Just the FAQs

If you are looking for numerical data on Mars — precise length of day, orbital shape, etc. — NASA has a useful Fact Sheet. Similarly, NASA also keeps a list of successful Mars missions, with summaries of each mission, its instruments, and its findings. What follows here are brief descriptive answers to some common questions, with occasional links to more detailed information.

How big is Mars?
Mars has roughly half of Earth’s diameter. But its surface area is about the same as all the land areas on Earth.

How far is Mars from Earth?
It depends. Because both planets are in motion as they orbit the Sun, the distance between them is always changing. When Mars is closest to Earth, it’s about 56 million kilometers (35 million miles) away.  When farthest, the distance is about 400 million kilometers (250 million miles). The differences mean that the time it takes for a radio command from Earth to reach a spacecraft at Mars (or for the spacecraft to send a reply to Earth) varies from about 3 minutes to more than 22 minutes.

How strong is gravity on Mars?
38% as strong as on Earth, so if you weigh 100 lbs here, you weigh 38 lbs on Mars.

How long is a day on Mars?
From one noon to the next, 24 hours 39 minutes 35 seconds. Scientists call this period a sol (rhymes with all). Sols are divided into 24 hours; minutes and seconds are expressed as decimals of an hour.

How long is year on Mars?
687 Earth days (669 sols), or 1.9 Earth years. Mars orbits about 1.5 times farther from the Sun than Earth.

Does Mars have seasons?
Yes, because its spin axis tilts 25° to its orbit around the Sun. (By coincidence, Earth’s tilts at about 23°.) However, because the Martian year is nearly twice as long as ours, the seasons there are similarly longer than those here.

The position of Mars in its orbit (or alternatively its season) is shown as its solar longitude, Ls 0° to 360°. (NASA/JPL)

What date is it on Mars?
The answer to this simple-looking question is more complicated than you’d expect! Because nobody lives on Mars (for now), Mars has no calendar like those we use on Earth. However, for the purpose of tracking long-term changes in climate, weather, and other variables, scientists created a running chronology of Mars Years (abbreviated as MY).  The counting started on April 11, 1955, when Mars Year 1 began. Mars Year 32 began August 1, 2013 and runs until June 18, 2015. (A useful conversion page is here.) Mars Years are divided into Mars months, but these vary in length because they are not counted by sols, but rather in terms of the planet’s annual orbit around the Sun. Each Mars month lasts for 30° of orbital solar longitude, so they number twelve in all, just as on Earth (although they are named only by number). Solar orbital longitude is given in degrees and abbreviated Ls (say “ell-sub-ess”). It’s measured from the start of spring in the northern hemisphere, shown as Ls=0°. However, because the Martian orbit is elliptical, the planet’s orbital speed varies continually throughout the year, and this varies the number of sols in each month. The longest (month 3) lasts 66.7 sols, while the shortest (month 9) lasts 46.1 sols.

How hot is it on Mars?
Ground temperatures range from 27 °C (about 81 °F) at the equator to −87 °C (−125 °F) at the polar caps. Because of Mars’ greater distance from the Sun, sunlight is only about about half as strong as at Earth. That plus the thin atmosphere (see next question) produce temperatures much colder on average than on Earth.

Would I need a spacesuit there?
Yes. The air is almost entirely (95%) carbon dioxide, making it unbreathable, and it’s also very thin, less than 1% as much air as on Earth. To get air as thin as the surface of Mars, you’d need to go up to an altitude of about 39,000 meters (130,000 feet) in our atmosphere.

How old is Mars?
Mars formed at the same time as Earth and the solar system, about 4.56 billion years ago. Scientists have divided Mars’ geologic history into several periods, each named for an area where rocks of that age are best shown. The periods are (from oldest to youngest): the Noachian, the Hesperian, and the Amazonian. (The period between the formation of Mars and the start of the Noachian is called the pre-Noachian.) The Noachian began about 4.1 billion years ago and lasted until 3.7 billion years ago. The Hesperian started then and continued until roughly 3.0 billion years ago. The Amazonian includes everything since the Hesperian ended — in other words, the Amazonian is still going on now. One way to keep these straight in your mind is remember that as you go back in time, they line up in alphabetical order: Amazonian, Hesperian, Noachian.

How old is that crater?
Scientists can date the surface on any moon or planet by counting how many impact craters they see in a given area. The more craters, the older the surface. But while that’s useful for comparing the relative ages of two or more areas, to find actual ages in years scientists need to bring rock samples back to Earth for study in a lab to measure the rocks’ natural radioactivity. Because radioactive elements decay at known rates, the ratios of various elements in a rock can show how many thousands or (more often) millions of years have passed since the rock was last melted or shocked by a meteorite impact. You can find out more about radiometric dating here.

Why does the northern half of Mars have so few craters?
The southern half of Mars shows more craters than the northern half because craters in the north were obliterated or buried by later sediments. Another difference between the two halves is that the northern plains lie on average about 5,000 meters (16,000 feet) lower than the southern highlands. A third difference is that the northern crust is only about half as thick as the crust in the south. Scientists call the edge between these two regions the crustal dichotomy. What caused the dichotomy has been actively debated ever since it was discovered in the 1970s, and the question remains unsolved. Most theories involve either one or more big impacts or currents of molten rock deep inside Mars. For more on the dichotomy question, see this page.

Does Mars have plate tectonics?
Not now, although it may have early in its history. The strongest evidence lies in striped patterns of magnetism in parts of the southern highlands. These resemble similar patterns on Earth’s sea floors that do result from plate tectonic motions.

Is there life on Mars?
This is one of the greatest scientific questions ever, but there’s no definitive answer. So far no spaceprobe has found evidence of any kind of life on Mars. However both Earth and Mars experienced similar conditions early in their history, and we know that life began here, so life may have begun on Mars also. If life did start there, then it may have died out if conditions became too harsh. Or life may still exist there, perhaps buried below the surface where conditions may be less hostile. We simply need to explore Mars much more completely to have any chance of answering this question.

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