The seventh inner moon of Neptune

Abstract

During its 1989 flyby, the Voyager 2 spacecraft imaged six small moons of Neptune, all with orbits well interior to that of the large, retrograde moon Triton¹. Along with a set of nearby rings, these moons are probably younger than Neptune itself; they formed shortly after the capture of Triton and most of them have probably been fragmented multiple times by cometary impacts^1-3. Here we report Hubble Space Telescope observations of a seventh inner moon, Hippocamp. It is smaller than the other six, with a mean radius of about 17 kilometres. We also observe Naiad, Neptune's innermost moon, which was last seen in 1989, and provide astrometry, orbit determinations and size estimates for all the inner moons, using an analysis technique that involves distorting consecutive images to compensate for each moon's orbital motion and that is potentially applicable to searches for other moons and exoplanets. Hippocamp orbits close to Proteus, the outermost and largest of these moons, and the orbital semimajor axes of the two moons differ by only ten per cent. Proteus has migrated outwards because of tidal interactions with Neptune. Our results suggest that Hippocamp is probably an ancient fragment of Proteus, providing further support for the hypothesis that the inner Neptune system has been shaped by numerous impacts.

Extended Data Fig. 1 |. Recovery of Naiad

Each panel shows a portion of an HST image after processing and coadding as described in the text. The location of Naiad in each panel is indicated by a small square; a closeup is inset at upper right. The outline of Neptune’s disk is indicated by a blue ellipse. a, View from Visit 01, orbit 1 of HST program GO-11656, obtained on 2009 August 19. It shows the first unambiguous detection of Naiad since the 1989 Voyager flyby of Neptune. b, View from Visit 08, orbit 2 of program GO-14217, taken on 2016 September 2.

Extended Data Fig. 2 |. Phase curves of Neptune’s inner moons

Each panel shows measurements of disk-integrated reflectance D vs. phase angle for one of Neptune’s inner moons through broad, visual filters. Error bars are ± 1σ. Colors indicate the instrument, filter and observing mode as defined in the legend. Solid lines are a least-squares linear fit to the data; dotted lines indicate the range of the uncertainty in the model, ± 1σ, as derived from the covariance matrix of each fit. The values in Table 1 correspond to the mean and uncertainty extrapolated to α = 0.

Extended Data Fig. 3 |. Deep searches for small moons

Each panel shows multiple HST images coadded into a “map” in which longitude increases from 0 to 360° along the horizontal axis and radial position is 0 to 400,000 km along the vertical axis. a, View derived from the five HST orbits of program GO-11656, obtained on 2009 August 19. b, View from the two orbits of Visit 03 in HST program GO-14217, taken on 2016 September 2.

Extended Data Fig. 4 |. Diagram of the Neptune system

All of the known features of the Neptune system interior to Triton are shown to scale. (Triton orbits about three times further out than Proteus.) Rings and arcs are shown in green. Moon shapes are indicated by red ellipses indicating their dimensions a × c, enlarged relative to their orbits by a factor of 20.

Extended Data Fig. 5 |. Image processing steps illustrated

a, Image icwp01n7q_flt.fits taken on 2016 August 31. b, The image after hot pixels and cosmic ray hits have been removed. c, The boolean mask where white indicates pixels that will be ignored from further analysis. d, The image after the mean of other images from the same HST visit have been averaged and subtracted. This step removes most of the glare. e, The image after an “unsharp mask” process involving the subtraction of a median-filtered version of the same image. The outline of Neptune’s disk is indicated by a blue ellipse in each panel.

Fig. 1 |. Detections of Hippocamp 2004–2016.

a, View from Visit 04 of GO-10398, showing the earliest detection on 2004 December 9. Neptune is behind the HRC occulting mask. b, Visit 08, GO-10398, 2005 May 12. c, View from the first orbit from Visit 01, GO-11656, 2009 August 19. The gray vertical band is due to Neptune’s saturation bloom. d, Visit 03, GO-14217, 2016 September 2. Panels a and b have been rotated 90° counterclockwise. A small square locates Hippocamp in each panel; a closeup is inset at upper right. Other moons and the outline of Neptune are identified.

Fig. 2 |. Image processing steps leading to the discovery of Hippocamp.

a, Image ib2e02ziq_flt, the first in a sequence of eight long exposures from the second HST orbit of Visit 02 in program GO-11656 (2009 August 19). b, Image ib2e02zmq_flt, taken 21 minutes later. Despina, Galatea, and Larissa have shifted noticeably in position. c, Image from panel a, transformed to match the geometry of the image in panel b. d, The result of coadding all eight images, revealing Hippocamp and Thalassa. The outline of Neptune’s disk, as distorted by the camera, is shown in each panel.