On this day in science history: Mars 5 launched

In 1973, the USSR launched Mars 5, on a Proton SL-12/D-1-e booster. It was one of several Soviet Mars probes - Mars 4, 5, 6, and 7 - launched in Jul-Aug 1973. The Mars 5 mission was to orbit Mars, which was achieved on 12 Feb 1974. Each orbit took about 25 hours. It was designed to return information on the composition, structure, and properties of the martian atmosphere and surface. However, after only 22 orbits, the mission ended prematurely due to loss of pressurization in the transmitter housing. Before the failure, data for a small portion of the martian southern hemisphere was captured with about 60 images forwarded over a nine day period. The probe also sent more measurements made by other instruments.

Mars in natural colour in 2007. By ESA - European Space Agency & Max-Planck Institute for Solar System Research for OSIRIS Team ESA/MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA [CC BY-SA 3.0-igo (http://creativecommons.org/licenses/by-sa/3.0-igo)], via Wikimedia Commons

Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.

The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature. Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Mars trojan.

There are ongoing investigations assessing the past habitability potential of Mars, as well as the possibility of extant life. Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is less than 1% of the Earth's, except at the lowest elevations for short periods. The two polar ice caps appear to be made largely of water. The volume of water ice in the south polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a depth of 11 meters (36 ft). In November 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region of Mars. 

The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior.

Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude reaches −2.91, which is surpassed only by Jupiter, Venus, the Moon, and the Sun. Optical ground-based telescopes are typically limited to resolving features about 300 kilometers (190 mi) across when Earth and Mars are closest because of Earth's atmosphere.

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Tipping points are real: Gradual changes in CO2 levels can induce abrupt climate changes

During the last glacial period, within only a few decades the influence of atmospheric CO2 on the North Atlantic circulation resulted in temperature increases of up to 10 degrees Celsius in Greenland - as indicated by new climate calculations from researchers at the Alfred Wegener Institute and the University of Cardiff. Their study is the first to confirm that there have been situations in our planet's history in which gradually rising CO2 concentrations have set off abrupt changes in ocean circulation and climate at "tipping points." These sudden changes, referred to as Dansgaard-Oeschger events, have been observed in ice cores collected in Greenland. The results of the study have just been released in the journal Nature Geoscience.

Ice core sample taken from drill. Photo by Lonnie Thompson, Byrd Polar Research Center, Ohio State University. [Public domain], via Wikimedia Commons

Previous glacial periods were characterised by several abrupt climate changes in the high latitudes of the Northern Hemisphere. However, the cause of these past phenomena remains unclear. In an attempt to better grasp the role of CO2 in this context, scientists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) recently conducted a series of experiments using a coupled atmosphere-ocean-sea ice model.

First author Xu Zhang explains: "With this study, we've managed to show for the first time how gradual increases of CO2 triggered rapid warming." This temperature rise is the result of interactions between ocean currents and the atmosphere, which the scientists used the climate model to explore. According to their findings, the increased CO2 intensifies the trade winds over Central America, as the eastern Pacific is warmed more than the western Atlantic. This is turn produces increased moisture transport from the Atlantic, and with it, an increase in the salinity and density of the surface water. Finally, these changes lead to an abrupt amplification of the large-scale overturning circulation in the Atlantic. "Our simulations indicate that even small changes in the CO2 concentration suffice to change the circulation pattern, which can end in sudden temperature increases," says Zhang.

Further, the study's authors reveal that rising CO2 levels are the dominant cause of changed ocean currents during the transitions between glacial and interglacial periods. As climate researcher Gerrit Lohmann explains, "We can't say for certain whether rising CO2 levels will produce similar effects in the future, because the framework conditions today differ from those in a glacial period. That being said, we've now confirmed that there have definitely been abrupt climate changes in the Earth's past that were the result of continually rising CO2 concentrations."

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https://www.sciencedaily.com/releases/2017/06/170623100414.htm

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On this day in science history: Pioneer 10 crossed the orbit of Pluto

In 1983, Pioneer 10, an American space probe, crossed the orbit of Pluto, the outermost planet, to continue its voyage into the universe beyond our solar system. This space exploration project was conducted by the NASA Ames Research Center in California, and the space probe was manufactured by TRW Inc.

