Brave New Worlds: Secrets of our solar system

The closer we look at the worlds that circle our sun, the more spectacular and mysterious they become. As growing numbers of space probes have succeeded in travelling to faraway destinations, they have turned distant and mysterious points of light into alien landscapes dotted with unanswered questions.

Scientists recently sent the Messenger spacecraft flying past Mercury to send back pictures of a never-seen-before hemisphere. But it is far from the only probe producing new and exciting footage of our solar system. An explosion in the number of craft launched into orbit has left an unprecedented number of them scattered across space, producing staggering images and data from across the length and breadth of our solar system, and even beyond.

Today, unmanned spacecraft are finally able to nestle close to the fiery heat of the sun, and fly through the solar system's cold and uncharted outer reaches. They can be found in orbit around five worlds, including the Earth and the Moon. They are plying the space-lanes between the planets, measuring thin swirling clouds of plasma gas and dust spilled from passing comets. They are en route to Pluto, the most distant planet, and are even hoping to land on the shifting sands of a comet. Never before in human history has a generation voyaged so much.

This, then, is a planet-by-planet guide to the secrets that we are finally uncovering about our solar system's past and present, and the ways in which the groundbreaking research of recent years might affect the future of mankind.

The Sun

The Sun dominates everything within our solar system, providing energy and stability to the Earth, and creating a gravitational centre-point for the planets to orbit around. But despite the fact that it underpins almost every aspect of our existence, there is still much about it that remains an enigma.

In an effort to unpick the Sun's mysteries, a flotilla of spacecraft is orbiting close by the Earth with their attention turned sunward. Their goal is to understand our Sun and to improve predictions of important solar events, such as when clouds called Coronal Mass Ejections are flung towards the Earth. These are clouds of magnetised gas ejected into space through eruptions from the solar surface that have masses of upwards of a few billion tons and travel up to 2,000 kilometres a second. They can be hazards for satellites, radio communications and power systems.

Among those on the look out are the probes of the Stereo mission. Two nearly identical spacecraft have been positioned just ahead of and just behind the Earth's orbit. Having slightly different lines of sight to the Sun's surface they can track the gas clouds and determine if they are heading Earthward. Recently Stereo captured the first ever images of a collision between an ejected gas cloud and a comet. When the cloud struck, the comet lost its tail.

Another such mission is the Japanese Hinode spacecraft, which has produced spectacularly detailed images showing sunspots and, in particular, the superhot gas that arches above them. Unprecedented detail can be seen in the fine so-called granulation on the surface of the Sun, where hot cells of gas rise to the surface where they cool before submerging.

Stereo: www.nasa.gov/mission_pages/stereo/main/index.html

Hinode: http://solarb.msfc.nasa.gov/

Mercury

Mercury has always been a strange, distant world. Named after the messenger of the gods, it has been far less studied than other planets because of the difficulty of observing it and actually getting there. It's the smallest in the solar system, taking only 88 days to circle the Sun, and its close orbit means that it is never far from the solar glare. Getting a probe to Mercury involves many years of celestial gymnastics and flybys of other planets along the way.

Recently, the Messenger probe flew past at a distance of just 150 miles, the first time in 33 years that a probe had visited the planet. Messenger produced some stunning images. The previous probe, Mariner 10 in 1975, was only able to see about half the planet.

At first sight, Mercury looks much like the Moon. But underneath the surface it is unique: it is dense and has an overlarge core, suggesting that the planet's outer layers were blasted away by a collision billions of years ago. When the solar system was young, the planet was bombarded by comets and asteroids that triggered magma eruptions that flooded the plains. Later, it began to cool and contract and its surface began to crack and form the ridges that snake over it.

Of particular interest among the Messenger images are those of the Caloris Basin, revealed in its entirety for the first time. Formed by a giant impact, it is 800 miles across and ringed by mountains up to 7,000ft high. It is named Caloris – Latin for heat – because the Sun is often directly overhead. That close, temperatures can reach 430C.

