The Martian Lake District

Three billion years ago, the red planet, Mars, was warm enough to sustain lakes of liquid water, according to satellite images just published in the journal Geology. Previously, astronomers had assumed that this period was simply too cold and arid for surface water.

Researchers at Imperial College London and University College London now suggest that during the Hesperian Epoch, the Martian surface around the equator was spotted with lakes, each approximately 20 kilometres across, formed from melted ice. Earlier studies had hinted at the warm and wet early history of Mars during the period 4 billion to 3.8 billion years ago, well before the Hesperian Epoch. Detailed images from NASA’s Mars Reconnaissance Orbiter, which is currently circling the planet, suggest that there were later warm and wet periods; age is determined by meteorite crater count. The evidence lies in several flat-floored depressions located above Ares Vallis, a giant gorge that runs 2000 km across the equator of Mars.

Martian lakes
Martian lakes

According to Nicholas Warner, of IC’s Department of Earth Science and Engineering, “Most of the research on Mars has focused on its early history and the recent past. Scientists had largely overlooked the Hesperian Epoch as it was thought that Mars was then a frozen wasteland. Excitingly, our study now shows that this middle period in Mars’ history was much more dynamic than we previously thought.”

Warner and colleagues, Sanjeev Gupta, Jung-Rack Kim, Shih-Yuan Lin, and Jan-Peter Muller, claim that there may have been increased volcanic activity, meteorite impacts or shifts in Mars’ orbit during this period, which could have warmed its atmosphere enough to melt ice. This in turn would have bolstered the greenhouse effect temporarily, trapping more heat from the sun and making the planet warm enough for liquid water to exist on its surface.

Until now, the Ares Vallis depressions have remained a mystery to scientists, although they suspected that their formation was due to sublimation of ice directly to water vapour. The loss of ice would have created cavities between the soil particles, which would have caused the ground beneath to subside.

Martian channels
Martian channels

The researchers have now discovered small, sinuous channels that connect the depressions, which they say could only have been formed by running water, essentially making the sublimation theory redundant. The team also compared the Mars images with images of thermokarst landscapes on Earth in places such as Siberia and Alaska. Thermokarst landscapes are areas where permafrost is melting, creating lakes that are interconnected by the same type of channels the team says exist on Mars. The team says that the melting ice created lakes that may have burst their banks allowing water to carve pathways through the frozen ground from higher lakes into lower-lying lakes.

UCL’s Muller who works at the Mullard Space Science Laboratory who carried out the 3D mapping of the Martian surface, explains how modelling with sub-metre resolution allowed the team to test their hypotheses much more rigorously than ever before.

Topographic image
Topographic image

One thing that the scientists do not yet know is how long the warm and wet period lasted during the Hesperian epoch or how long the lakes remained liquid. Nevertheless, the study may have implications for so-called “astrobiologists” looking for evidence of life on Mars. The team say these lake beds indicate regions on the planet that may have once been suitable for some form of microbial Martian life. As such, they represent good targets for future robotic missions seeking out ancient life on Mars.

LINKS

Geology, 2010, 38, 71-74
http://dx.doi.org/10.1130/G30579.1
Video flypast

Moon river?

The media was recently drenched with the idea that water had been found on the Moon, offering speculation as to our nearest neighbour offering an oasis-like site for a lunar base from which we could launch missions to Mars and beyond. The truth, if it is ever confirmed, is a little more subtle.

Is moisture on the Moon, simply wishing on a star? (Photo by David Bradley)
Is moisture on the Moon, simply wishing on a star? (Photo by David Bradley)

The Apollo missions of the 1970s had always hinted at the presence of water on the Moon, although its presence in samples brought back to earth was thought to be nothing more than contamination. In 1998, scientists announced that the Lunar Prospector spacecraft had detected 300 million tonnes of water on the moon and hinted that there may be as much as 6 billion tonnes. In July, an analysis of tiny beads of volcanic glass collected by two Apollo missions revealed water trapped inside, suggesting that the Moon’s water had not been entirely vaporized by the violent events that led to its formation. The discovery had implications for the volcanic origin of possible water reservoirs at the Moon’s poles.

However, new evidence released at the end of September based on data from India’s Chandrayaan-1 probe and the Deep Impact and Cassini missions suggests that there may well be some degree of hydration up there. Researchers in India and the US used data from NASA’s Moon Mineralogy Mapper, the M3, aboard the Chandrayyan-1 satellite, which was launched into orbit around the moon in October 2008 to reveal the presence of water on the moon. Chandrayaan’s mission ceased in August 2009.

