On mass,
composition, and temperature Neptune and Uranus are not far from being twins,
but it's not hard to tell the two apart in visible light. Neptune is a deep
shade of blue befitting a planet named after the god of the sea, while Uranus
is paler and slightly greenish. (If you're going to snigger, this article will
give you plenty of opportunities.) So, why do they look so different?
A new
paper undergoing peer review attributes the difference to a layer of initially
tiny methane ice particles that sink as they grow, eventually forming a sort of
snow that then re-evaporates.
All the
planets as far out to Saturn, and even some larger asteroids, have been
intensely studied at close range by orbiting space probes. The ice giants
Uranus and Neptune, on the other hand, have received nothing more than Voyager
2's flybys 36 and 33 years ago respectively to image them using technology from
the 1970s when the craft was launched. It's no surprise many astronomers are
pushing to find a way back.
In the
meantime, Professor Patrick Irwin of the University of Oxford and colleagues
have combined Voyager 2's data with measurements taken by Hubble and
ground-based telescopes to try to make sense of the pairs' atmospheres. They
have presented a model, available to read on arXiv that might explain the two
different shades.
Irwin and
co-authors conclude both planets have an atmosphere with a base pressure more
than 700 kiloPascals (seven times atmospheric pressure) composed of hydrogen
sulfide (the "rotten eggs" gas) ice and photochemical haze. Yes, as
Irwin previously showed, Uranus (and Neptune) smells like farts.
Above this
is a layer of photochemical haze at pressures 1-2 times sea level on Earth, the
authors conclude, which is in turn topped by a layer of the same composition
but lower pressure. The haze is produced in the planets' upper atmospheres
before sinking to concentrate around the point where methane condenses. When
these particles become seeds around which methane nucleates the combination
falls as snow until the particles are released to act as nuclei for H2S cloud
formation.
Snapped by Hubble in 2005, Uranus shows off its rings (left). In 2006, Hubble captured its banded structure and strange dark storm. Image credit: Credits: NASA, ESA, and M. Showalter (SETI Institute); Right: NASA, ESA, L. Sromovsky and P. Fry (U. Wisconsin), H. Hammel (Space Science Institute), and K. Rages (SETI Institute)
The
methane, which reflects blue light while absorbing red, accounts for the
dominant shade. Uranus has a thicker aerosol layer, causing the color to fade.
Neptune, on the other hand, has a thin layer of methane ice particles at a
pressure lower than at the top of Mount Everest that enhances the reflection of
the blue part of the spectrum.
Hubble
reveals Neptune has a dynamic atmosphere that changes over just a few days,
capturing its changing weather conditions in 1994. Image credit: NASA/JPL/STScI
Neptune in
particular is not uniformly colored, with Voyager 2 beaming back images of dark
spots, since also seen by Hubble. The authors attribute these to concentrations
of material in the deep aerosol layer.
Besides
getting to grips with the least-understood planets in our system, the work
could provide insight into the abundance of Neptune-sized worlds found orbiting
other stars.
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