When Earth formed some four and a half billion years ago, it initially had no moon, just as both Venus and Mercury are moonless. Not long after its formation, Earth was struck by a Mars-sized object. That collision knocked material off our young planet that quickly coalesced into the Moon. The collision caused Earth to spin quite rapidly, so fast that a day took only about 4 hours. The Moon was also much closer to Earth, less than a tenth of its current distance.
The Moon’s gravity created a large
tidal force on Earth, slowing its spin a bit each year until our day is now 24
hours long. As Earth’s day lengthened, the Moon moved farther away. Good for
the Moon. Had it remained that close, Earth’s gravitational tug on the Moon
would pull it toward our planet. At some point before it collided with Earth, the
tidal force on the Moon would have ripped it apart creating a ring of debris surrounding
our planet.
Mars possesses two tiny moons. Phobos
is only 14 miles across. Deimos is even smaller, a mere 8 miles. Both are
likely captured asteroids.
Phobos orbits Mars from a distance
of only 3,700 miles, less than half the diameter of Earth. It is the closest
known moon to its planet. It’s so close, Mars’ gravity is slowly pulling it even
closer. When it gets too close to Mars, the gravity of the Red Planet will rip
it apart.
Phobos is already shown signs of
this stress. Close-up photos by NASA probes show parallel grooves running
across the entire moon. When the Mars Reconnaissance Rover first photographed
Phobos and astronomers saw the grooves, they initially thought they were caused
by the impact of the large meteor that created Stickney Crater. The collision
was so powerful, it nearly shattered the small moon.
Stickney Crater and grooves on Phobos. Credit NASA and JPL.
Astronomers now realize that these
grooves are caused by the gravity from Mars creating a strain on Phobos so
strong it is having trouble holding itself together. Before long, in astronomical
terms, perhaps a few million years, Phobos will shatter, forming a ring around
Mars.
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NASA’s Artemis program plans to
send astronauts to the Moon by the middle of this decade. But that’s not the
end purpose of the Artemis program. Its ultimate goal is to land humans on
Mars. To that end, NASA has constructed a simulated Mars Habitat at the Johnson
Space Center in Houston. Later this year, four people will enter the simulator
and be locked in for a year. This is the first of three crews to stay in the
Mars habitat. These will simulate life on Mars, giving NASA valuable data for
when crews leave for the Red Planet.
Each month, I write an astronomy-related column piece for
the Oklahoman newspaper. After it is published there, I post that same column to my blog page.
This is reprinted by permission from the Oklahoman and www.Oklahoman.com.