Pioneer 10 was launched on March 2, 1972, by an Atlas-Centaur expendable vehicle from Cape Canaveral, Florida. Between July 15, 1972, and February 15, 1973, it became the first spacecraft to traverse the asteroid belt. Photography of Jupiter began on November 6, 1973, at a range of 25,000,000 kilometres (16,000,000 mi), and a total of about 500 images were transmitted. The closest approach to the planet was on December 4, 1973, at a range of 132,252 kilometres (82,178 mi). During the mission, the on-board instruments were used to study the asteroid belt, the environment around Jupiter, the solar wind, cosmic rays, and eventually the far reaches of the Solar System and heliosphere.

Artist's impression of Pioneer 10's flyby of Jupiter, by Rick Guidice [Public domain], via Wikimedia Commons

So, what do we know about Jupiter?

Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a giant planet with a mass one-thousandth that of the Sun, but two and a half times that of all the other planets in the Solar System combined. Jupiter and Saturn are gas giants; the other two giant planets, Uranus and Neptune are ice giants. Jupiter has been known to astronomers since antiquity. The Romans named it after their god Jupiter. When viewed from Earth, Jupiter can reach an apparent magnitude of −2.94, bright enough for its reflected light to cast shadows, and making it on average the third-brightest object in the night sky after the Moon and Venus.

Jupiter is primarily composed of hydrogen with a quarter of its mass being helium, though helium comprises only about a tenth of the number of molecules. It may also have a rocky core of heavier elements, but like the other giant planets, Jupiter lacks a well-defined solid surface. Because of its rapid rotation, the planet's shape is that of an oblate spheroid (it has a slight but noticeable bulge around the equator). The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding Jupiter is a faint planetary ring system and a powerful magnetosphere. Jupiter has at least 67 moons, including the four large Galilean moons discovered by Galileo Galilei in 1610. Ganymede, the largest of these, has a diameter greater than that of the planet Mercury.

Radio communications were lost with Pioneer 10 on January 23, 2003, because of the loss of electric power for its radio transmitter, with the probe at a distance of 12 billion kilometers (80 AU) from Earth.

Jupiter has been explored on several other occasions by robotic spacecraft, such as the Voyager flyby missions and later, the Galileo orbiter. In late February 2007, Jupiter was visited by the New Horizons probe, which used Jupiter's gravity to increase its speed and bend its trajectory en route to Pluto. The latest probe to visit the planet is Juno, which entered into orbit around Jupiter on July 4, 2016. Future targets for exploration in the Jupiter system include the probable ice-covered liquid ocean of its moon Europa.

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https://en.wikipedia.org/wiki/Pioneer_10

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Dwarf star 200 light years away contains life's building blocks

Many scientists believe the Earth was dry when it first formed, and that the building blocks for life on our planet - carbon, nitrogen and water - appeared only later as a result of collisions with other objects in our solar system that had those elements.

Today, a UCLA-led team of scientists reports that it has discovered the existence of a white dwarf star whose atmosphere is rich in carbon and nitrogen, as well as in oxygen and hydrogen, the components of water. The white dwarf is approximately 200 light years from Earth and is located in the constellation Boötes.

The Earth seen from Apollo 17. By NASA/Apollo 17 crew; taken by either Harrison Schmitt or Ron Evans [Public domain or Public domain], via Wikimedia Commons

Benjamin Zuckerman, a co-author of the research and a UCLA professor of astronomy, said the study presents evidence that the planetary system associated with the white dwarf contains materials that are the basic building blocks for life. And although the study focused on this particular star - known as WD 1425+540 - the fact that its planetary system shares characteristics with our solar system strongly suggests that other planetary systems would also.

"The findings indicate that some of life's important preconditions are common in the universe," Zuckerman said.

The scientists report that a minor planet in the planetary system was orbiting around the white dwarf, and its trajectory was somehow altered, perhaps by the gravitational pull of a planet in the same system. That change caused the minor planet to travel very close to the white dwarf, where the star's strong gravitational field ripped the minor planet apart into gas and dust. Those remnants went into orbit around the white dwarf - much like the rings around Saturn, Zuckerman said - before eventually spiraling onto the star itself, bringing with them the building blocks for life.