Messenger will make two more flybys, one later this year and one in 2009. Both will enable the probe to lose some speed so that it can be captured into orbit by Mercury in 2011, when it will survey and map the entire planet. The European Space Agency also has a Mercury probe planned; called BepiColombo, it will arrive in 2019.

Mercury Messenger: http://messenger.jhuapl.edu/

Venus

If you are awake at dawn, chances are that you will see bright Venus in the south-eastern sky. To the ancients, the planet's light had a unique purity that rendered it female, and unknowable. It's also the brightest object in the night sky after the Sun and Moon.

Shrouded in a layer of highly reflective clouds partly consisting of droplets of sulphuric acid, the surface of Venus has been mapped in the past 20 years only by a satellite, using radar. It found a relatively small number of craters. Astronomers believe this to be evidence of extensive volcanic activity which, every few hundred million years, melts the surface and wipes it clean of impact craters. Venus has several times as many volcanoes as Earth, and nearly 2,000 are more than 100km across.

Beneath its clouds, most of Venus's surface consists of smooth volcanic plains. Two highland regions make up the rest of its surface area, one in the northern hemisphere and the other just south of its equator. The northern continent, called Ishtar Terra, is about the size of Australia. Maxwell Montes, the highest mountain on Venus, is on Ishtar Terra, with its peak about 33,000ft above average surface elevation. The southern continent is called Aphrodite Terra. The larger of the two highland regions, it is roughly the size of South America. Much of it is covered by a network of fractures and faults.

The only spacecraft now in Venusian orbit is the European Space Agency's Venus Express. It has recorded abundant lightning in the high atmosphere. The existence of this lightning had been disputed since suspected bursts were detected by the Soviet Venera probes more than 30 years ago, but Venus Express detected whistler-mode waves – the signatures of lightning – and their intermittent appearance indicates a pattern associated with weather activity. The lightning rate is at least half of Earth's.

Venus Express has found evidence for past oceans, and discovered a huge double atmospheric vortex at the south pole of the planet. Studies show that Venus's atmosphere was once much more like Earth's and that there was probably much liquid water on the surface. A runaway greenhouse effect was caused by evaporation of water, which – added to Venus's carbon dioxide-rich atmosphere and thick clouds of sulphur dioxide – produced the strongest planetary warming in the solar system, pushing surface temperatures above 460C. This makes Venus's surface hotter than Mercury's, even though Venus is nearly twice Mercury's distance from the Sun and receives only 25 per cent of Mercury's solar radiation.

Venus Express: http://sci.esa.int/science-e/www/area/index.cfm?fareaid=64

The Moon

After decades of neglect, we are entering a new age of lunar exploration. This is much needed: despite the fact that scientists have studied hundreds of pounds of returned Moon rocks, many unanswered questions about it remain, and we are not even sure for certain where the Moon originally came from.

There are two probes in lunar orbit, and more to follow. The Japanese Kaguya was launched in September and is the largest lunar mission since the Apollo days. It's in a low orbit, 80 miles above the surface, and has deployed two small subsatellites. Its first batch of data is expected shortly, though it has already returned the first high-definition video from lunar orbit as it passed over the deep shadows of the lunar north pole, watching the blue-and-white Earth rise over the horizon. Japan is planning a follow-up to Kaguya, possibly before 2010. Looking further ahead, the Japanese Space Agency is thinking of a manned lunar landing in 2020 and a manned lunar base by 2030.

China launched its first moonprobe – Chang'e 1 – in October, releasing its first picture a month later. "I come with greetings from China," said a female voice that was programmed into the probe. One of its aims is to survey the lunar surface for the presence of helium-3, a form of helium rare on Earth that if found in sufficient quantities on the Moon would be a valuable resource to mine and ship to Earth for use in nuclear fusion reactors. China plans to land a rover on the moon in 2012, and to conduct a sample return mission in 2017. It has recently signed an agreement to work with Russia to eventually land astronauts there.

