Sky and Telescope - May 2018 - 11

MARS

Gardening on the
Red Planet
STUDENTS TAKING PART in the Red

Thumbs Mars Garden Project found
that, under certain conditions, Martian soil would grow nutritious plants
reasonably well, Edward Guinan (Villanova University) announced at the
American Astronomical Society meeting
in Washington, D.C.
Future Mars colonists will have to
grow their own food in order to survive.
But the Red Planet's surface receives less
than half the sunlight that Earth does,
as well as more ultraviolet radiation,
and its dirt is more iron-rich, particularly in iron oxides. (It's also rich in
poisonous perchlorates, which would

have to be removed before gardening.)
The students planted vegetable and
herb seedlings, such as lettuce, kale,
garlic, potatoes, and hops, in a simulant that's 90% like Martian soil. Then
they monitored the plants' growth in a
campus greenhouse, testing various soil
additives such as fertilizer.
Mixed greens such as lettuce and
kale did well, but potatoes did not. The
clay-like Martian simulant was so thick
that it crushed the growing taters. Most
plants fared better when the students
added ﬁller such as coffee grounds to
the Martian simulant, which ﬂuffed the
dirt up and allowed water to percolate
through to the roots.
■ CAMILLE M. CARLISLE

* To read the full story, visit https://

is.gd/marsgardening.

p Villanova students grew garlic, kale, sweet
potatoes, hops, lettuce, and other plants in
simulated Martian soil.

COSMOLOGY

Orderly Dwarf Galaxies Challenge Cosmological Wisdom

CEN TAURUS A: ESO; G A RDENING: VILL A NOVA UNIV ERSIT Y

IN THE FEBRUARY 2ND

Science, Oliver Müller
(University of Basel,
Switzerland) and colleagues report that satellite galaxies are moving
around the huge elliptical Centaurus A (NGC
5128) in an orderly
fashion. The ﬁnding
p Centaurus A
may challenge our
understanding of galaxy evolution.
Current cosmological theory predicts
that large galaxies should surround
themselves with a chaotic swarm of
dwarfs. Instead, out of the 16 Centaurus A satellites with known radial
velocities, 14 orbit in the same direction
along a broad plane that crosses perpendicular to the galaxy's famous dust
band. The result echoes similar ﬁndings
for our own Milky Way Galaxy, as well
as for the Andromeda Galaxy (M31).
Some theorists have proposed that
such order might come from gravitational interactions within our Local
Group of galaxies. But according to
Müller and his coauthors, the likelihood of ﬁnding just one example of

IN BRIEF

coordinated motion
among dwarf satellites
is smaller than 0.5% in
current cosmological
simulations. "Finding three such systems
in the nearby universe seems extremely
unlikely," they write.
However, as Eline
Tolstoy (University of
Groningen, The Netherlands) points
out, "It's still small number statistics."
Müller's team could only measure the
velocities of the brightest satellites,
which could skew the results. Moreover,
previous observations have shown that
Centaurus A has experienced a major
merger in its past, which might have
organized its satellites' orbits.
More complete observations of galaxies and their satellite systems can help
settle the issue, but the measurements
are difﬁcult to make. New data from
the European Space Agency's Gaia mission, to be released in late April, should
at least clarify the motions of dwarf
galaxies around the Milky Way.

New Definition for Biggest
Planets Proposed

■ GOVERT SCHILLING

■ SHANNON HALL

Scientists have long set the dividing line
between giant planets and brown dwarfs
at 13 times Jupiter's mass, the minimum
mass required to ignite deuterium fusion in an object's core. However, in the
January 20th Astrophysical Journal, Kevin
Schlaufman (Johns Hopkins University)
set a different upper boundary: between
4 and 10 times Jupiter's mass. The new
definition is based on how the objects
form: Giant planets grow from the bottom
up in a process called core accretion, so
they should form more easily in protoplanetary disks enriched with heavy elements.
Brown dwarfs grow from the top down,
collapsing directly from a cloud of gas,
which doesn't depend so much on heavy
elements. Indeed, Schlaufman found that,
in 146 carefully selected planetary systems, objects with masses less than 4 to
10 Jupiters tend to form around stars rich
with heavy elements, while objects with
more than 10 Jupiter masses form around
all kinds of stars. These results won't
reclassify any planets within our own solar
system, but they do have implications
for where we draw the line between giant
exoplanets and brown dwarfs. Read more
at https://is.gd/planetdeﬁnition.

s k y a n d t e l e s c o p e . c o m * M AY 2 0 1 8

11
http://www.is.gd/marsgardening http://www.is.gd/marsgardening https://www.is.gd/planetdefinition http://www.skyandtelescope.com
Sky and Telescope - May 2018

Table of Contents for the Digital Edition of Sky and Telescope - May 2018
