LECTURE 9: THE DWARF PLANET CERES
Ceres, also designated 1 Ceres (see minor planet names), is the smallest identified dwarf planet in the Solar System and the only one in the asteroid belt. It was discovered on January 1, 1801, by Giuseppe Piazzi,[15] and is named after the Roman goddess Ceres—the goddess of growing plants, the harvest, and of motherly love.
With a diameter of about 950 km, Ceres is by far the largest and most massive body in the asteroid belt, and contains approximately a third of the belt's total mass.[16] Recent observations have revealed that it is spherical, unlike the irregular shapes of smaller bodies with lower gravity.[8] The surface of Ceres is probably made of a mixture of water ice and various hydrated minerals like carbonates and clays.[9] Ceres appears to be differentiated into a rocky core and ice mantle.[3] It may harbour an ocean of liquid water, which makes it a target of current searches for extraterrestrial life.[16] Ceres may be surrounded by a tenuous atmosphere containing water vapour.[17]
Ceres' apparent magnitude ranges from 6.7 to 9.3, hence at its brightest is still too dim to be seen with the naked eye.[10] On September 27, 2007, NASA launched the Dawn Mission space probe to explore Vesta (2011-2012) and Ceres (2015).[18]
Ceres is the largest object in the asteroid belt, which lies between Mars and Jupiter.[9] The Kuiper belt is known to contain larger objects, including Pluto, 50000 Quaoar, and 90482 Orcus, while more distant Eris, in the scattered disc, is the largest of all these bodies.[34]
The mass of Ceres has been determined by analysis of the influence it exerts on small asteroids. Results obtained by different authors are slightly different.[35] The average of the three most precise values as of 2008 is approximately 9.4×1020 kg.[4][35] With this mass Ceres comprises about a third of the estimated total 3.0 ± 0.2 ×1021 kg mass of the asteroids in the solar system,[36] together totalling about 4% of the mass of the Moon. Ceres' size and mass are sufficient to give it a nearly spherical shape.[3] That is, it is close to hydrostatic equilibrium. In contrast, other large asteroids such as 2 Pallas,[37] 3 Juno,[38] and 4 Vesta[39] are known to be quite irregular.
The surface composition of Ceres is broadly similar to that of C-type asteroids.[9] However, some differences do exist. The ubiquitous features in the IR spectra of Ceres are that of hydrated materials, which indicates the presence of significant amounts of water in the interior of this body. Other possible surface constituents include iron-rich clays (cronstedtite) and carbonates (dolomite and siderite), which are common minerals in carbonaceous chondrite meteorites.[9] The spectral features of carbonates and clay are usually absent in the spectra of other C-type asteroids.[9] Sometimes Ceres is classified as G-type asteroid.[40]
The surface of Ceres is relatively warm. The maximum temperature with the Sun overhead was estimated from measurements to be 235 K (about −38 °C) on May 5, 1991.[13] Taking into account also the heliocentric distance at the time, this gives an estimated maximum of about 239 K at perihelion. There are some indications that Ceres may have a tenuous atmosphere and water frost on the surface.[17] Ultraviolet observations by IUE spacecraft detected statistically significant hydroxide water vapour near the Cererean north pole.[17]
Peter Thomas of Cornell University has proposed that Ceres has a differentiated interior;[3] its oblateness appears too small for an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle.[3] This mantle of thickness from 120 to 60 km could contain 200 million cubic kilometres of water (16–26% of Ceres by mass; 30–60% by volume), which is more than the amount of fresh water on the Earth.[41] This result is supported by the observations made by Keck telescope in 2002 and by evolutionary modelling.[4][42]
Only a few features have been unambiguously detected on the surface of Ceres. High resolution ultraviolet Hubble Space Telescope images taken in 1995 showed a dark spot on its surface which was nicknamed "Piazzi" in honour of the discoverer of Ceres.[40] This was thought to be a crater. Later near-infrared images with a higher resolution taken over a whole rotation with the Keck telescope using adaptive optics showed several bright and dark features moving with the dwarf planet's rotation.[43][4] Two dark features had circular shapes and are presumably craters; one of them was observed to have a bright central region, while another was identifyied with "Piazzi" feature.[43][4] More recent visible light Hubble Space Telescope images of a full rotation taken in 2003 and 2004 showed eleven recognizable surface features, the nature of which is currently unknown.[8][44] One of these features corresponds to the "Piazzi" feature observed earlier.[8]
These last observations also determined that Ceres' north pole points in the direction of right ascension 19 h 24 min (291°), declination +59°, in the constellation Draco. This means that Ceres' axial tilt is very small—about 3°.[3][8]
Ceres follows an orbit between Mars and Jupiter, within the main asteroid belt, with a period of 4.6 Earth years. The orbit is moderately inclined (i = 10.6° compared to 7° for Mercury and 17° for Pluto) and moderately eccentric (e = 0.08 compared to 0.09 for Mars).[1]
The diagram illustrates the orbits of Ceres (blue) and several planets (white/grey). The segments of orbits below the ecliptic are plotted in darker colours, and the orange plus sign is the Sun's location. The top left diagram is a polar view that shows the location of Ceres in the gap between Mars and Jupiter. The top right is a close-up demonstrating the locations of the perihelia (q) and aphelia (Q) of Ceres and Mars. The perihelion of Mars is on the opposite side of the Sun from those of Ceres and several of the large main belt asteroids, including 2 Pallas and 10 Hygiea. The bottom diagram is a perspective view showing the inclination of the orbit of Ceres compared to the orbits of Mars and Jupiter.
