Spectra Confirm Ice on Asteroid 24 Themis. How Did it Get There?

Humberto Campins of the University of Central Florida in Orlando and his colleagues recorded spectra of the asteroid 24 Themis over a seven-hour period, corresponding to 84 percent of the rotational period of the spinning rock. The spectra, taken with NASA’s Infrared Telescope Facility on Hawaii’s Mauna Kea, revealed the consistent presence of frozen water as different parts of the asteroid’s surface came into view ...

The finding corroborates earlier observations (SN Online: 7/18/08) of the same asteroid by Andrew S. Rivkin of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., and Joshua Emery of the University of Tennessee in Knoxville, who also used the Infrared Telescope Facility. Over several years, Rivkin and Emery had found evidence of frozen water in single spots on 24 Themis but had not studied the asteroid as it made one entire rotation. Together, the two teams’ findings reveal that the asteroid’s entire surface is coated with frozen water, Campins says.

The analyses of the sunlight reflected off the asteroid also show that organic compounds are widespread on the surface, he added, including polycyclic aromatic hydrocarbons, CH2 and CH3.

All our spectra confirm the shape and depth of the 3.1 micron absorption reported by Rivkin and Emery (2008) and show this absorption is present throughout the asteroid's rotation. The shape and wavelength of this absorption is interpreted by Rivkin and Emery (2008) as due to water ice, as opposed to hydrated minerals originally suggested by Lebofsky et al. (1990). Our spectra fit well the same water ice model used by Rivkin and Emery (2008), and are significantly different from spectra of hydrated silicates found in asteroids and meteorites. As Rivkin and Emery (2008) state, "If confirmed, a finding of water ice on Themis would be the first detection of ice on an asteroidal surface, and the first detection of water per se (as opposed to OH) on an asteroidal surface."

This likely presence of water ice opens up interesting possibilities that could transform current views of some asteroids. For example, since water ice is not stable on the surface of Themis at 3.2 AU over 4.5 Gy, what is its source?

At the asteroid’s average distance from the sun -- 3.2 times Earth’s distance to the sun-- frozen water on the surface would readily vaporize, noted Campins. That means the ice must be continually replenished...

One possibility is that ice lies buried several meters below the surface of the roughly 160-kilometer-wide asteroid and makes its way to the top when the asteroid is pummeled by space debris. Norbert Schörghofer of the University of Hawaii at Manoa proposed last year that ice can persist for billions of years just below the surface of a dusty space rock if the asteroid’s surface temperature is less than about 145 kelvins. The temperature of 24 Themis lies in that range.

The presence of frozen water on 24 Themis also suggests that some asteroids resemble comets, becoming active suddenly and venting material into space when pockets of ice vaporize, Campins noted.

As of 2008, three centaurs have been found to display cometary comas: Chiron, 60558 Echeclus, and 166P/NEAT. Chiron and Echeclus are therefore classified as both asteroids and comets. Other centaurs such as 52872 Okyrhoe are suspected of showing cometary activity. Any centaur that is perturbed close enough to the Sun is expected to become a comet.

...the temperature on the surface of Mercury can range to over 400 degrees Celsius [750 degrees Fahrenheit]. Water can't really sit there. This water is clearly there [in its atmosphere]. The very first time we took a whiff of the planet, it was right there.

...the process of chemical sputtering could create water where none existed before from the ingredients of solar wind and Mercury rock...

"The solar wind is highly ionized. Those are radicals -- they want to make connections with everything that they can. Imagine a solar wind hydrogen showing up and hitting the surface. It weathers whatever the mineral is, and steals an oxygen. If you do that, you get something like OH-, for example." OH-, also known as a hydroxyl group, would produce a peak at atomic mass 17 on the FIPS spectrum. "You can do it in reverse -- an oxygen from the solar wind can steal a hydrogen. The process is called chemical sputtering."

NASA scientists have discovered water molecules in the polar regions of the moon. Instruments aboard three separate spacecraft revealed water molecules in amounts that are greater than predicted, but still relatively small. Hydroxyl, a molecule consisting of one oxygen atom and one hydrogen atom, also was found in the lunar soil.

"The data from Cassini's VIMS instrument and M3 closely agree," said Roger Clark, a U.S. Geological Survey scientist in Denver and member of both the VIMS and M3 teams. "We see both water and hydroxyl. While the abundances are not precisely known, as much as 1,000 water molecule parts-per-million could be in the lunar soil. To put that into perspective, if you harvested one ton of the top layer of the moon's surface, you could get as much as 32 ounces of water."

Data indicate that water exists diffusely across the moon as hydroxyl or water molecules — or both — adhering to the surface in low concentrations. Additionally, there may be a water cycle in which the molecules are broken down and reformulated over a two week cycle, which is the length of a lunar day.

