Ganymede and Callisto are similar in size and are made of a similar mixture of ice and rock, but data from the Galileo and Voyager spacecraft show that they look different at the surface and on the inside. Just like Earth and Venus, Ganymede and Callisto are twins, and understanding how they were born the same and grew up to be so different is of tremendous interest to planetary scientists.

Ganymede and Callisto’s evolutionary paths diverged about 3.8 billion years ago during the Late Heavy Bombardment, the phase in lunar history dominated by large impact events. Impacts during this period melted Ganymede so thoroughly and deeply that the heat could not be quickly removed. All of Ganymede’s rock sank to its center the same way that all the chocolate chips sink to the bottom of a melted carton of ice cream. Callisto received fewer impacts at lower velocities and avoided complete melting. Ganymede is closer to Jupiter and therefore is hit by twice as many icy impactors as Callisto, and the impactors hitting Ganymede have a higher average velocity.

Image Credit: NOAA/GSD

(Source: swri.org)

Fire and Ice

Saturn’s largest and second largest moons, Titan and Rhea, appear to be stacked on top of each other in this true-color scene from NASA’s Cassini spacecraft.

Titan is likely differentiated into several layers with a 3,400-kilometre (2,100 mi) rocky center surrounded by several layers composed of different crystal forms of ice.Its interior may still be hot and there may be a liquid layer consisting of a “magma" composed of water and ammonia between the ice Ih crust and deeper ice layers made of high-pressure forms of ice.

Rhea is an ice-cold body of weak density (1.236 g/cm3), indicating that the moon consists of a rocky nucleus counting only for a third of the mass of Rhea, the rest being mainly some ice-cold water.

Credit: NASA/JPL-Caltech/SSI

Rhea: Saturn’s Mysterious Moon

Rhea, the second largest moon of Saturn, is a dirty snowball of rock and ice. The only moon with an oxygen atmosphere, thin though it may be, Rhea is one of the most heavily cratered satellites in the solar system.

A very faint oxygen atmosphere exists around Rhea, the first direct evidence of an oxygen atmosphere on a body other than Earth. The atmosphere is thin, with oxygen measuring about 5 trillion times less dense than that found on Earth. Oxygen could be released as the surface is irradiated by ions from Saturn’s magnetosphere. The source of the carbon dioxide is less clear, but could be the result of similar irradiation, or from dry ice much like comets.

On March 6, 2008, NASA announced that Rhea may have a tenuous ring system. This would mark the first discovery of rings about a moon. The rings’ existence was inferred by observed changes in the flow of electrons trapped by Saturn’s magnetic field as Cassini passed by Rhea. Dust and debris could extend out to Rhea’s Hill sphere, but were thought to be denser nearer the moon, with three narrow rings of higher density. The case for a ring was strengthened by the subsequent finding of the presence of a set of small ultraviolet-bright spots distributed along Rhea’s equator (interpreted as the impact points of deorbiting ring material).However, when Cassini made targeted observations of the putative ring plane from several angles, no evidence of ring material was found, but there’s still something around Rhea that is causing a strange, symmetrical structure in the charged-particle environment around Saturn’s second-largest moon.

Image credit: NASA/JPL-Caltech/SSI,Gordan Ugarkovic

Enceladus and Saturn

Cassini Narrow Angle camera (NAC) clear filter frame colorized to approximate the appearance of Saturn’s limb and emphasize Enceladus’ grayish color in contrast. 

Image credit: Gordan Ugarkovic

Enceladus and Saturn

Cassini Narrow Angle camera (NAC) clear filter frame colorized to approximate the appearance of Saturn’s limb and emphasize Enceladus’ grayish color in contrast.

Image credit: Gordan Ugarkovic

A simulation of Io transiting Jupiter as seen from the Earth. Io’s shadow is seen on the surface of Jupiter, leading Io slightly due to the sun and Earth not being in the same line.

A simulation of Io transiting Jupiter as seen from the Earth. Io’s shadow is seen on the surface of Jupiter, leading Io slightly due to the sun and Earth not being in the same line.

Fire and Ice

Saturn’s largest and second largest moons, Titan and Rhea, appear to be stacked on top of each other in this true-color scene from NASA’s Cassini spacecraft.

Titan is likely differentiated into several layers with a 3,400-kilometre (2,100 mi) rocky center surrounded by several layers composed of different crystal forms of ice.Its interior may still be hot and there may be a liquid layer consisting of a “magma" composed of water and ammonia between the ice Ih crust and deeper ice layers made of high-pressure forms of ice.

Rhea is an ice-cold body of weak density (1.236 g/cm3), indicating that the moon consists of a rocky nucleus counting only for a third of the mass of Rhea, the rest being mainly some ice-cold water.

Credit: NASA/JPL-Caltech/SSI

A simulation of what the Apollo 8 crew saw as the Earth rose above the lunar horizon during their fourth orbit around the Moon.

A simulation of what the Apollo 8 crew saw as the Earth rose above the lunar horizon during their fourth orbit around the Moon.

