|
Two Caltech/JPL scientists presented the discoveries of NASA/JPL’s most famous recent space probes at Caltech’s 2018 Alumni Seminar Day.
Water On Mars
Bethany Ehlmann, Professor of Planetary Science, summarized what NASA has discovered about water on Mars from its many space probes, some in orbit above the Red Planet and others on its surface.
Current surface conditions on Mars are cold and dry — the average temperature is 210 K (–113ºF), and the atmospheric pressure is about 0.6% of Earth’s.
Ehlmann compared the Martian surface to Earth’s Antarctic Dry Valleys, where liquid water forms lakes for one month each year. Mars is like that, she said, but drier. (I’ve been to Antarctica — it’s incredibly dry, drier than the Sahara Desert.)
Ehlmann said there are places on the Martian surface where water can remain “stable”, in one form or another, for periods of several weeks or perhaps even one year. But any exposed water, liquid or ice, eventually evaporates or sublimates into the atmosphere.
Ehlmann believes that far below its surface, Mars has over 200 lakes of liquid water.
All this, and more, leads NASA to believe that oceans covered parts of Mars between 4.1 and 3.1 billion years ago, about the same age as the earliest evidence of life on Earth.
A vital difference between the Blue Planet and the Red Planet — a difference that may have fostered life here and doomed life there — is that Earth has a very strong global magnetic field that shields our atmosphere from the solar wind. Without that shield, Mars lost its atmosphere and its water billions of years ago.
DAWN on Ceres & Vesta
Carol Raymond, Deputy Principal Investigator, reported on DAWN, which NASA launched in September 2007 to explore the two oldest and largest bodies in the main asteroid belt between Mars and Jupiter: Vesta and Ceres.
Ceres has 33% and Vesta has 9% of the asteroid belt’s total mass, which isn’t much — less than 0.05% of Earth’s mass.
DAWN has a huge solar panel array that powers its ion thruster, ionizing Xenon gas and accelerating those ions backwards to propel DAWN forwards. Electrons are injected into the accelerated beam to keep everything neutral. Ion propulsion is highly efficient, but has very low thrust — DAWN can go from 0 to 60 mph in 4 days. It launched with conventional chemical rockets, and uses ion propulsion only in deep space.
DAWN arrived at Vesta in July 2011, and explored this rocky asteroid for 14 months. After leaving Vesta, DAWN reached dwarf planet Ceres in March 2015, where it is still collecting data in a slowly shrinking orbit, intending to come within 22 miles of the surface.
Vesta has two enormous craters near its south pole: 250 mile-wide Veneneia and 310 mile-wide Rheasilvia, whose central peak rises 16 miles, a bit smaller than Olympus Mons on Mars, the largest mountain in the solar system. Scientists believe the impacts that created these craters melted most if not all of Vesta, and blasted about 1% of its total mass into space. This debris accounts for 5% of all meteorites found on Earth.
Ceres, on the other hand, is both rock and ice. It has a rigid, 24-mile thick crust and a solid core. Sandwiched between is an icy, salty, mushy ocean. Large impactors, rather than creating large surface craters, puncture the crust and penetrate deep into the mush.
Ceres seems to have formed from material that originated in the outer solar system, and somehow migrated inward into the asteroid belt.
DAWN found bright spots on Ceres’ asphalt-black surface comprised of sodium carbonates and aluminum salts, which are also found only on Earth, Europa, and Enceladus. These spots are due to cryovolcanism — volcanoes that spew not lava but a variety of ices from a briny subsurface ocean. A prominent example is 10-million-year old Ahuna Mons, which stands 2.4 miles high.
DAWN detected organic compounds that would be useful for living organisms. Raymond believes Ceres has (or had): (1) liquid water, (2) an energy source, and (3) the chemicals life requires. She wonders whether there ever were any Ceresians.
)
Best Regards,
Robert
July 24, 2018
Note: Previous newsletters can be found on my website
|