In the past, Ceres had been considered to be the largest member of an asteroid family.[45] These groupings of asteroids share similar orbital elements, which may indicate a common origin through an asteroid collision some time in the past. Ceres, however, was found to have spectral properties different from other members of the family, and so this grouping is now called the Gefion family, named after the lowest-numbered family member, 1272 Gefion.[45] Ceres appears to be merely an interloper in its own family, coincidentally having similar orbital elements but not a common origin.[46] Self rotation period of Ceres is 9 hours and 4 minutes.[6]
The observations imply that Ceres is a remaining protoplanet – planetary embryo, which formed 4.57 billion years ago in the asteroid belt.[47] While the majority of embryos (including all lunar- to Mars-sized bodies) were ejected from the Solar System by Jupiter or merged with other embryos to form terrestrial planets,[47] Ceres survived relatively intact.[42] Two other possible remaining protoplanets are Pallas and Vesta,[18] but they do not have relaxed shapes, in the case of Vesta perhaps only because it suffered a catastrophic impact after solidifying.[39]
Further evolution of Ceres was relatively simple. Heated by the energy of accretion and by decay of various radionuclides including, possibly, short-lived elements like Al26, Ceres differentiated into a rocky core and icy mantle soon after its formation.[8][42] This event caused resurfacing by the water volcanism and tectonics erasing many geological features.[42] However due to the fast depletion of heat sources Ceres cooled down quickly.[42][48] The ice on the surface gradually sublimated leaving behind various hydrated minerals like clays and carbonates.[9] Now Ceres is a geologically dead body, whose surface is being sculptured only by impacts.[8]
The existence of significant amounts of water ice in Ceres[3] raises a possibility that it has or had a layer of liquid water in the interior.[48][42] This hypothetical layer is often called an ocean.[9] The water layer is (or was) probably located between the rocky core and ice mantle like in Europa.[42] The existence of the ocean is more likely if ammonia or other antifreeze is dissolved in water.[42] The possible existence of liquid water inside Ceres makes it a target in the search for extraterrestrial life.[16]
When Ceres has an opposition near the perihelion, it can reach a visual magnitude of +6.7.[10] This is generally regarded as being just barely too dim to be seen with the naked eye, but under exceptional viewing conditions a very sharp-sighted person may be able to see this dwarf planet. The only asteroids that can reach so bright a magnitude are 4 Vesta, and, during rare oppositions near perihelion, 2 Pallas and 7 Iris.[49] 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 observation milestones for Ceres include:
- An occultation of a star by Ceres observed in Mexico, Florida and across the Caribbean on November 13, 1984.[50]
- Ultraviolet Hubble Space Telescope images with 50 km resolution taken in 1995.[51][40]
- Infrared images with 30 km resolution taken with the Keck telescope in 2002 using adaptive optics.[43]
- Visible light images with 30 km resolution (the best to date) taken using Hubble in 2003 and 2004.[44][8]
To date, no space probe has visited Ceres. However, NASA launched the Dawn Mission on 27 September 2007, which will explore the asteroid 4 Vesta in 2011 before arriving at Ceres in 2015.[18]
The mission profile calls for the Dawn Spacecraft to enter orbit around Ceres at an altitude of 5,900 km. After five months of study, the spacecraft will reduce the orbital distance to 1,300 km, then down to 700 km after another five months.[52] The spacecraft instrumentation includes a framing camera, a visual and infrared spectrometer, and a gamma-ray and neutron detector. These will be used to examine the dwarf planet's shape and element abundance.[18]
Radio signals from spacecraft in orbit around Mars and on its surface have been used to estimate the mass of Ceres from the perturbations induced by it onto the motion of Mars.[36]