HDR Lunar Corona

A corona is a diffraction pattern in the sky produced by light interacting with small water droplets in clouds. It has a huge range of brightness. Most scattered light goes into the bright central aureole but even that is dim compared with the moon itself. The first ring outside the aureole has only 1.75% of the forward-scattered light intensity. The second ring is 4X fainter still at 0.42% and so on. Our eyes cannot cope with the intensity range and we usually see no more than 2-3 rings. Cameras are worse and cannot capture the rings without grossly over-exposing the moon and central portions. Sometimes a fortuitous cloud does allow the lunar details to be preserved while capturing the outer coronal delicacy. In the clear air of the 2,070m high observatory, four rings were seen visually and even more are on the image.  High Dynamic Range (HDR) techniques were used to render the brightness range. Individual images were made with a 50mm lens on a Canon EOS 20D using exposures of 1/1000, ½, 1, 2.5, 5 and 10s. They were combined in PhotoShop CS (File > Automate > Merge to HDR) followed by levels and curves adjustment.

Image Credit: Boris Dmitriev & Stanislav Korotkiy, Special Astrophysical Observatory

HDR Lunar Corona

A corona is a diffraction pattern in the sky produced by light interacting with small water droplets in clouds. It has a huge range of brightness. Most scattered light goes into the bright central aureole but even that is dim compared with the moon itself. The first ring outside the aureole has only 1.75% of the forward-scattered light intensity. The second ring is 4X fainter still at 0.42% and so on.

Our eyes cannot cope with the intensity range and we usually see no more than 2-3 rings. Cameras are worse and cannot capture the rings without grossly over-exposing the moon and central portions. Sometimes a fortuitous cloud does allow the lunar details to be preserved while capturing the outer coronal delicacy.

In the clear air of the 2,070m high observatory, four rings were seen visually and even more are on the image.  High Dynamic Range (HDR) techniques were used to render the brightness range. Individual images were made with a 50mm lens on a Canon EOS 20D using exposures of 1/1000, ½, 1, 2.5, 5 and 10s. They were combined in PhotoShop CS (File > Automate > Merge to HDR) followed by levels and curves adjustment.

Image Credit: Boris Dmitriev & Stanislav Korotkiy, Special Astrophysical Observatory

Earthshine

Earthshine is sunlight that has reflected from the Earth onto the dark side of the Moon and back again to Earth. Here ‘‘dark side’’ refers to that portion of the lunar surface that, at any instant, faces the Earth but does not face the Sun. Earthshine can be best seen during the crescent phases (the 1-5 day period before or after a New Moon). During this time the sun is mostly behind the moon from our perspective and bathing the Earth in a lot of direct light that is reflected onto the shadowed parts of the moon.

Image credit: Csere Mihály

Earthshine

Earthshine is sunlight that has reflected from the Earth onto the dark side of the Moon and back again to Earth. Here ‘‘dark side’’ refers to that portion of the lunar surface that, at any instant, faces the Earth but does not face the Sun. Earthshine can be best seen during the crescent phases (the 1-5 day period before or after a New Moon). During this time the sun is mostly behind the moon from our perspective and bathing the Earth in a lot of direct light that is reflected onto the shadowed parts of the moon.

Image credit: Csere Mihály


Near-infrared color composite image of Saturn’s largest moon Titan taken with the Keck II adaptive optics system. Titan’s surface appears red, while haze layers at progressively higher altitudes in the atmosphere appear green and blue. 

Image Credit: W. M. Keck Observatory/SRI/New Mexico State University

Near-infrared color composite image of Saturn’s largest moon Titan taken with the Keck II adaptive optics system. Titan’s surface appears red, while haze layers at progressively higher altitudes in the atmosphere appear green and blue. 

Image Credit: W. M. Keck Observatory/SRI/New Mexico State University

(Source: keckobservatory.org)

Apollo 17 Site - 2D Zoom-In

Starting from a full moon image made from Apollo imagery then a zoom diving deep into the Taurus-Littrow Valley the landing site of Apollo 17. Pushing deeper into the animation reveals the Hubble Advanced Camera for Surveys data of the Apollo 17 Camelot crater.


Credit: NASA, ESA and G. Bacon (STScI)

Apollo 17 Site - 2D Zoom-In

Starting from a full moon image made from Apollo imagery then a zoom diving deep into the Taurus-Littrow Valley the landing site of Apollo 17. Pushing deeper into the animation reveals the Hubble Advanced Camera for Surveys data of the Apollo 17 Camelot crater.

Credit: NASA, ESA and G. Bacon (STScI)

Galileo view of an Earth-Moon conjunction

As Galileo receded from its second flyby of Earth on December 16 and 17, 1992, it captured this sequence of Earth rotating as the Moon zipped by on its orbit. There are 56 frames in total, each separated by 15 minutes, spanning about 14 hours.

Credit: NASA / JPL / Doug Ellison

Galileo view of an Earth-Moon conjunction

As Galileo receded from its second flyby of Earth on December 16 and 17, 1992, it captured this sequence of Earth rotating as the Moon zipped by on its orbit. There are 56 frames in total, each separated by 15 minutes, spanning about 14 hours.

