DRAMATIC CHANGE IN JUPITER'S GREAT RED SPOT FROM SPACECRAFT OBSERVATIONS

The red edges of Jupiter's Great Red Spot (GRS) have contracted from a longitude length of ∼21° during the Voyager flybys in 1979 to ∼155 in Hubble data acquired in 2012, shrinking on average 019/year over the modern era. Amateur observers documented a sudden decrease in longitudinal extent in early 2014, faster than the average rate of contraction, and Hubble Space Telescope (HST) time was granted to characterize the current state of the storm. In the 2014 Hubble data, the red edges of the spot span about 141 in longitude, a decrease of 14 (1760 km) in 21 months, a rate of change 4 times greater than typical since 1979. This change has happened without any visible extrinsic causes. Continuous monitoring by amateur observers showed no anomalous events in the surrounding region that might have affected the GRS, from mid-2011 to the end of 2013 (see the British Astronomical Association, Jupiter Section: http://www.britastro.org/jupiter/section_reports.htm).

Another notable characteristic of the 2014 images is the deep red color of the GRS, which is normally seen only during climatic cycles when the adjacent South Equatorial Belt (SEB) "fades" or whitens, such as during the Pioneer flybys in 1974 (Rogers 1995). Red colors on Jupiter are due to absorption at blue and green wavelengths, with whiter regions showing the least absorption at short wavelengths. Spectroscopy was performed using Hubble images from 1995 to 2014 to determine the visible-wavelength color of the darkest core of the storm, Figure 1. In this region, the reflectance spectrum (I/F) is fairly constant from 1995 to 2009 (Sanchez-Lavega et al. 2013; Simon et al. 2014). The greatest change in I/F of up to 0.08 can be seen near 410 nm when comparing data from 1996 and 2008. The data acquired in 2014, however, show that the I/F spectrum of the GRS is depressed by 0.1 to 0.15 at all wavelengths shorter than 500 nm, and by 0.05 near 630 nm. In addition, the I/F in the 890 nm methane gas absorption band has increased by ∼0.11 since 2009, using the same filter, implying the altitudes and/or the concentrations of upper tropospheric aerosols have increased (Perez-Hoyos et al. 2009). Spectra of other red regions on the planet do not show simultaneous increases in methane-band reflectivity and short-wavelength absorption, ruling out calibration issues.

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In addition to the spectral change in the core of the GRS, ratio images indicate that the spectral slope from 500 to 630 nm has steepened in 2014 compared with previous dates over most of the GRS. Figure 2 shows enhanced true color and the spectral slope from 631 to 502 nm in 2014, in comparison with similar filters in 2009. There is usually some variation in spectral slope across the GRS, as seen in 2009, but it is generally very muted when compared with a nearby cyclonic barge or small regions in the North Equatorial Belt (NEB), both of which are often redder than the GRS (Sanchez-Lavega et al. 2013; Simon et al. 2014). In 2014, however, the steepest slope is observed in the GRS core, and the entire GRS collar shows more 500 nm absorption than even dark regions of the NEB.

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The steeper slope from 500 to 630 nm indicates the presence of an additional absorption not normally seen within the GRS (Simon et al. 2014). Several mechanisms could be invoked, including increased UV irradiation of the color-producing compounds, but they must also explain the increased reflectance at 890 nm. The GRS changes are distinct from those observed in Oval BA in 2006, another large anticyclonic vortex, which reddened but maintained a constant 890 nm reflectivity over at least the 1995–2008 period (Wong et al. 2011). Sublimation of white ammonia ice layers mantling red cores was suggested to explain the color change of Oval BA (de Pater et al. 2010), but this mechanism is difficult to reconcile with the increased 890 nm reflectivity we observe at the GRS.

Although we do not know what compounds color Jupiter's clouds, all chromophore candidates show blue and/or green absorption, resulting in red-reflecting material. Recent laboratory work has suggested a combination of NH₃ and hydrocarbons may be photochemically processed by UV irradiation to generate a reddish material of similar spectral character to the GRS (Carlson et al. 2014). The spectral shape changes with exposure time, and longer irradiation results in depression of overall reflectance, as well as additional absorption near 500 nm. When the Hubble data are compared with these laboratory reflectance spectra (Figure 3), the 2014 data are show a best qualitative fit with the longest exposure time, while other dates are better fit with shorter exposures. Thus, one possible explanation for the 2014 spectrum is longer UV irradiation of aerosol particles containing NH₃ and C₂H₂, resulting in changed spectral slope and overall spectral depression, though other chromophores could also contribute (West et al. 1986). However, full chromophore comparison with laboratory work would require radiative transfer analysis to constrain the particle properties in the GRS clouds (Strycker et al. 2011); there are insufficient data from this image set to perform such modeling.

