[meteorite-list] Forgotten Moons: Phobos and Deimos Eat Mars' Celebrity Dust

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:39 2004
Message-ID: <200103131808.KAA14107_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/forgotten_moons_010313.html

Forgotten Moons: Phobos and Deimos Eat Mars' Celebrity Dust

By Robert Roy Britt
space.com
13 March 2001

With the recent surprise landing of a robotic probe on Asteroid 433 Eros and
missions scheduled regularly to focus on Mars, some say it's time to dust
off plans to send a spacecraft to the Red Planet's two mysterious moons.

Phobos and Deimos, shadowed by the Red Planet's celebrity status, have a
spotty history of exploration.

A 1988 unmanned Soviet mission to Phobos disappeared inexplicably. And a
robotic venture proposed to NASA, called Aladdin, did not make the cut in
1999 and is shelved for now. Two previous Mars missions -- the Viking
orbiter and Mars Global Surveyor -- went out of their way to obtain pictures
of Phobos and Deimos.

But NASA's Mars Odyssey, scheduled to launch in April, will give the moons
nary a wink or nod.

Researchers say that after back-to-back failed missions to Mars, pressure is
understandably intense to keep Odyssey focused on the primary objective.
NASA cannot afford to lose another spacecraft, especially on some scientific
side trip.

But those who study Phobos and Deimos see them as destinations, not detours.

"There is no object in the solar system that is not worth studying, but
those moons suffer from being so close to such an interesting planet, and
they've always taken a back seat," said Philip Christensen of Arizona State
University, who has used Mars Global Surveyor images to make limited studies
of Phobos.

Unsolved mysteries

In the 24 years since the Viking orbiter returned the first closeups of the
moons, revealing them to be odd shaped lumpy objects, scientists have
learned almost nothing about them, Christensen laments. Despite his own
research and efforts of others, "we're not progressing," he said.

Scientists still don't know if the moons were created along with the birth
of Mars, or if they are asteroids that were captured later. Their
composition remains unknown. Mars Global Surveyor passed within 200 miles
(320 kilometers) of Phobos, and showed that the moon had been pounded to
powder by countless collisions with smaller space rocks. But how, with
virtually no gravity, does Phobos hold on to this dusty debris?

Answers to these questions and others would improve basic models of the
formation and evolution of planets and of our solar system. And others want
to know if the moons are rich in valuable metals or substances that could be
used for spacecraft propulsion.

To get these answers, researchers say we need to chip away at the rocks.

"Asteroids and small moons are tough to study remotely," Christensen told
SPACE.com, "because they're so churned up and so bombarded by everything
from the solar wind to micrometeorites to large objects that slam into them.
And it's not clear what the outer meter or so of the surface is telling us
about the interior."

Make Deimos a Space Port

While Philip Christensen would be thrilled with almost any mission to Phobos
or Deimos, another long-time space innovator has specific plans for the
little moons. Fred Singer, president of the Science & Environmental Policy
Project, has argued for the past two years for a human mission to one of the
Martian satellites.

"They are neglected," Singer said in a telephone interview. "People have
talked only about Mars. To me, these moons are interesting in their own
right. That's not to say Mars is not interesting, but I think the moons
deserve equal consideration."

But who wants to be the first astronaut to say, "I landed on Phobos!"?

Actually, Singer contends that the best mission to Mars starts at Deimos,
the outermost of the two satellites. Take a travelling research vessel
stuffed with a half dozen humans and strap it to Deimos for six months, he
suggests. Then use the moon -- an odd-shaped object with an average diameter
of 8 miles (13 kilometers) that orbits 12,400 miles (20,000 kilometers) from
Mars -- as a shield against cosmic rays and solar storms.

Call it a space rock dock.

>From the relatively safe perch high above the Red Planet, the crew would
send robotic probes to the surface of Mars and control them in real-time
(whereas the distance between Earth and Mars creates a long communications
delay). And instead of enduring months of red dust on the harsh Martian
surface, a small team could make a quick weekend sortie to the surface to do
those things only humans can do.

Like making that first footprint in red dust.

But why dock at Deimos, instead of going straight to Mars?

"It's cheaper. It can be done faster, sooner," said Singer, who is also a
researcher at George Mason University and whose study of the orbit and
origin of Phobos and Deimos dates back to the 1960s. "It's safer, much
safer," he continues. "And you can get more information about Mars by
exploring it from Deimos than you can by landing on the surface."