The researchers think these events occurred relatively recently, perhaps in the past 100,000 years or so, said Edward Young, another co-author of the study and a UCLA professor of geochemistry and cosmochemistry. They estimate that approximately 30 percent of the minor planet's mass was water and other ices, and approximately 70 percent was rocky material.

The research suggests that the minor planet is the first of what are likely many such analogs to objects in our solar system's Kuiper belt. The Kuiper belt is an enormous cluster of small bodies like comets and minor planets located in the outer reaches of our solar system, beyond Neptune. Astronomers have long wondered whether other planetary systems have bodies with properties similar to those in the Kuiper belt, and the new study appears to confirm for the first time that one such body exists.

White dwarf stars are dense, burned-out remnants of normal stars. Their strong gravitational pull causes elements like carbon, oxygen and nitrogen to sink out of their atmospheres and into their interiors, where they cannot be detected by telescopes.

The research, published in the Astrophysical Journal Letters, describes how WD 1425+540 came to obtain carbon, nitrogen, oxygen and hydrogen. This is the first time a white dwarf with nitrogen has been discovered, and one of only a few known examples of white dwarfs that have been impacted by a rocky body that was rich in water ice.

"If there is water in Kuiper belt-like objects around other stars, as there now appears to be, then when rocky planets form they need not contain life's ingredients," said Siyi Xu, the study's lead author, a postdoctoral scholar at the European Southern Observatory in Germany who earned her doctorate at UCLA.

"Now we're seeing in a planetary system outside our solar system that there are minor planets where water, nitrogen and carbon are present in abundance, as in our solar system's Kuiper belt," Xu said. "If Earth obtained its water, nitrogen and carbon from the impact of such objects, then rocky planets in other planetary systems could also obtain their water, nitrogen and carbon this way."

A rocky planet that forms relatively close to its star would likely be dry, Young said.

"We would like to know whether in other planetary systems Kuiper belts exist with large quantities of water that could be added to otherwise dry planets," he said. "Our research suggests this is likely."

According to Zuckerman, the study doesn't settle the question of whether life in the universe is common.

"First you need an Earth-like world in its size, mass and at the proper distance from a star like our sun," he said, adding that astronomers still haven't found a planet that matches those criteria.

The researchers observed WD 1425+540 with the Keck Telescope in 2008 and 2014, and with the Hubble Space Telescope in 2014. They analyzed the chemical composition of its atmosphere using an instrument called a spectrometer, which breaks light into wavelengths. Spectrometers can be tuned to the wavelengths at which scientists know a given element emits and absorbs light; scientists can then determine the element's presence by whether it emits or absorbs light of certain characteristic wavelengths. In the new study, the researchers saw the elements in the white dwarf's atmosphere because they absorbed some of the background light from the white dwarf.

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https://www.sciencedaily.com/releases/2017/02/170209163852.htm

http://www.prlabs.co.uk/lab-supplies.php?N=carbon-capsule&Id=44459

https://en.wikipedia.org/wiki/Earth

How the Antarctic Ice Sheet is affecting climate change

Scientists have known for decades that small changes in climate can have significant impacts on the massive Antarctic Ice Sheet.

Now a new study suggests the opposite also is true. An international team of researchers has concluded that the Antarctic Ice Sheet actually plays a major role in regional and global climate variability - a discovery that may also help explain why sea ice in the Southern Hemisphere has been increasing despite the warming of the rest of the Earth.

Results of the study are being published this week in the journal Nature.

View of the Riiser-Larsen Ice Shelf in Antarctica. By Ben Holt  (NASA), via Wikimedia Commons

Global climate models that look at the last several thousand years have failed to account for the amount of climate variability captured in the paleoclimate record, according to lead author Pepijn Bakker, a former post-doctoral researcher at Oregon State University now with the MARUM Center for Marine Environmental Studies at the University of Bremen in Germany.

The research team's hypothesis was that climate modelers were overlooking one crucial element in the overall climate system - an aspect of the ocean, atmosphere, biosphere or ice sheets - that might affect all parts of the system.

"One thing we determined right off the bat was that virtually all of the climate models had the Antarctic Ice Sheet as a constant entity," Bakker said. "It was a static blob of ice, just sitting there. What we discovered, however, is that the ice sheet has undergone numerous pulses of variability that have had a cascading effect on the entire climate system."

