SUPPLEMENTARY INFORMATION

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1 doi:.38/nature149 1 Observation information This study examines 2 hours of data obtained between :33:42 and 12:46:28 Universal Time (UT) on April using the -metre Keck telescope. This dataset was obtained using the NIRSPEC spectrometer in high-resolution crossdispersed mode with a resolution of R 2,. The slit measures.432 width by 24 length with a pixel on the CCD corresponding to.144 squared on the sky. The spectra taken consist of a product of twelve -second integrations, creating exposures 6 seconds long. During one such exposure, Saturn rotates.7 CML, a negligible amount given that the width of the slit covers 2.6 CML. On the night of these observations, Saturn s northern hemisphere was tilted towards Earth with a sub-earth latitude of 8.2. The column of H 3 emission viewed at Saturn is a line-of-site column, so we see through more atmosphere than a column that is radially extruding from the planet. The Q(1, ) and R(2,2 ) spectral lines at m and 3.93 m also had a spectral background subtraction performed to improve the signal, as shown and explained in Fig. S1. The telescope movement during the observations has been corrected for by aligning each spectral image to the position of the main rings, which remain at a constant brightness and position on Saturn, such that Saturn s position on the CCD varied less than 1 latitude at any point along the noon meridian. The atmospheric seeing on the night was approximately.4, translating to an intensity smearing on the planet of 2 rising to 4, between the range of 28 and 2 latitude. To provide an accurate latitude mapping of the Q(1, ) line (in Fig. 2.), we selected 38 minutes of exposures which had the lowest seeing effect during the night, this was performed by selecting the spectral images in which Saturn s rings were the least spatially broadened by atmospheric attenuation. This was not performed to the same degree for the R(2,2 ) line, which includes 86 minutes of exposures to obtain a good signal-to-noise ratio, but this increases the error associates with latitude. The errors associated seeing are displayed in Fig. S2. 1

2 RESEARCH SUPPLEMENTARY INFORMATION Figure S1: Background subtraction regions. The Q(1, ) and R(2,2 )H 3 lines are shown within the highlighted blue regions, the backgrounds that were subtracted are shown as white boxes adjacent to the blue regions. These boxes show the approximate location of the pixels that were used to form Fig. 2. The Q(1, ) line background region is directly adjacent ( pixels away) whereas the R(2,2 ) background is taken from pixels away. All nearby regions that are not saturated by hydrocarbon reflection away from the poles yield similar profiles like that in Fig Degrees smeared by seeing Degrees latitude Figure S2: Latitudinal error as a function of latitude. Here, the x- and y- axes show latitude on Saturn and the corresponding error in latitude induced by the affect of atmospheric attenuation (seeing), respectively. The black and red lines indicate the northern and southern hemispheres. Note that the southern hemisphere is more susceptible to errors in seeing due to viewing geometry - the southern latitudes are represented by viewer pixels on the CCD

3 RESEARCH 2 Magnetic mapping information As referenced in the main text, we use the axisymmetric magnetic mapping model of Bunce et al. (8), with updated internal field coefficients from Burton et al. (). This field model contains all the relevant effects, including the full order-3 internal field, ring current, oblateness of the planet and a choosable height in the ionosphere to map latitudes on the planet via magnetic field lines to the equatorial plane. The spherical harmonic coefficients used were g1 = 21,136 nt, g2 = 1,26 nt and g3 = 2,219 nt. The height of the ionosphere was set to 1, km above the 1-bar level, where the peak H 3 density is approximately located. Saturn s planetary radius is 6,268 km. The sub-solar magnetopause distance was set to planetary radii from Saturn s center. The Z3 magnetic field model (Connerney et al., Zonal harmonic model of Saturn s magnetic field from Voyager 1 and 2, Nature, 298, 44-46, 1982) was also used (shown in Fig. S3 and S4), using the same parameters, and with the latest available coefficients: g1 = 21,248 nt, g2 = 1,613, g3 = 2683 (Dougherty et al., Cassini Magnetometer Observations During Saturn Orbit Insertion, Science, 37, , ). The differences between models are less than half that of the observational errors in latitude (see Fig. S2) such that these observations are unable to conclude the accuracy of either model. 2 1 Equatorial magnetic mapping [Z3 model] RSat A B C D < Major ring subdivisions > D C B A Water influx Ring gaps Polar* emission Instability peak Cassini division** Instability region *Auroral/mid latitude **Multiple gaps Figure S3: Fig. 2 cast in the Z3 field model mapping. This plot is the same as Fig. 2 but this time the coefficients of the Z3 model are used. 3