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Reddening mechanisms are all tied to dynamical changes in some manner, so the determination of variation in vertical circulation or other nearby flows is critical. The internal wind speed of the GRS can top 150 m s⁻¹ in a high-velocity collar that is smaller than the visual diameter of the storm (Simon-Miller et al. 2002; Asay-Davis et al. 2009; Shetty & Marcus 2010). Figure 4 shows a velocity field extracted using an automated method (AACIV; Asay-Davis et al. 2009) from the 2014 data. Correlations were calculated for box sizes of 5° in latitude/longitude, between pairs of velocity field advected images (Asay-Davis et al. 2009). The quality of the extracted velocity field was insufficient to accurately constrain the location and magnitude of the high-speed collar around the vortex due to limitations of the imaging data. Jupiter's relatively large distance from the Earth at the time of the observations resulted in 20% lower spatial resolution compared with prior HST analyses based on data taken near opposition. The shadow of Ganymede over the western side of the vortex in the later three frames resulted in only tie point pairs from short separations over large parts of the GRS, i.e., separations of about 43 min rather than 10 hr. The eastern area of later frames in both orbits also suffered from reduced contrast because the proximity to the limb of Jupiter resulted in a longer path length through overlying hazes and feature blurring. Within the limits of the analysis, we identify tangential velocities in the 100–150 m s⁻¹ range, with higher speeds at the north–south extrema of the vortex, where it interacts with zonal jets, but with larger uncertainties than typical. Thus, preliminary manual and automated measurements of cloud motions showed no significant increase in wind speed when compared with winds measured from Galileo, Cassini, and HST data from 1996 to 2006 (Simon-Miller et al. 2002; Asay-Davis et al. 2009; Shetty & Marcus 2010), implying an internal circulation period of about 3.2 days for a cloud to circumnavigate the high velocity collar.

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The GRS is centered near 221 S ± 02 planetographic latitude, nestled within alternating eastward and westward zonal jets. The westward jet at 195 S is deflected northward around the GRS, and a broad, double-peaked, eastward jet at 265 to 29° S is deflected southward, and have average wind speeds of −60 m s⁻¹ and 50 m s⁻¹, respectively. These zonal wind jets can typically vary by 5–10 m s⁻¹ over time and longitude depending on the cloud features present, although the 265 S jet may vary by more than 20 m s⁻¹ (Simon-Miller & Gierasch 2010; Asay-Davis et al. 2011). No secular change in jet speeds or locations can be correlated with the monotonically decreasing GRS area, although the temporal sampling of zonal wind fields is sparse (Simon-Miller & Gierasch 2010; Asay-Davis et al. 2011).

The latitudinal extent of the GRS has historically spanned between 105 and 11°, and is currently 94, the smallest size measured to date (Figure 5). There has been a slight overall trend toward latitude shrinkage of 004/year, significantly slower than in the longitude dimension, but the fastest change of 07 occurred between 2012 and 2014. Though the storm has a clearly diminishing length-to-width aspect ratio, from 1.8 in the Voyager era (and much greater historically) to 1.5 currently, the longitude and latitude spans over these dates have a correlation coefficient of 0.84.

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The sudden shrinkage of the GRS and its observed color change are likely to be related to changes in zonal jet interactions. The reduction in width is due to shifts of both the northern edge (latitude mean 1979–2012, 168 S ± 0.3; 2014, 174 S) and the southern edge (latitude mean 1979–2012, 274 S ± 0.2; 2014, 268 S). This causes reduced deflection of the zonal jets north and south of the GRS, altering the zonal wind shear environment sensed by the vortex. In addition, there is decreased interaction with small vortices carried by those wind jets; eddies enter the internal flow on the GRS's eastern side, usually in the southeast quadrant. Eddies originating on the westward zonal jet encounter the flow to the northwest of the GRS and join other eastward zonal jet eddies as they are pulled into the flow moving south of the GRS. Eddies on the northern (265 S) cusp of the eastward jet can enter the flow of the GRS after passing its southern edge, while those further south pass by without interaction; this may be the reason for the altered color in 2014. In particular, in 2014 the GRS is now sufficiently withdrawn from the eastward zonal jet cusp at 265 S that it does not ingest vortices carried by that zonal jet. Such small vortices often inject fresh white clouds as they enter the GRS flow and are sheared apart, as observed in movies and images captured by the Voyager, Cassini, and Hubble spacecraft.

A similar phenomenon occurs during SEB fading events, when there are no vortices being carried by the westward jet toward the GRS and its color intensifies (Peek 1958; Rogers 1995). In Figure 2, there is an absence in 2009 of cyclonic bright clouds northwest of the GRS in the SEB, as is typical to the start of a fading cycle (Rogers 1995). This proceeded as usual with reddening of the GRS in 2010. By 2014, the SEB showed normal activity and the GRS would have been expected to be paler; it was often quite pale in 2012, for example. Thus, the reddening of the GRS in 2014 was contrary to the usual pattern, as there was no unusual activity in the SEB to account for it.

It is not yet clear how large vortices such as the GRS interact energetically with the zonal wind jets. Which of these feeds energy or momentum into the other is uncertain and dependent on assumptions about the depth of the zonal wind jets. Nor is it understood what colors the clouds and why they can vary in spectral slope and absolute brightness, though the current spectrum may be indicating a change in colored-particle production. Further analyses of the temperatures, winds, and size of the GRS over time will provide interesting constraints to analytic models of geostrophic balance in the region. In addition, global circulation modeling of the GRS in its flow field for different dates will show which configurations are most stable to decay.

This work was based on observations made with the NASA/ESA Hubble Space Telescope under programs GO12045 and GO13631. Support for these programs was provided by NASA through grants from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. The changes were initially identified, and supporting data provided, by amateur observers, especially Damian Peach, and the JUPOS team, especially Michel Jacquesson.