Ceres
(Ceres : dwarf Planet in asteroid belt, atmosphere, internal structure,discovery,exploration)
Ceres (/ˈsɪəriːz/; minor-planet designation: 1 Ceres) is the
largest object in the asteroid belt that lies between the orbits of Mars and
Jupiter. Its diameter is approximately 945 kilometers (587 miles), making it
the largest of the minor planets within the orbit of Neptune. The 33rd-largest
known body in the Solar System, it is the only dwarf planet within the orbit of
Neptune. Composed of rock and ice, Ceres is estimated to compose approximately
one third of the mass of the entire asteroid belt. Ceres is the only object in
the asteroid belt known to be rounded by its own gravity (though detailed
analysis was required to exclude 4 Vesta). From Earth, the apparent magnitude
of Ceres ranges from 6.7 to 9.3, and hence even at its brightest it is too dim
to be seen with the naked eye except under extremely dark skies.
Ceres was the first asteroid to be discovered (by Giuseppe
Piazzi at Palermo on 1 January 1801). It was originally considered a planet,
but was reclassified as an asteroid in the 1850s after many other objects in
similar orbits were discovered.
Ceres appears to be differentiated into a rocky core and an
icy mantle, and may have a remnant internal ocean of liquid water under the
layer of ice. The surface is probably a mixture of water ice and various
hydrated minerals such as carbonates and clay. In January 2014, emissions of
water vapor were detected from several regions of Ceres. This was unexpected
because large bodies in the asteroid belt typically do not emit vapor, a hallmark
of comets.
The robotic NASA spacecraft Dawn entered orbit around Ceres
on 6 March 2015. Pictures with a resolution previously unattained were taken
during imaging sessions starting in January 2015 as Dawn approached Ceres,
showing a cratered surface. Two distinct bright spots (or high-albedo features)
inside a crater (different from the bright spots observed in earlier Hubble
images) were seen in a 19 February 2015 image, leading to speculation about a
possible cryovolcanic origin or outgassing. On 3 March 2015, a NASA
spokesperson said the spots are consistent with highly reflective materials
containing ice or salts, but that cryovolcanism is unlikely. However, on 2
September 2016, NASA scientists released a paper in Science that claimed that a
massive ice volcano called Ahuna Mons is the strongest evidence yet for the
existence of these mysterious ice volcanoes. On 11 May 2015, NASA released a
higher-resolution image showing that, instead of one or two spots, there are
actually several. On 9 December 2015, NASA scientists reported that the bright
spots on Ceres may be related to a type of salt, particularly a form of brine
containing magnesium sulfate hexahydrite (MgSO4•6H2O); the spots were also
found to be associated with ammonia-rich clays. In June 2016, near-infrared
spectra of these bright areas were found to be consistent with a large amount
of sodium carbonate (Na
2CO
3), implying that recent geologic activity was probably
involved in the creation of the bright spots.
In October 2015, NASA released a true color portrait of
Ceres made by Dawn. In February 2017, organics were reported to have been
detected on Ceres in Ernutet crater (see image).
Discovery
Piazzi's book Della scoperta del nuovo pianeta Cerere
Ferdinandea outlining the discovery of Ceres, dedicated the new
"planet" to Ferdinand I of the Two Sicilies.
Johann Elert Bode, in 1772, first suggested that an
undiscovered planet could exist between the orbits of Mars and Jupiter. Kepler
had already noticed the gap between Mars and Jupiter in 1596. Bode based his
idea on the Titius–Bode law which is a now-discredited hypothesis that was
first proposed in 1766. Bode observed that there was a regular pattern in the
semi-major axes of the orbits of known planets, and that the pattern was marred
only by the large gap between Mars and Jupiter. The pattern predicted that the
missing planet ought to have an orbit with a semi-major axis near 2.8
astronomical units (AU). William Herschel's discovery of Uranus in 1781 near
the predicted distance for the next body beyond Saturn increased faith in the
law of Titius and Bode, and in 1800, a group headed by Franz Xaver von Zach,
editor of the Monatliche Correspondenz, sent requests to twenty-four
experienced astronomers (whom he dubbed the "celestial police"),
asking that they combine their efforts and begin a methodical search for the
expected planet. Although they did not discover Ceres, they later found several
large asteroids.