The Antarctic Ice Sheet, in fact, has demonstrated dynamic behavior over the past 8,000 years, according to Andreas Schmittner, a climate scientist in Oregon State's College of Earth, Ocean, and Atmospheric Sciences and co-author on the study.

"There is a natural variability in the deeper part of the ocean adjacent to the Antarctic Ice Sheet - similar to the Pacific Decadal Oscillation, or El Niño/La Niña but on a time scale of centuries - that causes small but significant changes in temperatures," Schmittner said. "When the ocean temperatures warm, it causes more direct melting of the ice sheet below the surface, and it increases the number of icebergs that calve off the ice sheet."

Those two factors combine to provide an influx of fresh water into the Southern Ocean during these warm regimes, according to Peter Clark, a paleoclimatologist in OSU's College of Earth, Ocean, and Atmospheric Sciences and co-author on the study.

"The introduction of that cold, fresh water lessens the salinity and cools the surface temperatures, at the same time, stratifying the layers of water," Clark said. "The cold, fresh water freezes more easily, creating additional sea ice despite warmer temperatures that are down hundreds of meters below the surface."

The discovery may help explain why sea ice has expanded in the Southern Ocean despite global warming, the researchers say. The same phenomenon doesn't occur in the Northern Hemisphere with the Greenland Ice Sheet because it is more landlocked and not subject to the same current shifts that affect the Antarctic Ice Sheet.

"One message that comes out of this study is that the Antarctic Ice Sheet is very sensitive to small changes in ocean temperatures, and humans are making the Earth a lot warmer than it has been," Bakker said.

Sediment cores from the sea floor around Antarctica contain sand grains delivered there by icebergs calving off the ice sheet. The researchers analyzed sediments from the last 8,000 years, which showed evidence that many more icebergs calved off the ice sheet in some centuries than in others. Using sophisticated computer modeling, the researchers traced the variability in iceberg calving to small changes in ocean temperatures.

The Antarctic Ice Sheet covers an area of more than 5 million square miles and is estimated to hold some 60 percent of all the fresh water on Earth. The east part of the ice sheet rests on a major land mass, but in West Antarctica, the ice sheet rests on bedrock that extends into the ocean at depths of more than 2,500 meters, or more than 8,000 feet, making it vulnerable to disintegration.

Scientists estimate that if the entire Antarctic Ice Sheet were to melt, global sea levels would rise some 200 feet.

Other authors on the study include Nicholas Golledge of Victoria University of Wellington in New Zealand and Michael Weber of the University of Bonn in Germany.

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Hubble spots possible water plumes erupting on Jupiter's moon Europa

Astronomers using NASA's Hubble Space Telescope have imaged what may be water vapor plumes erupting off the surface of Jupiter's moon Europa. This finding bolsters other Hubble observations suggesting the icy moon erupts with high altitude water vapor plumes.

The observation increases the possibility that missions to Europa may be able to sample Europa's ocean without having to drill through miles of ice.

"Europa's ocean is considered to be one of the most promising places that could potentially harbor life in the solar system," said Geoff Yoder, acting associate administrator for NASA's Science Mission Directorate in Washington. "These plumes, if they do indeed exist, may provide another way to sample Europa's subsurface."

Jupiter. By NASA, ESA, and A. Simon (Goddard Space Flight Center) [Public domain], via Wikimedia Commons

The plumes are estimated to rise about 125 miles (200 kilometers) before, presumably, raining material back down onto Europa's surface. Europa has a huge global ocean containing twice as much water as Earth's oceans, but it is protected by a layer of extremely cold and hard ice of unknown thickness. The plumes provide a tantalizing opportunity to gather samples originating from under the surface without having to land or drill through the ice.

The team, led by William Sparks of the Space Telescope Science Institute (STScI) in Baltimore observed these finger-like projections while viewing Europa's limb as the moon passed in front of Jupiter.

The original goal of the team's observing proposal was to determine whether Europa has a thin, extended atmosphere, or exosphere. Using the same observing method that detects atmospheres around planets orbiting other stars, the team realized if there was water vapor venting from Europa's surface, this observation would be an excellent way to see it.

