NASA Cassini: Seasonal changes on Saturn

A blue hue appears in Saturn's southern hemisphere as winter approaches. Credit: NASA/JPL-Caltech/SSI

Three of the most recent images show Saturn’s largest moon, Titan, and the recently discovered vortex in the atmosphere at the south pole.

The visible light cameras on Cassini have been monitoring a strange yellow haze in the detached haze layer at Titan’s south pole since 27 March.

The visual and infrared spectrometers then detected clouds building up in the region on 22 May.

When Cassini flew by Titan on June 27, 2012, it was found that the vortex is spinning faster than the moon itself is rotating.

The views of the vortex are only possible because Cassini is on a new orbit that is tilted, which enables it to get a better look at the polar regions of both Saturn and its moons.

"Cassini has been in orbit now for the last eight years, and despite the fact that we can't know exactly what the next five years will show us, we can be certain that whatever it is will be wondrous," explains Carolyn Porco.

The south polar vortex is visible in this image. Credit: NASA/JPL-Caltech/SSI 

Cassini is currently in its second mission extension, which has been called the Solstice Mission, and monitoring the seasonal changes on the ringed planet is one of the main objectives at the moment.

"It is so fantastic to experience, through the instruments of Cassini, seasonal changes in the Saturn system," said depute project scientist Amanda Hendrix.

"Some of the changes we see in the data are completely unexpected, while some occur like clockwork on a seasonal timescale. It's an exciting time to be at Saturn."

NASA Mars HiRISE Image: Late Springtime Defrosting of Northern Dunes

This observation shows dunes in the Martian north polar sand sea (commonly referred to as the "north polar erg") in the process of defrosting.

Every winter, dunes and other surfaces at these northern latitudes are coated with several tens of centimeters of carbon dioxide frost and ice, plus a minor amount of water frost.

Details of this process are particularly visible this subimage. The white material is fine grained frost.

The dark, splotchy tones on the dunes may be deposits of particulates deposited from carbon dioxide "geysers" or relatively thick deposits of carbon dioxide ice. The more brownish colors represent defrosted areas. Polygonal patterns on the surface of the dunes are probably cracks in overlying carbon dioxide ice.

Landslides on the dunes' lee slopes are apparent,with a morphology consistent with fluidization from carbon dioxide frost. This and other areas of the north polar region are being investigated by HiRISE to compare to changes in past years.

Read more about Mars Seasonal changes

Earth's Seasonal Change: Not-So-Equal Equinox

The seasons on Earth will officially change Wednesday, heralding their shifting nature with an astronomical feat: the autumnal equinox.


On Sept. 22, at 11:09 p.m. EDT (8:09 p.m. PDT), the fall season will begin in the Northern Hemisphere while the Earth's Southern Hemisphere residents ring in their spring. This date – one of two each year – is called an equinox, from the Latin for "equal night," alluding to the fact that day and night are then of equal length worldwide but this is not necessarily so.

The not-so-equal equinox


The definition of the equinox as being a time of equal day and night is a convenient oversimplification.


For one thing, it treats night as simply the time the sun is beneath the horizon, and completely ignores twilight. If the sun were nothing more than a point of light in the sky and if Earth lacked an atmosphere, then at the time of an equinox the sun would indeed spend one half of its path above the horizon and one half below.


But in reality, atmospheric refraction raises the sun's disk by more than its own apparent diameter while it is rising or setting. Thus, when we see the sun as a reddish-orange ball just sitting on the horizon, we're looking at an optical illusion. [Top 10 Extreme Planet Facts]


It is actually completely below the horizon. So from our point of view, the day on an equinox appears longer than it actually is. This illusion means that the appearance of equal day and night, from a skywatcher's view, will come several days later.


In addition to refraction hastening sunrise and delaying sunset, there is another factor that makes daylight longer than night at an equinox: Sunrise and sunset are defined as the times when the first or last speck of the sun's upper limb is visible above the horizon – not the center of the disk.


This is why, when you check your newspaper's almanac or weather page on Wednesday of this week to look up the times of local sunrise and sunset, you'll notice that the duration of daylight from sunrise to sunset still lasts a bit more than 12 hours – not exactly 12 as the term "equinox" suggests.