[meteorite-list] Dawn Journal - December 29, 2014

[Ron Baalke]

http://dawnblog.jpl.nasa.gov/2014/12/29/dawn-journal-december-29/

Dawn Journal 
by Marc Rayman
December 29, 2014

 
Pardawn Me, Dear Readers,

Far away from Earthlings who look forward to a new year, Dawn looks forward 
to a new world. On the far side of the sun, the interplanetary explorer 
is closing in on Ceres, using its advanced ion propulsion system to match 
solar orbits with the dwarf planet.

Since breaking out of orbit around the giant protoplanet Vesta in September 
2012, the spaceship has patiently flown in interplanetary cruise. That 
long mission phase is over, and now Dawn is starting the Ceres chapter 
of its extraordinary extraterrestrial expedition. Configured for its approach 
phase, the craft is following a new and carefully designed course described 
in detail last month. In March it will slip ever so gracefully into orbit 
for an ambitious and exciting exploration of the alien world ahead.

Over the past year, we have provided previews of the major activities 
during all the phases of Dawn's mission at Ceres. This month, let's take 
a look at Ceres itself, an intriguing and mysterious orb that has beckoned 
for more than two centuries. Now, finally, after so long, Earth is answering 
the cosmic invitation, and an ambassador from our planet is about to take 
up permanent residence there. Over the course of Dawn's grand adventure, 
our knowledge will rocket far, far beyond all that has been learned before.

There can be two accounts of Ceres: its own history, which dates back 
to near the dawn of the solar system almost 4.6 billion years ago, and 
its history in the scope of human knowledge, which is somewhat shorter. 
Both are rich topics, with much more than we can cover here (or in the 
first log for this entire mission), but let's touch on a few tidbits. 
We begin with the latter history.

In 1800, the known solar system contained seven planets: Mercury, Venus, 
Earth (home to some of our readers), Mars, Jupiter, Saturn and Uranus. 
This reflected a new and sophisticated scientific understanding, because 
Uranus had first been noticed in telescopes not long before, in 1781. 
(The other planets had been known to ancient sky watchers.) Even before 
William Herschel's fortuitous sighting of a planet beyond Saturn, astronomers 
had wondered about the gap between Mars and Jupiter and speculated about 
the possibility of a planet there. Although some astronomers had searched, 
their efforts had not yielded a new planet.

Astronomer Giuseppe Piazzi was not looking for a planet on Jan. 1, 1801, 
but he spotted an unfamiliar dot of light that moved slowly among the 
stars. He named it for Ceres, the Roman goddess of agriculture, and if 
you had cereal this morning, you have already had an etymological connection 
with the goddess.

The Dawn project worked with the International Astronomical Union (IAU) 
to formalize a plan for names on Ceres that builds upon and broadens Piazzi's 
theme. Craters will be named for gods and goddesses of agriculture and 
vegetation from world mythology. Other features will be named for agricultural 
festivals.

Because Ceres was fainter than the other known planets, it was evident 
that it was smaller. Nevertheless, many astronomers considered it to be 
a planet too.

It is worth noting the significance of this. Modern astronomy had chanced 
upon only one other planet, so Piazzi's discovery was A Big Deal. When 
a new chemical element was found a couple of years later, it was named 
cerium in tribute to the new planet Ceres. (Uranus had been similarly 
honored with the 1789 naming of uranium. That element's peculiar property 
of emitting radiation would not be known for another century.)

In the six years following the discovery of Ceres, three more bodies were 
detected orbiting between Mars and Jupiter. (One of them is Vesta, now 
known in spectacular detail thanks to Dawn's extensive exploration in 
2011-2012.) There then ensued a gap of more than 38 years before another 
was found, so for well over a generation, the sun's family of planets 
was unchanged.

So if you had been reading about all this 200 years ago, there would have 
been at least two important differences from now. One is that your Internet 
connection would have been considerably slower. The other is that you 
might have learned in school or elsewhere that Ceres was a planet.

In 1846, a planet was discovered beyond Uranus, and we call it Neptune. 
Nothing else of comparable size has subsequently been seen in our solar 
system.

With scientific knowledge and technology progressing in the middle of 
the nineteenth century, new objects were glimpsed between Mars and Jupiter. 
As more and more were seen over the years, what we now know as the main 
asteroid belt was gradually recognized. Terminology changed too. One of 
the great strengths of science is that it advances, and sometimes we have 
to modify our vocabulary to reflect the improved, refined view of the 
universe.

By the time Pluto was sighted in 1930, Ceres had long been known as a 
"minor planet' and an 'asteroid.' For a while thereafter, Pluto enjoyed 
planetary status similar to what Ceres had had. In fact, in 1940, scientists 
named two more additions to the periodic table of the elements neptunium 
and plutonium. While the histories are not identical, there is a certain 
parallel, with more and more objects in Pluto's part of the solar system 
later being found. Terminology changed again: Pluto was subsumed into 
the new category of "dwarf planets" defined by the IAU in 2006. Ceres 
was the first body to be discovered that met the criteria established 
by the IAU, and Pluto was the second. (Spacecraft are now on their way 
to both dwarf planets: Dawn to orbit Ceres 214 years after its discovery 
and the wonderful New Horizons mission to fly past Pluto 85 years after 
it was found.)