"The atmosphere of an extrasolar planet blocks some of the starlight that is behind it," Sparks explained. "If there is a thin atmosphere around Europa, it has the potential to block some of the light of Jupiter, and we could see it as a silhouette. And so we were looking for absorption features around the limb of Europa as it transited the smooth face of Jupiter."

In 10 separate occurrences spanning 15 months, the team observed Europa passing in front of Jupiter. They saw what could be plumes erupting on three of these occasions.

This work provides supporting evidence for water plumes on Europa. In 2012, a team led by Lorenz Roth of the Southwest Research Institute in San Antonio, detected evidence of water vapor erupting from the frigid south polar region of Europa and reaching more than 100 miles (160 kilometers) into space. Although both teams used Hubble's Space Telescope Imaging Spectrograph instrument, each used a totally independent method to arrive at the same conclusion.

"When we calculate in a completely different way the amount of material that would be needed to create these absorption features, it's pretty similar to what Roth and his team found," Sparks said. "The estimates for the mass are similar, the estimates for the height of the plumes are similar. The latitude of two of the plume candidates we see corresponds to their earlier work."

But as of yet, the two teams have not simultaneously detected the plumes using their independent techniques. Observations thus far have suggested the plumes could be highly variable, meaning that they may sporadically erupt for some time and then die down. For example, observations by Roth's team within a week of one of the detections by Sparks' team failed to detect any plumes.

If confirmed, Europa would be the second moon in the solar system known to have water vapor plumes. In 2005, NASA's Cassini orbiter detected jets of water vapor and dust spewing off the surface of Saturn's moon Enceladus.

Scientists may use the infrared vision of NASA's James Webb Space Telescope, which is scheduled to launch in 2018, to confirm venting or plume activity on Europa. NASA also is formulating a mission to Europa with a payload that could confirm the presence of plumes and study them from close range during multiple flybys.

"Hubble's unique capabilities enabled it to capture these plumes, once again demonstrating Hubble's ability to make observations it was never designed to make," said Paul Hertz, director of the Astrophysics Division at NASA Headquarters in Washington. "This observation opens up a world of possibilities, and we look forward to future missions - such as the James Webb Space Telescope - to follow up on this exciting discovery."

The work by Sparks and his colleagues will be published in the Sept. 29 issue of The Astrophysical Journal.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (the European Space Agency.) NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. STScI, which is operated for NASA by the Association of Universities for Research in Astronomy in Washington, conducts Hubble science operations.

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The chemistry of ice cream

Who doesn’t love ice cream? Especially when the sun is shining! But what is the science that lies behind ice cream making? Have a look at this graphic which takes a look at some of the ingredients that go into ice cream, and the important role they play in creating the finished product.

Graphic: Compound Interest 

Before the development of modern refrigeration, ice cream was a luxury reserved for special occasions. Making it was quite laborious; ice was cut from lakes and ponds during the winter and stored in holes in the ground, or in wood-frame or brick ice houses, insulated by straw. Many farmers and plantation owners, including U.S. Presidents George Washington and Thomas Jefferson, cut and stored ice in the winter for use in the summer. Frederic Tudor of Boston turned ice harvesting and shipping into a big business, cutting ice in New England and shipping it around the world.

Ice cream was made by hand in a large bowl placed inside a tub filled with ice and salt. This was called the pot-freezer method. French confectioners refined the pot-freezer method, making ice cream in a sorbetière (a covered pail with a handle attached to the lid). In the pot-freezer method, the temperature of the ingredients is reduced by the mixture of crushed ice and salt. The salt water is cooled by the ice, and the action of the salt on the ice causes it to (partially) melt, absorbing latent heat and bringing the mixture below the freezing point of pure water. The immersed container can also make better thermal contact with the salty water and ice mixture than it could with ice alone.

The hand-cranked churn, which also uses ice and salt for cooling, replaced the pot-freezer method. The exact origin of the hand-cranked freezer is unknown, but the first U.S. patent for one was #3254 issued to Nancy Johnson on 9 September 1843. The hand-cranked churn produced smoother ice cream than the pot freezer and did it quicker. Many inventors patented improvements on Johnson's design.

