快猫短视频

Tucson, Arizona

IN ITS YOUTH the Red Planet was very different from the frozen, barren desert we have seen on our TV screens courtesy of Pathfinder. The winding river valleys, shorelines and flood plains formed in those early times are clearly visible from space: proof that liquid water once flowed on Mars. It must have been warmer, too. And the planet鈥檚 atmosphere must have been thicker to trap heat from the Sun and create enough pressure to prevent water from boiling away.

So what went wrong? Why did the Martian atmosphere vanish and what happened to the water that once flowed across its surface? Did the planet die from natural causes, or was it a case of suicide, or even murder?

快猫短视频s hope to get some answers from the Mars Global Surveyor spacecraft, which entered orbit around Mars last week. Soon it will be photographing, measuring and studying the surface of Mars. But this is just the beginning of a much longer, more detailed mission. The Surveyor will enter a new phase of operation three years from now as a communications satellite for a Mars lander and two 鈥減enetrators鈥� now under construction. Long after it has finished its main mission, the Surveyor will listen for radio signals from the ground, amplify them and retransmit them to Earth.

The mission designed to take advantage of the Surveyor鈥檚 services-the Mars Volatiles and Climate Surveyor (MVACS)-will be launched in January 1999. It will land at the edge of the southern polar cap of Mars and study the climate, the soil and atmosphere, looking for the telltale evidence that will explain why Mars is now a dead planet.

There are three main theories and together they read like the plot of a detective novel. The first possibility is that Mars was murdered. The surface of Mars is scarred with craters from collisions with comets and asteroids. The blast from such impacts can blow away the atmosphere above the crater. And with enough large impacts, a substantial fraction of the atmosphere could be eroded. It would have been a slow, drawn-out death by suffocation.

Sucked away

Another possibility is suicide. Mars would have needed a thick atmosphere of a greenhouse gas like carbon dioxide to keep it warm enough for liquid water to flow over its surface. And what little atmosphere it has today is mainly CO₂. Much of its CO₂ may have reacted with Martian silicate rocks to form chemicals called carbonates. If there was no other source of CO₂, the atmosphere would gradually have been sucked into the surface of the planet.

A similar process occurs on Earth, but here there is a carbon cycle to maintain the status quo. CO₂ is removed from the atmosphere by photosynthesis in plants. Carbonates such as limestone or chalk form from the shells of dead sea creatures that fall to the ocean floor, removing their carbon from the cycle. But CO₂ is also pumped back into the atmosphere by volcanic activity. The net effect is that CO₂ levels remain roughly constant.

The evidence for the suicide scenario comes from Martian meteorites that have landed on Earth. Small amounts of carbonates have been found in these rocks, and some scientists believe that this supports the claim that life once existed on Mars. But carbonates can form by purely physical processes too-as in the reaction between CO₂ and silicates-if the conditions are right. Either way, once CO₂ was removed from the atmosphere, it would not have been replaced since Mars is not volcanically active.

The final possibility is that the Red Planet died of natural causes. Mars has an orbit that is more eccentric than the Earth鈥檚, so the amount of sunlight it receives during the year varies hugely. The axis of the planet may also have tilted sharply in the past so that winters were very severe. Combinations of changes in tilt and orbit, the so-called Milankovitch cycles, are believed to be responsible for the onset of ice ages on Earth. Mars may be in the middle of a similar ice age in which much of its CO₂ atmosphere has condensed to form permafrost at the poles or beneath the entire surface. This would be hidden from orbiting spacecraft.

Martian ice ages may be self-perpetuating. A thinner atmosphere would be less effective at transporting heat from the equator to the poles. Because of this, the poles would stay cold, keeping the CO₂ locked up even when a change in the planet鈥檚 eccentricity and tilt made the winters less severe. This may have locked Mars into a permanent ice age.

The only way to decide which of these scenarios is most likely is to land a craft at the poles, or at least nearby, to look for evidence. This is just what the next mission will do. Images from Mariner 9, which orbited Mars in 1971, and the Viking orbiters from the mid-1970s show strange layered terrain at the edge of the south polar cap. These formations can be traced for hundreds of kilometres and scientists believe they are made of dust and ice, laid down in layers as periods of cold alternated with warmer conditions. A record of the Martian climate may be written in the geology of the area in the same way that the climate record on Earth is reflected in ocean sediments, ice cores and tree rings.

In May 1999, MVACS will land at a latitude of 72掳 South, at the edge of the southern polar cap. It will study these strange layers, searching for carbonates and ice in the soil. In addition, it will monitor the seasonal CO₂ and water content of the atmosphere.

