Astronomy Now Home
Home Magazine Sky Chart Resources Store

On Sale Now!



The September 2014 issue of Astronomy Now is on sale! Order direct from our store (free 1st class post & to UK addresses). The Astronomy Now iPad/iPhone editions are now available worldwide on the App Store.



Top Stories



Earthshine used to test life detection method
...By imagining the Earth as an exoplanet, scientists observing our planet's reflected light on the Moon with ESO's Very Large Telescope have demonstrated a way to detect life on other worlds...
  READ MORE

Solid buckyballs discovered in space
...Astronomers using NASA’s Spitzer Space Telescope have detected a particular type of molecule, given the nickname “buckyball”, in a solid form for the first time...
  READ MORE

Steamy water-world gets the Hubble treatment
...Hubble Space Telescope observations of a 7 Earth-mass planet find an unusual water-rich world swathed in a thick, steamy atmosphere...
  READ MORE








Lake levels change on Titan
DR EMILY BALDWIN
ASTRONOMY NOW
Posted: 16 July 2010


Bookmark and Share

For the first time, the levels of liquid in Titan’s lakes have been found to fall and rise with the seasons, just like on Earth.

Titan, Saturn’s large, planet-like moon, is the only location in the Solar System – aside from the Earth of course – that has a hydrological cycle. While Earth’s hydrological cycle runs on water, Titan’s distance from the Sun makes for much chillier conditions, and a hydrological cycle that is based on liquid methane, ethane and propane.

Synthetic Aperture Radar (SAR) map of Ontario Lacus, the largest lake in Titan's southern hemisphere. Radar altimeter tracks show that Ontario lies in a shallow regional basin. The early (June 2005) and subsequent (June/July 2009) outlines of the lake are shown in cyan and blue, respectively. During the four-year observation period the lake receded by ~10 km at places, consistent with an average depth reduction of ~1 m/yr. Inset; Region A with contours of constant distance from shoreline. Image: Radar Science Team, NASA/JPL/Caltech

Now, after analysing Cassini spacecraft data collected over a period of four years, scientists have found that the depth of liquid in Titan’s southern hemisphere lakes have dropped at a rate of about one metre per year. For the case of Ontario Lacus (named for its similar size to Lake Ontario on Earth), the scientists found that between June 2005 and July 2009, a period of time that covers the transition between mid-summer and autumn on Titan, the shoreline receded by around ten kilometres.

The results are based on radar altimetry and Synthetic Aperture Radar (SAR) data, which measures the roughness of a surface such that a flat, smooth surface such as a lake appears dark, while rough features (such as mountains) appear bright. By combining the SAR and radar altimetry data, the scientists could build up a picture of the absorptive properties of the liquid. “It argues that the liquids are relatively pure hydrocarbons made up of methane and ethane and not a gunky tar," says Oded Aharonson, associate professor of planetary science at Caltech.

"The liquid is not highly attenuating," explains Alexander G. Hayes, also from Caltech, "which means it is fairly clear to radar energy – that is, transparent, like liquid natural gas. Because of this, radar can see through the liquid in Titan's lakes to a depth of several metres. Then the radar hits the floor, and bounces back. Or, if the lake is deeper than a few metres, the radar is completely absorbed, producing a 'black' signature."

Ephemeral lake observations in Titan's south polar region near (60S,150E). The blinking image shows partially filled lakes (outlined in cyan) disappearing between images obtained in December 2007 (Cassini pass T39) and May 2009 (T55). Models of the change in radar brightness suggest that the amount of liquid loss is ~1 m/yr, consistent with the analysis of shoreline recession at Ontario Lacus. Image: Cassini Radar Science Team, NASA/JPL/Caltech

Taking into account the liquid’s optical properties, the researchers could then focus on the depth of the lake. "We were able to determine the bathymetry of the lake out to a depth of about eight metres," says Hayes, who adds that the lake is shallowest and most gently sloped along its southern edge, in areas where sediment is accumulating. Along its eastern and northern shore, where it hits a mountain range, the slope of the lake is somewhat steeper, which Hayes calls the ‘beachhead’.

Comparing images of Ontario Lacus separated by four years revealed that the lake had shrunk. "The extent to which the lake has receded is related to the slope – i.e., where the lake is shallow, the liquid will have receded more," says Hayes. "This allows us to deduce the vertical height by which the lake depth has dropped, which is about one metre per year."

The rate of evaporation of methane from nearby lakes was also determined by comparing the radar signatures from images taken in December 2007 and May 2009, in terms of how ‘dark’ the signature appeared. In all the lakes the radar darkness decreased or disappeared entirely, translating as a reduction of liquid. "We got the same result: one metre per year of liquid loss," says Aharonson.

As yet, no analogous changes have been noted in the northern hemisphere lakes, which is now entering spring. "We would expect it will happen, but we don't know how it would manifest in the data if the lakes in the north are significantly deeper," says Aharonson "We'll continue to look for this effect with future radar images, to disentangle the seasonal variations from longer-term climate variations we previously have proposed."

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.
 GET YOUR COPY

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
 GET YOUR COPY

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
 GET YOUR COPY


HOME | NEWS ARCHIVE | MAGAZINE | SOLAR SYSTEM | SKY CHART | RESOURCES | STORE | SPACEFLIGHT NOW

© 2014 Pole Star Publications Ltd.