4 RESEARCH SUPPLEMENTARY INFORMATION Burton et al Connerney et al Ring transparency Saturn radii (R/Rs) Figure S4: Fig. 3 cast in the Z3 field model mapping. This plot is the same as Fig. 3 but here the coefficients of the Z3 model are used. 3 Data To investigate the temporal evolution of the observations, we divide the dataset into 3 1 hour rolling bins, the resulting intensity against latitude profiles are displayed in Figures S, S6 and S7: each bin represents 1 hour of the 2 hour dataset - the first, middle and final hour. We have found that the same patterns are found in the data in all bins, and that it is significantly clearer in Fig. S1. The structure appears to diminish with time, and it is believed an increase in the atmospheric seeing is the cause, but do not have a reliable log of atmospheric seeing other than at the start of the observations to verify this. We conclude that a combination of a large quantity of exposures and excellent atmospheric conditions are needed to resolve these features with high structural detail. If atmospheric seeing is too large, the small features seen will appear to smear in the spatial component. If an exposure is too short, the signal from the features will be too weak relative to the planets background emission to show any structure. Past observations have lower resolution both spectrally and spatially. However, it is possible to see some features align with ring gaps in the previous highest resolution dataset as shown in Fig. S8. This clearly shows emission associated with the Cassini division, but the other peaks at lower latitude remain the subject for debate. As proof that Saturn was expected to exhibit a uniform brightness, we provide an intensity profile of Jupiter and Saturn together, as seen in Fig. S9. These data were obtained with the 3-metre NASA infrared telescope facility (IRTF) on 28 August, providing a similar spatial resolution on Jupiter as the W.M. Keck telescope does for Saturn. The Jovian observations took place between 11:7 and 11:9 UT and covered 219- CML, comparable to the exposure length obtained in Fig. 2 (accounting for the different light-gathering power of both telescopes)

5 RESEARCH 2 1 Equatorial magnetic mapping RSat Mid-high lat. emission Ring feature Ring matter Multiple gaps Instability region Longitude co-added: degrees Figure S: Temporal bin 1. The first temporal binning of data from the dataset displayed in Fig. 2, this represents approximately 1-hour of co-added data. Here, the Q(1, ) (blue) and R(2,2 )(black)h 3 emission lines are plotted as intensity as a function of spatial position in both planetocentric latitude and equatorial mapping coordinates. Even with the reduction in total exposure time, most features can still be seen - the emission at low latitudesisnot uniform.

6 RESEARCH SUPPLEMENTARY INFORMATION 2 1 Equatorial magnetic mapping RSat Mid-high lat. emission Ring feature Ring matter Cassini division Instability radius Longitude co-added: 1-17 degrees Figure S6: Temporal bin 2. Same as Fig. S3 but representing the co-addition of the central hour of the dataset, this overlaps the first and the third temporal bin. Again, most features can still be seen, although the relative strengths appear slightly shifted. Equatorial magnetic mapping RSat Mid-high lat. emission Ring feature Ring matter Cassini division Instability radius Longitude co-added: degrees Figure S7: Temporal bin 3. Same as Fig. S3 but representing the co-addition of the final hour of the dataset. This plot is somewhat smeared in the spatial component, most likely due to an increase in atmospheric turbulence - i.e. seeing during the night of the observation, which acts to displace features in the northsouth direction, thus causing the intensity peaks to become broad. The smeared plotting does however still display a large variation in the same pattern as the other bins, highlighting the non-uniformity of the low latitude region. 6

7 RESEARCH Figure S8: Comparison to previous Saturn observations. This shows pole-to-pole H 3 intensity (arbitrary units, dark grey line) of as a function of latitude and arcseconds subtended in Earth s sky. This shows data taken by the NASA IRTF on 7- February and consists of 8 hours of exposures. Positive latitudes indicate northern hemisphere values. The emission has been line-of-sight corrected. The colour scale is similar to that of Fig. 2, differing in that the position of the rings is denoted by the green shaded area enclosed by dashed green lines. In the red lines located at north and south, higher intensity is seen - this maps to the Cassini division, whilst ring gaps mapped to lower latitudes are debatably of higher intensity. Intensity (arbitrary units) Jupiter Saturn Spatial scale (pixel) Figure S9: Jupiter and Saturn comparison. The Q(1, )H 3 emission lines for Jupiter and Saturn are plotted as intensity as a function of spatial scale. Like Fig. 2, the northern hemisphere is to the left and the peak intensity has been cropped at the poles on either side. Between 6 and 8 pixels on the x-axis, the equatorial emission is removed due to the rings reflection at Saturn. Saturn s H 3 intensity is significantly more variable than Jupiters at low latitudes, with Jupiter s emission showing a smooth curvature. The red and grey shading on the emission lines indicate the 1-sigma error ranges for Saturn and Jupiter, respectively