Later this year, India's Chandrayaan-1 will be launched, containing instruments from the United States and Europe. The Indian Space Research Organisation also plans to undertake manned space exploration in the next decade and to have a person walk on the moon by 2020 (although it must be said this is optimistic given India's lack of space technology expertise and budget).

The US Lunar Reconnaissance Orbiter (LRO), also due for launch this year, will take high-resolution imagery of the Moon's surface to prepare for manned landings planned for 2017. It will also seek to answer one of the Moon's greatest mysteries: is there ice hidden in the shadows at the lunar poles? Measurements from orbit suggest that ice from bombarding comets could have been caught in the "cold trap" conditions in the dark craters. If there is ice, then it would be a valuable resource for a lunar base.

Russia, meanwhile, intends to resume its Luna-Glob project, an unmanned lander and orbiter slated to launch in 2012. Germany announced in March 2007 that it will launch a lunar orbiter in 2012 and a British Moon mission is also seeking funding and support.

Kaguya: www.selene.jaxa.jp/index_e.htm

Chang'e 1:www.cnsa.gov.cn/n615709/n772514/n772543/93747.html

Mars

The Romans thought it was like a drop of blood hanging in the sky, and named it after their god of war. In reality, Mars is a small, rocky world that has as much land area as the Earth and a thin atmosphere that supports clouds and dust storms. It has many Earth-like aspects: extinct volcanoes, valleys, shifting deserts, polar ice-caps. Olympus Mons is the highest known mountain in the solar system, and Valles Marineris is the largest canyon.

For many scientists, Mars is the first new world, our primary space exploration goal. It is currently prominent in the night sky, rising from the north-east nearly two hours before sunset and high in the sky by 11pm.

The planet is hosting three functional orbiting spacecraft: Mars Odyssey, Mars Express, and the Mars Reconnaissance Orbiter. A fourth probe, the Mars Global Surveyor, suffered a malfunction last summer and no longer works. The Martian surface is also home to the two Mars Exploration Rovers (Spirit and Opportunity).

Evidence gathered by these and other missions suggests that Mars was once covered in water and there are indications that small geyser-like eruptions still occur. Observations by Mars Global Surveyor suggest that parts of the southern polar ice-cap have been receding. Mars, it seems, has its own problems with global warming.

The Mars Odyssey spacecraft has just discovered entrances to seven possible caves on the slopes of a volcano, a find that is fuelling interest in potential underground habitats for primitive forms of life.

In September 2006, the Opportunity rover reached the rim of Victoria Crater (below) in Meridiani Planum and transmitted the first views of the 2,000ft-wide crater including a remarkable dune field. Craters are important for scientists as they allow access to layers under the surface and a means to analyse Mars's geological history.

After a planet-wide dust storm that delayed entry into Victoria for six weeks, Opportunity entered at a point called Duck Bay. It is now gingerly making its way down the crater, collecting data from rock layers and looking at the band of light-coloured rocks beneath the rim. Scientists planned to have the rover head directly downhill to a rock target nicknamed Ronov, but had to change plans when engineers determined that the route would tilt the rover too much.

Recently, the craft in orbit watched a dust storm engulf Mars. They detected that it warmed the atmosphere but cooled the surface. It's good that can't happen on Earth; it would plunge us into twilight for months, with catastrophic results for ecosystems.

Mars: http://mpfwww.jpl.nasa.gov/

Jupiter

The largest world in our solar system, Jupiter has always been regarded as king of the night sky. Even though Venus is brighter, only Jupiter is able to lord it over the dark midnight skies. The planet has been out of close surveillance since the Galileo probe burned up in its atmosphere in the autumn of 2003. There are currently no plans to send a probe back and astronomers have to be satisfied with monitoring the planet from the Hubble Space Telescope.