One of the astronomers selected for the search was Giuseppe
Piazzi, a Catholic priest at the Academy of Palermo, Sicily. Before receiving
his invitation to join the group, Piazzi discovered Ceres on 1 January 1801. He
was searching for "the 87th of the
Catalogue of the Zodiacal stars of Mr la Caille", but found that "it
was preceded by another". Instead of a star, Piazzi had found a moving
star-like object, which he first thought was a comet. Piazzi observed Ceres a
total of 24 times, the final time on 11 February 1801, when illness interrupted
his observations. He announced his discovery on 24 January 1801 in letters to
only two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode of
Berlin. He reported it as a comet but "since its movement is so slow and
rather uniform, it has occurred to me several times that it might be something
better than a comet". In April, Piazzi sent his complete observations to
Oriani, Bode, and Jérôme Lalande in Paris. The information was published in the
September 1801 issue of the Monatliche Correspondenz.
By this time, the apparent position of Ceres had changed
(mostly due to Earth's orbital motion), and was too close to the Sun's glare
for other astronomers to confirm Piazzi's observations. Toward the end of the
year, Ceres should have been visible again, but after such a long time it was
difficult to predict its exact position. To recover Ceres, Carl Friedrich
Gauss, then 24 years old, developed an efficient method of orbit determination.
In only a few weeks, he predicted the path of Ceres and sent his results to von
Zach. On 31 December 1801, von Zach and Heinrich W. M. Olbers found Ceres near
the predicted position and thus recovered it.
The early observers were only able to calculate the size of
Ceres to within an order of magnitude. Herschel underestimated its diameter as
260 km in 1802, whereas in 1811 Johann Hieronymus Schröter overestimated it as
2,613 km.
Internal structure
Diagram showing a possible internal structure of Ceres
Ceres' oblateness is consistent with a differentiated body,
a rocky core overlain with an icy mantle. This 100-kilometer-thick mantle
(23%–28% of Ceres by mass; 50% by volume) contains up to 200 million cubic
kilometers of water, which would be more than the amount of fresh water on
Earth. This result is supported by the observations made by the Keck telescope
in 2002 and by evolutionary modeling. Also, some characteristics of its surface
and history (such as its distance from the Sun, which weakened solar radiation
enough to allow some fairly low-freezing-point components to be incorporated
during its formation), point to the presence of volatile materials in the
interior of Ceres. It has been suggested that a remnant layer of liquid water
may have survived to the present under a layer of ice.
Shape and gravity field measurements by Dawn confirm Ceres
is a body in hydrostatic equilibrium with partial differentiation and isostatic
compensation, with a mean moment of inertia of 0.37 (which is similar to that
of Callisto at ~0.36). The densities of the core and outer layer are estimated
to be 2.46–2.90 and 1.68–1.95 g/cm3, with the latter being about 70–190 km
thick. Only partial dehydration of the core is expected. The high density of
the outer layer (relative to water ice) reflects its enrichment in silicates
and salts. Ceres is the smallest object confirmed to be in hydrostatic
equilibrium, being 600 km smaller and less than half the mass of Saturn's moon
Rhea, the next smallest such object. Modeling has suggested Ceres could have a
small metallic core from partial differentiation of its rocky fraction.
Atmosphere
There are indications that Ceres has a tenuous water vapor
atmosphere outgassing from water ice on the surface.
Surface water ice is unstable at distances less than 5 AU
from the Sun, so it is expected to sublime if it is exposed directly to solar
radiation. Water ice can migrate from the deep layers of Ceres to the surface,
but escapes in a very short time. As a result, it is difficult to detect water
vaporization. Water escaping from polar regions of Ceres was possibly observed
in the early 1990s but this has not been unambiguously demonstrated. It may be
possible to detect escaping water from the surroundings of a fresh impact
crater or from cracks in the subsurface layers of Ceres. Ultraviolet
observations by the IUE spacecraft detected statistically significant amounts
of hydroxide ions near Ceres' north pole, which is a product of water vapor
dissociation by ultraviolet solar radiation.