The scientists were looking for a signature of water in the craters near the poles, but found evidence for water instead on the sunlit portions of the moon. This was certainly unexpected and the science team from M3 looked and re-looked at their data for several months.

the findings show that not only is the moon hydrated, the process that makes it so is a dynamic one that is driven by the daily changes in solar radiation hitting any given spot on the surface.

The sun might also have something to do with how the water got there.

The rocks and regolith that make up the lunar surface are about 45 percent oxygen (combined with other elements as mostly silicate minerals). The solar wind — the constant stream of charged particles emitted by the sun — are mostly protons, or positively charged hydrogen atoms.

If the charged hydrogens, which are traveling at one-third the speed of light, hit the lunar surface with enough force, they break apart oxygen bonds in soil materials, Taylor, the M3 team member suspects. Where free oxygen and hydrogen exist, there is a high chance that trace amounts of water will form.

Another option is that an action similar to the recent findings of water on the Moon, where solar wind interacts with a rocky body without an atmosphere to create H2O and OH molecules. Without an atmosphere, the body is exposed to solar wind, which includes hydrogen ions. The hydrogen is able to interact with oxygen in surface of the asteroid to create water molecules.

The flaw in the conventional approach is that only gas-phase chemical reactions and reactions induced by solar radiation (photolysis) are considered. The far more energetic molecular and atomic reactions due to plasma discharge sputtering of an electrically charged comet nucleus are not even contemplated [see below]. Yet this model solves many comet mysteries that are seldom mentioned.

The hydroxyl radical, OH, is the most abundant cometary radical. It is detected in the coma at some distance from the comet nucleus, where it is assumed that water (H2O) is broken down by solar UV radiation to form OH, H and O. It is chiefly the presence of this radical that leads to estimates of the amount of water ice sublimating from the comet nucleus. The comas of O and OH are far less extensive than the H coma but have comparable density.

The negatively charged oxygen atom, or negative oxygen ion, has been detected close to cometary nuclei. And the spectrum of neutral oxygen (O) shows a "forbidden line" indicative of the presence of an "intense" electric field. The discovery at comet Halley of negative ions puzzled investigators because they are easily destroyed by solar radiation. They wrote, "an efficient production mechanism, so far unidentified, is required to account for the observed densities." And the intense electric field near the comet nucleus is inexplicable if it is merely an inert body ploughing through the solar wind.

Let's see how the electrical model of comets explains these mysteries. The electric field near the comet nucleus is expected if a comet is a highly negatively charged body, relative to the solar wind. Cathode sputtering of the comet nucleus will strip atoms and molecules directly from solid rock and charge them negatively. So the presence of negative oxygen and other ions close to the comet nucleus is to be expected. Negative oxygen ions will be accelerated away from the comet in the cathode jets and combine with protons from the solar wind to form the observed OH radical at some distance from the nucleus.

The important point is that the OH does not need to come from water ice on, or in, the comet. Of course, some water is likely to be present on a comet or asteroid. It depends upon their parent body.

Copious X-rays were discovered by accident coming from a comet. No one expected them from an inert body rushing through the solar wind. An ad hoc explanation was devised that required protons from the Sun to combine with electrons from the comet. No one sensed the irony. that moving protons combining with electrons is the defining characteristic of an electric current flowing between the Sun and the comet.

X-rays are generated naturally by high-voltage discharges.

A comet's tail arises from the interaction between the electric charge of the comet and the solar discharge plasma. The comet spends most of its time far from the Sun, where the plasma charge density and voltage with respect to the Sun is low. The comet moves slowly and it easily accumulates enough charge to balance the ambient voltage.

As the comet approaches the Sun, the nucleus moves at a furious speed through regions of increasing charge density and voltage. The comet's surface charge and internal polarization, developed in deep space, respond to the new environment by forming cathode jets and a visible plasma sheath, or coma. The strong electric field in the comet's plasma sheath generates x-rays.

A charged comet nucleus moving through the solar plasma will form a number of plasma sheaths or double layers, where the comet's electric field is concentrated. The Giotto mission to comets Halley and Grigg-Skjellerup were surprised by their sharp plasma boundaries and one "mystery" boundary. There should be no mystery when they are considered as electrical plasma sheaths and not simply mechanical "bow shocks." The so-called "near nucleus ion pile-up" is simply a manifestation of a cometary plasma sheath, or double layer. It is a region with a strong electric field and consequently capable of generating strong x-rays. The cometary x-rays were found coming from a region that didn't conform to simple hydrodynamic collision calculations. It was remarked that it was like finding the shockwave from a supersonic aircraft several kilometres to one side of the aircraft. A plasma sheath is controlled by electromagnetic forces and is not expected to conform to bow-shock physics.