For one thing, the scheme would avoid the need to build a costly Mars base,
required to shield the crew from deadly radiation. And sidling a large
spacecraft up to a low-gravity moon is a lot less costly than setting down
on Mars, a maneuver that requires a lot more fuel, adding costly weight to a
mission.

And Singer says that from Deimos, scientists could deploy a series of
robotic probes to the Martian surface just as easily as they could from
Mars. Each probe's destination and mission would be determined by what was
learned from previous probes.

Lesson from NEAR

The NEAR spacecraft landing on Asteroid Eros in February illustrates the
potential of a mission to Phobos and Deimos, Singer says.

But NEAR was just a robot. Isn't sending humans an incredible undertaking?

"Phobos and Deimos are much easier to get to than Eros, and much easier to
land on," Singer said, "so it is a very viable proposition."

A landing would be aided by the fact that the moons are locked into stable
orbits and rotations. Eros, on the other hand, was tumbling through space
less predictably and made a tricky target.

Meanwhile, the crew could do some serious science on Deimos itself. Singer
points out that Phobos and Deimos are similar to Eros in size and
appearance. "One wonders if they have any connection," he said.

And now that there is some data on Eros' composition, thanks to NEAR, a
comparison could be made. If the objects are similar, it would be strong
evidence that Phobos and Deimos are in fact asteroids that were snagged into
orbit by Mars' gravitational tug.

Nothing like being out there

Christensen, a geologist by training, says putting humans on Deimos makes
sense, and scientists could "learn a lot" about Mars by studying the Red
Planet from above.

Chipping away at Deimos wouldn't be a bad thing, either.

"We may never really know the story of these moons, and what they're made of
and what their interior is like, until we go and land on them," Christensen
said. "There's nothing like being out there with a hammer and a hand lens
and doing real geology, being able to pick up the rock and look at it."

Cool Phobos Facts and Images

Phobos orbits just 3,700 miles (6,000 kilometers) from Mars, on average. No
moon in our solar system is closer to its host planet. And it is getting
closer.

The 14-mile- (23-kilometer-) wide moon is snuggling up to Mars at a rate of
6 feet (1.8 meters) every 100 years. In about 50 million years, the moon
will crash into Mars. Or, scientists say, the gravitational force of the Red
Planet might tear the moon apart. If that happens, the shreds of Phobos
could form a ring around Mars, similar to the rings around Saturn.

An impact long ago left a big crater on Phobos -- 6 miles (10 kilometers)
wide -- called Stickney. Streaks of material ejected by the impact can be
seen across the moon's surface. Stickney appears to be filled with dust, and
boulders look to be sliding down its sloped surface.

Like Mars' other moon, Deimos, Phobos has a thick layer of regolith, or dust
and rock. It is thought to be especially thick on Phobos -- up to 330 feet
(100 meters). Researchers think the regolith was created when other space
rocks slammed into the moon, pounding it into powder. But scientists are
stumped as to how the material stuck to an object that has almost no
gravity.

Scientists say that when the Sun shines on Phobos, the temperature is not
unlike a pleasant winter day in Chicago. But just around the corner, on the
dark side of the tiny moon, the thermometer can plunge to an extreme Arctic
minus 170 degrees Fahrenheit (minus 112 Celsius).

>From Mars, Phobos would appear about one-third as big as Earth's moon. But
since it isn't round, lovers on Mars would stand hand-in-hand, staring up at
a shiny potato. More about Phobos in our Reference Section.

Cool Deimos Facts and Images

Deimos is the smaller and more distant of the two Martian moons, with an
average diameter of 8 miles (13 kilometers) and an orbit that is 12,400
miles (20,000 kilometers) away, on average. On the surface of Deimos, the
acceleration of gravity is less than 0.1 percent that of Earth. But like
Phobos, Deimos has been able to develop landforms, like craters and rims,
similar to those found on larger objects.

Deimos' craters lack the grooves and ridges seen on Phobos. Typically when a
space rock hits another object, surface material is thrown up and out of the
resulting crater, scientists say. The material usually falls back to the
surface surrounding the crater.

However, these "ejecta deposits" are not seen on Deimos, while they do
appear on Phobos. Researchers speculate that Deimos' gravity is so low that
the ejecta escaped to space. But still there seems to be that coating of
dust.

Of the two Martian moons, Deimos has a much smoother surface. This may be
because the deposit of dust and other fine material continued after most of
the moons impact craters were formed.

>From Mars, Deimos is so far away and so small that it would appear as no
more than a very bright star in the night Martian sky.
Received on Tue 13 Mar 2001 01:08:43 PM PST