We discussed this new nomenclature in some detail shortly after it was 
adopted. We understand that the designation then, as now, is controversial 
among some scientists and the public, and there are strong emotions on 
this topic. We will not delve into it here (nor in the blog comments below), 
preferring instead to focus on the extraordinary successes of science, 
the great power of the scientific method and the thrill of bold adventures 
far from home. The Dawn team remains both unperturbed and confident in 
what to call this intriguing and alluring world: we call it "Ceres." And 
our goal is to develop that faint smudge of light amidst the stars into 
a richly detailed portrait.

One of the advances of science was the recognition that Ceres really is 
entirely different from typical residents of the main asteroid belt. It 
is a colossus! There are millions upon millions of asteroids, and yet 
Ceres itself contains roughly 30 percent of the mass in that entire vast 
region of space. By the way, Vesta, the second most massive body there, 
constitutes about eight percent of the asteroid belt's mass. It is remarkable 
that Dawn will single-handedly explore around 40 percent of the asteroid 
belt's mass.

With an equatorial diameter of about 605 miles (975 kilometers), a value 
that Dawn will refine very soon, Ceres is the largest body between the 
sun and Pluto that a spacecraft has not yet visited. It is occasionally 
described as being comparable in size to Texas, which is like comparing 
a basketball to a flat sheet of paper. Ceres has a surface area 38 percent 
of that of the continental United States, or more than 10 times the area 
of Texas. (Nevertheless, until Dawn shows evidence to the contrary, we 
will assume Texas has more rodeos.) It is nearly a third of the area of 
Europe and larger than the combined lands of France, Germany, Italy, Norway, 
Spain, Sweden and the United Kingdom. Such a large place offers the promise 
of tremendous diversity and many marvelous and exciting sights to behold. 
Earth is about to be introduced to a fascinating new world.

How did Ceres come to be? And why is that being phrased as a question 
instead of a more declarative introduction to the history and nature of 
this dwarf planet? For that matter, why is this paragraph composed exclusively 
of questions? At least this sentence isn't a question, right? OK, really, 
shouldn't we stay more on topic?

At the dawn of the solar system almost 4.6 billion years ago, the young 
sun was surrounded by a swirling cloud of dust and gas. Sometimes some 
particles would happen to hit and stick together. Then more and more and 
more particles would stick to them, and eventually these agglomerations 
would grow so large that their gravity would pull in even more material. 
It was through mechanisms like this that the planets formed.

But when massive Jupiter developed, its powerful gravity terminated the 
growth of objects nearby, leaving bits and pieces as asteroids. Ceres 
and Vesta, already sizable by then, might have grown to become even larger, 
each incorporating still more of the nearby material, had Jupiter not 
deprived them of such an opportunity. Not having made it to full planetary 
proportions, Ceres and Vesta are known as protoplanets, and studying them 
provides scientists with insight into the largest building blocks of planets 
and into worlds that are intriguing in their own rights.

Ceres apparently formed far enough from the sun under conditions cool 
enough for it to hang on to water molecules. Indeed, scientists have good 
reason to believe that water (mostly in the form of ice) may make up an 
astonishing 30 percent of its mass. Ceres may contain more water than 
Mars or any other body in the inner solar system except Earth. (Comets, 
of course, have high proportions of water too, but they are so minuscule 
compared to this behemoth that each one harbors a quite negligible amount 
of water when measured against Ceres’ huge inventory.)

Although some of the moons of the outer planets also are ice and rock, 
and they display very interesting characteristics to the impressive and 
capable spacecraft that have flown past (in some cases repeatedly, as 
the craft orbited the host planet), no probe has had the capability to 
linger in orbit around any of them. Dawn's in-depth exploration of Ceres 
will yield more detailed and complete views than we have obtained of any 
icy moon.

Radioactive elements incorporated into Ceres when it was forming would 
supply it with some heat, and its great bulk would provide thermal insulation, 
so it would take a very long time for the heat to escape into space. The 
sun, faraway though it is, adds still more heat. As a result, there may 
be some water warm enough to be liquid. (The concentration of any chemical 
impurities in the water that affect its freezing point, as salt does, 
may make an important difference in how much is liquid.) This distant, 
alien world may have lakes or even oceans of liquid water deep underground. 
What a fantastic possibility!

There will be no liquid on the frigid surface. Even ice on the surface, 
exposed to the cold vacuum of space, would sublimate before long. But 
ice could be just beneath the surface, perhaps well less than a yard (a 
meter) deep.

Ceres then may have a thin, dusty crust over a mantle rich in ice that 
might be more than 60 miles (100 kilometers) thick. Its warmer core is 
likely composed mostly of rock.

