For millennia, people on Earth have watched comets in the sky. Many
ancient cultures saw comets as the harbingers of doom, but today
scientists know that comets are really frozen balls of dust, gas, and
rock and may have been responsible for delivering water to planets like
Earth billions of years ago.
While comets are inherently interesting, they can also provide information about other aspects of our Solar System.
More specifically, comets can be used as laboratories to study the
behavior of the stream of particles flowing away from the Sun, known as
the solar wind.
Recently, astronomers announced the results of a study using data collected with NASA's Chandra X-ray Observatory of two comets -- C/2012 S1 (also known as "Comet ISON") and C/2011 S4 ("Comet PanSTARRS").
Chandra observed these two comets in 2013 when both were relatively
close to Earth, about 90 million and 130 million miles for Comets ISON
and PanSTARRS respectively. These comets arrived in the inner Solar
System after a long journey from the Oort cloud, an enormous cloud of
icy bodies that extends far beyond Pluto's orbit.
The graphics show the two comets in optical images taken by an
astrophotographer, Damian Peach, from the ground during the comets'
close approach to the sun that have been combined with data from the
Digitized Sky Survey to give a larger field of view. (The greenish hue
of Comet ISON is attributed to particular gases such as cyanogen, a gas
containing carbon and nitrogen, escaping from the comet's nucleus.)
The insets show the X-rays detected by Chandra from each comet. The different shapes of the X-ray emission (purple) from the two comets indicate differences in the solar wind
at the times of observation and the atmospheres of each comet. Comet
ISON, on one hand, shows a well-developed, parabolic shape, which
indicates that the comet had a dense gaseous atmosphere. On the other
hand, Comet PanSTARRS has a more diffuse X-ray haze, revealing an
atmosphere with less gas and more dust.
Scientists have determined that comets produce X-ray emission when
particles in the solar wind strike the atmosphere of the comet. Although
most of the particles in the solar wind are hydrogen and helium atoms,
the observed X-ray emission is from "heavy" atoms (that is, elements
heavier than hydrogen and helium, such as carbon and oxygen). These
atoms, which have had most of their electrons stripped away, collide
with neutral atoms in the comet's atmosphere. In a process called "charge exchange,"
an electron is exchanged between one of these neutral atoms, usually
hydrogen, and a heavy atom in the solar wind. After such a collision, an
X-ray is emitted as the captured electron moves into a tighter orbit.
The Chandra data allowed scientists to estimate the amount of carbon
and nitrogen in the solar wind, finding values that agree with those
derived independently using other instruments such as NASA's Advanced
Composition Explorer (ACE). New measurements of the amount of neon in
the solar wind were also obtained.
The detailed model developed to analyze the Chandra data on comets
ISON and PanSTARRS demonstrates the value of X-ray observations for
deriving the composition of the solar wind. The same techniques can be
used, together with Chandra data, to investigate interactions of the
solar wind with other comets, planets, and the interstellar gas.
A paper describing these results appeared in the February 20th, 2016 issue of The Astrophysical Journal and is available online.
The authors are Bradford Snios and Vasili Kharchenko (University of
Connecticut), Carey Lisse (Johns Hopkins University), Scott Wolk
(Harvard-Smithsonian Center for Astrophysics), Konrad Dennerl (Max
Planck Institute for Extraterrestrial Physics) and Michael Combi
(University of Michigan).
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for NASA's Science Mission Directorate in NASA’s Chandra X-ray Observatory,
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.
Fast Facts for ISON:
Scale: Main image is about 40 arcmin across. (X-ray image: 3.7 arcmin)
Category: Solar System
Observation Dates: 13 Oct - 6 Nov 2013
Observation Time: 10 hours
Obs. IDs: 15673-15675, 16493-16495
Instrument: ACIS
References: Snios, B. et al, 2016, ApJ, 818, 199; arXiv:1601.06622
Color Code: X-ray (Purple); Optical (Red, Green, Blue)
Distance Estimate: 0.95 Astronomical Units (AU)
Source: NASA’s Chandra X-ray Observatory
