Sky & Telescope - July 2023 - 4

SPECTRUM by Peter Tyson
The Case for Uranus
LOOK AT THE IMAGE BELOW. It's a view of Uranus that Voyager 2,
the only spacecraft we've ever sent near the seventh planet, beamed
back to us in 1986. The pale-turquoise orb looks beautiful but
bland, just a featureless marble with little to draw the eye.
Now look at the Hubble telescope image that opens our Uranus
feature on page 14. Bland? Featureless? Hardly. Taken only a dozen years after
Voyager 2's flyby, in 1998, this false-color infrared portrait reveals at a glance
how mistaken that impression of dullness is. (For equally eye-opening Webb
telescope images of Uranus and its moons, see https://is.gd/WebbUranus.)
Voyager 2 passed the planet during Uranus's solstice, when continuous sunlight
on the southern hemisphere triggered the formation of a methane haze
that obscured everything. But as the Hubble picture shows, even halfway to the
next equinox Uranus's atmosphere displayed loads more
character than seen in the Voyager 2 shot.
In the image on page 14, green and blue regions
Mysterious world:
Uranus captured by
Voyager 2 in 1986
indicate where the Uranian skies are relatively clear and
sunlight penetrates deep into the atmosphere, while
yellow and gray areas capture where solar rays reflect
off higher haze or cloud layers. Orange and red colors
denote high-altitude clouds, some racing along at more
than 500 kilometers per hour (300 mph).
But these features are almost ho-hum compared to
others that Uranus boasts. As seen in the Hubble image,
which shows the planet in its proper orientation to the ecliptic plane, Uranus
lies on its side. Unlike all the other planets, it rotates at a nearly 90° angle from
the plane of its orbit, rolling like a cylinder rather than spinning like a top.
This odd tilt generates the most extreme seasons in the solar system: In
Uranus's 84-year orbit of the Sun, its summer and winter each last roughly 21
years. Day and night as we know them only occur in the planet's spring and
fall, when they cycle every 17 hours.
There are more quirks, which Emily Lakdawalla explores in her feature. As
she passionately argues, these idiosyncrasies combine to make a compelling case
for sending a major mission to Uranus. She's not alone in this wish: Planetary
scientists are calling for a flagship mission, akin ideally to the Cassini mission
to Saturn, that will boldly go where we've gone only once before.
The Kepler mission revealed that a majority of exoplanets may be ice giants
between a few times Earth's mass and roughly that of Uranus or Neptune. To
have any hope of understanding those extrasolar ice giants,
let alone our own, we need to send a flagship mission to
Uranus. Your move, NASA.
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Sky & Telescope - July 2023

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