We may have to go no farther than the next planet out to find out if life exists or ever existed-beyond Earth. Mars has seemingly always been the subject of fascination, from ancient times, when it was seen as a cosmic incarnation of the Roman god of war, to the twentieth century, when many imagined the planet to be the abode of Percival Lowell’s desperate canal builders.

Mars is a small planet, about half the diameter of Earth and only about 15 percent of its volume. For further reference, the surface area of Mars is about the same as the surface area of all of the continents on Earth. On average,the planet orbits about 50 percent farther from the Sun than we do. The thin Martian carbon dioxide atmosphere (only 1 percent as thick as Earth’s) can’t trap much heat, so the surface is very cold.

Daytime temperatures near the equator rarely rise above the freezing point of water, and nights near the poles routinely drop down to the freezing point of carbon dioxide (which is 150 kelvins, or about -190°F). Today Mars is a dusty world in a deep freeze.

And yet, spacecraft images, meteorites from Mars, and other data over nearly 50 years have shown that Mars is the most Earthlike place in the solar system (besides Earth itself), and that during its first few billion years, the Red Planet may have been a much warmer and wetter world.
What happened? Possibilities include gradual cooling of the planet’s core and solar wind or catastrophic impact destruction of the atmosphere. Determining how and why the planet’s climate changed so dramatically is a hot topic of research.

We’ve learned enough about the Mars of 3 or 4 billion years ago to know that parts of the surface and subsurface were habitable to life.
The next 50 years of Mars exploration will be all about expanding the search for habitable environments there and finding out if any were-or still are-inhabited.



Hydrogen is the lightest of all the elements. Each of its atoms has just one electron moving around one proton. Hydrogen accounts for about three-fourths of the mass of all the atoms we can see in the universe. On Earth, it is one of the elements in water (H2O), but as a gas it is only found in very small traces in our atmosphere. Since hydrogen burns easily in air and creates almost no pollution, it could become a fuel of the future.

Hydrogen is about 14 times lighter than air, so a balloon filled with hydrogen floats upward like a bubble in the heavier air around it. Many years ago, hydrogen was used in huge balloons and airships, lifting them up into the air. Since hydrogen catches fire easily when mixed with air, and even explodes, airships today use safer gases.

Hydrogen burns by combining with oxygen to produce water. A spacecraft’s rocket uses liquid hydrogen, along with liquid oxygen, as a fuel. These two elements are mixed in the spacecraft’s main rocket engines and then ignited.

As soon as hydrogen burns, it explodes, generating a thrust (a push with force) that lifts the spacecraft into space.

hydrogen could one day replace gasoline and diesel as a vehicle fuel. It can be combined with oxygen from air inside a fuel cell, generating electricity for an electric motor. Hydrogen-fueled cars produce only water in their exhaust.

In the 1920s and 1930s, gigantic hydrogen-filled airships carried passengers across continents and oceans. Then, in 1937, the hydrogen in the German airship Hindenburg caught fire, probably from a stray spark. The massive fireball and explosion killed 36 people. Since then, airships have mostly used other, less flammable gases.