M3 uses reflectance spectrometry to determine the content of minerals in the thin layer of upper soil on the surface of the moon. The data revealed the presence of chemical bonds between hydrogen and oxygen atoms, like those found between the oxygen atom and its attendant hydrogen atoms in H2O.

However, the next generation of lunar astronauts are not likely to sip from moon springs or splash their silvery boots in lunar puddles because revelations of chemical bonds between hydrogen and oxygen atoms is indicative of water molecules but is even more indicative of hydroxyl ions (OH). It could be that good, old-fashioned H2O forms only when the solar wind doth blow and brings with it hydrogen atoms that can combine with the hydroxyl radicals forming “H+OH” (H2O). It may be that less than a litre of actual water is present per tonne of rock spread across the surface to a depth of a few centimetres and present as water of hydration of the minerals from which the rock is composed.

The rocks and soils that comprise the lunar surface contain about 45 percent oxygen, mostly in the form of silicate minerals. The constant deluge of hydrogen atoms from the solar wind could readily pull oxygen and hydroxyl from the soil and form water molecules on the fly, especially given the hydrogen ions are moving at one third the speed of light when they hit.

Taylor and other M3 team members believe their findings will be of particular significance as mankind continues to plan for a return to the moon. The maps created by M3 could provide mission planners with locations prime for extraction of needed water from the lunar soil.

Following the lunar announcement, Jim Bell, President of The Planetary Society, said: “The possible presence of minor amounts of hydrated material on the Moon is intriguing, though the findings still need to be confirmed by other methods and other investigators. Chandrayaan is another great example of the power and value of international collaboration in space exploration, and The Planetary Society congratulates the entire Chandrayaan, Deep Impact and Cassini teams.”

Researchers still hope to find liquid water at the bottom of the deepest, darkest lunar craters at depths that never see sunlight nor feel the solar wind. Such, hopefully, icy depths are akin to the cold places on the planet Mars where evidence of water ice has been found.

LINKS
Science, 2009, in press
Chandrayaan-1 site

Over and Oort on the comet’s tale

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An enormous asteroid or comet smashing into the Earth 65 million years ago killed off the dinosaurs. But, according to a new study by US scientists, published in the wake of an impact event on Jupiter, cometary collisions with Earth probably didn’t cause any more than one other extinction event during life’s history.

US researchers have looked to the Oort Cloud to help them work out just how many comets may have struck the earth during the time complex life has existed. The Oort Cloud is a remnant of the nebula from which the solar system formed 4.5 billion years ago. It starts about 93 billion miles from the sun (a thousand times as far from the sun as the Earth) and stretches to an incredible three light years away. It is thought that the Oort Cloud contains billions of comets but most are so small and distant that we will never see them.

Now, Thomas Quinn, Nathan Kaib and colleagues at the University of Washington, have used computer models to simulate the evolution of comet clouds in the solar system over the last 1.2 billion years. They found that even outside the periods of comet showers, the inner Oort Cloud was a major source of long-period comets. Some of these can cross Earth’s path but the simulations show that no more than two or three comets could have struck the Earth even during the most powerful comet shower of the last 500 million years.

Astronomers have known for some time that the inner solar system of rocky planets – Earth, Mars, Venus, and Mercury – is protected from comets by the gas giants Saturn and Jupiter. The enormous gravitational fields of these planets essentially eject comets into interstellar space or act as traps for them. This latter point was reinforced on 20th July when a huge scar appeared on Jupiter’s surface, likely evidence of a cometary impact.

There are about 3,200 known long-period comets of which Hale-Bopp is perhaps the most famous as it was visible with the naked eye for much of 1996 and 1997. Some long-period comets take thousands if not millions of years to make a single orbit of the sun. Comet Halley by contrast, which reappears every 75 years, is a short-period comet, which originate in a different part of the solar system known as the Kuiper Belt and are much more predictable. The computer simulation by Quinn and colleagues assumed that the inner Oort Cloud is the only source of potentially dangerous long-period comets.

The long-period comet 2001 RX14
The long-period comet 2001 RX14

“For the past 25 years, the inner Oort Cloud has been considered a mysterious, unobserved region of the solar system capable of providing bursts of bodies that occasionally wipe out life on Earth,” Quinn explains, “We have shown that comets already discovered can actually be used to estimate an upper limit on the number of bodies in this reservoir.”

With three major impacts taking place almost simultaneously, researchers had suggested that the minor extinction event of 40 million years ago resulted from a comet shower. Kaib and Quinn’s research implies that if that relatively minor extinction event was caused by a comet shower, then that was probably the most-intense comet shower since the fossil record began.

Further resources

Science Express, 2009, Jul 30, online

Tom Quinn’s home page

Comet shower movie from Nathan Kaib