Meet Hippocamp, Neptune’s Smallest Moon

February 20, 2019 - Comment

Though first recorded by Hubble in 2004, subsequent imagery reveals that this tiny, innermost moon of Neptune probably had had a violent historical past. A composite of three Hubble House Telescope pictures from November 6, 2004, reveals the faint blip at decrease left of a small satellite tv for pc. An enlarged view is at


Though first recorded by Hubble in 2004, subsequent imagery reveals that this tiny, innermost moon of Neptune probably had had a violent historical past.

Hubble view of Hippocamp

A composite of three Hubble House Telescope pictures from November 6, 2004, reveals the faint blip at decrease left of a small satellite tv for pc. An enlarged view is at higher proper. Initially designated S/2004 N 1, it is now named Hippocamp. The 4 different faint moons have been found by Voyager 2 in 1989. Neptune’s shiny disk is hidden behind a dark-gray masks.
Mark Showalter / SETI Institute

An article printed within the February 21st challenge of Nature, authored by Mark Showalter (SETI Institute) and three colleagues, is titled “The seventh internal moon of Neptune.” From the title alone, you may conclude that these observers have introduced the invention of a tiny new physique circling Planet Eight.

Not so, nevertheless. The Worldwide Astronomical Union really introduced the moon’s discovery again in July 2013, when it carried the short-term designation S/2004 N 1 (as a result of it was first recorded on pictures taken in 2004, proven at proper). You may learn all about it right here.

What’s new in Nature is the physique’s now-official title, the novel “stacking” method utilized by the group in its discovery, and what this asteroid-size object is telling us in regards to the historical past of the Neptune’s system.

First, the title: Showalter has chosen Hippocamp, the title of a legendary sea creature sometimes portrayed with the top and torso of a horse (generally with wings) and the tail of a fish. It properly matches the IAU’s naming scheme for Neptunian moons, involving Greek and Roman mythology of the seas, as Hippocampus is the genus for seahorses. “I am a scuba diver and am very keen on seahorses,” he tells Sky & Telescope. “That is the actual motive I appreciated the title.”

Showalter has a knack for monitoring down faint outer-planet our bodies — he’d already chalked up weirdly formed Pan (30 km) inside Saturn’s A hoop, Mab (25 km) and Cupid (18 km) round Uranus, and the elongated moonlets Kerberos (19 × 10) and Styx (16 × 9) round Pluto. His key to a hit is a method acquainted to beginner astrophotographers.

You may recall that Voyager 2 found six small moons contained in the orbit of Triton when it flew previous Neptune in 1989. However Hippocamp was too small and escaped detection. Showalter looked for extra internal moons with devoted Hubble House Telescope runs in 2004 and 2005. Orbital movement inside about 200,000 km of the planet is so fast that the HST pictures needed to be not than about 5 minutes to keep away from extreme smear. However these quick photographs would not be delicate sufficient to document one thing as small as Hippocamp.

Inner moons of Neptune

This diagram reveals the relative distances of Neptune’s seven internal moons and its rings from the planet.
Nature

However something orbiting very near Neptune absolutely could be in a round orbit within the planet’s equatorial aircraft, with simply predictable movement from body to border. So Showalter digitally offset every Hubble picture to match the expected movement and “stacked” them. Doing this, Hippocamp repeatedly popped into view. He used this similar trick, likewise yielding optimistic detections, with follow-up Hubble runs in 2009 and 2016. (Curious, his group ran the orbital movement of Hippocamp again in time to see if, maybe, this little physique may need been lurking unseen in a Voyager picture. It wasn’t.)

An additional profit of those searches is refined orbits for Neptune’s close-in moons. For instance, 75-km-wide Naiad hadn’t been seen (not less than with certainty) since Voyager 2’s flyby. Showlater’s group did spot it — however in a spot diametrically reverse the place it ought to have been, based mostly on the presumed orbit. Thalassa (92 km) proved much less difficult however was nonetheless displaced 19° from its predicted orbital longitude. The group additionally stories that no different moons bigger than 24 km probably lie inside 200,000 km of Neptune or bigger than 20 km farther out.

Hippocamp’s Violent Historical past

Curiously, Hippocamp has an orbit located simply 12,000 km inside that of Proteus, the biggest and most huge of Voyager 2’s six discoveries with an estimated diameter of 407 km. Tidal interactions with Neptune are slowly driving Proteus outward, that means that it was a lot nearer to Hippocamp prior to now. So are these two associated ultimately?

Given the prevalence of orbital resonances amongst outer-planet our bodies, dynamicists may need anticipated to see such relationships amongst Neptune’s tightly spaced internal moons. Nonetheless, Showalter and his group have used the Hubble knowledge to refine the orbits of all seven — and no resonances of any sort have turned up

Proteus bears a big crater known as Pharos. It is not less than 230 km throughout — so big, in comparison with the moon itself, that its formation will need to have come near shattering Proteus. Hippocamp, by comparability, represents about 2% of the mass that might have been ejected in the course of the Pharos affect. So fairly plausibly Hippocamp assembled from fragments of Proteus. In reality, Showalter’s group calculates that impacts with interloping comets ought to have shattered Hippocamp 9 instances over the previous four billion years, solely to drag itself again collectively every time. (These a number of reassemblies additionally neatly clarify why Hippocamp’s orbit is neither eccentric nor inclined, regardless of the numerous gravitational affect of Proteus.)

Given all of the “lacking mass” from the Pharos affect, possibly Hippocamp began out bigger and has regularly been whittled all the way down to its current dimension. Even so, its continued survival appears exceptional — possibly, if the IAU had allowed it, Showalter may need thought-about the title “Phoenix” as an alternative!



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