Sky and Telescope - December 2016 - 22

Solar System Expedition

NASA / JPL-CALTECH / UCLA / MPS / DLR / IDA

Pockmark Pitfalls

SOMETHING BLUE This false-color rendering of Ceres
highlights diﬀerences in surface composition. The crater
Occator and its spots appear just left of center.

exposed to the cold of space, the top would have frozen,
encasing the dwarf planet in a shell of ice. Through a
combination of blasting by impacts and sublimation,
more than 100 km of ice could have been lost in the geologically brief span of 200 million years. As Ceres aged,
it would have cooled, because radioactive elements in its
interior decayed and provided less and less heat, making it more diﬃcult for underground water to stay above
the freezing point. But before it froze, the water would
have provided a medium for complex chemical reactions,
creating a rich inventory of minerals.
Although ice is certainly far more prevalent, could
liquid water persist underground even today? Scientists
don't know. The loss of heat over time depends not only
on the speciﬁc supply of radioactive elements Ceres
started with but also on how the heat moved. Salt, for
example, conducts heat very poorly, so it could store thermal energy longer than rock. Also, the freezing point
of water depends on what chemicals are dissolved in it.
As one case in point, ammoniated salts make eﬀective
antifreeze. Scientists are using the extensive data from
Dawn to reﬁne mathematical models to gain insight into
the interior conditions in the present and in the past.

It does not require mathematical models to see that
Ceres has many craters. Or does it? To most people, the
presence of craters on an ancient, airless world is not
surprising. We know the solar system is replete with
asteroids and comets that leave scars on the larger surfaces they strike.
But scientists did not expect to see so many craters
on Ceres. In the darkness before Dawn, the reasoning
was that if Ceres' crust were composed largely of ice (an
assumption based principally on the dwarf planet's low
density), then craters would disappear over geological
time scales. As the ice ﬂowed, albeit more slowly than
glacial speeds, the surface would gradually relax, much
as your skin relaxes after pressure from a ﬁ ngernail is
removed and the imprint eventually vanishes.
But Dawn has found areas on Ceres so heavily cratered, they are said to be saturated. That is, there are as
many craters as it's possible for there to be. This may
seem naïve. After all, no matter how many craters there
are, if a small asteroid slams into Ceres, it will form at
least one more. But a region saturated in craters has
so many that any new impact is as likely to obliterate
existing craters as it is to make new ones. Even in the
rough-and-tumble main asteroid belt, it takes a long
time to accumulate that many craters, so the process of
relaxation on Ceres must be slower than expected. That
is likely because of the abundance of salts and rock in
that subsurface ice, making the combination stiﬀer than
a more ice-rich layer would be.
But the mathematical models for how many craters
should have formed at diﬀerent sizes show there actually is a deﬁcit of large ones. Based on what we found at
Vesta, Ceres should have pockmarks up to sizes comparable with its diameter (roughly 900 km). Yet it has only
16 craters larger than 100 km, about 40% of the number
expected. The two largest Cerean craters have diameters
of 280 km (Kerwan) and 260 km (Yalode). In contrast,
Dawn found a gigantic Vestan basin (Rheasilvia) more
than 500 km in diameter and another (Veneneia)
400 km across overlapping at the south pole.
The absence of large Cerean craters could be the
result of the loss of an exterior, icy shell or some internal
processes that erase them. Scientists can use their

Dawn's Primary Mission Orbits Around Ceres
Mapping Orbit

Dates (2015)

Altitude in km (mi)

Resolution in meters (ft) per pixel

April 23-May 9

13,600 (8,400)

1,300 (4,200)

24×

June 6-30

4,400 (2,700)

410 (1,400)

73×

Aug 17-Oct 23

1,470 (915)

140 (450)

217×

Dec 16-Sept 2, 2016

385 (240)

35 (120)

830×

December 2016 sky & telescope

Resolution compared to Hubble

Sky and Telescope - December 2016

Table of Contents for the Digital Edition of Sky and Telescope - December 2016