A recent analysis of Hubble's 2007 observations showed two giant plumes moving through the planet's atmosphere. Scientists believe that such outbursts indicate that the planet's considerable internal heat reserves play a significant role in generating weather patterns. However, we do have had a recent close view of Jupiter obtained when the New Horizons spacecraft passed the planet in March last year en route to distant Pluto. The probe skimmed past, taking a peek at its cloudscape. Strangely, it showed a quieter Jupiter. In the image it returned, the south pole is seen capped by a haze of small particles. Away from the pole there is an anticyclonic vortex with thunderheads at its core, and further north there are tendrils of straggling, disorganised storms leading to a cloud-free region before the cloud and winds increase as you approach the equator.

New Horizons took images of one of Jupiter's major moons – Io, the most volcanically active body in the solar system. In the image, Io shows a beautiful display – a 190-mile plume of sulphur from the Tvashtar Patera volcano. A mile-high curtain of lava is erupting from its volcanic vent as a lake of lava spills over the landscape, coating it orange and yellow.

New Horizons has now passed Jupiter and entered hibernation mode for the long haul to Pluto.

New Horizons at Jupiter: http://pluto.jhuapl.edu/news_center/news/100907.htm

Saturn

With its magnificent ring system, Saturn is perhaps the most engaging planet for scientists to observe – yet it is also only very slowly yielding its secrets. Scientists are uncertain how Saturn's iconic rings were formed, but tend to speculate that they were the result of the break-up of a moon that got too close, or leftover fragments that never formed a moon at all.

In July 2004, the Cassini spacecraft flew through a gap in the rings and went into orbit around Saturn, the first spacecraft ever to do so. Thanks to that continuing expedition, we have learnt more about this world in the past few years than in the past few centuries, and have been able to witness some of the grandest sights in the solar system.

Cassini has seen the rings from various aspects, against the dark sky and against the clouds of Saturn. They've been revealed to be far more detailed than we imagined. Occasionally, Saturn's moons are seen, sometimes in pairs alongside the rings – a sight of unparalleled grandeur.

Three new moons were discovered. They're very small, and were named Methone, Pallene and Polydeuces. Later, another was discovered in a gap in the rings and called Daphnis. The only other known moon inside Saturn's ring system is called Pan. Recently, scientists discovered a storm at the south pole of Saturn with a distinct eyewall – a characteristic of a hurricane on Earth never before seen on another planet. Unlike a hurricane, the storm appears to be stationary at the pole. It is 5,000 miles across and 45 miles high, and is packing winds of 350mph.

Cassini: http://saturn.jpl.nasa.gov/home/index.cfm

Uranus

Gas giant Uranus has only been visited once, by Voyager 2 in 1986. It has methane and water clouds in its atmosphere, which seems to be the coldest in the solar system. There are no plans to go back.

www.nineplanets.org/uranus.html

Neptune

Voyager 2 was also the only spacecraft to reach Neptune, in 1989. The planet proved similar to Uranus but bluer, although Voyager found a huge dark spot in its southern hemisphere. However, recent observations from the Hubble Space Telescope show that it has vanished. Studies exist for more probes to Neptune, but there are no definite plans.

www.nineplanets.org/neptune.html

Pluto

Almost lost among the stars in this image sent back by the New Horizons spacecraft is tiny Pluto, which recently suffered the indignity of being relegated from its status as a planet and is now known as 134340 Pluto – the second-largest known dwarf planet in the solar system (after a more distant body called Eris), and the 10th-largest body observed directly orbiting the Sun.

The New Horizons image (below), taken from a distance of 6.7 billion miles, will be used to aid the spacecraft's navigation as it closes in on the unexplored planet for a mid-2015 flyby.

The mission very nearly didn't happen. In August 1992, Nasa scientist Robert Staehle telephoned Pluto's discoverer Clyde Tombaugh, requesting permission to visit his planet. "I told him he was welcome to it," Tombaugh later recalled, "though he's got to go one long, cold trip." But, in 2000, Nasa cancelled the mission, apparently for financial reasons.

After an intense political battle, a revised mission to Pluto, dubbed New Horizons, was launched in 2006. Project leader S Alan Stern later confirmed that some of the ashes of Clyde Tombaugh, who died in 1997, had been placed aboard the spacecraft.

New Horizons: http://pluto.jhuapl.edu/

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