01/11/2006
Mars and Moon - Past and Present
Last week I mentioned that fraudulent fellow named Hwang and dashed hopes for a major breakthrough in stem cell research. When I wrote the column, I hadn’t read my December Scientific American, which named Hwang “Researcher of the Year” for 2005. In his Week in Review columns on this Web site, Brian Trumbore often says to wait 24 hours; in science it takes a bit longer, say a month! This week, let’s be more positive and consider some scientific ventures that have far exceeded expectations, the spacecraft and rovers either circling or crawling on the surface of Mars. The international teams working with NASA/JPL and the European Space Agency (ESA) have reasons to be very proud of their achievements. Visits to the Web sites of ESA and JPL provided much of the material below.
The rovers Spirit and Opportunity have been wandering around Mars for two years, much longer than they had been expected to survive. Opportunity has traveled 4 miles, up and down slopes and in and out of situations where its wheels were stuck, thanks to its expert drivers back on Earth. Today, the hardy little traveler is headed for Victoria Crater, a half mile wide crater where layers of Mars’ geological past are exposed. Both Spirit and Opportunity have delivered on the hope that they would confirm the presence of water in Mars’ past. They identified hydrated minerals that contain molecules of water in their structure and found areas that are thought to have been pools of water a long time ago.
There was a downside to these findings, however. The minerals that were found were of a type that must have formed from highly acidic pools of water, not the sort of condition favorable for life. Although, here on Earth, various kinds of organisms have been shown to thrive under surprisingly hot and acidic conditions, scientists would feel more comfortable if there had been a period on Mars that was not so acidic.
Here’s where ESA and its Mars Express orbiter comes into the picture. Mars Express contains two instruments of note here – OMEGA and MARSIS. With both infrared and visible light capabilities, OMEGA can both see and identify the minerals that appear on the surface of Mars. OMEGA sees a lot of hydrated sulfates that are formed from those acidic solutions. But last March the OMEGA team revealed that OMEGA had also spotted outcroppings of hydrated minerals known as “phyllosilicates”; one example of a phyllosilicate is clay. Clays form, not from acidic solutions, but from neutral or alkaline solutions.
At a meeting last month of the American Geophysical Union, the OMEGA team announced they had found layers of the clays underneath layers of the sulfates. They say that their findings are consistent with a scenario in which, around 3.8 billion years ago, there was lots of water that reacted with the rocks/minerals on Mars to form the clays. There was mud on Mars. If life did form on Mars, this would have been the most propitious time. It also would have been about the same time that life first appeared on Earth. Later, on Mars, conditions became more hostile and the acidic times prevailed.
According to an article by George Musser in the December Scientific American, Diana Blaney of JPL reported at a meeting in September of the American Astronomical Society that Spirit had looked at a rock dubbed Independence and found it to be claylike. Why the keen interest in clays, aside from the fact that they typically form in neutral or alkaline, not acidic solutions? I think that I’ve mentioned in the past that some workers have shown that clays promote reactions that form chemical compounds necessary for life and that clays may even provide templates for the formation of structures resembling cell walls.
What about water on Mars today? If astronauts ever do arrive on Mars, it would be great if they didn’t have to carry all the water to satisfy their needs. A known supply of water would certainly come into play in the selection of a landing site, I should think. Here’s where ESA’s MARSIS comes in. MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Imaging) has been used to study both the Martian atmosphere and what lies under the Martian surface. By studying the echoes of returning radar waves, the researchers have found hidden impact zones that formed when Mars was struck by space objects early on in its history. These impact zones were filled in with material from winds, lava flows, etc.
The MARSIS team found evidence that the impact basins may be filled in partially with water-ice mixtures. However, more spectacular was what they found when they looked in areas around Mars’ north pole. Radar reflections showed a likely layer of nearly pure water-ice that is over a half-mile thick. If I went to Mars, I sure would try to land somewhere near that big chunk of ice. Without having to carry extra water, there might be room for the ingredients for a celebratory Scotch or bourbon on the rocks after the long trip from Earth!
Another orbiter is NASA’s Mars Global Explorer. Michael Malin, of Malin Space Science Systems in San Diego, has a camera onboard the Explorer. His camera revealed a mystery that was discussed at the December meeting. There are hundreds of craters on Mars that are filled with some kind of sediment; some are filled to overflowing. The mystery is not only where did the sediment come from but in many of them sediment has been removed so the there are mounds of sediment in the craters. One striking example is the nearly 100-mile wide Henry Crater near Mars’ equator.
The Henry crater has a mound of sediment that is over a half mile high, higher than the rim of the crater. The mound of sediment has distinct layers that match the layers of sediment sticking to the walls of the crater. Was the sediment laid down by wind, seas or what? Has wind been responsible for removing the sediment, forming the mounds? Nobody at the meeting had a solution. Hopefully, the Mars Reconnaissance Orbiter, scheduled to arrive over Mars in March with a higher resolution camera, will provide clues to solve this mystery.
Although not related to the Mars, a Mars-size object is believed to have hit Earth early on in its history, the result being that debris from the collision ended up orbiting Earth and forming our Moon. In the collision, Earth is thought to have picked up perhaps 10 percent of its mass from the Mars-size intruder. Although other objects have struck and are still striking our planet, this collision is thought to be the last truly monumental batch of material incorporated into our Earth. Thorsten Kleine in Germany and his colleagues from Germany, Switzerland and the UK have an article in the December 9 Science that sheds light on when this huge event took place.
Initially, both the Moon and Earth were molten and what Kleine and coworkers have done is to date the time when the molten magma cooled down and crystallized. By dating that time, they can also come up with a date for Earth’s last big grab of material. I won’t go into detail on their work except to say that it involves measuring the amounts of different isotopes of the metals hafnium, tungsten and tantalum in samples of metal and rocks supplied by NASA. I assume that the samples were collected by our astronauts back in the days when we were walking on the Moon. The researchers’ results indicate the moon crystallized 4.53 billion years ago, the big collision taking place some 30 to 50 million years after the formation of the solar system.
December’s been a good month for learning about the past and present of our planet and two of our solar system companions.
Allen F. Bortrum
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