In early 2014, using data from the Herschel Space
Observatory, it was discovered that there are several localized (not more than
60 km in diameter) mid-latitude sources of water vapor on Ceres, which each
give off approximately 1026 molecules (or 3 kg) of water per second. Two
potential source regions, designated Piazzi (123°E, 21°N) and Region A (231°E,
23°N), have been visualized in the near infrared as dark areas (Region A also
has a bright center) by the W. M. Keck Observatory. Possible mechanisms for the
vapor release are sublimation from approximately 0.6 km2 of exposed surface
ice, or cryovolcanic eruptions resulting from radiogenic internal heat or from
pressurization of a subsurface ocean due to growth of an overlying layer of
ice. Surface sublimation would be expected to be lower when Ceres is farther
from the Sun in its orbit, whereas internally powered emissions should not be
affected by its orbital position. The limited data available was more
consistent with cometary-style sublimation; however, subsequent evidence from
Dawn strongly suggests ongoing geologic activity could be at least partially
responsible.
Studies using Dawn's gamma ray and neutron detector (GRaND)
reveal that Ceres is accelerating electrons from the solar wind regularly;
although there are several possibilities as to what is caus
ing this, the most accepted is that these electrons are
being accelerated by collisions between the solar wind and a tenuous water
vapor exosphere.
In 2017, Dawn confirmed that Ceres has a transient
atmosphere that appears to be linked to solar activity. Ice on Ceres can
sublimate when energetic particles from the Sun hit exposed ice within craters.
Origin and evolution
Ceres is possibly a surviving protoplanet (planetary
embryo), which formed 4.57 billion years ago in the asteroid belt. Although the
majority of inner Solar System protoplanets (including all lunar- to Mars-sized
bodies) either merged with other protoplanets to form terrestrial planets or
were ejected from the Solar System by Jupiter, Ceres is thought to have
survived relatively intact. An alternative theory proposes that Ceres formed in
the Kuiper belt and later migrated to the asteroid belt. The discovery of
ammonia salts in Occator crater supports an origin in the outer Solar System.
Another possible protoplanet, Vesta, is less than half the size of Ceres; it
suffered a major impact after solidifying, losing ~1% of its mass.
The geological evolution of Ceres was dependent on the heat
sources available during and after its formation: friction from planetesimal
accretion, and decay of various radionuclides (possibly including short-lived
extinct radionuclides such as aluminium-26). These are thought to have been
sufficient to allow Ceres to differentiate into a rocky core and icy mantle
soon after its formation. This process may have caused resurfacing by water
volcanism and tectonics, erasing older geological features. Ceres's relatively
warm surface temperature implies that any of the resulting ice on its surface
would have gradually sublimated, leaving behind various hydrated minerals like
clay minerals and carbonates.
Today, Ceres has become considerably less geologically
active, with a surface sculpted chiefly by impacts; nevertheless, evidence from
Dawn reveals that internal processes have continued to sculpt Ceres's surface
to a significant extent, in stark contrast to Vesta and of previous
expectations that Ceres would have become geologically dead early in its
history due to its small size. The presence of significant amounts of water ice
in its composition and evidence of recent geological resurfacing, raises the
possibility that Ceres has a layer of liquid water in its interior. This
hypothetical layer is often called an ocean. If such a layer of liquid water
exists, it is hypothesized to be located between the rocky core and ice mantle
like that of the theorized ocean on Europa. The existence of an ocean is more
likely if solutes (i.e. salts), ammonia, sulfuric acid or other antifreeze
compounds are dissolved in the water.
Observation
When Ceres has an opposition near the perihelion, it can
reach a visual magnitude of +6.7. This is generally regarded as too dim to be
seen with the naked eye, but under exceptional viewing conditions a very
sharp-sighted person may be able to see it. The only other asteroids that can
reach a similarly bright magnitude are 4 Vesta, and, during rare oppositions
near perihelion, 2 Pallas and 7 Iris. At a conjunction Ceres has a magnitude of
around +9.3, which corresponds to the faintest objects visible with 10×50
binoculars. It can thus be seen with binoculars whenever it is above the
horizon of a fully dark sky.
Some notable observations and milestones for Ceres include:
• 1984
November 13: An occultation of a star by Ceres observed in Mexico, Florida and
across the Caribbean.
