GETTING THE WORD OUT

THIS WEEK
NASA/JPL-CALTECH/UCLA/MPS/DLR/IDA/PSI/LPI
Bright spots and
ice dazzle on Ceres
Jacob Aron, The Woodlands, Texas
LET’S take a look inside. The latest
from NASA’s Dawn probe, which
has been in orbit around the dwarf
planet Ceres since April last year,
suggests this tiny world has water
ice on the surface and craters that
provide a window on its interior.
A spectacular view of the
beguiling bright spots at the heart
of the 92-kilometre-wide Occator
crater from just 375 kilometres up,
shown above, was just one of the
latest findings unveiled by the
mission team at the Lunar and
Planetary Science Conference
in The Woodlands, Texas, on
22 March.
These spots have perplexed
researchers since Dawn first
reached Ceres, the largest object
in the asteroid belt. Probing their
nature could give clues to Ceres’s
interior. In the new image, Dawn
has seen colour variations across
the surface of the bright regions.
These wouldn’t be visible to the
human eye, but reflect possible
differences in the composition
of the material seen by Dawn.
This close-in view reveals that
the central bright spot actually
sits within a 10-kilometre-wide
depression inside the crater.
At the centre of that is a small
mound. “We’re starting to see
how complex the distribution of
the bright material is,” said Carol
Raymond of NASA’s Jet Propulsion
Laboratory in California.
How this arrangement formed
is still a mystery. Timothy Bowling
at the University of Chicago
presented a possible explanation,
“Seeing water ice anywhere
on Ceres is a surprise – it
is warm enough at the
surface for it to evaporate”
in which a meteorite smacked
into Ceres, exposing icy material
from as much as 40 kilometres
below the surface and heating it up.
As this material settled into the
Occator crater we see today, the
water would evaporate, leaving
bright salt and minerals behind.
The floor of Occator is also
riddled with fractures that
seem to be older than the crater
itself. These could have provided
an escape route for material
How to stack
oranges in 24
dimensions
IT’S a tight squeeze. Mathematicians
have proved that they know the
best way to pack spheres in eight
and 24 dimensions – the first time
this problem has been solved in a
new dimension for almost 20 years.
“I think they’re fantastic results. I’m
excited that this has been done at last,”
says Thomas Hales at the University
of Pittsburgh, Pennsylvania.
The sphere-packing problem asks
a deceptively simple question: what
–Beguiling brightness– arrangement crams the most spheres
into a limited volume? It is easy to
beneath the surface.
describe, but difficult to prove.
The team behind Dawn has also In 1611, Johannes Kepler suggested
made an unexpected discovery: that the best arrangement for
water ice hiding in a crater. The 3-dimensional spheres like oranges
10-kilometre-wide Oxo crater is a pyramid. But it took until 1998
seems to have formed relatively for Hales to publish a proof – and
recently. The find comes from another 16 years to formally verify it.
spectral data taken during Dawn’s Meanwhile, mathematicians have
higher orbit in June last year.
been gunning for higher dimensions.
Seeing water ice anywhere on Now, Maryna Viazovska at Humboldt
Ceres is a surprise as it is warm University of Berlin has proved that
enough at the surface for any
a grid called the E8 lattice is the best
ice to evaporate into space. That packing in eight dimensions (arxiv.
means it must have been exposed org/abs/1603.04246v1). Almost
recently, said Jean-Philippe
immediately after, she teamed up
Combe of the Bear Fight Institute with other researchers to prove that
in Winthrop, Washington. “This a related arrangement called the
area is possibly a cold trap where Leech lattice is best in 24 dimensions
H 2
0-rich materials could be
(arxiv.org/abs/1603.06518v1).
preserved for at least some time,” The fact that these dimensions
he said.
were next to fall is no coincidence.
So where has this water
For reasons we don’t understand,
come from? Models of Ceres’s such lattices don’t show up in other
formation, along with the
dimensions. But they were suspected
sightings of bright spots like the to be the most efficient arrangements
ones at Occator, suggest the dwarf in the dimensions they apply to.
planet has an icy sub-surface layer “These are unbelievably good
that is mixed up with salt and packings,” says Henry Cohn at
rock. The ice at Oxo could have Microsoft Research New England in
been exposed by a landslide,
Cambridge, Massachusetts. “The
or dug up by a meteorite.
spheres in these dimensions fit
Dawn will continue to gather perfectly, in ways that don’t happen
data about Ceres until August at in other dimensions.”
least, and possibly into next year. Packing spheres in 24 dimensions
But some questions about this isn’t just a mathematical game. The
enigmatic world may not
problem has applications in wireless
be answered until a repeat visit. communication, and has been used to
“It would be nice to land there, communicate with spacecraft in the
wouldn’t it?” said Raymond. ■ distant solar system. Lisa Grossman ■
12 | NewScientist | 2 April 2016