As heat dissipated from Ceres’ interior over the eons, it may have undergone 
convection, with the warmer material rising and cooler material sinking 
very slowly. This is reminiscent of what occurs in pot of heated water 
and in Earth's interior. Even if it did occur at some time in Ceres' history, 
it probably is not happening any longer, as too much heat would have been 
lost by now, so there would not be enough left to power the upward movement 
of warm material. But the convective process might have written its signature 
in structures or minerals left behind when ice sublimated after being 
pushed to the surface. Dawn's photos of geological features and measurements 
of the composition may provide a window to forces in the interior of the 
protoplanet sometime in its past.

Even if convection is no longer occurring, Ceres is not entirely static. 
We have very tantalizing information from a marvelously productive far-infrared 
space telescope named for the only known astronomer who found a planet 
before Piazzi made his discovery. The Herschel Space Observatory recently 
detected a tiny amount of water vapor emanating from the distant dwarf 
planet. Scientists do not know how the water vapor makes it into space. 
It might be from ice sublimating (possibly following a powerful impact 
that exposed subsurface ice) or perhaps from geysers or even erupting 
cryovolcanoes ('cold volcanoes') powered by heat that Ceres has retained 
since its formation. In any case, Herschel saw water, albeit in very, 
very small quantity.

It is not certain whether water vapor is there all the time. It is unknown 
whether, for example, it depends on solar heating and hence where Ceres 
is in its somewhat elliptical orbit around the sun (not as circular as 
Earth's orbit but more circular than Mars'), which requires 4.6 years 
to complete.

Even if the water vapor is present during Dawn's 1.3-year primary mission 
in orbit, it would be extremely difficult to detect. Herschel made its 
findings when our ship was already far, far from Earth, well along its 
interplanetary itinerary. The probe's sensors were designed for studying 
the solid surfaces of airless bodies, not an exceedingly tenuous veil 
of water molecules. For context, the water vapor Herschel measured is 
significantly less dense than Earth's atmosphere is even far above the 
International Space Station, which orbits in what most people consider 
to be the vacuum of space. Dawn will not need windshield wipers! Nevertheless, 
as we saw in February, the Dawn team, ever creative and dedicated to squeezing 
as much out of the mission as possible, investigated techniques this year 
that might be effective in searching for an exceptionally thin vapor. 
They have augmented the plan with many hours of observations of the space 
above Ceres when the spacecraft is over the night side during its first 
science orbit in April and May at an altitude of 8,400 miles (13,500 kilometers). 
It is possible that if there is some water vapor, the instruments may 
pick up a faint signature in the sunlight that passes through it.

Regardless of the possibility of detecting traces of water from Ceres, 
Dawn will focus its measurements on the uncharted surface and the interior, 
as it did at Vesta. Vesta displayed landscapes battered by craters from 
impacts during more than 4.5 billion years in the rough and tumble asteroid 
belt. Ceres has spent most or all of its history also in the asteroid 
belt, but it is possible it will not show its age so clearly. Ice, although 
very hard at such low temperatures, is not as hard as rock. So it may 
be that the surface gradually "relaxes" after an impact, just as your 
skin restores its shape after pressure has been removed. Craters older 
than a few tens of millions of years may have slowly disappeared. (That 
may sound old, but it is a small fraction of Ceres' lifetime.) Near the 
poles, where it is colder so ice is harder, the scars of impact craters 
may be preserved for longer.

Ceres has more than water-ice and rock. It probably contains organic materials, 
some produced by chemical processes with the minerals already there and 
some delivered by asteroids that fell to its surface. This is noteworthy, 
because water and organic chemicals are ingredients for life. The combination 
of Ceres' internal heat and the weak but persistent heating from the sun 
provides energy, which also is essential for life. Even if the possibility 
of life itself there is extremely remote (and it is beyond Dawn's capability 
to detect), the conditions for "prebiotic" chemistry would be tremendously 
interesting. That is why, as we explained in August, we want to protect 
the special environment on the ground from contamination by the terrestrial 
chemicals in our orbiting spacecraft.

While there is more known about Ceres, there is much, much more that is 
unknown. Dawn seeks to discover many of the secrets of this unfamiliar, 
fascinating member of the solar system family. One of the measures of 
its success would be if, upon answering many of our questions about Ceres, 
we are left with even more questions. Now on the threshold of an old world 
which will be new to us, we do not have long to wait for the great rewards 
of new knowledge, new insight, new thrills and new mysteries to solve.

Dawn is 382,000 miles (614,000 kilometers) from Ceres, or 1.6 times the 
average distance between Earth and the moon. It is also 3.77 AU (351 million 
miles, or 564 million kilometers) from Earth, or 1,500 times as far as 
the moon and 3.84 times as far as the sun today. Radio signals, traveling 
at the universal limit of the speed of light, take one hour and three 
minutes to make the round trip.