If aliens took a low-budget tour of our galaxy (19,362 planets in 3 days!) and only had time to visit one person on Earth, what would they think we were like? If they visited your grandmother, they might conclude people were wise and kind, but not very strong (Why do humans put their medicine in bottles they can't open?). If they saw someone's year-old sister, they might think people were curious and full of energy, but not capable of speech (at least not anything that made much sense). If they encountered someone's older brother, they might think all our time and energy is spent on something called "basketball."
Aliens really need to visit more than one person to get a good idea of what we're like. The same applies to us as we explore other planets. That's why NASA is so interested in understanding the Earth and comparing it to Mars and other worlds in our solar system as part of the new Vision for Space Exploration.
If Martians existed, they may be green -- with envy of Earth. From space, Earth is a blue-white gem on black velvet, with abundant liquid water. Closer inspection of this gem finds it bursting with life, much of it microbial!
About half the size of Earth, Mars is reddish brown (officially "ochre"), revealing a planet-wide desert beneath wisps of water ice clouds. Its thin atmosphere is frigid, and most of the planet's water is locked in ice or mixed with frozen ground. With no known liquid water and no protection from the Sun's ultraviolet radiation due to a thin atmosphere without an ozone layer, the Martian surface is believed to be inhospitable to life as we know it. But who really knows? We are learning of life's limits here on Earth today and there have been MANY surprises. It turns out that our home planet is a microbial universe, with simple bacteria living happily within rocks.
On a galactic tour, Mars would be easy to pass by, except a closer look reveals whispers of long-forgotten Earth features in the frigid deserts, such as those resembling dried-up riverbeds and deltas. That indicates Mars was once much warmer and wetter, perhaps because its atmosphere was thicker billions of years ago. But did Mars once have life? The key Martian mystery is how the Martian paradise was lost or hidden. What happened to the air and water, and why did Mars go into a deep freeze, or did it? Indeed, today we know Mars experiences dramatic climate changes, much more intense than those we know of here on Earth.
Earth has also gone through dramatic climate changes, as evidence for a series of ice ages indicates. By carefully studying how the climate functions on both worlds, we can understand why some worlds remain havens for life, and others appear to have dried up and frozen over.
A close look at Earth gives clues to the Martian mystery. If aliens visited special dry valleys in Antarctica, they would see cold and dry deserts like those on Mars. If they plunged from space down to a remarkable region in Antarctica, called the Don Juan Pond, they could visit one of the saltiest, coldest bodies of water on Earth. For the most part, the pond remains liquid despite temperatures below the normal freezing point of water because of its high salt content. This pond is of particular interest because it represents one possibility for the kinds of extreme conditions where life might exist elsewhere in the solar system: it's chemically stressful due to the high concentrations of salt and minerals and the extreme cold. If life is able to survive there, it might exist in similar places elsewhere.
Evidence for relatively recent volcanic activity on Mars indicates that its interior is still warm from the heat of its formation and from the processes that separate a planetary interior into fundamental layers (i.e., crust, mantle, core). If this is so, it may be warm enough for liquid water to exist at different depths beneath the Martian surface. On Earth, scientists have discovered bacteria in rock miles deep. If microscopic life ever existed on the Martian surface, it might have retreated below as the planet's surface froze, and might still exist in salty reservoirs (aquifers) beneath shifting Martian drifts of dust and talcum-powder-like sand.
Ice talks. Tiny air bubbles trapped in glaciers and polar caps are pristine samples of ancient atmospheres, and as ice ebbs and flows over the landscape, it writes a record of climate change. Both Earth and Mars have polar caps, and detailed study of these areas will allow us to compare how the climate evolved and varied on these worlds.
Earth has an invisible shield that protects it from space radiation and the solar wind, a thin, high-speed stream of electrified gas that blows constantly from the Sun. This shield is the Earth's internal magnetic field. Mars does not have a large-scale, internal magnetic field like Earth, but there is evidence, in the form of magnetized Martian rocks, that it had one long ago. We know that the solar wind can erode away an unprotected planet's atmosphere over billions of years, if the atmosphere is not continually replaced by frequent volcanic activity and other photochemical processes. Could this be one reason why the Martian atmosphere is so thin today? We need to explore both planets in more detail to be sure. Furthermore, giant impact events can also erode planetary atmospheres over time.