Several cameras on board MVACS will provide much of the data. First, a small descent imager will take photographs as the craft comes in to land. A stereo camera, built at the University of Arizona in Tucson and similar in design to the one carried by Pathfinder now on the Martian surface, will survey the landing site and monitor weather. MVACS also carries another camera mounted on the end of its robot arm. This will take close-up images of the soil and its layering.

The robot arm will also place samples of Martian soil in a small oven known as the Thermal and Evolved Gas Analyzer (TEGA)-a pair of small metal tubes wrapped in platinum wire heaters. The sample occupies one of the tubes, which is heated at a constant rate, along with the other, empty tube. The instrument monitors the temperature of the tubes: any deviation between them will indicate that a phase change is taking place in the sample, the melting of ice, for example, or a carbonate rock decomposing.

Any gases given off by the sample will be flushed through a laser spectrometer. This contains two tunable laser diodes, which emit infrared light over a small range of wavelengths according to the amount of current passed through them. Since gases such as water vapour and CO₂ absorb infrared light at specific frequencies it will be possible to measure how much of each is present. 鈥淥ne of the big mysteries of Mars is where is all the water and CO₂. TEGA may allow us to see if the soil can be where all the water and CO₂ is hiding,鈥� says Bill Boynton, a planetary scientist at the University of Arizona who leads the TEGA team.

Spotting oxygen is important too. Much of the planet鈥檚 water may have been broken down into hydrogen and oxygen by sunlight. The hydrogen would have escaped into space, but the heavier and more reactive oxygen atoms may have been absorbed by the soil. There is some evidence for this already. The Viking landers found that the soil gave off CO₂ when nutrients were added. This could indicate life, but Viking also discovered that no organic molecules were present, and life without organic compounds seems unlikely. An alternative explanation is that Martian soil contains a highly reactive oxidising agent-something you might expect if it had reacted with atmospheric oxygen in the past. An oxidising soil would decompose nutrients and would destroy any other organic compounds. However, the story may be different beneath the surface, out of reach of an oxygen-rich atmosphere. If TEGA finds no oxidising agents beneath the surface, says Boynton, this might suggest that organic molecules or even microbial life could survive there.

Weather eye

The lander will provide detailed weather reports. As well as pressure, temperature and wind speed sensors, the lander has a laser spectrometer to determine the composition of the atmosphere by measuring the amount of light it reflects and absorbs. One important measurement will be which isotopes of carbon, hydrogen and oxygen are present in CO₂ and water vapour. Heavier isotopes are less likely to escape into space, so their relative abundance shows how fast the atmosphere has escaped in the past.

The mission will also test a pair of torpedo-shaped 鈥減enetrators鈥�, which will be dropped before the main lander enters the atmosphere. Each weighs about 2.5 kilograms and is protected from burning up by ceramic heat shields. They will hit the ground 100 kilometres north of the MVACS lander site at a speed of about 190 metres per second-enough to bury the penetrator to a depth of up to 2 metres.

Each will carry an accelerometer to record the atmospheric drag and the impact with the surface. This will tell researchers whether the surface is layered. A small drill will scrape a sample of soil into a thimble-sized metal cup. This will then be heated and a tiny spectrometer the size of a postage stamp will look for any water that is produced.

None of this information can be transmitted back to Earth from beneath the surface, so the tail of each penetrator is designed to detach on impact and reel out a cable connected to the main body. The tail carries a pressure sensor and small antenna which will radio the data to the Mars Global Surveyor spacecraft orbiting above. With pressure sensors on each penetrator and the lander, it may be possible to monitor the passage of storm systems over the surface.

Penetrating insights

The penetrators should continue working for a couple of days, and maybe longer, if their batteries survive the impact. It鈥檚 a big 鈥渋f鈥�-the penetrators will strike the surface with a force equivalent to 80 000 times their weight here on Earth. No penetrator has successfully reached another planet or moon. The Japanese space agency ISAS plans to send penetrators to the Moon next year and the Russian space agency鈥檚 Mars 96 mission carried two, but they were lost along with the rest of the mission when its booster misfired. While the latest attempt may provide valuable data, its main objective is to show that penetrator technology works.

Meanwhile, the MVACS lander should continue collecting data for much longer, up to seven months during the Martian summer in 1999. During that time it should provide vital clues to the planet鈥檚 cause of death. But as winter sets in and the Sun sinks low on the horizon, MVAC鈥檚 solar panels will stop producing power to operate the lander. As CO₂ frost begins to form around it, MVACS will fall silent.

But this may not be the end. 快猫短视频s hope that the following spring, almost 400 days later, MVACS will come back to life. By that time, the next salvo of Mars missions will be on its way.