question: why do comets look like barren deserts in close-up pics when they're described as dirty snowballs?
comment: Nearly all of the water, ice and other volatiles on a comet are BENEATH the surface. So ya, that's basically it! That's the main straight forward answer to your question, as to why you don't see water-ice in most upclose photos of comets. --------------------------------- However, just to go into a bit more deeply So... as the comet comes soaring into the inner solar system (where Earth is), is does the following: - At a certain point it crosses over the "frost-line" of our solar system. - The frost-line is the point that is close enough to the sun to begin heating and warming the surface, along with a few layers directly beneath the surface, to the point where ice melts, and water begins to evaporate/vaporize. - So at that point, that's when the water and other volatiles start to vaporize, and begin escaping from beneath the surface. - Sometimes they can't escape right away from below the surface and get trapped or blocked, and form pockets of gas. - The pockets of gas then build up more and more pressure, then burst upwards, causing a small eruption on the surface, and a jet of gas. - These jets of gas/volatiles can even act like mini rocket engines and change the speed, course of the comet! - Anyways as the comet gets closer and closer to the sun, all these vaporizing volatiles begin to leave a hazy cloudy trail behind. That's why you get these beautiful hazy tails with comets. - NEXT, the comet swings around the sun, and then begins moving further and further away from the sun. - It then goes back across the frost-line, freezing in a deep cold state, so that no more water and volatiles are escaping very much anymore. - And so, if you visit the comet in the cold part of the frost-line border, then all you will see is a rocky surface, because on it's last approach to the sun, the sun vaporized all the surface ice, and the stuff directly below. - IN THE END, that means gradually with each crossing of the frost line and approach to the sun, the comet will loose more and more of its water/ice/volatiles. - So it's hazy comet tail will become less and less spectacular and impressive with each future orbital pass.
passages: Typical comets have much bigger orbits and have a lot of ice and dust that make for their big tails.
   Section::::Physical characteristics.:Nucleus.
  
  The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are popularly described as "dirty snowballs" after Fred Whipple's model. However, many comets have a higher dust content, leading them to be called "icy dirtballs". Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.
   In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's "Giotto" probe and the Soviet Union's "Vega 1" and "Vega 2") that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.
   Measurements carried out by the "Philae" lander on 67P/Churyumov–Gerasimenko comet, indicate that the dust layer could be as much as thick. Beneath that is hard ice, or a mixture of ice and dust. Porosity appears to increase toward the center of the comet. While most scientists thought that all the evidence indicated that the structure of nuclei of comets is processed rubble piles of smaller ice planetesimals of a previous generation, the "Rosetta" mission dispelled the idea that comets are "rubble piles" of disparate material.
  
  Section::::Structure.:Splitting.
   Until 2016, the only known example of a rock comet was 3200 Phaethon, which has a perihelion (closest point to the Sun) closer than any other named asteroid (0.14 AU). It has been known to brighten on occasions, implying outgassing, and has been observed ejecting dust by the STEREO spacecraft. In 2016, observations of 322P/SOHO by the Spitzer Space Telescope showed it to most likely be a rock comet as well, which would make it the lowest-perihelion "asteroid", with a perihelion of 0.054 AU, even though it was initially identified as a comet.
   By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's "Giotto" probe and the Soviet Union's "Vega 1" and "Vega 2") that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.
   Section::::See also.
  
  BULLET::::- Centaur (minor planet)
  
  BULLET::::- Extinct comet
  
  Section::::External links.
  
  BULLET::::- Proper elements of active asteroids at Asteroid Families Portal
  
  BULLET::::- Henry Hsieh's Main-Belt Comets page has extensive details on Main-belt comets
  
  BULLET::::- David Jewitt. Main Belt Comets
  
  BULLET::::- Planetary Society article on MBCs
  
  BULLET::::- Discussion of possible differences in characteristics of the water in MBCs and other comets
  
  BULLET::::- YouTube Interview with David Jewitt (discussion on main-belt comets starts around 9 minutes into video)
  
  BULLET::::- Impact trigger mechanism diagram by David Jewitt
  
  BULLET::::- Comet-like appearance of (596) Scheila
  
  BULLET::::- Project T3: Finding Comets in the Asteroid Population
   Rock comet
  
  A rock comet is a rare type of small Solar System body that exhibits features of both a comet and an asteroid, mainly in that it outgasses material primarily made up of grains of rock.
  
  Rock comets, unlike other comets, which outgas primarily ice, have a nucleus made of rock. As a result, their outgassing can be unpredictable.
   The sky of a comet changes dramatically as it nears the Sun. During perihelion, a comet's ices begin to sublime from its surface, forming tails of gas and dust, and a coma. An observer on a comet nearing the Sun might see the stars slightly obscured by a milky haze, which could create halo effects around the Sun and other bright objects.
  
  Section::::Extrasolar planets.
  
