[meteorite-list] Astronomers Predict that Pluto Has a Ring

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 9 Aug 2011 16:37:04 -0700 (PDT)
Message-ID: <201108092337.p79Nb4jT019600_at_zagami.jpl.nasa.gov>

http://www.technologyreview.com/blog/arxiv/27064/?p1=blogs

Astronomers Predict that Pluto Has a Ring
Technology Review (MIT)
August 8, 2011

Until recently, the only ring in the Solar System was Saturn's. But in
1960s and 70s, astronomers discovered rings around Uranus and Neptune.
Meanwhile, the Voyager 1spacecraft sent back images of Jupiter's ring.

To be sure, these rings are much less impressive than Saturn's but the
implications are clear: rings seem much more common than astronomers
once thought. Perhaps they are even the norm.

And that raises an interesting question: could Pluto possibly have a ring?

The observational evidence is that Pluto does not have a ring. The best
images are from the Hubble Space Telescope and they show nothing.

But today, Pryscilla Maria Pires dos Santos and pals at UNESP-Sao Paulo
State University in Brazil say that Pluto ought to have a ring after
all, but one that is too faint for Hubble to spot.

Their conclusion comes from modelling the way that micrometeorite
impacts on Pluto's satellites, Nix and Hydra, ought to send dust into
orbit about the dwarf planet.

This dust inevitably spirals into Pluto and its satellites because of
its interaction with the solar wind. In this way, the dust is removed
from orbit.

But that doesn't mean it can't form a ring. The important question is
whether the dust can be replaced as quickly as it is removed.

Pires dos Santos and co calculate that the dust initially forms a ring
about 16,000 km wide, encompassing the orbits of both Nix and Hydra.
However, the solar wind then removes about 50 per cent of the dust
within a year.

However, that still leaves enough to form a ring, albeit it an extremely
faint one. "A tenuous ring...can be maintained by the dust particles
released from the surfaces of Nix and Hydra," say Pires dos Santos and co.

They calculate that its transparency (or optical depth) has a value of
10^-11. By comparison, the main ring of Uranus has a transparency of
between 0.5 and 2.5.

Hubble ought to be able to to see a ring around Pluto with a
transparency of about 10^-5 so it's no surprise that it hasn't seen the
ring that the Brazilian team predict. There's no way to see such a ring
directly from Earth.

Fortunately, there is a way to settle the matter.

The New Horizons spacecraft is currently on its way to Pluto, equipped
not with a camera capable of seeing the ring but with a dust counter
that should do the trick instead. If this probe finds itself in the
lightest of dust clouds when it arrives on 14 July 2015, we'll finally
know for sure.

Ref: arxiv.org/abs/1108.0712 <http://arxiv.org/abs/1108.0712>: Small
Particles In Pluto's Environment: Effects Of The Solar Radiation Pressure
Received on Tue 09 Aug 2011 07:37:04 PM PDT