Credit: NASA / JPL / Doug Ellison

How Many Mega Impacts Have Whacked Earth?

If you enjoy leaves turning, pumpkin patches and other rites of autumn, thank the giant impact event early in Earth’s history which knocked our planet’s axis off kilter, creating the seasons we that know and love today. But was there just one planet-tilting impact?
University of Western Ontario geologist Grant Young thinks there were two, separated by billions of years. He has been studying rocks from the Ediacaran Period, 540 to 635 million years ago and says he sees signs of massive changes to Earth’s seasons and climate that can best be explained by a axis-shifting collision of a small planetary body into the ocean about 570 million years ago.
That’s long after the famous smash up with a Mars-sized body that is credited with creating the Moon around 4 billion years ago and giving Earth its mild tilt and modern seasons. It was also at the time Earth was seeing the earliest animals, or metazoans, come into being.
The scenario Young has presented in the October issue of GSA Today is that there was first the collision that created the Moon. But instead of that giving Earth its current tilt (which wobbles a bit, and is currently at 23.5 degrees), that first event knocked the planet over almost on its side. That orientation would give the poles a temperate climate without nights for half the year and the equator much less sunshine all year round.
Full Article


Credit: Larry O’Hanlon

How Many Mega Impacts Have Whacked Earth?

If you enjoy leaves turning, pumpkin patches and other rites of autumn, thank the giant impact event early in Earth’s history which knocked our planet’s axis off kilter, creating the seasons we that know and love today. But was there just one planet-tilting impact?

University of Western Ontario geologist Grant Young thinks there were two, separated by billions of years. He has been studying rocks from the Ediacaran Period, 540 to 635 million years ago and says he sees signs of massive changes to Earth’s seasons and climate that can best be explained by a axis-shifting collision of a small planetary body into the ocean about 570 million years ago.

That’s long after the famous smash up with a Mars-sized body that is credited with creating the Moon around 4 billion years ago and giving Earth its mild tilt and modern seasons. It was also at the time Earth was seeing the earliest animals, or metazoans, come into being.

The scenario Young has presented in the October issue of GSA Today is that there was first the collision that created the Moon. But instead of that giving Earth its current tilt (which wobbles a bit, and is currently at 23.5 degrees), that first event knocked the planet over almost on its side. That orientation would give the poles a temperate climate without nights for half the year and the equator much less sunshine all year round.

Full Article

Credit:

The Moon from Zond 8

Which moon is this? Earth’s. Our Moon’s unfamiliar appearance is due partly to an unfamiliar viewing angle as captured by a little-known spacecraft — the Soviet Union’s Zond 8 that circled the Moon in October of 1970. Pictured above, the dark-centered circular feature that stands out near the top of the image is Mare Orientale, a massive impact basin formed by an ancient collision with anasteroid. Mare Orientale is surrounded by light colored and highly textured highlands. Across the image bottom lies the dark and expansive Oceanus Procellarum, the largest of the dark (but dry) maria that dominate the side of the Moon that always facestoward the Earth. Originally designed to carry humans, robotic Zond 8 came within 1000 km of the lunar surface, took about 100 detailed photographs on film, and returned them safely to Earth within a week.

Credit: Galspace

The Moon from Zond 8

Which moon is this? Earth’s. Our Moon’s unfamiliar appearance is due partly to an unfamiliar viewing angle as captured by a little-known spacecraft — the Soviet Union’s Zond 8 that circled the Moon in October of 1970. Pictured above, the dark-centered circular feature that stands out near the top of the image is Mare Orientale, a massive impact basin formed by an ancient collision with anasteroidMare Orientale is surrounded by light colored and highly textured highlands. Across the image bottom lies the dark and expansive Oceanus Procellarum, the largest of the dark (but dry) maria that dominate the side of the Moon that always facestoward the Earth. Originally designed to carry humans, robotic Zond 8 came within 1000 km of the lunar surface, took about 100 detailed photographs on film, and returned them safely to Earth within a week.

Credit: Galspace


Enceladus nightshine & plumes




Narrow-angle false color composite taken on 2006-03-22 using infrared (IR3), green and ultraviolet (UV3) filters. Because the far infrared filter was used, at wavelengths in which Saturn is less bright than in the green filter, the saturnlit portion of Enceladus looks greenish here.
The rings present their unlit side with the F ring showing a brilliant white color while the main rings are darker and more brownish. A hint of the G ring can be seen beyond the F ring.





Credit: Gordan Ugarkovic

Narrow-angle false color composite taken on 2006-03-22 using infrared (IR3), green and ultraviolet (UV3) filters. Because the far infrared filter was used, at wavelengths in which Saturn is less bright than in the green filter, the saturnlit portion of Enceladus looks greenish here.

The rings present their unlit side with the F ring showing a brilliant white color while the main rings are darker and more brownish. A hint of the G ring can be seen beyond the F ring.