  Section::::Extrasolar planets.:Basics.
   The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0%; by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.
   Cometary nuclei are among the darkest objects known to exist in the Solar System. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.5–3.0% of the light that falls on it; by comparison, fresh asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
   Some main-belt comets display a cometary dust tail only for a part of their orbit near perihelion. This strongly suggests that volatiles at their surfaces are sublimating, driving off the dust. Activity in 133P/Elst–Pizarro is recurrent, having been observed at each of the last three perihelia. The activity persists for a month or several out of each 5-6 year orbit, and is presumably due to ice being uncovered by minor impacts in the last 100 to 1000 years. These impacts are suspected to excavate these subsurface pockets of volatile material helping to expose them to solar radiation.
   BULLET::::- Stardust. Data from the "Stardust" mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been "born in fire", at extremely high temperatures of over . Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk. As a result, comets also contain crystalline grains that formed in the early, hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials". These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.
   Section::::Causes.:Size and activity of the nucleus.
  
  Cometary nuclei vary in size from a few hundreds of metres across or less to many kilometres across. When they approach the Sun, large amounts of gas and dust are ejected by cometary nuclei, due to solar heating. A crucial factor in how bright a comet becomes is how large and how active its nucleus is. After many returns to the inner Solar System, cometary nuclei become depleted in volatile materials and thus are much less bright than comets which are making their first passage through the Solar System.
   In April 2011, scientists from the University of Arizona discovered evidence for the presence of liquid water in comet Wild 2. They have found iron and copper sulfide minerals that must have formed in the presence of water. The discovery shatters the existing paradigm that comets never get warm enough to melt their icy bulk. In the spring of 2014, the recovery of particles of interstellar dust from the Discovery program's Stardust mission was announced.
  
  Section::::Sample return.:Spacecraft location.
   Section::::Comets.
  
  Observational evidence suggest that the cometary nucleus may not be a well-consolidated single body, but may instead be a loosely bound agglomeration of smaller fragments, weakly bonded and subject to occasional or even frequent disruptive events, although the larger cometary fragments are expected to be primordial condensations rather than collisionally derived debris as in the asteroid case. However, "in situ" observations by the "Rosetta" mission, indicates that it may be more complex than that.
  
  Section::::Moons.
   The main-belt comets are unique in that they have flat (within the plane of the planets' orbits), approximately circular (small eccentricity), asteroid-like orbits, and not the elongated, often tilted orbits characteristic of all other comets. Because (118401) LINEAR can generate a coma (produced by vapour boiled off the comet), it must be an icy asteroid. When a typical comet approaches the Sun, its ice heats up and sublimates (changes directly from ice to gas), venting gas and dust into space, creating a tail and giving the object a fuzzy appearance. Far from the Sun, sublimation stops, and the remaining ice stays frozen until the comet's next pass close to the Sun. In contrast, objects in the asteroid belt have essentially circular orbits and are expected to be mostly baked dry of ice by their confinement to the inner Solar System (see extinct comet).
   Section::::Ontario Major Intermediate B.
   Near-Earth comets are much less common than asteroids; , according to statistics maintained by CNEOS, 18,837 NEOs were discovered: 107 near-Earth comets and 18,730 near-Earth asteroids. There are 1,929 NEOs that are classified as potentially hazardous asteroids (PHAs). Comets typical properties are cause for various issues, including that they typically are very dark (in astronomical speak, they have a low albedo) which can hamper taking accurate measurements; P/2016 BA14 reflected approximately 3% of visible light which is darker then fresh asphalt.
   The temperature of the asteroid belt varies with the distance from the Sun. For dust particles within the belt, typical temperatures range from 200 K (−73 °C) at 2.2 AU down to 165 K (−108 °C) at 3.2 AU However, due to rotation, the surface temperature of an asteroid can vary considerably as the sides are alternately exposed to solar radiation and then to the stellar background.
  
  Section::::Characteristics.:Main-belt comets.
   NASA's scientists reported that the rays coming off the rough ends consist of hundreds of tons of fluffy ice and dust chunks – the largest particles are of golf ball to basketball-size – and they are ejected by jets of carbon dioxide. The scientists also said that this is the first time that comet activity powered by sublimation of frozen carbon dioxide is observed as the comet nears the sun; the ice within the comet must be primordial, dating from the beginnings of the solar system.
  
  Section::::See also.
  
  BULLET::::- List of periodic comets
   During deep minima, the carbon-related spectral bands become less prominent, and some emission lines are seen. In typical R CrB variables, the spectra at minimum show many strong emission lines of metals, but in DY Persei only a few are detected. A broad emission line of neutral sodium is seen, together with possible emission from neutral calcium and carbon.
  
  Section::::Properties.
   Results from the "Rosetta" and "Philae" spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on "Rosetta" determined that electrons (within above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.
  
  Comets already visited are:
  
  BULLET::::- Halley's Comet
  
  BULLET::::- Tempel 1 (also hit with impactor)
  
  BULLET::::- 19P/Borrelly
   In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope but these detections have been questioned. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.
   Scientists have long wondered how tiny silicate crystals, which need high temperatures to form, have found their way into frozen comets, born in the very cold environment of the Solar System's outer edges. The crystals would have begun as non-crystallized, amorphous silicate particles, part of the mix of gas and dust from which the Solar System developed. This mystery has deepened with the results of the "Stardust" sample return mission, which captured particles from Comet Wild 2. Many of the Stardust particles were found to have formed at temperatures in excess of 1000 K.
  