume, and an unusually bright area on the surface. The "plume" feature, with a signal level of just 5 DN, is not observed in subsequent images. Its location in the corner of the Voyager vidicon image, where noise and distortion are most severe, suggests that it is a camera artifact.

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[TheAnt]

Despite the result from the Hubble telescope, a check of old data by Cassini failed to reveal any plumes at Europa.

[Vultur]

Despite the result from the Hubble telescope, a check of old data by Cassini failed to reveal any plumes at Europa.

That looks like it's only for one brief period (the flyby), though, right? So that doesn't really cast doubt on the Hubble results since it's not for the same time -- does it?

[tedstryk]

I've reviewed virtually every image from Galileo, Voyager, Pioneer, Cassini, and New Horizons. Nothing except for a Voyager image that depends on one pixel, and that is almost certainly noise.

[vjkane]

That looks like it's only for one brief period (the flyby), though, right? So that doesn't really cast doubt on the Hubble results since it's not for the same time -- does it?

I only read the press accounts on this paper, but with that in mind: If plumes are a regular occurrence, then there should be enhanced plasma or gas cloud centered on Europa's orbit. This study used Cassini's observations to look for such a cloud and didn't find it. However, at least one other group doesn't think that the Cassini measurements would be conclusive and reportedly are working on a rebuttal.

I've been following this subject since the first announcement. One thing to keep in mind is that the Hubble measurements were at the limit of detection, and every subsequent analysis has emphasized that the plumes might not have existed. If they did exist, it is possible that the occur infrequently. In an analogy that I've read from the science teams that have looked at this, each of Io's volcanoes erupts infrequently. Because Io is so extremely heated by Jupiter's gravitational tug, there's usually some volcano going off. Even so, there are periods of much greater and much less volcanic activity. By this reasoning, the lesser gravitational tug on Europa might create plumes only in limited areas and there might be months or years or more between eruptions.

It may be that we'll have to wait for JUICE to arrive to get definitive answers. The one solid recommendation from the Europa Clipper Science Definition Team is that a mission oriented towards Europa plumes is very premature to consider, but that the Clipper mission should carry a suite of instruments that could search for and study plumes (much like the broad range of instruments on Cassini has allowed detailed studies of the totally unexpected Enceladus plumes).

[nprev]

I wouldn't characterize Enceladus' plumes as 'completely unexpected', though; the moon's association with the E-ring was strongly inferred prior to Cassini, and in hindsight it's about as robust an emission torus as anyone could hope for. Something was clearly going on there but the mechanism was not known.

By analogy, any eruptive activity on Europa must be both extremely transient and volumetrically small given the absence of anything remotely comparable to the E-ring. I wonder if the isolated Hubble observation could be best explained as the aftermath of an impact?

[TheAnt]

That looks like it's only for one brief period (the flyby), though, right? So that doesn't really cast doubt on the Hubble results since it's not for the same time -- does it?

Yes its ofc data only for the time of the flyby, even so Cassini did not look for any active plumes but for any resulting cloud and the latter would hang around a lot longer.

@Vjkane: I am firmly with nprev here, the plumes of Enceladus were actively searched after, and it took quite a while before we got that first backlit image showing them clearly. That something have been going on at Encladus have been known at least since the data from the Voyagers did show something were going on at this moon.
A rebuttal is a good sign, hardly anyhing in the scientific world is accepted before someone else have poked your hypothesis hard with a pointy stick. =)

[Explorer1]

If it was an impact that caused the plumes, would JUICE or Europa Clipper see any evidence? Considering that its effects were visible from Earth, there would have to be a rather large crater, right? Obviously only speculation at this point...

[katodomo]

According to its Yellow Book (page 26), JUICE will have a resolution of 500-1000 m/pixel for regional features and under 50 m/pixel for selected local features during the flybys.

[nprev]

Might not have been a particularly large impactor; I'd guess something like a 10m rock. Depending on the inbound trajectory the relative velocity might've been quite high that close to Jupiter so a lot of water vapor could result from a hit & probably the impactor would vaporize as well. Has the data been examined for spectroscopic peaks in typical meteoritic elements like carbon, silicon, iron, etc. in addition to water?

[rlorenz]

@Vjkane: I am firmly with nprev here, the plumes of Enceladus were actively searched after, and it took quite a while before we got that first backlit image showing them clearly. That something have been going on at Encladus have been known at least since the data from the Voyagers did show something were going on at this moon.

Just to enlarge on this, the Cassini Phase A report in 1988 clearly shows the degree of interest, declaring Iapetus and Enceladus the two icy satellites of particular interest, noting (section 3.4.2)
'The strange appearance of Enceladus surface and the location of the ephemeral E-ring may be a coincidence, but there is a strong probability that they are connected in some manner, possibly through current eruptive activity on Enceladus'

[Bjorn Jonsson]

If it was an impact that caused the plumes, would JUICE or Europa Clipper see any evidence? Considering that its effects were visible from Earth, there would have to be a rather large crater, right? Obviously only speculation at this point...

A new and relatively small crater could only be detected if it formed in the terrain that was well imaged by Galileo.

Plumes similar to Enceladus' plumes would be ~70 km high when scaled to Europa's gravity so they wouldn't be too difficult to image from a Jupiter-orbiting spacecraft. JUICE should have no problems imaging them from e.g. Ganymede's orbit if they are near the limb but one possible problem is that any plumes might be smaller than this and they could be episodic. For imaging you want high-phase imaging and possibly a violet or UV filter. There aren't that many good high-phase images of Europa but as Ted indicated, any plumes can be ruled out in all of them except maybe for one Voyager image where noise is by far the most likely reason.

[TheAnt]

Just to enlarge on this, the Cassini Phase A report in 1988 clearly shows the degree of interest, declaring Iapetus and Enceladus the two icy satellites of particular interest, noting (section 3.4.2)
'The strange appearance of Enceladus surface and the location of the ephemeral E-ring may be a coincidence, but there is a strong probability that they are connected in some manner, possibly through current eruptive activity on Enceladus'

Thank you for pinpointing that one rlorenz.
And yes carefully phrased as it should be. =)
It also serves as a good example that some researchers get one aha moment prior to a mission or a flyby since their minds start to deal with whatever might turn up in that area of space. And it is also a good thing for planning the observations (we've seen some such also for Pluto now which might or not turn out correct when New Horizons get a closer look). =)