In March of 2015, NASA’s Dawn mission arrived around Ceres, a protoplanet that is the largest object in the Asteroid Belt. Along with Vesta, the Dawn mission seeks to characterize the conditions and processes of the early Solar System by studying some of its oldest objects. One thing Dawn has determined since its arrival is that water-bearing minerals are widespread on Ceres, an indication that the protoplanet once had a global ocean.
Naturally, this has raised
many questions, such as what happened to this ocean, and could Ceres still have
water today? Towards this end, the Dawn mission team recently conducted two
studies that shed some light on these questions. Whereas the former used
gravity measurements to characterize the interior of the protoplanet, the
latter sought to determine its interior structure by studying its topography.
The first study, titled
“Constraints on Ceres’ internal structure and evolution from its shape and
gravity measured by the Dawn spacecraft“, was recently published in the Journal
of Geophysical Research. Led by Anton Ermakov, a postdoctoral researcher at
JPL, the team also consisted of researchers from the NASA’s Goddard Space
Flight Center, the German Aerospace Center, Columbia University, UCLA and MIT.
Together, the team relied on
gravity measurements of the protoplanet, which the Dawn probe has been
collecting since it established orbit around Ceres. Using the Deep Space
Network to track small changes in the spacecraft’s orbit, Ermakov and his
colleagues were able to conduct shape and gravity data measurements of Ceres to
determine the internal structure and composition. What they found was that
Ceres shows signs of being geologically active; if not today, than certainly in
the recent past.
A view of Ceres in natural colour, pictured by the Dawn spacecraft in May 2015. Credit: NASA/ JPL/Planetary Society/Justin Cowart |
This is indicated by the
presence of three craters – Occator, Kerwan and Yalode – and Ceres’ single tall
mountain, Ahuna Mons. All of these are associated with “gravity anomalies”,
which refers to discrepancies between the way scientists have modeled Ceres’
gravity and what Dawn observed in these four locations.
The team concluded that
these four features and other outstanding geological formations, are therefore
indications of cryovolcanism or subsurface structures. What’s more, they
determined that the crust’s density was relatively low, being closer to that of
ice than solid rock. This, however, was
inconsistent with a previous study performed by Dawn guest investigator Michael
Bland of the U.S. Geological Survey.
Bland’s study, which was
published in Nature Geoscience back in 2016, indicated that ice is not likely
to be the dominant component of Ceres strong crust, on a count of it being too
soft.
Naturally, this raises the
question of how the crust could be light as ice in terms of density, but also
much stronger. To answer this, the second team attempted to model how Ceres’
surface evolved over time.
Their study, titled “The
Interior Structure of Ceres as Revealed by Surface Topography and Gravity“, was
published in the journal Earth and Planetary Science Letters. Led by Roger Fu,
an assistant professor with the Department of Earth, Atmospheric and Planetary
Sciences at MIT, this team consisted of members from Virginia Tech, Caltech,
the Southwest Research Institute (SwRI), the US Geological Survey, and the
INAF.
Together, they investigated
the strength and composition of Ceres’ crust and deeper interior by studying
the dwarf planet’s topography. By modeling how the protoplanet’s crust flows,
Fu and colleagues determined that it is likely a mixture of ice, salts, rock,
and likely clathrate hydrate. This type of structure, which is composed of a
gas molecule surrounded by water molecules, is 100 to 1,000 times stronger than
water ice.
This high-strength crust,
they theorize, could rest on a softer layer that contains some liquid. This
would have allowed Ceres’ topography to deform over time, smoothing down
features that were once more pronounced. It would also account for its possible
ancient ocean, which would have frozen and become bound up with the crust.
Gravity measurements of Ceres, which provided hints about its internal structure. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA |
Nevertheless, some of its
water would still exist in a liquid state underneath the surface.
This theory is consistent
with several thermal evolution models which were published before the Dawn
mission arrived at Ceres. These models contend that Ceres’ interior contains
liquid water, similar to what has been found on Jupiter’s moon Europa and
Saturn’s moon Enceladus. But in Ceres’ case, this liquid could be what is left
over from its ancient ocean rather than the result of present-day geological
activity in the interior.
Taken together, these
studies indicate that Ceres has had a long and turbulent history. While the
first study found that Ceres’ crust is a mixture of ice, salts and hydrated
materials – which represents most of its ancient ocean – the second study
suggests there is a softer layer beneath Ceres’ rigid surface crust, which
could be the signature of residual liquid left over from the ocean.
As Julie Castillo-Rogez, the
Dawn project scientist at JPL and a co-author on both studies, explained, “More
and more, we are learning that Ceres is a complex, dynamic world that may have
hosted a lot of liquid water in the past, and may still have some underground.”
On October 19, 2017, NASA
announced that the Dawn mission would be extended until its fuel runs out,
which is expected to happen in the latter half of 2018. This extension means
that the Dawn probe will be in orbit around Ceres as it goes through perihelion
in April 2018. At this time, surface ice will start to evaporate to form a
transient atmosphere around the body.
During this period and long
after, the spacecraft is likely to remain in a stable orbit around Ceres, where
it will continue to send back information on this protoplanet/large asteroid.
What it teaches us will also
go a long way towards informing our understanding of the early Solar System and
how it evolved over the past few billion years.
In the future, it is possible
that a mission will be sent to Ceres that is capable of landing on its surface
and exploring its topography directly. With any luck, future missions will also
be able to explore the interior of Ceres, and other “ocean worlds” like Europa
and Enceladus, and find out what lurks beneath their icy surfaces!
Via Universetoday