• 1995 June
25: Ultraviolet Hubble Space Telescope images with 50-kilometer resolution.
• 2002:
Infrared images with 30-km resolution taken with the Keck telescope using
adaptive optics.
• 2003 and
2004: Visible light images with 30-km resolution (the best prior to the Dawn
mission) taken using Hubble.
• 2012
December 22: Ceres occulted the star TYC 1865-00446-1 over parts of Japan,
Russia, and China. Ceres' brightness was magnitude 6.9 and the star, 12.2.
• 2014:
Ceres was found to have an atmosphere with water vapor, confirmed by the
Herschel space telescope.
• 2015: The
NASA Dawn spacecraft approached and orbited Ceres, sending detailed images and
scientific data back to Earth.
Exploration
Artist's conception of Dawn, travelling from Vesta to Ceres
In 1981, a proposal for an asteroid mission was submitted to
the European Space Agency (ESA). Named the Asteroidal Gravity Optical and Radar
Analysis (AGORA), this spacecraft was to launch some time in 1990–1994 and
perform two flybys of large asteroids. The preferred target for this mission
was Vesta. AGORA would reach the asteroid belt either by a gravitational
slingshot trajectory past Mars or by means of a small ion engine. However, the
proposal was refused by ESA. A joint NASA–ESA asteroid mission was then drawn
up for a Multiple Asteroid Orbiter with Solar Elec
tric Propulsion (MAOSEP), with one of the mission profiles
including an orbit of Vesta. NASA indicated they were not interested in an asteroid
mission. Instead, ESA set up a technological study of a spacecraft with an ion
drive. Other missions to the asteroid belt were proposed in the 1980s by
France, Germany, Italy, and the United States, but none were approved.
Exploration of Ceres by fly-by and impacting penetrator was the second main
target of the second plan of the multiaimed Soviet Vesta mission, developed in
cooperation with European countries for realisation in 1991–1994 but canceled
due to the Soviet Union disbanding.
First asteroid image (Ceres and Vesta) from Mars – viewed by
Curiosity (20 April 2014)
In the early 1990s, NASA initiated the Discovery Program,
which was intended to be a series of low-cost scientific missions. In 1996, the
program's study team recommended as a high priority a mission to explore the
asteroid belt using a spacecraft with an ion engine. Funding for this program
remained problematic for several years, but by 2004 the Dawn vehicle had passed
its critical design review.
It was launched on 27 September 2007, as the space mission
to make the first visits to both Vesta and Ceres. On 3 May 2011, Dawn acquired
its first targeting image 1.2 million kilometers from Vesta. After orbiting
Vesta for 13 months, Dawn used its ion engine to depart for Ceres, with
gravitational capture occurring on 6 March 2015 at a separation of 61,000 km,
four months prior to the New Horizons flyby of Pluto.
Dawn's mission profile calls for it to study Ceres from a
series of circular polar orbits at successively lower altitudes. It entered its
first observational orbit ("RC3") around Ceres at an altitude of
13,500 km on 23 April 2015, staying for only approximately one orbit (fifteen
days). The spacecraft will subsequently reduce its orbital distance to 4,400 km
for its second observational orbit ("survey") for three weeks, then
down to 1,470 km ("HAMO;" high altitude mapping orbit) for two months
and then down to its final orbit at 375 km ("LAMO;" low altitude
mapping orbit) for at least three months. The spacecraft instrumentation
includes a framing camera, a visual and infrared spectrometer, and a gamma-ray
and neutron detector. These instruments will examine Ceres' shape and elemental
composition. On 13 January 2015, Dawn took the first images of Ceres at
near-Hubble resolution, revealing impact craters and a small high-albedo spot
on the surface, near the same location as that observed previously. Additional
imaging sessions, at increasingly better resolution took place on 25 January,
4, 12, 19, and 25 February, 1 March, and 10 and 15 April.
Dawn's arrival in a stable orbit around Ceres was delayed
after, close to reaching Ceres, it was hit by a cosmic ray, making it take
another, longer route around Ceres in back, instead of a direct spiral towards
it.
The Chinese Space Agency is designing a sample retrieval
mission from Ceres that would take place during the 2020s.
No comments:
Post a Comment