Earth and Mars had violent births; their own gravity allowed them to grow by pulling in surrounding rubble and asteroids from the nascent solar system, which slammed into their surfaces and excavated craters the size of small continents. The Martian deserts of today are pockmarked with craters, scars from billions of years of cosmic collisions similar to those mostly hidden and obscured on present-day Earth.
Some Martian craters reveal a vast number of layers in the Martian crust, thus offering a rare glimpse into the history and structure of the planet's uppermost layers. In other words, it's a cross section of the planet's skin, and therefore extremely valuable for researchers to discover many key aspects about the planet's history of deposition and erosion, processes often profoundly affected by the presence of liquid water.
Layering is often associated with deposition by water here on Earth and the layers within these craters on Mars may provide clues to a time in the Martian past when water-lain sediments filled up such craters and perhaps formed shallow salty seas or even small oceans. Retreating seas such as the Aral Sea are excellent examples here on Earth of environments similar to how the Mars of the past may have operated.
But craters all over the planet are important to Mars scientists. The record of impacts found near the poles suggests that there may be a buried icescape covering much of the planet as evidenced by erosion and ejecta patterns that do not seem to behave in ways we would expect from ordinary rocky planetary crusts.
Since all the worlds in the solar system formed the same way, you would expect to see many impact craters on Earth, but obvious impact craters are relatively rare. This is because Earth is so alive "geologically"; water erosion, wind erosion, and shifting landscapes from plate tectonics and volcanism erase craters before they are very old. Only sophisticated techniques, including those used by remote sensing instruments on board NASA satellites, can help pick them out. Exploring fresh craters on Earth gives scientists confidence that their interpretation of impact features on alien worlds is correct. Remote sensing has been used here on Earth to target field "geophysical" surveys, including drilling, to understand the collision history of our own planet and to characterize the potential threat from asteroid and comet collisions. In addition, the incredible energies associated with impacts into frozen crusts (on Earth and Mars) can clearly result in temporary bodies of water, melted from the collision.
Is Mars "dead", or is it only sleeping? There are tantalizing hints that Mars might have brief returns to the warm and wet condition of its youth in geologically recent times. Could this be is the result of occasional volcanic activity that temporarily thickens the atmosphere? Or perhaps due to release of subsurface waters from aquifers at times when the Martian climate is more hospitable to liquid water? Even if this does not happen naturally, humans might be able to awaken Mars locally, as they venture there to search for clues to past or present Martian life.
If the Martian were atmosphere slightly thicker, perhaps as a consequence of a comet impacting the surface and releasing its gases and water vapor, Mars may temporarily warm up enough to allow for liquid water to be stable. The slightly thicker atmosphere would act like a blanket, trapping more of the Sun's heat and allowing frozen Martian water and carbon dioxide to evaporate or sublimate. This will make the atmosphere even denser, which would allow it to become warmer still in a self-reinforcing cycle. Similar cycles probably have occurred due to climate changes caused by the natural variation in the tilt of the Martian axis of rotation.
Imagine a time when Martian climatic conditions allowed for it to rain on a parched Martian plain. Imagine desert cliffs, which have known only an icy, thin wind, echoing with the pitter-patter of rain, forming shallow salty seas... perhaps reawakening long dormant habitats. Imagine human explorers on Mars uncovering such past records themselves, ultimately seeking fossil evidence of ancient Martian life, just as the great archeologists of Earth came upon the civilizations of the past?
This is exploration at its most thrilling; understanding other worlds that may have been habitable for life, and it is part of the NASA vision of searching for life to the Universe. However, to make this a reality, we need a detailed understanding of how hospitable planetary environments and climates operate. There is only one example to learn from our own Earth.
While NASA prepares to deploy a new family of probes around the fourth planet, it's exciting to consider that it already maintains a powerful and diverse fleet around the third planet. NASA's Earth Observing fleet of vehicles constitutes a major milestone in the history of Earth science, facilitating kinds of wide-scale and synergistic research endeavors that until the last decade have been impossible to even consider.