In Europe and early America, ice cream was made and sold by small businesses, mostly confectioners and caterers. Jacob Fussell of Baltimore, Maryland was the first to manufacture ice cream on a large scale. Fussell bought fresh dairy products from farmers in York County, Pennsylvania, and sold them in Baltimore. An unstable demand for his dairy products often left him with a surplus of cream, which he made into ice cream. He built his first ice cream factory in Seven Valleys, Pennsylvania, in 1851. Two years later, he moved his factory to Baltimore. Later, he opened factories in several other cities and taught the business to others, who operated their own plants. Mass production reduced the cost of ice cream and added to its popularity.

The development of industrial refrigeration by German engineer Carl von Linde during the 1870s eliminated the need to cut and store natural ice, and, when the continuous-process freezer was perfected in 1926, commercial mass production of ice cream and the birth of the modern ice cream industry was underway.

In modern times, a common method for producing ice cream at home is to use an ice cream maker, an electrical device that churns the ice cream mixture while cooled inside a household freezer. Some more expensive models have an inbuilt freezing element. A newer method is to add liquid nitrogen to the mixture while stirring it using a spoon or spatula for a few seconds; a similar technique, advocated by Heston Blumenthal as ideal for home cooks, is to add dry ice to the mixture while stirring for a few minutes. Some ice cream recipes call for making a custard, folding in whipped cream, and immediately freezing the mixture. Another method is to use a pre-frozen solution of salt and water, which gradually melts as the ice cream freezes.

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http://www.compoundchem.com/2015/07/14/ice-cream/

https://en.wikipedia.org/wiki/Ice_cream

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Why does salt melt ice?

­If you live in a place that has lots of snow and ice in the winter, then you have probably seen the highway department spreading salt on the road to melt the ice.­ You may have also used salt on ice when making home-made ice cream. Salt lowers the freezing/melting point of water, so in both cases the idea is to take advantage of the lower melting point.

 Ice forms when the­ temperature of water reaches 32 degrees Fahrenheit (0 degrees Celsius). When you add salt, that temperature drops: A 10-percent salt solution freezes at 20 F (-6 C), and a 20-percent solution freezes at 2 F (-16 C).

On a roadway, this means that if you sprinkle salt on the ice, you can melt it. The salt dissolves into the liquid water in the ice and lowers its freezing point.

If you ever watch salt melting ice, you can see the dissolving process happen -- the ice immediately around the grain of salt melts, and the melting spreads out from that point. If the temperature of the roadway is lower than 15 F or so, then the salt really won't have any effect -- the solid salt cannot get into the structure of the solid water to start the dissolving process. In that case, spreading sand over the top of the ice to provide traction is a better option.

When you are making ice cream, the temperature around the ice cream mixture needs to be lower than 32 F if you want the mixture to freeze. Salt mixed with ice creates a brine that has a temperature lower than 32 F. When you add salt to the ice water, you lower the melting temperature of the ice down to 0 F or so. The brine is so cold that it easily freezes the ice cream mixture.

Two things happen when ice and water are placed in contact:

• Molecules on the surface of the ice escape into the water (melting), and
• molecules of water are captured on the surface of the ice (freezing).

When the rate of freezing is the same as the rate of melting, the amount of ice and the amount of water won't change on average (although there are short-term fluctuations at the surface of the ice). The ice and water are said to be in dynamic equilibrium with each other. The balance between freezing and melting can be maintained at 0°C, the melting point of water, unless conditions change in a way that favours one of the processes over the other.

The balance between freezing and melting processes can easily be upset. If the ice/water mixture is cooled, the molecules move slower. The slower-moving molecules are more easily captured by the ice, and freezing occurs at a greater rate than melting.

Conversely, heating the mixture makes the molecules move faster on average, and melting is favoured.  Adding salt to the system will also disrupt the equilibrium. Consider replacing some of the water molecules with molecules of some other substance. The foreign molecules dissolve in the water, but do not pack easily into the array of molecules in the solid. This leads to fewer water molecules on the liquid side because the some of the water has been replaced by salt. The total number of waters captured by the ice per second goes down, so the rate of freezing goes down. The rate of melting is unchanged by the presence of the foreign material, so melting occurs faster than freezing. That's why salt melts ice.

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http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml