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Discovery of Neptune's Inner Moon Hippocamp† with the Hubble

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Discovery of Neptune's Inner Moon Hippocamp† with the Hubble 1 Discovery of Neptune’s Inner Moon Hippocamp† with the Hubble 2 Space Telescope 3 M. R. Showalter1*, I. de Pater2*, J. J. Lissauer3* & R. S. French1* 4 5 1SETI Institute, 189 Bernardo Avenue, Mountain View, CA 94043, USA. 6 2Department of Astronomy, University of California, Berkeley, CA 94720, USA. 7 3NASA Ames Research Center, Moffett Field, CA 94035, USA. 8 9 During its 1989 flyby, the Voyager 2 spacecraft imaged six small, inner moons of Neptune, 10 all orbiting well interior to large, retrograde moon Triton1. The six, along with a set of 11 nearby rings, are probably much younger than Neptune itself. They likely formed during 12 the capture of Triton and have been fragmented multiple times since then by cometary 13 impacts1,2. Here we report on the discovery of a seventh inner moon, Hippocamp, found in 14 images taken by the Hubble Space Telescope (HST) during 2004–2016. It is smaller than 15 the other six, with a mean radius R » 16 km. It was not detected in the Voyager images. We 16 also report on the recovery of Naiad, Neptune’s innermost moon, seen for the first time 17 since 1989. We provide new astrometry, orbit determinations, and size estimates for all the 18 inner moons to provide context for these results. The analysis techniques developed to 19 detect such small, moving targets are potentially applicable to other searches for moons 20 and exoplanets. Hippocamp orbits just 12,000 km interior to Proteus, the outermost and 21 largest of the inner moons. This places it within a zone that should have been cleared by 22 Proteus as the larger moon migrated away from Neptune via tidal interactions. We suggest 23 that Hippocamp is most likely a fragment of Proteus, providing further support for the † Note to editor and reviewers: The name “Hippocamp” was recently approved by the IAU but has not yet been publicly announced. The designation “Neptune XIV” is expected to be approved during the IAU General Assembly in mid-August. *e-mail: [email protected]; [email protected]; [email protected]; [email protected] 1 24 hypothesis that the inner Neptune system is relatively young and has been shaped by 25 numerous impacts. 26 Hippocamp, also designated S/2004 N 1 and Neptune XIV, was discovered in 2013 during a 27 re-analysis of lengthy HST exposures of the Neptune system3. We found it in images taken 28 during 2004–2005 and 2009. HST images obtained in 2016 confirm the discovery (Fig. 1). 29 The long delay between image acquisition and discovery arose because of the specialized 30 image processing techniques required. To detect a small moon in an image, motion smear should 31 be limited to the scale of the point spread function (PSF). Neptune’s inner moons orbit at speeds 32 up to 12 km/s, whereas the projected scale of Hubble’s PSF is a few thousand km; this limits 33 exposure times to 200–300 s before smear dominates and signal-to-noise ceases to grow. 34 However, using HST’s widest filters, the detection of an object such as Hippocamp, with V 35 magnitude ~ 26 3, requires ~ 30 minutes of continuous integration, which is far longer than the 36 smear limit. Thus, it might appear that HST is incapable of detecting such a faint, moving target. 37 We addressed this problem by introducing a distortion model to the Neptune images. Our 38 procedure was to derive a pair of functions r(x) and q(x), which return orbital radius and inertial 39 longitude as a function of 2-D pixel coordinate x. The inverse function x(r,q) could also be 40 readily defined. We derived the mean motion function n(r) from Neptune’s gravity field 4 41 including its higher moments J2 and J4 . One can use these functions to transform an image taken 42 at time t0 to match the appearance of another image at a time Dt later by relocating each pixel x0 43 in the original image to a new location x1: 44 x1 = x(r(x0), q(x0) + n(r(x0)) ´ Dt) . [1] 45 This transformation makes it possible to distort a sequence of images so that they all match the 46 geometry of one frame; any moon on a prograde, circular, equatorial orbit will appear at fixed 47 pixel coordinates. After the transformations, the images can be coadded so that much longer 48 effective exposure times are obtained. Fig. 2 illustrates the technique. This procedure, originally 49 developed to study Neptune’s arcs, revealed Hippocamp. (Note that this procedure neglects the 50 sub-pixel correction for each moon’s Laplace plane; that topic is discussed further below.) 51 The geometric transformation creates a spiral pattern that winds tighter with decreasing r. 52 (Fig. 2c). The method fails when adjacent pixels sheer to the point that individual PSFs become 2 53 severely distorted. For the inner moons of Neptune, this limits the coadding of images to those 54 that have been obtained within the same HST orbit. The total available integration time in one 55 HST orbit is ~ 30 minutes, meaning that Hippocamp is just above the theoretical limit of what 56 HST can detect in the inner Neptune system. The supplemental data for this paper includes all of 57 the images used in this investigation at various levels of processing. The supplemental video 58 shows one sequence of images in which Hippocamp is visible to the eye without distortion or 59 coadding. 60 This same image analysis technique has also revealed Naiad (Extended Data Fig. 1). 61 Identifying Naiad was challenging because its orbit differed substantially from that predicted by 62 the latest reference ephemeris5; in 2016, Naiad falls nearly 180° away from its predicted location. 63 Nevertheless, the astrometry from HST and Voyager is consistent with uniform, near-circular 64 motion if one allows for a one-sigma increase in Naiad’s Voyager-derived mean motion6; see 65 Extended Data Table 1. The very large ephemeris error implies that reported detections of Naiad 66 from the W. M. Keck Telescope in 20027 were misidentifications. A 20° error in the predicted 67 orbit of Thalassa5 suggest that it may also have been misidentified in the same data set. 68 Determining the orbits of Hippocamp and Naiad entailed solving simultaneously for the 69 orbits of all Neptune’s inner moons. Table 1 lists all our derived orbital elements: n = mean 70 motion; a = semimajor axis; e = eccentricity; i = inclination; l0 = mean longitude at epoch; v0 = 71 longitude of pericenter at epoch; W0 = longitude of ascending node at epoch; v¢ = apsidal 72 precession rate; W¢ = nodal regression rate. Each orbit is defined relative to its local Laplace 73 plane; this plane nearly aligns with Neptune’s equator for the innermost moons, but tilts toward 74 the plane of Triton’s orbit for larger a. The calculated angle of this tilt, y, is listed in Table 1. All 75 of the Laplace planes share a common ascending node, which coincides with the descending 76 node of Triton’s orbit. 77 Supplementary Table 1 lists all the images used in this analysis. Extended Data Tables 2–4 78 and Supplementary Table 2 contain all of our astrometry. Orbits have been determined only from 79 HST data 2004–2016; for moons other than Hippocamp, more precise orbits could be obtained 80 by also including prior detections from Voyager- and Earth-based telescopes8. Nevertheless, our 3 81 orbital elements for the four largest moons are in extremely close agreement with prior 82 determinations4–6; see the Methods section and Extended Data Table 4 for further details. 83 Table 1 also lists the disk-integrated photometry of Neptune’s inner moons as obtained 84 through broad visual filters. Again, our results agree with earlier Voyager and HST photometry9. 85 The other inner moons all have albedos in the range 0.09 ± 0.019, so if Hippocamp’s albedo is 86 similar, then R = 16 ± 1 km. Our photometry does not have sufficient accuracy to constrain the 87 moon’s shape or phase function. 88 We can rule out the existence of any additional moons interior to the orbit of Proteus that are 89 more than half as bright as Hippocamp. Beyond the orbit of Proteus, our images are freer from 90 Neptune’s glare and orbital motion is slower, making it possible to add together larger sets of 91 images (Extended Data Fig. 2). Implant tests within these images indicate that a moon ~ 30% as 92 bright as Hippocamp would generally be visible beyond Proteus, but none have been seen. Our 93 orbital coverage is generally complete out to a » 200,000 km and about 2/3 complete out to a » 94 300,000 km. Our search for retrograde, equatorial moons also yielded negative results. 95 We can extrapolate the orbit of Hippocamp back to the time of the Voyager 2 flyby (August 96 25, 1989) with a precision of ± 1.5° in orbital longitude. This information, combined with 97 knowledge of Voyager’s trajectory and its camera pointing, enabled us to identify where the 98 moon might have appeared in the Voyager images. The orbital uncertainty corresponds to a few 99 tens of pixels in images from the narrow-angle camera. Extended Data Table 5 lists the most 100 sensitive candidate images. All are clear-filter images taken by the narrow-angle camera. Any 101 prediction that fell within 200 pixels of the field of view is listed. This same procedure 102 accurately predicted all the best images of Neptune’s other inner moons.
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